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1. Midplane neutral density profiles in the National Spherical Torus Experiment

Author(s):
Stotler, D.; F. Scotti; R.E. Bell; A. Diallo; B.P. LeBlanc; M. Podesta; A.L. Roquemore; P.W. Ross
Abstract:
Atomic and molecular density data in the outer midplane of NSTX [Ono et al., Nucl. Fusion 40, 557 (2000)] are inferred from tangential camera data via a forward modeling procedure using the DEGAS 2 Monte Carlo neutral transport code. The observed Balmer-b light emission data from 17 shots during the 2010 NSTX campaign display no obvious trends with discharge parameters such as the divertor Balmer-a emission level or edge deuterium ion density. Simulations of 12 time slices in 7 of these discharges produce molecular densities near the vacuum vessel wall of 2–8 􏰁 10^17 m􏰈3 and atomic densities ranging from 1 to 7 􏰁 10^16 m􏰈3; neither has a clear correlation with other parameters. Validation of the technique, begun in an earlier publication, is continued with an assessment of the sensitivity of the simulated camera image and neutral densities to uncertainties in the data input to the model. The simulated camera image is sensitive to the plasma profiles and virtually nothing else. The neutral densities at the vessel wall depend most strongly on the spatial distribution of the source; simulations with a localized neutral source yield densities within a factor of two of the baseline, uniform source, case. The uncertainties in the neutral densities associated with other model inputs and assumptions are􏰅 50%.
Type:
Dataset
Issue Date:
August 2015

2. Observation of quasi-coherent edge fluctuations in Ohmic plasmas on NSTX

Author(s):
Banerjee, S.; A. Diallo; S.J. Zweben
Abstract:
A quasi-coherent mode with frequency f = 40 kHz is observed in Ohmic plasmas in NSTX with the gas puff imaging diagnostic (GPI). This mode is located predominantly just inside the separatrix, with a maximum fluctuation amplitude similar to that of the broadband turbulence in the same frequency range. The quasi-coherent mode has a poloidal wavelength 16 cm and a poloidal velocity 49 km/s in the electron diamagnetic direction, which are similar to the characteristics expected from a linear drift-wave like mode in the edge.
Type:
Dataset
Issue Date:
April 2016

3. Modeling of resistive plasma response in toroidal geometry using an asymptotic matching approach

Author(s):
Z. R. Wang; A. H. Glasser; D. Brennan; Y. Q. Liu; J-K. Park
Abstract:
The method of solving linear resistive plasma response, based on the asymptotic matching approach, is developed for full toroidal tokamaks by upgrading the Resistive DCON code [A.H. Glasser, Z.R. Wang and J.-K. Park, Physics of Plasmas, \textbf{23}, 112506 (2016)]. The derived matching matrix, asymptotically matching the outer and inner regions, indicates that the applied three dimension (3-D) magnetic perturbations contribute additional small solutions at each resonant surface due to the toroidal coupling of poloidal modes. In contrast, the resonant harmonic only affects the corresponding resonant surface in the cylindrical plasma. Since the solution of ideal outer region is critical to the asymptotic matching and is challenging to be solved in the toroidal geometry due to the singular power series solution at the resonant surfaces, systematic verification of the outer region $\Delta^\prime$ matrix is made by reproducing the well known analytical $\Delta^{\prime}$ result in [H.P. Furth, P.H. Rutherford and H. Selberg, The Physics of Fluids, \textbf{16}, 1054-1063 (1073)] as well as by making a quantitative benchmark with the PEST3 code [A. Pletzer and R.L. Dewar, J. Plasma Physics, \textbf{45}, 427-451 (1991)]. Finally, the reconstructed numerical solution of resistive plasma response from the toroidal matching matrix is presented. Comparing with the ideal plasma response, the global structure of the response can be affected by the small finite island at the resonant surfaces.
Type:
Dataset
Issue Date:
October 2020

4. Lower Hybrid Drift Waves During Guide Field Reconnection

Author(s):
Yoo, Jongsoo; Jeong-Young, Ji; M. V., Ambat; Shan, Wang; Hantao, Ji; Jenson, Lo; Bowen, Li; Yang, Ren; J., Jara-Almonte; William, Fox; Masaaki, Yamada; Andrew, Alt; Aaron, Goodman
Abstract:
Digital data for figures used in Lower Hybrid Drift Waves During Guide Field Reconnection, Geophysical Research Letters, 47, e2020GL087192, 2020.
Type:
Dataset
Issue Date:
29 October 2020

5. Implementation of higher-order velocity mapping between marker particles and grid in the particle-in-cell code XGC

Author(s):
Mollen Albert; Adams Mark F.; Knepley Matthew G.; Hager Robert; Chang C. S.
Abstract:
The global total-f gyrokinetic particle-in-cell code XGC, used to study transport in magnetic fusion plasmas or to couple with a core gyrokinetic code while functioning as an edge gyrokinetic code, implements a 5-dimensional (5D) continuum grid to perform the dissipative operations, such as plasma collisions, or to exchange the particle distribution function information with a core code. To transfer the distribution function between marker particles and a rectangular 2D velocity-space grid, XGC employs a bilinear mapping. The conservation of particle density and momentum is accurate enough in this bilinear operation, but the error in the particle energy conservation can become undesirably large and cause non-negligible numerical heating in a steep edge pedestal. In the present work we update XGC to use a novel mapping technique, based on the calculation of a pseudo-inverse, to exactly preserve moments up to the order of the discretization space. We describe the details of the implementation and we demonstrate the reduced interpolation error for a tokamak test plasma by using 1st- and 2nd-order elements with the pseudo-inverse method and comparing to the bilinear mapping.
Type:
Dataset
Issue Date:
March 2021

7. M3D-C1 simulations of the plasma response to RMPs in NSTX-U single-null and snowflake divertor configurations

Author(s):
Canal, G.P.; Ferraro, N.M.; Evans, T.E.; Osborne, T.H.; Menard, J.E.; Ahn, J.-W.; Maingi, R.; Wingen, A.; Ciro, D.; Frerichs, H.; Schmitz, O.; Soukhanovskii, V.; Waters, I.
Abstract:
Non-axisymmetric control coils and the so-called snowflake divertor configuration are two potential solutions proposed to solve two separate outstanding issues on the path towards self-sustained burning plasma operations, namely the transient energy bursts caused by edge localized modes and the steady state heat exhaust problem. In a reactor, these two proposed solutions would have to operate simultaneously and it is, therefore, important to investigate their compatibility and to identify possible conflicts that could prevent them from operating simultaneously. In this work, single- and two-fluid resistive magnetohydrodynamic calculations are used to investigate the effect of externally applied magnetic perturbations on the snowflake divertor configuration. The calculations are based on simulated NSTX-U plasmas and the results show that additional and longer magnetic lobes are created in the null-point region of the snowflake configuration, compared to those in the conventional single-null. The intersection of these longer and additional lobes with the divertor plates are expected to cause more striations in the particle and heat flux target profiles. In addition, the results indicate that the size of the magnetic lobes, in both single-null and snowflake configurations, are more sensitive to resonant magnetic perturbations than to non-resonant magnetic perturbations. The results also suggest that lower values of current in non-axisymmetric control coils would be required to suppress edge localized modes in plasmas with the snowflake configuration.
Type:
Dataset
Issue Date:
July 2017

8. Overview of NSTX Upgrade Initial Results and Modelling Highlights

Author(s):
Menard, J.E.; Allain, J.P.; Battaglia, D.J.; Bedoya, F.; Bell, R.E.; Belova, E.; Berkery, J.W.; Boyer, M.D.; Crocker, N.; Diallo, A.; Ebrahimi, F.; Ferrraro, N.; Fredrickson, E.; Frerichs, H.; Gerhardt, S.; Gorelenkov, N.; Guttenfelder, W.; Heidbrink, W.; Kaita, R.; Kaye, S.M.; Kriete, D.M.; Kubota, S.; LeBlanc, B.P.; Liu, D.; Lunsford, R.; Mueller, D.; Myers, C.E.; Ono, M.; Park, J.-K.; Podesta, M.; Raman, R.; Reinke, M.; Ren, Y.; Sabbagh, S.A.; Schmitz, O.; Scotti, F.; Sechrest, Y.; Skinner, C.H.; Smith, D.R.; Soukhanovskii, V.; Stoltzfus-Dueck, T.; Yuh, H.; Wang, Z.; Waters, I.; Ahn, J.-W.; Andre, R.; Barchfeld, R.; Beiersdorfer, P.; Bertelli, N.; Bhattacharjee, A.; Boyle, D.; Brennan, D.; Buttery, R.; Capece, A.; Canal, G.; Canik, J.; Chang, C.S.; Darrow, D.; Delgado-Aparicio, L.; Domier, C.; Ethier, S.; Evans, T.; Ferron, J.; Finkenthal, M.; Fonck, R.; Gan, K.; Gates, D.; Goumiri, I.; Gray, T.; Hosea, J.; Humphreys, D.; Jarboe, T.; Jardin, S.; Jaworski, M.A.; Koel, B.; Kolemen, E.; Ku, S.; LaHaye, R.J.; Levinton, F.; Luhmann Jr., N.; Maingi, R.; Maqueda, R.; McKee, G.; Meier, E.; Myra, J.; Perkins, R.; Poli, F.; Rhodes, T.; Riquezes, J.; Rowley, C.; Russell, D.; Schuster, E.; Stratton, B.; Stutman, D.; Taylor, G.; Tritz, K.; Wang, W.; Wirth, B.; Zweben, S.J.
Abstract:
The National Spherical Torus Experiment (NSTX) has undergone a major upgrade, and the NSTX Upgrade (NSTX-U) Project was completed in the summer of 2015. NSTX-U first plasma was subsequently achieved, diagnostic and control systems have been commissioned, H-Mode accessed, magnetic error fields identified and mitigated, and the first physics research campaign carried out. During 10 run weeks of operation, NSTX-U surpassed NSTX-record pulse-durations and toroidal fields, and high-performance ~1MA H-mode plasmas comparable to the best of NSTX have been sustained near and slightly above the n=1 no-wall stability limit and with H-mode confinement multiplier H98y2 above 1. Transport and turbulence studies in L-mode plasmas have identified the coexistence of at least two ion-gyro-scale turbulent micro-instabilities near the same radial location but propagating in opposite (i.e. ion and electron diamagnetic) directions. These modes have the characteristics of ion-temperature gradient and micro-tearing modes, respectively, and the role of these modes in contributing to thermal transport is under active investigation. The new second more tangential neutral beam injection was observed to significantly modify the stability of two types of Alfven Eigenmodes. Improvements in offline disruption forecasting were made in the areas of identification of rotating MHD modes and other macroscopic instabilities using the Disruption Event Characterization and Forecasting (DECAF) code. Lastly, the Materials Analysis and Particle Probe (MAPP) was utilized on NSTX-U for the first time and enabled assessments of the correlation between boronized wall conditions and plasma performance. These and other highlights from the first run campaign of NSTX-U are described.
Type:
Dataset
Issue Date:
October 2017

9. Hydrogen Retention in Lithium on Metallic Walls from “In Vacuo” Analysis in LTX and Implications for High-Z Plasma-Facing Components in NSTX-U

Author(s):
Kaita, R.; Lucia, M.; Allain, J. P.; Bedoya, F.; Capece, A.; Jaworski, M.; Koel, B. E.; Majeski, R.; Roszell, J.; Schmitt, J.; Scotti, F.; Skinner, C. H.; Soukhanovskii, V.
Abstract:
The application of lithium to plasma-facing components (PFCs) has long been used as a technique for wall conditioning in magnetic confinement devices to improve plasma performance. Determining the characteristics of PFCs at the time of exposure to the plasma, however, is difficult because they can only be analyzed after venting the vacuum vessel and removing them at the end of an operational period. The Materials Analysis and Particle Probe (MAPP) addresses this problem by enabling PFC samples to be exposed to plasmas, and then withdrawn into an analysis chamber without breaking vacuum. The MAPP system was used to introduce samples that matched the metallic PFCs of the Lithium Tokamak Experiment (LTX). Lithium that was subsequently evaporated onto the walls also covered the MAPP samples, which were then subject to LTX discharges. In vacuo extraction and analysis of the samples indicated that lithium oxide formed on the PFCs, but improved plasma performance persisted in LTX. The reduced recycling this suggests is consistent with separate surface science experiments that demonstrated deuterium retention in the presence of lithium oxide films. Since oxygen decreases the thermal stability of the deuterium in the film, the release of deuterium was observed below the lithium deuteride dissociation temperature. This may explain what occurred when lithium was applied to the surface of the NSTX Liquid Lithium Divertor (LLD). The LLD had segments with individual heaters, and the deuterium-alpha emission was clearly lower in the cooler regions. The plan for NSTX-U is to replace the graphite tiles with high-Z PFCs, and apply lithium to their surfaces with lithium evaporation. Experiments with lithium coatings on such PFCs suggest that deuterium could still be retained if lithium compounds form, but limiting their surface temperatures may be necessary.
Type:
Dataset
Issue Date:
2016

10. Advances in boronization on NSTX-Upgrade

Author(s):
Skinner, C.H.; Bedoya, F.; Scotti, F.; Allain, J.P.; Blanchard, W.; Cai, D.; Jaworski, M.; Koel, B.E.
Abstract:
Boronization has been effective in reducing plasma impurities and enabling access to higher density, higher confinement plasmas in many magnetic fusion devices. The National Spherical Torus eXperiment, NSTX, has recently undergone a major upgrade to NSTX-U in order to develop the physics basis for a ST-based Fusion Nuclear Science Facility (FNSF) with capability for double the toroidal field, plasma current, and NBI heating power and increased pulse duration from 1–1.5 s to 5–8 s. A new deuterated tri-methyl boron conditioning system was implemented together with a novel surface analysis diagnostic. We report on the spatial distribution of the boron deposition versus discharge pressure, gas injection and electrode location. The oxygen concentration of the plasma facing surface was measured by in-vacuo XPS and increased both with plasma exposure and with exposure to trace residual gases. This increase correlated with the rise of oxygen emission from the plasma.
Type:
Dataset
Issue Date:
January 2017

12. Laboratory study of the failed torus mechanism in arched, line-tied, magnetic flux ropes

Author(s):
Alt, Andrew; Ji, Hantao; Yoo, Jongsoo; Bose, Sayak; Goodman, Aaron; Yamada, Masaaki
Abstract:
Coronal mass ejections (CMEs) are some of the most energetic and violent events in our solar system. The prediction and understanding of CMEs is of particular importance due to the impact that they can have on Earth-based satellite systems, and in extreme cases, ground-based electronics. CMEs often occur when long-lived magnetic flux ropes (MFRs) anchored to the solar surface destabilize and erupt away from the Sun. One potential cause for these eruptions is an ideal magnetohydrodynamic (MHD) instability such as the kink or torus instability. Previous experiments on the Magnetic Reconnection eXperiment (MRX) revealed a class of MFRs that were torus-unstable but kink-stable, which failed to erupt. These “failed-tori” went through a process similar to Taylor relaxation where the toroidal current was redistributed before the eruption ultimately failed. We have investigated this behavior through additional diagnostics that measure the current distribution at the foot points and the energy distribution before and after an event. These measurements indicate that ideal MHD effects are sufficient to explain the energy distribution changes during failed torus events. This excludes Taylor relaxation as a possible mechanism of current redistribution during an event. A new model that only requires non-ideal effects in a thin layer above the electrodes is presented to explain the observed phenomena. This work broadens our understanding of the stability of MFRs and the mechanism behind the failed torus through the improved prediction of the torus instability and through new diagnostics to measure the energy inventory and current profile at the foot points.
Type:
Dataset
Issue Date:
2023

13. Microtearing Instabilities and Electron Thermal Transport in Low and High Collisionality NSTX Discharges

Author(s):
Rafiq T; Kaye S; Guttenfelder W; Weiland J; Schuster E; Anderson J; Luo L;
Abstract:
Microtearing mode (MTM) real frequency, growth rate, magnetic fluctuation amplitude and resulting electron thermal transport are studied in systematic NSTX scans of relevant plasma parameters. The dependency of the MTM real frequency and growth rate on plasma parameters, suitable for low and high collision NSTX discharges, is obtained by using the reduced MTM transport model [T. Rafiq, et al., Phys. Plasmas 23, 062507 (2016)]. The plasma parameter dependencies are compared and found to be consistent with the results obtained from MTM using the Gyrokinetic GYRO code. The scaling trend of collision frequency and plasma beta is found to be consistent with the global energy confinement trend observed in the NSTX experiment. The strength of the magnetic fluctuation is found to be consistent with the gyrokinetic estimate.In earlier studies, it was found that the version of the Multi-Mode (MM) anomalous transport model, which did not contain the effect of MTMs, provided an appropriate description of the electron temperature profiles in standard tokamak discharges and not in spherical tokamaks. When the MM model, which involves transport associated with MTMs, is incorporated in the TRANSP code and is used in the study of electron thermal transport in NSTX discharges, it is observed that the agreement with the experimental electron temperature profile is substantially improved.
Type:
Dataset
Issue Date:
February 2021

14. Feedback control design for non-inductively sustained scenarios in NSTX-U using TRANSP

Author(s):
Boyer, M.D.; Andre, R.G.; Gates, D.A.; Gerhardt, S.P.; Menard, J.E.; Poli, F.M.
Abstract:
This paper examines a method for real-time control of non-inductively sustained scenarios in NSTX-U by using TRANSP, a time-dependent integrated modeling code for prediction and interpretive analysis of tokamak experimental data, as a simulator. The actuators considered for control in this work are the six neutral beam sources and the plasma boundary shape. To understand the response of the plasma current, stored energy, and central safety factor to these actuators and to enable systematic design of control algorithms, simulations were run in which the actuators were modulated and a linearized dynamic response model was generated. A multi-variable model-based control scheme that accounts for the coupling and slow dynamics of the system while mitigating the effect of actuator limitations was designed and simulated. Simulations show that modest changes in the outer gap and heating power can improve the response time of the system, reject perturbations, and track target values of the controlled values.
Type:
Dataset
Issue Date:
June 2017

16. The updated ITPA global H-mode confinement database: description and analysis

Author(s):
Verdoolaege, G.; Kaye, S.M.; Angioni, C.; Kardaunn, O.W.J.F.; Maslov, M.; Romanelli, M.; Ryter, F.; Thomsen, K.
Abstract:
The multi-machine ITPA Global H-mode Confinement Database has been upgraded with new data from JET with the ITER-like wall and ASDEX Upgrade with the full tungsten wall. This paper describes the new database and presents results of regression analysis to estimate the global energy confinement scaling in H-mode plasmas using a standard power law. Various subsets of the database are considered, focusing on type of wall and divertor materials, confinement regime (all H-modes, ELMy H or ELM-free) and ITER-like constraints. Apart from ordinary least squares, two other, robust regression techniques are applied, which take into account uncertainty on all variables. Regression on data from individual devices shows that, generally, the confinement dependence on density and the power degradation are weakest in the fully metallic devices. Using the multi-machine scalings, predictions are made of the confinement time in a standard ELMy H-mode scenario in ITER. The uncertainty on the scaling parameters is discussed with a view to practically useful error bars on the parameters and predictions. One of the derived scalings for ELMy H-modes on an ITER-like subset is studied in particular and compared to the IPB98(y,2) confinement scaling in engineering and dimensionless form. Transformation of this new scaling from engineering variables to dimensionless quantities is shown to result in large error bars on the dimensionless scaling. Regression analysis in the space of dimensionless variables is therefore proposed as an alternative, yielding acceptable estimates for the dimensionless scaling. The new scaling, which is dimensionally correct within the uncertainties, suggests that some dependencies of confinement in the multi- machine database can be reconciled with parameter scans in individual devices. This includes vanishingly small dependence of confinement on line-averaged density and normalized plasma pressure (β), as well as a noticeable, positive dependence on effective atomic mass and plasma triangularity. Extrapolation of this scaling to ITER yields a somewhat lower confinement time compared to the IPB98(y, 2) prediction, possibly related to the considerably weaker dependence on major radius in the new scaling (slightly above linear). Further studies are needed to compare more flexible regression models with the power law used here. In addition, data from more devices concerning possible ‘hidden variables’ could help to determine their influence on confinement, while adding data in sparsely populated areas of the parameter space may contribute to further disentangling some of the global confinement dependencies in tokamak plasmas.
Type:
Dataset
Issue Date:
March 2021

17. Electron heating and energy inventory during asymmetric reconnection in a laboratory plasma

Author(s):
Yoo, Jongsoo; Na, Byungkeun; Jara-Almonte, Jonathan; Yamada, Maasaki; Ji, Hantao; Roytershteyn, V.; Argall, M. R.; Fox, W.; Chen, Li-Jen
Abstract:
Electron heating and the energy inventory during asymmetric reconnection are studied in the laboratory plasma with a density ratio of about 8 across the current sheet. Features of asymmetric reconnection such as the large density gradients near the low-density-side separatrices, asymmetric in-plane electric field, and bipolar out-of-plane magnetic field are observed. Unlike the symmetric case, electrons are also heated near the low-density-side separatrices. The measured parallel electric field may explain the observed electron heating. Although large fluctuations driven by lower-hybrid drift instabilities are also observed near the low-density-side separatrices, laboratory measurements and numerical simulations reported here suggest that they do not play a major role in electron energization. The average electron temperature increase in the exhaust region is proportional to the incoming magnetic energy per an electron/ion pair but exceeds scalings of the previous space observations. This discrepancy is explained by differences in the boundary condition and system size. The profile of electron energy gain from the electric field shows that there is additional electron energy gain associated with the electron diamagnetic current besides a large energy gain near the X-line. This additional energy gain increases electron enthalpy, not the electron temperature. Finally, a quantitative analysis of the energy inventory during asymmetric reconnection is conducted. Unlike the symmetric case where the ion energy gain is about twice more than the electron energy gain, electrons and ions obtain a similar amount of energy during asymmetric reconnection.
Type:
Dataset
Issue Date:
August 2017

18. Blob Structure and Motion in the Edge and SOL of NSTX

Author(s):
Zweben, S.J.; J.R. Myra; W.M. Davis; D.A. D'Ippolito; T.K. Gray; S.M. Kaye; B.P. LeBlanc; R.J. Maqueda; D.A. Russell; D.P. Stotler
Abstract:
Blob analysis dataset
Type:
Dataset
Issue Date:
January 2016

19. Quasi-linear gyrokinetic predictions of the Coriolis momentum pinch in NSTX

Author(s):
Guttenfelder W.; S.M. Kaye; Y. Ren; W. Solomon; R.E. Bell; J. Candy; S.P. Gerhardt; B.P. LeBlanc; H. Yuh
Abstract:
This paper presents quasi-linear gyrokinetic predictions of the Coriolis momentum pinch for low aspect-ratio NSTX H-modes where previous experimental measurements were focused. Local, linear calculations predict that in the region of interest (just outside the mid-radius) of these relatively high-beta plasmas, profiles are most unstable to microtearing modes that are only effective in transporting electron energy. However, sub-dominant electromagnetic and electrostatic ballooning modes are also unstable, which are effective at transporting energy, particles and momentum. The quasi-linear prediction of transport from these weaker ballooning modes, assuming they contribute transport in addition to that from microtearing modes in a nonlinear turbulent state, leads to a very small or outward convection of momentum, inconsistent with the experimentally measured inward pinch, and opposite to predictions in conventional aspect ratio tokamaks. Additional predictions of a low beta L-mode plasma, unstable to more traditional electrostatic ion temperature gradient-trapped electron mode instability, show that the Coriolis pinch is inward but remains relatively weak and insensitive to many parameter variations. The weak or outward pinch predicted in NSTX plasmas appears to be at least partially correlated to changes in the parallel mode structure that occur at finite beta and low aspect ratio, as discussed in previous theories. The only conditions identified where a stronger inward pinch is predicted occur either in the purely electrostatic limit or if the aspect ratio is increased. As the Coriolis pinch cannot explain the measured momentum pinch, additional theoretical momentum transport mechanisms are discussed that may be potentially important.
Type:
Dataset
Issue Date:
April 2016

20. Energetic particle optimization of quasi-axisymmetric stellarator equilibria

Author(s):
LeViness, Alexandra; Schmitt, John; Lazerson, Samuel; Bader, Aaron; Faber, Benjamin; Hammond, Kenneth; Gates, David
Abstract:
An important goal of stellarator optimization is to achieve good confinement of energetic particles such as, in the case of a reactor, alphas created by Deuterium-Tritium (D-T) fusion. In this work, a fixed-boundary stellarator equilibrium was re-optimized for energetic particle confinement via a two-step process: first, by minimizing deviations from quasi-axisymmetry (QA) on a single flux surface near the mid-radius, and secondly by maintaining this improved quasi-axisymmetry while minimizing the analytical quantity ΓC , which represents the angle between magnetic flux surfaces and contours of J||, the second adiabatic invariant. This was performed multiple times, resulting in a group of equilibria with significantly reduced energetic particle losses, as evaluated by Monte Carlo simulations of alpha particles in scaled-up versions of the equilibria. This is the first time that energetic particle losses in a QA stellarator have successfully been reduced by optimizing ΓC . The relationship between energetic particle losses and metrics such as QA error (Eqa) and ΓC in this set of equilibria were examined via statistical methods and a nearly linear relationship between volume-averaged ΓC and prompt particle losses was found.
Type:
Dataset
Issue Date:
2022

21. Observation of synergy between lower hybrid waves at two frequencies in EAST

Author(s):
Choi, W.; Poli, F. M.; Li, M. H.; Baek, S. G.; Gorenlenkova, M.; Ding, B. J.; Gong, X. Z.; Chan, A.; Duan, Y. M.; Hu, J. H.; Lian, H.; Lin, S. Y.; Liu, H. Q.; Qian, J. P.; Wallace, G.; Wang, Y. M.; Zang, Q.; Zhao, H. L.
Abstract:
Synergistic effects between two frequencies of lower hybrid (LH) waves—operating at 2.45 and 4.6 GHz—were observed in experiment on EAST for the first time. At low density (n_e,lin ≈ 2.0 × 10^19m^−3), simultaneous injection of a 65/35 mix of 2.45 GHz/4.6 GHz power achieved an LHCD efficiency that was 25% higher than what should be expected from the linear combination of the two sources. The experiment was interpreted with time-dependent simulations, using the equilibrium and transport solver TRANSP, coupled with the ray-tracing code GENRAY and the Fokker-Planck solver CQL3D. For each discharge, profiles of current and hard x-ray from simulation and measurement agree within uncertainties. An examination of the electron distribution function indicates that the LH synergy is supported by the increased width of the LH resonance plateau in the simultaneous injection case compared to independent injection.
Type:
Dataset
Issue Date:
June 2021

22. Stabilizing effect of resistivity towards ELM-free H-mode discharge in lithium-conditioned NSTX

Author(s):
Banerjee, D.; Zhu, P.; Maingi, R.
Abstract:
Linear stability analysis of the national spherical torus experiment (NSTX) Li-conditioned ELM-free H-mode equilibria is carried out in the context of the extended magneto-hydrodynamic (MHD) model in NIMROD. The purpose is to investigate the physical cause behind edge localized mode (ELM) suppression in experiment after the Li-coating of the divertor and the first wall of the NSTX tokamak. Besides ideal MHD modeling, including finite-Larmor radius effect and two-fluid Hall and electron diamagnetic drift contributions, a non-ideal resistivity model is employed, taking into account the increase of Z eff after Li-conditioning in ELM-free H-mode. Unlike an earlier conclusion from an eigenvalue code analysis of these equilibria, NIMROD results find that after reduced recycling from divertor plates, profile modification is necessary but insufficient to explain the mechanism behind complete ELMs suppression in ideal two-fluid MHD. After considering the higher plasma resistivity due to higher Z eff , the complete stabilization could be explained. A thorough analysis of both pre-lithium ELMy and with-lithium ELM-free cases using ideal and non-ideal MHD models is presented, after accurately including a vacuum-like cold halo region in NIMROD to investigate ELMs.
Type:
Dataset
Issue Date:
July 2017

24. Energy Exchange Dynamics across L-H Transitions in NSTX

Author(s):
Diallo, A.; Banerjee, S.; Zweben, S.; Stoltzfus-Dueck, T.
Abstract:
We studied the energy exchange dynamics across the low-to-high-confinement (L-H) in NSTX discharges using the gas-puff imaging (GPI) diagnotic. The investigation focused on the energy exchange between flows and turbulence, to help clarify the mechanism of the L-H transition. We apply this study to three type of heating schemes, including a total of 17 shots from the NSTX 2010 campaign run. Results show that the edge fluctuation characteristics (fluctuation levels, radial and poloidal correlation lengths) measured using GPI do not vary just prior to the H-mode transition, but change after the transition. Using a velocimetry approach (orthogonal-programming decomposition), velocity fields of a 24 $\times$ 30 cm GPI view during the L-H transition were obtained with good spatial ($\sim$1 cm) and temporal ($\sim$2.5 $\mu$s) resolutions. Analysis using these velocity fields shows that the production term is systematically negative just prior to the L-H transition indicating transfer from mean flows to turbulence, which is inconsistent with the predator-prey paradigm. Moreover, using the inferred absolute value of the production term, an estimate of the L-H transition duration is found to be 25 ms, which is much larger than the measured duration. These discrepancies are further reinforced by consideration of the ratio between the kinetic energy in the mean flow to the thermal free energy, which is estimated to be much less than 1, suggesting again that turbulence depletion mechanism may not be playing an important role in the transition to the H-mode. Although the Reynolds work is too small to directly deplete the turbulent free energy reservoir, order-of-magnitude analysis shows that the Reynolds stress may still make a non-negligible contribution to the observed poloidal flows.
Type:
Dataset
Issue Date:
June 2017

26. Time dependent analysis of visible helium line-ratios for electron temperature and density diagnostic using synthetic simulations on NSTX-U

Author(s):
Munoz Burgos, J.M.; Barbui, T.; Schmitz, O.; Stutman, D.; Tritz, K.
Abstract:
Helium line-ratios for electron temperature (Te) and density (ne) plasma diagnostic in the Scrape-Off-Layer (SOL) and Edge regions of tokamaks are widely used. Due to their intensities and proximity of wavelengths, the singlet 667.8 and 728.1 nm, and triplet 706.5 nm visible lines have been typically preferred. Time- dependency of the triplet line (706.5 nm) has been previously analyzed in detail by including transient effects on line-ratios during gas-puff diagnostic applications. In this work, several line-ratio combinations within each of the two spin systems are analyzed with the purpose of eliminating transient effects to extend the application of this powerful diagnostic to high temporal resolution characterization of plasmas. The analysis is done using synthetic emission modeling and diagnostic for low electron density NSTX SOL plasma conditions for several visible lines. This analysis employs both quasi-static equilibrium and time-dependent models in order to evaluate transient effects of the atomic population levels that may affect the derived electron temperatures and densities as a helium gas-puff penetrates the plasma. Ratios between the most intense lines are usually preferred due to their higher signal to noise ratio. The analysis of a wider range of spectral lines will help to extend this powerful diagnostic to experiments where the wavelength range of the measured spectra may be constrained either by limitations of the spectrometer, or by other conflicting lines from different ions.
Type:
Dataset
Issue Date:
November 2016

27. Plasma boundary shape control and real-time equilibrium reconstruction on NSTX-U

Author(s):
Boyer, M.; Battaglia, D.; Mueller, D.; Eidietis, N.; Erickson, K.; Ferron, J.; Gates, D.; Gerhardt, S.; Johnson, R.; Kolemen, E.; Menard, J.; Myers, C.; Sabbagh, S.; Scotti, F.; Vail, P.
Abstract:
The upgrade to the National Spherical Torus eXperiment (NSTX-U) included two main improvements: a larger center-stack, enabling higher toroidal field and longer pulse duration, and the addition of three new tangentially aimed neutral beam sources, which increase available heating and current drive, and allow for flexibility in shaping power, torque, current, and particle deposition profiles. To best use these new capabilities and meet the high-performance operational goals of NSTX-U, major upgrades to the NSTX-U Control System (NCS) hardware and software have been made. Several control algorithms, including those used for real-time equilibrium reconstruction and shape control, have been upgraded to improve and extend plasma control capabilities. As part of the commissioning phase of first plasma operations, the shape control system was tuned to control the boundary in both inner-wall limited and diverted discharges. It has been used to accurately track the requested evolution of the boundary (including the size of the inner gap between the plasma and central solenoid, which is a challenge for the ST configuration), X-point locations, and strike point locations, enabling repeatable discharge evolutions for scenario development and diagnostic commissioning.
Type:
Dataset
Issue Date:
March 2018

28. Enhanced Pedestal H-mode at low edge ion collisionality on NSTX

Author(s):
Battaglia, D.J.; Guttenfelder, W.; Bell, R.E.; Diallo, A.; Ferraro, N.;, Fredrickson, E.; Gerhardt, S.P.; Kaye, S.M.; Maingi, R.; Smith, D.R.
Abstract:
The Enhanced Pedestal (EP) H-mode regime is an attractive wide-pedestal ELM-free high-betap scenario for NSTX-U and next-step devices as it achieves enhanced energy confinement (H98y,2 > 1.5), large normalized pressure (betaN > 5) and significant bootstrap fraction (f_BS > 0.6) at I_p/B_T = 2 MA/T. This regime is realized when the edge ion collisionality becomes sufficiently small that a positive feedback interaction occurs between a reduction in the ion neoclassical energy transport and an increase in the particle transport from pressure-driven edge instabilities. EP H-mode was most often observed as a transition following a large ELM in conditions with low edge neutral recycling. It is hypothesized that the onset of pressure-driven instabilities prior to the full recovery of the neutral density leads to a temporary period with elevated ion temperature gradient that triggers the transition to EP H-mode. Linear CGYRO and M3D-C1 calculations are compared to beam emission spectroscopy (BES) and magnetic spectroscopy in order to describe the evolution of the edge particle transport mechanisms during the ELM recovery and the saturated EP H-mode state. The observations are consistent with the hypothesis that the onset of pressure-driven edge instabilities, such as the KBM and kink-peeling, can be responsible for the increased particle transport in EP H-mode.
Type:
Dataset
Issue Date:
June 2020

29. Kinetic neoclassical calculations of impurity radiation profiles

Author(s):
Stotler, D.P.; Battaglia, D.J.; Hager, R.; Kim, K.; Koskela, T.; Park, G.; Reinke, M.L.
Abstract:
Modifications of the drift-kinetic transport code XGC0 to include the transport, ionization, and recombination of individual charge states, as well as the associated radiation, are described. The code is first applied to a simulation of an NSTX H-mode discharge with carbon impurity to demonstrate the approach to coronal equilibrium. The effects of neoclassical phenomena on the radiated power profile are examined sequentially through the activation of individual physics modules in the code. Orbit squeezing and the neoclassical inward pinch result in increased radiation for temperatures above a few hundred eV and changes to the ratios of charge state emissions at a given electron temperature. Analogous simulations with a neon impurity yield qualitatively similar results.
Type:
Dataset
Issue Date:
2017

30. Reduced Model for Direct Induction Startup Scenario Development on MAST-U and NSTX-U

Author(s):
Battaglia, D.J.; Thornton, A.J.; Gerhardt, S.P.; Kirk, A.; Kogan, L; Menard, J.E.
Abstract:
A reduced semi-empirical model using time-dependent axisymmetric vacuum field calculations is used to develop the prefill and feed-forward coil current targets required for reliable direct induction (DI) startup on the new MA-class spherical tokamaks, MAST-U and NSTX-U. The calculations are constrained by operational limits unique to each device, such as the geometry of the conductive elements and active coils, power supply specifications and coil heating and stress limits. The calculations are also constrained by semi-empirical models for sufficient breakdown, current drive, equilibrium and stability of the plasma developed from a shared database. A large database of DI startup on NSTX and NSTX-U is leveraged to quantify the requirements for achieving a reliable breakdown (Ip ~ 20 kA). It is observed that without pre-ionization, STs access the large E/P regime at modest loop voltage (Vloop) where the electrons in the weakly ionized plasma are continually accelerating along the open field lines. This ensures a rapid (order millisecond) breakdown of the neutral gas, even without pre-ionization or high-quality field nulls. The timescale of the initial increase in Ip on NSTX is reproduced in the reduced model provided a mechanism for impeding the applied electric field is included. Most discharges that fail in the startup phase are due to an inconsistency in the evolution of the plasma current (Ip) and equilibrium field or loss of vertical stability during the burn-through phase. The requirements for the self-consistent evolution of the fields in the weakly and full-ionized plasma states are derived from demonstrated DI startup on NSTX, NSTX-U and MAST. The predictive calculations completed for MAST-U and NSTX-U illustrate that the maximum Ip ramp rate (dIp/dt) in the early startup phase is limited by the voltage limits on the poloidal field coils on MAST-U and passive vertical stability on NSTX-U.
Type:
Dataset
Issue Date:
August 2019

31. Scenario Development During Commissioning Operations on the National Spherical Torus Experiment Upgrade

Author(s):
Battaglia, D.J.; Boyer, M.D.; Gerhardt, S.; Mueller, D.; Myers, C.E.; Guttenfelder, W.; Menard, J.E.; Sabbagh, S.A.; Scotti, F.; Bedoya, F.; Bell, R.E.; Berkery, J.W.; Diallo, A.; Ferraro, N.; Jaworski, M.A.; Kaye, S.M.; LeBlanc, B.P.; Ono, M.; Park, J.-K.; Podesta, M.; Raman, R.; Soukhanovskii, V.A.
Abstract:
The National Spherical Torus Experiment Upgrade (NSTX-U) will advance the physics basis required for achieving steady-state, high-beta, and high-confinement conditions in a tokamak by accessing high toroidal field (1 T) and plasma current (1.0 - 2.0 MA) in a low aspect ratio geometry (A = 1.6 - 1.8) with flexible auxiliary heating systems (12 MW NBI, 6 MW HHFW). This paper describes progress in the development of L- and H-mode discharge scenarios and the commissioning of operational tools in the first ten weeks of operation that enable the scientific mission of NSTX-U. Vacuum field calculations completed prior to operations supported the rapid development and optimization of inductive breakdown at different values of ohmic solenoid current. The toroidal magnetic field (B_T0 = 0.65 T) exceeded the maximum values achieved on NSTX and novel long-pulse L-mode discharges with regular sawtooth activity exceeded the longest pulses produced on NSTX (tpulse > 1.8s). The increased flux of the central solenoid facilitated the development of stationary L-mode discharges over a range of density and plasma current (Ip). H-mode discharges achieved similar levels of stored energy, confinement (H98y,2 > 1) and stability (beta_N/beta_N-nowall > 1) compared to NSTX discharges for Ip < 1 MA. High-performance H-mode scenarios require an L-H transition early in the Ip ramp-up phase in order to obtain low internal inductance (li) throughout the discharge, which is conducive to maintaining vertical stability at high elongation (kappa > 2.2) and achieving long periods of MHD quiescent operations. The rapid progress in developing L- and H-mode scenarios in support of the scientific program was enabled by advances in real-time plasma control, efficient error field identification and correction, effective conditioning of the graphite wall and excellent diagnostic availability.
Type:
Dataset
Issue Date:
April 2018

32. Suppression of Alfvén modes on NSTX-U with outboard beam injection

Author(s):
Fredrickson, E.D.; Belova, E.V.; Battaglia, D.J.; Bell, R.E.; Crocker, N.A.; Darrow, D.S.; Diallo, A.; Gerhardt, S.P.; Gorelenkov, N.N.; LeBlanc, B.P.; Podesta, M.
Abstract:
In this paper we present data from experiments on NSTX-U where it is shown for the first time that small amounts of high pitch-angle beam ions can strongly suppress the counter-propagating Global Alfvén Eigenmodes (GAE). GAE have been implicated in the redistribution of fast ions and modification of the electron power balance in previous experiments on NSTX. The ability to predict the stability of Alfvén modes, and developing methods to control them, is important for fusion reactor like the International Tokamak Experimental Reactor (ITER) which are heated by a large population of non-thermal, super-Alfvénic ions consisting of fusion generated alphas and beam ions injected for current profile control. We present a qualitative interpretation of these observations using an analytic model of the Doppler-shifted ion-cyclotron resonance drive responsible for GAE instability which has an important dependence on k⊥ρL. A quantitative analysis of this data with the HYM stability code predicts both the frequencies and instability of the GAE prior to, and suppression of the GAE after the injection of high pitch-angle beam ions.
Type:
Dataset
Issue Date:
June 2017

33. Impact of edge harmonic oscillations on the divertor heat flux in NSTX

Author(s):
Gan, Kaifu; Gray, Travis; Zweben, Stewart; Eric, Fredrickson; Maingi, Rajesh; Battaglia, Devon; McLean, Adam; Wirth, Brian
Abstract:
All the data was uploaded with .cvs file, we have not uploaded the figure 1 data since it is just photo show field of view of IR and GPI diagnostic.
Type:
Dataset
Issue Date:
6 December 2021

34. Global modeling of wall material migration following boronization in NSTX-U

Author(s):
Nichols, J.H.; Jaworski, M.A.; Skinner, C.H.; Bedoya, F.; Scotti, F.; Soukhanovskii, V.A.; Schmid, K.
Abstract:
Boronization is commonly utilized in tokamaks to suppress intrinsic impurities, most notably oxygen from residual water vapor. However, this is a temporary solution, as oxygen levels typically return to pre-boronization levels following repeated plasma exposure. The global impurity migration model WallDYN has been applied to the post-boronization surface impurity evolution in NSTX-U. A “Thin Film Model” has been incorporated into WallDYN to handle spatially inhomogeneous conditioning films of varying thicknesses, together with an empirical boron conditioning model for the NSTX-U glow discharge boronization process. The model qualitatively reproduces the spatial distribution of boron in the NSTX-U vessel, the spatially-resolved divertor emission pattern, and the increase in oxygen levels following boronization. The simulations suggest that oxygen is primarily sourced from wall locations without heavy plasma flux or significant boron deposition, namely the lower and upper passive plates and the lower private flux zone.
Type:
Dataset
Issue Date:
March 2019

35. Plasma measurements of the Fe XVII L-shell emission and blending with F VIII and F IX

Author(s):
Beiersdorfer, P.; Lepson, J.K.; Gu, M.F.; Bitter, M.
Abstract:
We measured the L-shell emission spectrum of Fe XVII in a low-density, low-gradient magnetically confined laboratory plasma that contains predominantly C, O, Fe, and Ni as trace elements and find excellent agreement with the relative spectral emission obtained in solar and astrophysical observations. However, we obtained spectra that appear to have an usually large 1s^22s^22p^5_{1/2}3d_{3/2} --> 1s^22s^22p^6 Fe XVII resonance transition, commonly labeled 3C, from hot plasmas that also contain F. The wavelength of the Ly-alpha feature of F IX is coincident with the wavelength of the Fe XVII line 3C within one part in 538, and its flux, therefore, enhances the Fe XVII resonance line. Moreover, the resonance and forbidden lines of F VIII are close to the 3s --> 2p transitions in Fe XVII, and may further alter the inferred apparent Fe XVII line ratios, particularly in spectrometers with moderate spectral resolution. The enhanced emission of line 3C, thus, can serve as a new spectral diagnostic for the detection of fluorine in astrophysical plasmas.
Type:
Dataset
Issue Date:
November 2017

36. Three New Extreme Ultraviolet Spectrometers on NSTX-U for Impurity Monitoring

Author(s):
Weller, M.E.; Beiersdorfer, P.; Soukhanovskii, V.; Magee, E.W.; Scotti, F.
Abstract:
Three extreme ultraviolet (EUV) spectrometers have been mounted on the National Spherical Torus Experiment-Upgrade (NSTX-U). All three are flat-field grazing-incidence spectrometers and are dubbed X-ray and Extreme Ultraviolet Spectrometer (8 ñ 70 ≈), Long-Wavelength Extreme Ultraviolet Spectrometer (190 ñ 440 ≈), and Metal Monitor and Lithium Spectrometer Assembly (MonaLisa, 50 ñ 220 ≈). XEUS and LoWEUS were previously implemented on NSTX to monitor impurities from low- to high-Z sources and to study impurity transport while MonaLisa is new and provides the system increased spectral coverage. The spectrometers will also be a critical diagnostic on the planned laser blow-off (LBO) system for NSTX-U, which will be used for impurity edge and core ion transport studies, edge-transport code development, and benchmarking atomic physics codes.
Type:
Dataset
Issue Date:
November 2016

37. A Reduced Resistive Wall Mode Kinetic Stability Model for Disruption Forecasting

Author(s):
Berkery, J.W.; Sabbagh, S.A.; Bell, R.E.; Gerhardt, S.P.; LeBlanc, B.P.
Abstract:
Kinetic modification of ideal stability theory from stabilizing resonances of mode-particle interaction has had success in explaining resistive wall mode (RWM) stability limits in tokamaks. With the goal of real-time stability forecasting, a reduced kinetic stability model has been implemented in the new Disruption Event Characterization and Forecasting (DECAF) code, which has been written to analyze disruptions in tokamaks. The reduced model incorporates parameterized models for ideal limits on beta, a ratio of plasma pressure to magnetic pressure, which are shown to be in good agreement with DCON code calculations. Increased beta between these ideal limits causes a shift in the unstable region of delta W_K space, where delta W_K is the change in potential energy due to kinetic effects that is solved for by the reduced model, such that it is possible for plasmas to be unstable at intermediate beta but stable at higher beta. Gaussian functions for delta W_K are defined as functions of E cross B frequency and collisionality, with parameters reflecting the experience of the National Spherical Torus Experiment (NSTX). The reduced model was tested on a database of discharges from NSTX and experimentally stable and unstable discharges were separated noticeably on a stability map in E cross B frequency, collisionality space. The reduced model only failed to predict an unstable RWM in 15.6% of cases with an experimentally unstable RWM and performed well on predicting stability for experimentally stable discharges as well.
Type:
Dataset
Issue Date:
May 2017

38. Effect of progressively increasing lithium conditioning on edge transport and stability in high triangularity NSTX H-modes

Author(s):
Maingi, R.; Canik, J.M.; Bell, R.E.; Boyle, D.P.; Diallo, A.; Kaita, R.; Kaye, S.M.; LeBlanc, B.P.; Sabbagh, S.A.; Scotti, F.; Soukhanovskii, V.A.
Abstract:
A sequence of H-mode discharges with increasing levels of pre-discharge lithium evaporation (�dose�) was conducted in high triangularity and elongation boundary shape in NSTX. Energy confinement increased, and recycling decreased with increasing lithium dose, similar to a previous lithium dose scan in medium triangularity and elongation plasmas. Data-constrained SOLPS interpretive modeling quantified the edge transport change: the electron particle diffusivity decreased by 10-30x. The electron thermal diffusivity decreased by 4x just inside the top of the pedestal, but increased by up to 5x very near the separatrix. These results provide a baseline expectation for lithium benefits in NSTX-U, which is optimized for a boundary shape similar to the one in this experiment.
Type:
Dataset
Issue Date:
August 2016

39. Initial operation of the NSTX-Upgrade real-time velocity diagnostic

Author(s):
Podesta, M; Bell, R.E.
Abstract:
A real-time velocity (RTV) diagnostic based on active charge-exchange recombination spectroscopy is now operational on the National Spherical Torus Experiment-Upgrade (NSTX-U) spherical torus (Menard et al 2012 Nucl. Fusion 52 083015). The system has been designed to supply plasma velocity data in real time to the NSTX-U plasma control system, as required for the implementation of toroidal rotation control. Measurements are available from four radii at a maximum sampling frequency of 5 kHz. Post-discharge analysis of RTV data provides additional information on ion temperature, toroidal velocity and density of carbon impurities. Examples of physics studies enabled by RTV measurements from initial operations of NSTX-U are discussed.
Type:
Dataset
Issue Date:
November 2016

40. Initial transport and turbulence analysis and gyrokinetic simulation validation in NSTX-U L-mode plasmas

Author(s):
Guttenfelder, W.; Kaye, S.M.; Kreite, D.M.; Bell, R.E.; Diallo, A.; LeBlanc, B.P.; McKee, G.R.; Podesta, M.; Sabbagh, S.A.; Smith, D.R.
Abstract:
Transport analysis, ion-scale turbulence measurements, and initial linear and nonlinear gyrokinetic simulations are reported for a transport validation study based on low aspect ratio NSTX-U L-mode discharges. The relatively long, stationary L-modes enabled by the upgraded centerstack provide a more ideal target for transport validation studies that were not available during NSTX operation. Transport analysis shows that anomalous electron transport dominates energy loss while ion thermal transport is well described by neoclassical theory. Linear gyrokinetic GYRO analysis predicts that ion temperature gradient (ITG) modes are unstable around normalized radii $\rho$=0.6-0.8, although $E\timesB$ shearing rates are larger than the linear growth rates over much of that region. Deeper in the core ($\rho$=0.4-0.6), electromagnetic microtearing modes (MTM) are unstable as a consequence of the relatively high beta and collisionality in these particular discharges. Consistent with the linear analysis, local, nonlinear ion-scale GYRO simulations predict strong ITG transport at $\rho$=0.76, whereas electromagnetic MTM transport is important at $\rho$=0.47. The prediction of ion-scale turbulence is consistent with 2D beam emission spectroscopy (BES) that measures the presence of broadband ion-scale fluctuations. Interestingly, the BES measurements also indicate the presence of bi-modal poloidal phase velocity propagation that could be indicative of two different turbulence types. However, in the region between ($\rho$=0.56, 0.66), ion-scale simulations are strongly suppressed by the locally large $E\timesB$ shear. Instead, electron temperature gradient (ETG) turbulence simulations predict substantial transport, illustrating electron-scale contributions can be important in low aspect ratio L-modes, similar to recent analysis at conventional aspect ratio. However, agreement within experimental uncertainties has not been demonstrated, which requires additional simulations to test parametric sensitivities. The potential need to include profile-variation effects (due to the relatively large value of $\rho_*$=$\rho_i$/a at low aspect ratio), including electromagnetic and possibly multi-scale effects, is also discussed.
Type:
Dataset
Issue Date:
March 2019

41. The Role of Recti ed Currents in Far-Field RF Sheaths and in SOL Losses of HHFW Power on NSTX

Author(s):
Perkins, R.J.; Hosea, J.C.; Jaworski, M.A.; Bell, R.E.; Bertelli, N.; Kramer, G.J.; Roquemore, L.; Taylor, G.; Wilson, J.R.
Abstract:
Radio-frequency (RF) rectification is an important sheath phenomenon for wave heating of plasma in fusion devices and is proposed to be the mechanism responsible for converting highharmonic fast-wave (HHFW) power in the National Spherical Torus eXperiment (NSTX) into a heat ux to the divertor. RF rectification has two aspects: current rectification and voltage recti- fication, and, while the latter is emphasized in many application, we demonstrate the importance of current rectification in analysis of the NSTX divertor during HHFW heating. When rectified currents are accounted for in first-principle models for the heat ux to the tiles, we predict a sizeable enhancement for the heat ux in the presence of an RF field: for one case studied, the predicted heat ux increases from 0:103 MW=m2 to 0:209 MW=m2. We also demonstrate how this rectification scales with injected HHFW power by tracking probe characteristics during a HHFW power ramp; the rectified current may be clamped at a certain level. This work is important for minimizing SOL losses of HHFW power in NSTX-U but may also have implications for near-field studies of ICRF antennae: ignoring rectified current may lead to underestimated heat uxes and overestimated rectified voltages.
Type:
Dataset
Issue Date:
May 2017

42. Density perturbation mode structure of high frequency compressional and global Alfvén eigenmodes in the National Spherical Torus Experiment using a novel reflectometer analysis technique

Author(s):
Crocker, N.A.; Kubota, S.; Peebles, W.A.; Rhodes, T.L.; Fredrickson, E.D.; Belova, E.; Diallo, A.; LeBlanc, B.P.; Sabbagh, S.A.
Abstract:
Reflectometry measurements of compressional (CAE) and global (GAE) Alfvén eigenmodes are analyzed to obtain the amplitude and spatial structure of the density perturbations associated with the modes. A novel analysis technique developed for this purpose is presented. The analysis also naturally yields the amplitude and spatial structure of the density contour radial displacement, which is found to be 2–4 times larger than the value estimated directly from the reflectometer measurements using the much simpler ‘mirror approximation’. The modes were driven by beam ions in a high power (6 MW) neutral beam heated H-mode discharge (#141398) in the National Spherical Torus Experiment. The results of the analysis are used to assess the contribution of the modes to core energy transport and ion heating. The total displacement amplitude of the modes, which is shown to be larger than previously estimated (Crocker et al 2013 Nucl. Fusion 53 43017), is compared to the predicted threshold (Gorelenkov et al 2010 Nucl. Fusion 50 84012) for the anomalously high heat diffusion inferred from transport modeling in similar NSTX discharges. The results of the analysis also have strong implications for the energy transport via coupling of CAEs to kinetic Alfvén waves seen in simulations with the Hybrid MHD code (Belova et al 2015 Phys. Rev. Lett. 115 15001). Finally, the amplitudes of the observed CAEs fall well below the threshold for causing significant ion heating by stochastic velocity space diffusion (Gates et al 2001 Phys. Rev. Lett. 87 205003).
Type:
Dataset
Issue Date:
November 2017

43. Analytic stability boundaries for compressional and global Alfven eigenmodes driven by fast ions. II. Interaction via Landau resonance.

Author(s):
Lestz, J.B.; Gorelenkov, N.N.; Belova, E.V.; Tang, S.X.; Crocker, N.A.
Abstract:
Conditions for net fast ion drive are derived for beam-driven, co-propagating, sub-cyclotron compressional (CAE) and global (GAE) Alfven eigenmodes driven by the Landau resonance with super-Alfvenic fast ions. Approximations applicable to realistic neutral beam distributions and mode characteristics observed in spherical tokamaks enable the derivation of marginal stability conditions for these modes. Such conditions successfully reproduce the stability boundaries found from numerical integration of the exact expression for local fast ion drive/damping. Coupling between the CAE and GAE branches of the dispersion due to finite \omega/\omega_{ci} and k_\parallel/k_\perp is retained and found to be responsible for the existence of the GAE instability via this resonance. Encouraging agreement is demonstrated between the approximate stability criterion, simulation results, and a database of NSTX observations of co-CAEs.
Type:
Dataset
Issue Date:
January 2020

44. Energetic-particle-modified global Alfven eigenmodes

Author(s):
Lestz, J.B.; Belova, E.V.; Gorelenkov, N.N.
Abstract:
Fully self-consistent hybrid MHD/particle simulations reveal strong energetic particle modifications to sub-cyclotron global Alfven eigenmodes (GAE) in low-aspect ratio, NSTX-like conditions. Key parameters defining the fast ion distribution function -- the normalized injection velocity v_0/v_A and central pitch -- are varied in order to study their influence on the characteristics of the excited modes. It is found that the frequency of the most unstable mode changes significantly and continuously with beam parameters, in accordance with the Doppler-shifted cyclotron resonances which drive the modes, and depending most substantially on v_0/v_A. This unexpected result is present for both counter-propagating GAEs, which are routinely excited in NSTX, and high frequency co-GAEs, which have not been previously studied. Large changes in frequency without clear corresponding changes in mode structure could indicate the existence of a new energetic particle mode, referred to here as an energetic-particle-modified GAE (EP-GAE). Additional simulations conducted for a fixed MHD equilibrium demonstrate that the GAE frequency shift cannot be explained by the equilibrium changes due to energetic particle effects.
Type:
Dataset
Issue Date:
December 2017

45. Hybrid simulations of sub-cyclotron compressional and global Alfven Eigenmode stability in spherical tokamaks

Author(s):
Lestz, J.B.; Belova, E.V.; Gorelenkov, N.N
Abstract:
A comprehensive numerical study has been conducted in order to investigate the stability of beam-driven, sub-cyclotron frequency compressional (CAE) and global (GAE) Alfven Eigenmodes in low aspect ratio plasmas for a wide range of beam parameters. The presence of CAEs and GAEs has previously been linked to anomalous electron temperature profile flattening at high beam power in NSTX experiments, prompting further examination of the conditions for their excitation. Linear simulations are performed with the hybrid MHD-kinetic initial value code HYM in order to capture the general Doppler-shifted cyclotron resonance that drives the modes. Three distinct types of modes are found in simulations -- co-CAEs, cntr-GAEs, and co-GAEs -- with differing spectral and stability properties. The simulations reveal that unstable GAEs are more ubiquitous than unstable CAEs, consistent with experimental observations, as they are excited at lower beam energies and generally have larger growth rates. Local analytic theory is used to explain key features of the simulation results, including the preferential excitation of different modes based on beam injection geometry and the growth rate dependence on the beam injection velocity, critical velocity, and degree of velocity space anisotropy. The background damping rate is inferred from simulations and estimated analytically for relevant sources not present in the simulation model, indicating that co-CAEs are closer to marginal stability than modes driven by the cyclotron resonances.
Type:
Dataset
Issue Date:
March 2021

46. Statistical properties of magnetic structures and energy dissipation during turbulent reconnection in the Earth's magnetotail

Author(s):
Bergstedt, K.; Ji, H.; Jara-Almonte, J.; Yoo, J.; Ergun, R. E.; Chen, L.-J.
Abstract:
We present the first statistical study of magnetic structures and associated energy dissipation observed during a single period of turbulent magnetic reconnection, by using the in situ measurements of the Magnetospheric Multiscale mission in the Earth's magnetotail on 26 July 2017. The structures are selected by identifying a bipolar signature in the magnetic field and categorized as plasmoids or current sheets via an automated algorithm which examines current density and plasma flow. The size of the plasmoids forms a decaying exponential distribution ranging from subelectron up to ion scales. The presence of substantial number of current sheets is consistent with a physical picture of dynamic production and merging of plasmoids during turbulent reconnection. The magnetic structures are locations of significant energy dissipation via electric field parallel to the local magnetic field, while dissipation via perpendicular electric field dominates outside of the structures. Significant energy also returns from particles to fields.
Type:
Dataset
Issue Date:
14 September 2020

47. Application of Benchmarked Kinetic Resistive Wall Mode Stability Codes to ITER, Including Additional Physics

Author(s):
Berkery, J.W.; Wang, Z.R.; Sabbagh, S.A.; Liu, Y.Q.; Betti, R.; Guazotto, L.
Abstract:
Leading resistive wall mode (RWM) stability codes MARS-K [Y. Liu, et al., Phys. Plasmas 15, 112503 (2008)] and MISK [B. Hu, et al., Phys. Plasmas 12, 057301 (2005)] have been previously benchmarked. The benchmarking has now been extended to include additional physics, and used to project the stability of ITER in a realistic operating space. Due to ITER's relatively low plasma rotation and collisionality, collisions and non-resonance rotational effects were both found to have little impact on stability, and these non-resonance rotational effects also will not self-consistently affect the ITER RWM eigenfunction. Resonances between thermal ions and electrons and the expected level of ITER toroidal rotation were found to be important to stability, as were alpha particles, which are not in rotational resonance. MISK calculations show that without alpha particles, ITER is projected to be unstable to the RWM, but the expected level of alphas is calculated to provide a sufficient level of stability.
Type:
Dataset
Issue Date:
October 2017

48. Full-wave simulations of ICRF heating regimes in toroidal plasmas with non-Maxwellian distribution functions

Author(s):
Bertelli, N; Valeo, E.J.; Green, D.L.; Gorelenkova, M.; Phillips, C.K.; Podesta, M.; Lee, J.P.; Wright, J.C.; Jaeger, E.
Abstract:
At the power levels required for significant heating and current drive in magnetically-confined toroidal plasma, modification of the particle distribution function from a Maxwellian shape is likely [T. H. Stix, Nucl. Fusion, 15 737 (1975)], with consequent changes in wave propagation and in the location and amount of absorption. In order to study these effects computationally, both the finite-Larmor-radius and the high-harmonic fast wave (HHFW), versions of the full-wave, hot-plasma toroidal simulation code TORIC [M. Brambilla, Plasma Phys. Control. Fusion 41, 1 (1999) and M. Brambilla, Plasma Phys. Control. Fusion 44, 2423 (2002)], have been extended to allow the prescription of arbitrary velocity distributions of the form f(v||, v_perp, psi , theta). For hydrogen (H) minority heating of a deuterium (D) plasma with anisotropic Maxwellian H distributions, the fractional H absorption varies significantly with changes in parallel temperature but is essentially independent of perpendicular temperature. On the other hand, for HHFW regime with anisotropic Maxwellian fast ion distribution, the fractional beam ion absorption varies mainly with changes in the perpendicular temperature. The evaluation of the wave-field and power absorption, through the full wave solver, with the ion distribution function provided by either aMonte-Carlo particle and Fokker-Planck codes is also examined for Alcator C-Mod and NSTX plasmas. Non-Maxwellian effects generally tends to increase the absorption with respect to the equivalent Maxwellian distribution.
Type:
Dataset
Issue Date:
May 2017

49. Study of Stark Broadening of Krypton Helium-beta Lines and Estimation of Electron Density and Temperature in NIF Compressed Capsules

Author(s):
Hill, K. W.; Gao, L.; Kraus, B. F.; Bitter, M.; Efthimion, P. C.; Pablant, N. A.; Schneider, M. B.; Thorn, D. B.; Chen, H.; Kauffman, R. L.; Liedahl, D. A.; MacDonald, M. J.; MacPhee, A. G.; Scott, H. A.; Stoupin, S.; Doron, R.; Stambulchik, E.; Maron, Y.; Lahmann, B.
Abstract:
Numerical data used to draw the figures in the manuscript
Type:
Dataset
Issue Date:
6 June 2022

50. Initial Results from Boron Powder Injection Experiments in WEST Lower Single Null L-mode Plasmas

Author(s):
Bodner, Grant; Gallo, Alberto; Diallo, Ahmed; Lunsford, Robert; Moreau, Philippe; Nagy, Alex; Pellissier, Francis-Pierre; Guillemaut, Christophe; Gunn, James; Bourdelle, Clarisse; Desgranges, Corinne; Manas, Pierre; Bortolon, Alessandro; Klepper, Christopher; Tsitrone, Emanuelle; Unterberg, Ezekial; Vermare, Laure
Abstract:
Using a recently installed impurity powder dropper (IPD), boron powder (< 150 μm) was injected into lower single null (LSN) L-mode discharges in WEST. IPDs possibly enable real-time wall conditioning of the plasma-facing components and may help to facilitate H-mode access in the full-tungsten environment of WEST. The discharges in this experiment featured Ip = 0.5 MA, BT = 3.7 T, q95 = 4.3, tpulse = 12–30 s, ne,0 ~ 4×1019 m-2, and PLHCD ~ 4.5 MW. Estimates of the deuterium and impurity particle fluxes, derived from a combination of visible spectroscopy measurements and their corresponding S/XB coefficients, showed decreases of ~ 50% in O+, N+, and C+ populations during powder injection and a moderate reduction of these low-Z impurities (~ 50%) and W (~ 10%) in the discharges that followed powder injection. Along with the improved wall conditions, WEST discharges with B powder injection observed improved confinement, as the stored energy WMHD, neutron rate, and electron temperature Te increased significantly (10–25% for WMHD and 60–200% for the neutron rate) at constant input power. These increases in confinement scale up with the powder drop rate and are likely due to the suppression of ion temperature gradient (ITG) turbulence from changes in Zeff and/or modifications to the electron density profile.
Type:
Dataset
Issue Date:
2022

51. Wall conditioning and ELM mitigation with boron nitride powder injection in KSTAR

Author(s):
Gilson, Erik; Lee, H; Bortolon, A; Choe, W; Diallo, A; Hong, SH; Lee, HM; Maingi, R; Mansfield, DK; Nagy, A; Park, SH; Song, IW; Song, JI; Yun, SW; Nazikian, R
Abstract:
Results from KSTAR powder injection experiments, in which tens of milligrams of boron nitride (BN) were dropped into low-power H-mode plasmas, show an improvement in wall conditions in subsequent discharges and, in some cases, a reduction or elimination of edge-localized modes (ELMs). Injected powder is distributed by the plasma flow and is deposited on the wall and, over the course of several discharges, was observed to gradually reduce recycling by 33%, and decrease both the ELM amplitude and frequency. This is the first demonstration of the use of BN for ELM mitigation. In all of these experiments, an Impurity Powder Dropper (IPD) was used to introduce precise, controllable amounts of the materials into ELMy H-mode KSTAR discharges. The plasma duration was between 10 s and 15 s, 𝐼𝑝 = 500 kA, 𝐵𝑇 = 1.8 T, 𝑃NBI = 1.6 MW, and 𝑃ECH = 0.6 MW. Plasma densities were between 2 and 3 × 1019 m−3. In all cases, the pre-fill and startup gas-fueling was kept constant, suggesting that the decrease in baseline D𝛼 emission is in fact due to a reduction in recycling. The results presented herein highlight the viability of powder injection for intra-shot and between-shot wall conditioning.
Type:
Dataset
Issue Date:
September 2021

52. Modeling of Lithium Granule Injection in NSTX with M3D-C1

Author(s):
Fil, A.; Kolemen, E.; Bortolon, A.; Ferraro, N.; Jardin, S.; Parks, P.B.; Lunsford, R.; Maingi, R.
Abstract:
In this paper we present initial simulations of pedestal control by Lithium Granule Injection (LGI) in NSTX. A model for small granule ablation has been implemented in the M3D-C1 code [1], allowing the simulation of realistic Lithium granule injections. 2D simulations in NSTX L-mode and H-mode plasmas are done and the effect of granule size, injection angle and velocity on the pedestal gradient increase are studied. For H-mode cases, the amplitude of the local pressure perturbation caused by the granules is highly dependent on the solid granule size. In our simulations, reducing the granule injection velocity allows one to inject more particles at the pedestal top.
Type:
Dataset
Issue Date:
January 2017

53. Multi-species impurity granule injection and mass deposition projections in NSTX-U discharges Authors

Author(s):
Lunsford, R.; Bortolon, A.; Roquemore, A.L.; Mansfield, D.K.; Jaworski, M.A.; Kaita, R.; Maingi, R.; Nagy, A.
Abstract:
By employing a neutral gas shielding (NGS) model to characterize impurity granule injection the pedestal atomic deposition for three different species of granule: lithium, boron, and carbon are determined. Utilizing the duration of ablation events recorded on experiments performed at DIII-D to calibrate the NGS model we are able to quantify the ablation rate and mass deposition location with respect to the plasma density profile. The species specific granule shielding constant is then used to model granule ablation within NSTX-U discharges. Simulations of 300, 500 and 700 micron diameter granules injected at 50 m/sec are presented for NSTX-U L-mode type plasmas as well as H-mode discharges with low natural ELM frequencies. Additionally, ablation calculations of 500 micron granules of each species are presented at velocities ranging from 50 � 150 m/sec. In H-mode type discharges these simulations show that the majority of the injected granule is ablated within or just past the steep gradient region of the discharge. At this radial position, the perturbation to the background plasma generated by the ablating granule can lead to conditions advantageous for the rapid triggering of an ELM crash event.
Type:
Dataset
Issue Date:
July 2017

54. Type-I ELM mitigation by continuous lithium granule gravitational injection into the upper tungsten divertor in EAST

Author(s):
Sun, Zhen; Yuzhong, Qian; Maingi, Rajesh; Wang, Yifeng; Wang, Yumin; Nagy, Alex; Tritz, Kevin; Lunsford, Robert; Gilson, Erik; Zuo, Guizhong; Xu, Wei; Huang, Ming; Meng, Xiancai; Mansfield, Dennis K.; Zang, Qing; Zhu, Xiang; Lin, Xin; Liu, Haiqing; Duan, Yanmin; Zhang, Ling; Lyu, Bo; Liu, Yong; Wang, Liang; Bortolon, Ale; Xu, Guosheng; Gong, Xianzu; Hu, Jiansheng
Abstract:
Large edge-localized modes (ELMs) were mitigated by gravitational injection of lithium granules into the upper X-point region of the EAST device with tungsten plasma-facing components. The maximum ELM size was reduced by ~ 70% in high βN H-mode plasmas. Large ELM stabilization was sustained for up to about 40 energy confinement times, with constant core radiated power and no evidence of high-Z or low-Z impurity accumulation. The lithium granules injection reduced the edge plasma pedestal density and temperature and their gradients, due to increased edge radiation and reduced recycling from the plasma-facing components. Ideal stability calculations using the ELITE code indicate that the stabilization of large ELMs correlates with improved stability of intermediate-n peeling-ballooning modes, due to reduced edge current resulting from the profile changes. The pedestal pressure reduction was partially offset by a core density increase, which resulted in a modest ~ 7% drop in core stored energy and normalized energy confinement time. We surmise that the remnant small ELMs are triggered by the penetration of multiple Li granules just past the separatrix, similar to small ELMs triggered by deuterium pellet [S. Futatani et al., Nucl. Fusion 54 (2014) 073008]. This study extends previous ELM elimination with Li powder injection [R. Maingi et al., Nucl. Fusion 58 (2018) 024003] in EAST because 1) use of small, dust-like powder and the related potential health hazards were eliminated, and 2) use of macroscopic granules should be more applicable to future devices, due to deeper penetration than dust particles, e.g. inside the separatrix with velocities ~ 10 m/s in EAST.
Type:
Article
Issue Date:
April 2021

55. Fusion Nuclear Science Facilities and Pilot Plants Based on the Spherical Tokamak

Author(s):
Menard, J.E.; Brown, T.; El-Guebaly, L.; Boyer, M.; Canik, J.; Colling, B.; Raman, R.; Wang, Z.; Zhai, Y.; Buxton, P.; Covele, B.; D'Angelo, C.; Davis, A.; Gerhardt, S.; Gryaznevich, M.; Harb, M.; Hender, T.C.; Kaye, S.; Kingham, D.; Kotschenreuther, M.; Mahajan, S.; Maingi, R.; Marriott, E.; Meier, E.T.; Mynsberge, L.; Neumeyer, C.; Ono, M.; Park, J.-K.; Sabbagh, S.A.; Soukhanovskii, V.; Valanju, P.; Woolley, R.
Abstract:
A Fusion Nuclear Science Facility (FNSF) could play an important role in the development of fusion energy by providing the nuclear environment needed to develop fusion materials and components. The spherical torus/tokamak (ST) is a leading candidate for an FNSF due to its potentially high neutron wall loading and modular configuration. A key consideration for the choice of FNSF configuration is the range of achievable missions as a function of device size. Possible missions include: providing high neutron wall loading and fluence, demonstrating tritium self-sufficiency, and demonstrating electrical self-sufficiency. All of these missions must also be compatible with a viable divertor, first-wall, and blanket solution. ST-FNSF configurations have been developed simultaneously incorporating for the first time: (1) a blanket system capable of tritium breeding ratio TBR approximately 1, (2) a poloidal field coil set supporting high elongation and triangularity for a range of internal inductance and normalized beta values consistent with NSTX/NSTX-U previous/planned operation, (3) a long-legged divertor analogous to the MAST-U divertor which substantially reduces projected peak divertor heat-flux and has all outboard poloidal field coils outside the vacuum chamber and superconducting to reduce power consumption, and (4) a vertical maintenance scheme in which blanket structures and the centerstack can be removed independently. Progress in these ST-FNSF missions vs. configuration studies including dependence on plasma major radius R0 for a range 1m to 2.2m are described. In particular, it is found the threshold major radius for TBR = 1 is R0 greater than or equal to 1.7m, and a smaller R0=1m ST device has TBR approximately 0.9 which is below unity but substantially reduces T consumption relative to not breeding. Calculations of neutral beam heating and current drive for non-inductive ramp-up and sustainment are described. An A=2, R0=3m device incorporating high-temperature superconductor toroidal field coil magnets capable of high neutron fluence and both tritium and electrical self-sufficiency is also presented following systematic aspect ratio studies.
Type:
Dataset
Issue Date:
October 2016

56. Design and simulation of the snowflake divertor control for NSTX-U

Author(s):
Vail, P. J.; Boyer, M. D.; Welander, A. S.; Kolemen, E.; U.S. Department of Energy contract number DE-AC02-09CH11466
Abstract:
This paper presents the development of a physics-based multiple-input-multiple-output algorithm for real-time feedback control of snowflake divertor (SFD) configurations on the National Spherical Torus eXperiment Upgrade (NSTX-U). A model of the SFD configuration response to applied voltages on the divertor control coils is first derived and then used, in conjunction with multivariable control synthesis techniques, to design an optimal state feedback controller for the configuration. To demonstrate the capabilities of the controller, a nonlinear simulator for axisymmetric shape control was developed for NSTX-U which simultaneously evolves the currents in poloidal field coils based upon a set of feedback-computed voltage commands, calculates the induced currents in passive conducting structures, and updates the plasma equilibrium by solving the free-boundary Grad-Shafranov problem. Closed-loop simulations demonstrate that the algorithm enables controlled operations in a variety of SFD configurations and provides capabilities for accurate tracking of time-dependent target trajectories for the divertor geometry. In particular, simulation results suggest that a time-varying controller which can properly account for the evolving SFD dynamical response is not only desirable but necessary for achieving acceptable control performance. The algorithm presented in this paper has been implemented in the NSTX-U Plasma Control System in preparation for future control and divertor physics experiments.
Type:
Dataset
Issue Date:
April 2019

57. Real-time capable modeling of neutral beam injection on NSTX-U using neural networks

Author(s):
Boyer, M.D.; Kaye, S.; Erickson, K.
Abstract:
A new model of heating, current drive, torque and other effects of neutral beam injection on NSTX-U that uses neural networks has been developed. The model has been trained and tested on the results of the Monte Carlo code NUBEAM for the database of experimental discharges taken during the first operational campaign of NSTX-U. By projecting flux surface quantities onto empirically derived basis functions, the model is able to efficiently and accurately reproduce the behavior of both scalars, like the total neutron rate and shine through, and profiles, like beam current drive and heating. The model has been tested on the NSTX-U real-time computer, demonstrating a rapid execution time orders of magnitude faster than the Monte Carlo code that is well suited for the iterative calculations needed to interpret experimental results, optimization during scenario development activities, and real-time plasma control applications. Simulation results of a proposed design for a nonlinear observer that embeds the neural network calculations to estimate the poloidal flux profile evolution, as well as effective charge and fast ion diffusivity, are presented.
Type:
Dataset
Issue Date:
February 2019

58. Supplemental material for: Verification, validation, and results of an approximate model for the stress of a Tokamak toroidal field coil at the inboard midplane

Author(s):
Swanson, CPS; Kahn, S; Rana, C; Titus, PH; Brooks, AW; Guttenfelder, W; Zhai, Y; Brown, TG; Menard, JE
Abstract:
This is the supplemental material for the manuscript "Verification, validation, and results of an approximate model for the stress of a Tokamak toroidal field coil at the inboard midplane" submitted to Fusion Engineering and Design. This material includes PDF writeups of the derivations of the axisymmetric extended plane strain model, the elastic properties smearing model, and 20+ MATLAB scripts and functions which implement the model and generate the figures in the paper.
Type:
collection, Dataset, and Software
Issue Date:
2022

59. Modelling of Ablatant Deposition from Electromagnetically Driven Radiative Pellets for Disruption Mitigation Studies

Author(s):
Lunsford, Robert; Raman, Roger; Brooks, Arthur; Ellis, Robert A.; Lay, W-S;
Abstract:
The Electromagnetic Particle Injector (EPI) concept is advanced through the simulation of ablatant deposition into ITER H-mode discharges with calculations showing penetration past the H-mode pedestal for a range of injection velocities and granule sizes concurrent with the requirements of disruption mitigation. As discharge stored energy increases in future fusion devices such as ITER, control and handling of disruption events becomes a critical issue. An unmitigated disruption could lead to failure of the plasma facing components resulting in financially and politically costly repairs. Methods to facilitate the quench of an unstable high current discharge are required. With the onset warning time for some ITER disruption events estimated to be less than 10 ms, a disruption mitigation system needs to be considered which operates at injection speeds greater than gaseous sound speeds. Such an actuator could then serve as a means to augment presently planned pneumatic injection systems. The EPI uses a rail gun concept whereby a radiative payload is delivered into the discharge by means of the JxB forces generated by an external current pulse, allowing for injection velocities in excess of 1 km/s. The present status of the EPI project is outlined, including the addition of boost magnetic coils. These coils augment the self-generated rail gun magnetic field and thus provide a more efficient acceleration of the payload. The coils and the holder designed to constrain them have been modelled with the ANSYS code to ensure structural integrity through the range of operational coil cu
Type:
Dataset
Issue Date:
June 2019

60. Geometric concepts for stellarator permanent magnet arrays

Author(s):
Hammond, K. C.; Zhu, C.; Brown, T.; Corrigan, K.; Gates, D. A.; Sibilia, M.
Abstract:
The development of stellarators that use permanent magnet arrays to shape their confining magnetic fields has been a topic of recent interest, but the requirements for how such magnets must be shaped, manufactured, and assembled remain to be determined. To address these open questions, we have performed a study of geometric concepts for magnet arrays with the aid of the newly developed MAGPIE code. A proposed experiment similar to the National Compact Stellarator Experiment (NCSX) is used as a test case. Two classes of magnet geometry are explored: curved bricks that conform to a regular grid in cylindrical coordinates, and hexahedra that conform to the toroidal plasma geometry. In addition, we test constraints on the magnet polarization. While magnet configurations constrained to be polarized normally to a toroidal surface around the plasma are unable to meet the required magnetic field parameters when subject to physical limitations on the strength of present-day magnets, configurations with unconstrained polarizations are shown to satisfy the physics requirements for a targeted plasma.
Type:
Dataset
Issue Date:
July 2020

61. Fusion Pilot Plant performance and the role of a Sustained High Power Density tokamak

Author(s):
Menard, Jonathan; Grierson, Brian; Brown, Tom; Rana, Chirag; Zhai, Yuhu; Poli, Francesca; Maingi, Rajesh; Guttenfelder, Walter; Snyder, Philip
Abstract:
Recent U.S. fusion development strategy reports all recommend that the U.S. should pursue innovative science and technology to enable construction of a Fusion Pilot Plant (FPP) that produces net electricity from fusion at low capital cost. Compact tokamaks have been proposed as a means of potentially reducing the capital cost of a fusion pilot plant. However, compact steady-state tokamak FPPs face the challenge of integrating a high fraction of self-driven current with high core confinement, plasma pressure, and high divertor parallel heat flux. This integration is sufficiently challenging that a dedicated sustained-high-power-density (SHPD) tokamak facility is proposed by the U.S. community as the optimal way to close this integration gap. Performance projections for the steady-state tokamak FPP regime are presented and a preliminary SHPD device with substantial flexibility in lower aspect ratio (A=2-2.5), shaping, and divertor configuration to narrow gaps to a FPP is described.
Type:
Dataset
Issue Date:
January 2022

62. Elemental and topographical imaging of microscopic variations in deposition on NSTX-U and DIII-D samples2

Author(s):
C.H. Skinner, C.P. Chrobak, R. Kaita, B.E.Koel
Abstract:
Abstract: Tokamak plasma facing components have surface roughness that can cause microscopic spatial variations in erosion and deposition and hence influence material migration, erosion lifetime, dust and tritium accumulation, and plasma contamination. However high spatial resolution measurements of deposition on the scale of the surface roughness have been lacking to date. We will present elemental images of graphite samples from NSTX-U and DIII-D DiMES experiments performed with a Scanning Auger Microprobe at sub-micron resolution that show strong microscopic variations in deposition and correlate this with 3D topographical maps of surface irregularities. The NSTX-U samples were boronized and exposed to deuterium plasmas and the DiMES samples had localized Al and W films and were exposed to dedicated helium plasmas. Topographical maps of the samples were performed with a 3D confocal optical microscope and compared to the elemental deposition pattern. The results revealed localized deposition concentrated in areas shadowed from the ion flux, incident in a direction calculated (for the DiMES case) by taking account of the magnetic pre-sheath.
Type:
Dataset
Issue Date:
April 2019

63. Linear gyrokinetic simulations of microinstabilities within the pedestal region of H-mode NSTX discharges in a highly shaped geometry

Author(s):
Coury, M.; Guttenfelder, W.; Mikkelsen, D.; Canik, J.; Canal, G.; Diallo, A.; Kaye, S.; Kramer, G.; Maingi, R.
Abstract:
Linear (local) gyrokinetic predictions of edge microinstabilities in highly shaped, lithiated and non-lihiated NSTX discharges are reported using the gyrokinetic code GS2. Microtearing modes dominate the non-lithiated pedestal top. The stabilization of these modes at the lithiated pedestal top enables the electron temperature pedestal to extend further inwards, as observed experimentally. Kinetic ballooning modes are found to be unstable mainly at the mid-pedestal of both types of discharges, with unstable trapped electron modes nearer the separatrix region. At electron wavelengths, ETG modes are found to be unstable from mid-pedestal outwards for eta(e,exp)~2.2, with higher growth rates for the lithiated discharge. Near the separatrix, the critical temperature gradient for driving ETG modes is reduced in the presence of lithium, reflecting the reduction of the lithiated density gradients observed experimentally. A preliminary linear study in the edge of non-lithiated discharges shows that the equilibrium shaping alters the electrostatic modes stability, found more unstable at high plasma shaping.
Type:
Dataset
Issue Date:
June 2016

64. Exploration of magnetic perturbation effects on advanced divertor configurations in NSTX-U

Author(s):
Frerichs, H.; Waters, I.; Schmitz, O.; Canal, G.P.; Evans, T.E.; Feng, Y.; Soukhanovskii, V.A.
Abstract:
The control of divertor heat loads - both steady state and transient - remains a key challenge for the successful operation of ITER and FNSF. Magnetic perturbations provide a promising technique to control ELMs (transients), but understanding their detailed impact is difficult due to their symmetry breaking nature. One approach for reducing steady state heat loads are so called 'advanced divertors' which aim at optimizing the magnetic field configuration: the snowflake and the (super-)X-divertor. It is likely that both concepts - magnetic perturbations and advanced divertors - will have to work together, and we explore their interaction based on the NSTX-U setup. An overview of different divertor configurations under the impact of magnetic perturbations is presented, and the resulting impact on plasma edge transport is investigated with the EMC3-EIRENE code. Variations in size of the magnetic footprint of the perturbed separatrix are found, which is related to the level of flux expansion on the divertor target. Non-axisymmetric peaking of the heat flux related to the perturbed separatrix is found at the outer strike point, but only in locations where flux expansion is not too large.
Type:
Dataset
Issue Date:
June 2016

65. ELM frequency enhancement and discharge modification through lithium granule injection into EAST H-modes

Author(s):
Lunsford; Hsu, J.S.; Sun, Z.; Maingi, R.; Mansfield, D.K.; Xu, W.; Zuo, G.Z.; Huang, M.; Diallo, A.; Osborne, T.; Tritz, K.; Canik, J.; Meng, X.C.; Gong, X.Z.; Wan, B.N.; Li, J.G.; EAST Team
Abstract:
The injection of impurity granules into fusion research discharges can serve as a catalyst for ELM events. For sufficiently low ELM frequencies, and granule sizes above a threshold, this can result in full control of the ELM cycle, referred to as ELM pacing. For this research, we extend the investigation to conditions where the natural ELM frequency is too high for ELM pacing to be realized. Utilizing multiple sizes of lithium granules and classifying their effects by granule size, we demonstrate that ELM mitigation through frequency multiplication can be used at ELM triggering rates that nominally make ELM pacing unrealizable. We find that above a size threshold, injected granules promptly trigger ELMs and commensurately enhance the ELM frequency . Below this threshold size, injection of an individual granule does not always lead to the prompt triggering of an ELM; however, collective ablation in the edge pedestal region does enhance the ELM frequency. Specifically, Li granules too small to individually trigger ELMs were injected into EAST H-mode discharges at frequencies up to 2.3 kHz; collectively the granules were observed to enhance the natural ELM frequency up to 620 Hz, resulting in a ~2.4x multiplication of the natural ELM frequency and a 50% decrease of the ELM size.
Type:
Dataset
Issue Date:
October 2018

66. Phase coherence of parametric-decay modes during high-harmonic fast-wave heating in the National Spherical Torus Experiment

Author(s):
Carlsson, J.; Wilson, J.R.; Hosea, J.; Greenough, N.; Perkins, R.
Abstract:
Third-order spectral analysis, in particular the auto bicoherence, was applied to probe signals from high-harmonic fast-wave heating experiments in the National Spherical Torus Experiment. Strong evidence was found for parametric decay of the 30 MHz radio-frequency (RF) pump wave, with a low-frequency daughter wave at 2.7 MHz, the local majority-ion cyclotron frequency. The primary decay modes have auto bicoherence values around 0.85, very close to the theoretical value of one, which corresponds to total phase coherence with the pump wave. The threshold RF pump power for onset of parametric decay was found to be between 200 kW and 400 kW.
Type:
Dataset
Issue Date:
June 2016

67. Effects of Axial Boundary Conductivity on a Free Stewartson-Shercliff Layer

Author(s):
Caspary, Kyle J.; Choi, Dahan; Ebrahimi, Fatima; Gilson, Erik P.; Goodman, Jeremy; Ji, Hantao
Abstract:
The effects of axial boundary conductivity on the formation and stability of a magnetized free Stewartson-Shercliff layer (SSL) in a short Taylor-Couette device are reported. As the axial field increases with insulating endcaps, hydrodynamic Kelvin-Helmholtz-type instabilities set in at the SSLs of the conducting fluid, resulting in a much reduced flow shear. With conducting endcaps, SSLs respond to an axial field weaker by the square root of the conductivity ratio of endcaps to fluid. Flow shear continuously builds up as the axial field increases despite the local violation of the Rayleigh criterion, leading to a large number of hydrodynamically unstable modes. Numerical simulations of both the mean flow and the instabilities are in agreement with the experimental results.
Type:
Dataset
Issue Date:
2018

68. Identification of a non-axisymmetric mode in laboratory experiments searching for standard magnetorotational instability

Author(s):
Wang, Yin; Gilson, Erik P.; Ebrahimi, Fatima; Goodman, Jeremy; Caspary, Kyle J.; Winarto, Himawan W.; Ji, Hantao
Abstract:
This dataset provides the source data of figures in the main text of the paper "Identification of a non-axisymmetric mode in laboratory experiments searching for standard magnetorotational instability" accepted by Nature Communications.
Type:
Dataset
Issue Date:
2022

69. A novel scheme for error field correction in permanent magnet stellarators

Author(s):
Rutkowski, Adam; Hammond, Kenneth; Zhu, Caoxiang; Gates, David; Chambliss, Amelia
Abstract:
Stellarators offer a promising path towards fusion reactors, but their design and construction are complicated by stringent tolerance requirements on highly complex 3D coils. A potential way to simplify the engineering requirements for stellarators is to use simple planar toroidal field coils along with permanent magnet arrays to generate shaping fields. In order to ensure sufficient field accuracy while minimizing engineering complexity and system cost, new techniques are required to correct the field produced by the permanent magnet arrays to within requirements set by plasma physics. This work describes a novel correction method developed for this purpose. This analysis is applied to the design of a quasi-axisymmetric stellarator that employs a combination of permanent magnets and planar toroidal field coils to generate its magnetic field. Analysis techniques and initial results using the method for error correction on a proposed permanent magnet stellarator are shown, and it is demonstrated that the method successfully meets the design requirements of the project.
Type:
Dataset
Issue Date:
7 December 2022

70. Effects of collisional ion orbit loss on neoclassical tokamak radial electric fields

Author(s):
Zhu, Hongxuan; Stoltzfus-Dueck, T; Hager, R; Ku, S; Chang, C. S.
Abstract:
Ion orbit loss is considered important for generating the radially inward electric field Er in a tokamak edge plasma. In particular, this effect is emphasized in diverted tokamaks with a magnetic X point. In neoclassical equilibria, Coulomb collisions can scatter ions onto loss orbits and generate a radially outward current, which in steady state is balanced by the radially inward current from viscosity. To quantitatively measure this loss-orbit current in an edge pedestal, an ion-orbit-flux diagnostic has been implemented in the axisymmetric version of the gyrokinetic particle-in-cell code XGC. As the first application of this diagnostic, a neoclassical DIII-D H-mode plasma is studied using gyrokinetic ions and adiabatic electrons. The validity of the diagnostic is demonstrated by studying the collisional relaxation of Er in the core. After this demonstration, the loss-orbit current is numerically measured in the edge pedestal in quasisteady state. In this plasma, it is found that the radial electric force on ions from Er approximately balances the ion radial pressure gradient in the edge pedestal, with the radial force from the plasma flow term being a minor component. The effect of orbit loss on Er is found to be only mild.
Type:
Dataset
Issue Date:
2022

71. Electromagnetic total-f algorithm for gyrokinetic particle-in-cell simulations of boundary plasma in XGC

Author(s):
Hager, Robert; Ku, Seung-Hoe; Sharma, Amil Y.; Churchill, Randy Michael; Chang, C. S.; Scheinberg, Aaron
Abstract:
The simplified delta-f mixed-variable/pull-back electromagnetic simulation algorithm implemented in XGC for core plasma simulations by Cole et al. [Phys. Plasmas 28, 034501 (2021)] has been generalized to a total-f electromagnetic algorithm that can include, for the first time, the boundary plasma in diverted magnetic geometry with neutral particle recycling, turbulence and neoclassical physics. The delta-f mixed-variable/pull-back electromagnetic implementation is based on the pioneering work by Kleiber and Mischenko et al. [Kleiber et al., Phys. Plasmas 23, 032501 (2016); Mishchenko et al., Comput. Phys. Commun. 238, 194 (2019)]. An electromagnetic demonstration simulation is performed in a DIII-D-like, H-mode boundary plasma, including a corresponding comparative electrostatic simulation, which confirms that the electromagnetic simulation is necessary for a higher fidelity understanding of the electron particle and heat transport even at the low-beta pedestal foot in the vicinity of the magnetic separatrix.
Type:
Dataset
Issue Date:
21 November 2022

72. Global gyrokinetic study of shaping effects on electromagnetic modes at NSTX aspect ratio with ad hoc parallel magnetic perturbation effects

Author(s):
Sharma, A. Y.; Cole, M. D. J.; Görler, T.; Chen, Y.; Hatch, D. R.; Guttenfelder, W.; Hager, R.; Sturdevant, B. J.; Ku, S.; Chang, C. S.
Abstract:
Plasma shaping may have a stronger effect on global turbulence in tight-aspect-ratio tokamaks than in conventional-aspect-ratio tokamaks due to the higher toroidicity and more acute poloidal asymmetry in the magnetic field. In addition, previous local gyrokinetic studies have shown that it is necessary to include parallel magnetic field perturbations in order to accurately compute growth rates of electromagnetic modes in tight-aspect-ratio tokamaks. In this work, the effects of elongation and triangularity on global, ion-scale, linear electromagnetic modes are studied at NSTX aspect ratio and high plasma beta using the global gyrokinetic particle-in-cell code XGC. The effects of compressional magnetic perturbations are approximated via a well-known modification to the particle drifts that was developed for flux-tube simulations [N. Joiner et al., Phys. Plasmas 17, 072104 (2010)], without proof of its validity in a global simulation. Magnetic equilibria are re-constructed for each distinct plasma profile that is used. Coulomb collision effects are not considered. Within the limitations imposed by the present study, it is found that linear growth rates of electromagnetic modes (collisionless microtearing modes and kinetic ballooning modes) are significantly reduced by NSTX-like shaping. For example, growth rates of kinetic ballooning modes at high beta are reduced to the level of that of collisionless trapped electron modes.
Type:
Dataset
Issue Date:
2022

73. Gyrokinetic understanding of the edge pedestal transport driven by resonant magnetic perturbations in a realistic divertor geometry

Author(s):
Hager, R.; Chang, C. S.; Ferraro, N. M.; Nazikian R.
Abstract:
Self-consistent simulations of neoclassical and electrostatic turbulent transport in a DIII-D H-mode edge plasma under resonant magnetic perturbations (RMPs) have been performed using the global total-f gyrokinetic particle-in-cell code XGC, in order to study density-pump out and electron heat confinement.The RMP field is imported from the extended magneto-hydrodynamics (MHD) code M3D-C1, taking into account the linear two-fluid plasma response.With both neoclassical and turbulence physics considered together, the XGC simulation reproduces two key features of experimentally observed edge transport under RMPs: increased radial particle transport in the pedestal region that is sufficient to account for the experimental pump-out rate, and suppression of the electron heat flux in the steepest part of the edge pedestal.In the simulation, the density fluctuation amplitude of modes moving in the electron diamagnetic direction increases due to interaction with RMPs in the pedestal shoulder and outward, while the electron temperature fluctuation amplitude decreases.
Type:
Dataset
Issue Date:
June 2020

74. Neutral recycling effects on ITG turbulence

Author(s):
Stotler, D.P.; Lang, J.; Chang, C.S.; Churchill, R.M.; Ku, S.-H.
Abstract:
The effects of recycled neutral atoms on tokamak ion temperature gradient (ITG) driven turbulence have been investigated in a steep edge pedestal, magnetic separatrix configuration, with the full-f edge gryokinetic code XGC1. Ion temperature gradient turbulence is the most fundamental and robust edge plasma instability, having a long radial correlation length and an ability to impact other forms of pedestal turbulence. The neutral atoms enhance the ITG turbulence, first, by increasing the ion temperature gradient in the pedestal via the cooling effects of charge exchange and, second, by a relative reduction in the ExB shearing rate.
Type:
Dataset
Issue Date:
August 2017

75. Elemental and topographical imaging of microscopic variations in deposition on NSTX-U and DIII-D samples

Author(s):
Skinner, C.H.; Chrobak, C.P.; Kaita, R.; Koel, B.E.
Abstract:
Tokamak plasma facing components have surface roughness that can cause microscopic spatial variations in erosion and deposition and hence influence material migration, erosion lifetime, dust and tritium accumulation, and plasma contamination. However high spatial resolution measurements of deposition on the scale of the surface roughness have been lacking to date. We will present elemental images of graphite samples from NSTX-U and DIII-D DiMES experiments performed with a Scanning Auger Microprobe at sub-micron resolution that show strong microscopic variations in deposition and correlate this with 3D topographical maps of surface irregularities. The NSTX-U samples were boronized and exposed to deuterium plasmas and the DiMES samples had localized Al and W films and were exposed to dedicated helium plasmas. Topographical maps of the samples were performed with a 3D confocal optical microscope and compared to the elemental deposition pattern. The results revealed localized deposition concentrated in areas shadowed from the ion flux, incident in a direction calculated (for the DiMES case) by taking account of the magnetic pre-sheath.
Type:
Dataset
Issue Date:
October 2018

76. Deep convolutional neural networks for multi-scale time-series classification and application to disruption prediction in fusion devices

Author(s):
Churchill, R.M; the DIII-D team
Abstract:
The multi-scale, mutli-physics nature of fusion plasmas makes predicting plasma events challenging. Recent advances in deep convolutional neural network architectures (CNN) utilizing dilated convolutions enable accurate predictions on sequences which have long-range, multi-scale characteristics, such as the time-series generated by diagnostic instruments observing fusion plasmas. Here we apply this neural network architecture to the popular problem of disruption prediction in fusion tokamaks, utilizing raw data from a single diagnostic, the Electron Cyclotron Emission imaging (ECEi) diagnostic from the DIII-D tokamak. ECEi measures a fundamental plasma quantity (electron temperature) with high temporal resolution over the entire plasma discharge, making it sensitive to a number of potential pre-disruptions markers with different temporal and spatial scales. Promising, initial disruption prediction results are obtained training a deep CNN with large receptive field ({$\sim$}30k), achieving an $F_1$-score of {$\sim$}91\% on individual time-slices using only the ECEi data.
Type:
Dataset
Issue Date:
October 2019

77. Vertical forces during VDEs in an ITER plasma and the role of halo currents

Author(s):
Clasuer, C; Jardin, S; Ferraro, N
Abstract:
Vertical displacement events (VDEs) can occur in elongated tokamaks causing large currents to flow in the vessel and other adjacent metallic structures. To better understand the potential magnitude of the associated forces and the role of the so called ``halo currents'' on them, we have used the M3D-C1 code to simulate potential VDEs in ITER. We used actual values for the vessel resistivity and pre-quench temperatures and, unlike most of the previous studies, the halo region is naturally formed by triggering the thermal quench with an increase in the plasma thermal conductivity. We used the 2D non-linear version of the code and vary the post-thermal quench thermal conductivity profile as well as the boundary temperature in order to generate a wide range of possible cases that could occur in the experiment. We also show that, for a similar condition, increasing the halo current does not increase the total force on the wall since it is offset by a decrease in the toroidal contribution.
Type:
Dataset
Issue Date:
February 2020

81. Initial operation and data processing on a system for real-time evaluation of Thomson scattering signals on the Large Helical Device

Author(s):
Hammond, K. C.; Laggner, F. M.; Diallo, A.; Doskoczynski, S.; Freeman, C.; Funaba, H.; Gates, D.A.; Rozenblat, R.; Tchilinguirian, G.; Xing, Z.; Yamada, I.; Yasuhara, R.; Zimmer, G.; Kolemen, E.
Abstract:
A scalable system for real-time analysis of electron temperature and density based on signals from the Thomson scattering diagnostic, initially developed for and installed on the NSTX-U experiment, was recently adapted for the Large Helical Device (LHD) and operated for the first time during plasma discharges. During its initial operation run, it routinely recorded and processed signals for four spatial points at the laser repetition rate of 30 Hz, well within the system's rated capability for 60 Hz. We present examples of data collected from this initial run and describe subsequent adaptations to the analysis code to improve the fidelity of the temperature calculations.
Type:
Dataset
Issue Date:
2021

82. Modeling of a Laser-Induced Rydberg Spectroscopy diagnostic for Direct Measurement of the Local Electric Field in the Edge Region of NSTX/NSTX-U

Author(s):
Reymond, L.; Diallo, A.; Vekselman, V.
Abstract:
We discuss a novel diagnostic allowing direct measurements of the local electric field in the edge region in NSTX/NSTX-U. This laser based diagnostic's principle consists of depleting the naturally populated $n=3$ level to a Rydberg state --sensitive to electric fields-- that will result in a suppression of part of the $D_{\alpha}$ emission. We refer to this approach as Laser-Induced Rydberg Spectroscopy (LIRyS). It is shown that the local electric field can be measured through the Stark induced resonances observed as dips in the $D_\alpha$ emission. Using forward-modeling of simulated absorption spectra, we show precisions reaching \SI{\pm 2}{\kilo\volt\per\meter} in regions with a local electric field of \SI{15}{\kilo\volt\per\meter}.
Type:
Dataset
Issue Date:
July 2018

83. Formation of solitary zonal structures via the modulational instability of drift waves

Author(s):
Zhou, Yao; Zhu, Hongxuan; Dodin, I. Y.
Abstract:
{\rtf1\ansi\ansicpg1252\cocoartf1561\cocoasubrtf600 {\fonttbl\f0\fswiss\fcharset0 Helvetica;} {\colortbl;\red255\green255\blue255;\red0\green0\blue0;} {\*\expandedcolortbl;;\cssrgb\c0\c0\c0;} \margl1440\margr1440\vieww10800\viewh8400\viewkind0 \pard\tx887\tx1775\tx2662\tx3550\tx4438\tx5325\tx6213\tx7101\tx7988\tx8876\tx9764\tx10651\tx11539\tx12427\tx13314\tx14202\tx15090\tx15977\tx16865\tx17753\tx18640\tx19528\tx20416\tx21303\tx22191\tx23079\tx23966\tx24854\tx25742\tx26629\tx27517\tx28405\tx29292\tx30180\tx31067\tx31955\tx32843\tx33730\tx34618\tx35506\tx36393\tx37281\tx38169\tx39056\tx39944\tx40832\tx41719\tx42607\tx43495\tx44382\tx45270\tx46158\tx47045\tx47933\tx48821\tx49708\tx50596\tx51484\tx52371\tx53259\tx54147\tx55034\tx55922\tx56810\tx57697\tx58585\tx59472\tx60360\tx61248\tx62135\tx63023\tx63911\tx64798\tx65686\tx66574\tx67461\tx68349\tx69237\tx70124\tx71012\tx71900\tx72787\tx73675\tx74563\tx75450\tx76338\tx77226\tx78113\tx79001\tx79889\tx80776\tx81664\tx82552\tx83439\tx84327\tx85215\tx86102\tx86990\tx87877\tx88765\slleading20\pardirnatural\partightenfactor0 \f0\fs38 \cf2 The dynamics of the radial envelope of a weak coherent drift wave is approximately governed by a nonlinear Schr\'f6dinger equation, which emerges as a limit of the modified Hasegawa\'97Mima equation. The nonlinear Schr\'f6dinger equation has well-known soliton solutions, and its modulational instability can naturally generate solitary structures. In this paper, we demonstrate that this simple model can adequately describe the formation of solitary zonal structures in the modified Hasegawa\'97Mima equation, but only when the amplitude of the coherent drift wave is relatively small. At larger amplitudes, the modulational instability produces stationary zonal structures instead. Furthermore, we find that incoherent drift waves with beam-like spectra can also be modulationally unstable to the formation of solitary or stationary zonal structures, depending on the beam intensity. Notably, we show that these drift waves can be modeled as quantumlike particles (\'93driftons\'94) within a recently developed phase-space (Wigner\'97Moyal) formulation, which intuitively depicts the solitary zonal structures as quasi-monochromatic drifton condensates. Quantumlike effects, such as diffraction, are essential to these condensates; hence, the latter cannot be described by wave-kinetic models that are based on the ray approximation.\ }
Type:
Dataset
Issue Date:
June 2019

84. Nonlinear saturation and oscillations of collisionless zonal flows

Author(s):
Zhu, Hongxuan; Zhou, Yao; Dodin, I. Y.
Abstract:
In homogeneous drift-wave (DW) turbulence, zonal flows (ZFs) can be generated via a modulational instability (MI) that either saturates monotonically or leads to oscillations of the ZF energy at the nonlinear stage. This dynamics is often attributed as the predator-prey oscillations induced by ZF collisional damping; however, similar dynamics is also observed in collisionless ZFs, in which case a different mechanism must be involved. Here, we propose a semi-analytic theory that explains the transition between the oscillations and saturation of collisionless ZFs within the quasilinear Hasegawa-Mima model. By analyzing phase-space trajectories of DW quanta (driftons) within the geometrical-optics (GO) approximation, we argue that the parameter that controls this transition is N ~ \gamma_MI/\omega_DW, where \gamma_MI is the MI growth rate and \omega_DW is the linear DW frequency. We argue that at N << 1, ZFs oscillate due to the presence of so-called passing drifton trajectories, and we derive an approximate formula for the ZF amplitude as a function of time in this regime. We also show that at N >~ 1, the passing trajectories vanish and ZFs saturate monotonically, which can be attributed to phase mixing of higher-order sidebands. A modification of N that accounts for effects beyond the GO limit is also proposed. These analytic results are tested against both quasilinear and fully-nonlinear simulations. They also explain the earlier numerical results by Connaughton et al. [J. Fluid Mech. 654, 207 (2010)] and Gallagher et al. [Phys. Plasmas 19, 122115 (2012)] and offer a revised perspective on what the control parameter is that determines the transition from the oscillations to saturation of collisionless ZFs.
Type:
Dataset
Issue Date:
May 2019

85. Theory of the tertiary instability and the Dimits shift from reduced drift-wave models

Author(s):
Zhu, Hongxuan; Zhou, Yao; Dodin, I. Y.
Abstract:
Tertiary modes in electrostatic drift-wave turbulence are localized near extrema of the zonal velocity $U(x)$ with respect to the radial coordinate $x$. We argue that these modes can be described as quantum harmonic oscillators with complex frequencies, so their spectrum can be readily calculated. The corresponding growth rate $\gamma_{\rm TI}$ is derived within the modified Hasegawa--Wakatani model. We show that $\gamma_{\rm TI}$ equals the primary-instability growth rate plus a term that depends on the local $U''$; hence, the instability threshold is shifted compared to that in homogeneous turbulence. This provides a generic explanation of the well-known yet elusive Dimits shift, which we find explicitly in the Terry--Horton limit. Linearly unstable tertiary modes either saturate due to the evolution of the zonal density or generate radially propagating structures when the shear $|U'|$ is sufficiently weakened by viscosity. The Dimits regime ends when such structures are generated continuously.
Type:
Dataset
Issue Date:
January 2020

86. Wave-kinetic approach to zonal-flow dynamics: recent advances

Author(s):
Zhu, Hongxuan; Dodin, I. Y.
Abstract:
Basic physics of drift-wave turbulence and zonal flows has long been studied within the framework of wave-kinetic theory. Recently, this framework has been re-examined from first principles, which has led to more accurate yet still tractable "improved" wave-kinetic equations. In particular, these equations reveal an important effect of the zonal-flow "curvature" (the second radial derivative of the flow velocity) on dynamics and stability of drift waves and zonal flows. We overview these recent findings and present a consolidated high-level picture of (mostly quasilinear) zonal-flow physics within reduced models of drift-wave turbulence.
Type:
Dataset
Issue Date:
March 2021

87. Theory of the tertiary instability and the Dimits shift within a scalar model

Author(s):
Zhu, Hongxuan; Zhou Yao; Dodin, I.Y.
Abstract:
The Dimits shift is the shift between the threshold of the drift-wave primary instability and the actual onset of turbulent transport in magnetized plasma. It is generally attributed to the suppression of turbulence by zonal flows, but developing a more detailed understanding calls for consideration of specific reduced models. The modified Terry--Horton system has been proposed by St-Onge [J. Plasma Phys. {\bf 83}, 905830504 (2017)] as a minimal model capturing the Dimits shift. Here, we use this model to develop an analytic theory of the Dimits shift and a related theory of the tertiary instability of zonal flows. We show that tertiary modes are localized near extrema of the zonal velocity $U(x)$, where $x$ is the radial coordinate. By approximating $U(x)$ with a parabola, we derive the tertiary-instability growth rate using two different methods and show that the tertiary instability is essentially the primary drift-wave instability modified by the local $U''$. Then, depending on $U''$, the tertiary instability can be suppressed or unleashed. The former corresponds to the case when zonal flows are strong enough to suppress turbulence (Dimits regime), while the latter corresponds to the case when zonal flows are unstable and turbulence develops. This understanding is different from the traditional paradigm that turbulence is controlled by the flow shear $U'$. Our analytic predictions are in agreement with direct numerical simulations of the modified Terry--Horton system.
Type:
Dataset
Issue Date:
June 2020

88. A software package for plasma facing component analysis and design: the Heat flux Engineering Analysis Toolkit (HEAT)

Author(s):
Looby, Tom; Reinke, Matthew; Wingen, Andreas; Menard, Jonathan; Gerhardt, Stefan; Gray, Travis; Donovan, David; Unterberg, Ezekial; Klabacha, Jonathan; Messineo, Mike
Abstract:
The engineering limits of plasma facing components (PFCs) constrain the allowable operational space of tokamaks. Poorly managed heat fluxes that push the PFCs beyond their limits not only degrade core plasma performance via elevated impurities, but can also result in PFC failure due to thermal stresses or melting. Simple axisymmetric assumptions fail to capture the complex interaction between 3D PFC geometry and 2D or 3D plasmas. This results in fusion systems that must either operate with increased risk or reduce PFC loads, potentially through lower core plasma performance, to maintain a nominal safety factor. High precision 3D heat flux predictions are necessary to accurately ascertain the state of a PFC given the evolution of the magnetic equilibrium. A new code, the Heat flux Engineering Analysis Toolkit (HEAT), has been developed to provide high precision 3D predictions and analysis for PFCs. HEAT couples many otherwise disparate computational tools together into a single open source python package. Magnetic equilibrium, engineering CAD, finite volume solvers, scrape off layer plasma physics, visualization, high performace computing, and more, are connected in a single web-based user interface. Linux users may use HEAT without any software prerequisites via an appImage. This manuscript introduces HEAT, discusses the software architecture, presents first HEAT results, and outlines physics modules in development.
Type:
Dataset
Issue Date:
March 2021

89. Machine Learning Characterization of Alfvénic and Sub-Alfvénic Chirping and Correlation With Fast-Ion Loss at NSTX

Author(s):
Woods, B. J. Q.; Duarte, V. N.; Fredrickson, E. D.; Gorelenkov, N. N.; Podestà, M.; Vann, R. G. L.
Abstract:
Abrupt large events in the Alfvenic and sub-Alfvenic frequency bands in tokamaks are typically correlated with increased fast-ion loss. Here, machine learning is used to speed up the laborious process of characterizing the behavior of magnetic perturbations from corresponding frequency spectrograms that are typically identified by humans. The analysis allows for comparison between different mode character (such as quiescent, fixed frequency, and chirping, avalanching) and plasma parameters obtained from the TRANSP code, such as the ratio of the neutral beam injection (NBI) velocity and the Alfven velocity (v_inj./v_A), the q-profile, and the ratio of the neutral beam beta and the total plasma beta (beta_beam,i / beta). In agreement with the previous work by Fredrickson et al., we find a correlation between beta_beam,i and mode character. In addition, previously unknown correlations are found between moments of the spectrograms and mode character. Character transition from quiescent to nonquiescent behavior for magnetic fluctuations in the 50200-kHz frequency band is observed along the boundary v_phi ~ (1/4)(v_inj. - 3v_A), where v_phi is the rotation velocity.
Type:
Dataset
Issue Date:
December 2019

90. Calibrationless rotating Lorentz-force flowmeters for low flow rate applications

Author(s):
Hvasta, M. G.; Dudt, D.; Fisher, A. E.; Kolemen, E.
Abstract:
A 'weighted magnetic bearing' has been developed to improve the performance of rotating Lorentz-force flowmeters (RLFFs). Experiments have shown that the new bearing reduces frictional losses within a double-sided, disc-style RLFF to negligible levels. Operating such an RLFF under 'frictionless' conditions provides two major benefits. First, the steady-state velocity of the RLFF magnets matches the average velocity of the flowing liquid at low flow rates. This enables an RLFF to make accurate volumetric flow measurements without any calibration or prior knowledge of the fluid properties. Second, due to minimized frictional losses, an RLFF is able to measure low flow rates that cannot be detected when conventional, high-friction bearings are used. This paper provides a brief background on RLFFs, gives a detailed description of weighted magnetic bearings, and compares experimental RLFF data to measurements taken with a commercially available flowmeter.
Type:
Dataset
Issue Date:
29 May 2018

91. Phase space effects on fast ion distribution function modeling in tokamaks

Author(s):
Podesta, M.; M. Gorelenkova; E.D. Fredrickson; N.N. Gorelenkov
Abstract:
Integrated simulations of tokamak discharges typically rely on classical physics to model energetic particle (EP) dynamics. However, there are numerous cases in which energetic particles can suffer additional transport that is not classical in nature. Examples include transport by applied 3D magnetic perturbations and, more notably, by plasma instabilities. Focusing on the effects of instabilities, ad-hoc models can empirically reproduce increased transport, but the choice of transport coefficients is usually somehow arbitrary. New approaches based on physics-based reduced models are being developed to address those issues in a simplified way, while retaining a more correct treatment of resonant wave-particle interactions. The kick model implemented in the tokamak transport code TRANSP is an example of such reduced models. It includes modifications of the EP distribution by instabilities in real and velocity space, retaining correlations between transport in energy and space typical of resonant EP transport. The relevance of EP phase space modifications by instabilities is first discussed in terms of predicted fast ion distribution. Results are compared with those from a simple, ad-hoc diffusive model. It is then shown that the phase-space resolved model can also provide additional insight into important issues such as internal consistency of the simulations and mode stability through the analysis of the power exchanged between energetic particles and the instabilities.
Type:
Dataset
Issue Date:
April 2016

92. Large-volume flux closure during plasmoid-mediated reconnection in coaxial helicity injection

Author(s):
Ebrahimi, F.; Raman, R.
Abstract:
A large-volume flux closure during transient coaxial helicity injection (CHI) in NSTX-U is demonstrated through resistive magnetohydrodynamics (MHD) simulations. Several major improvements, including the improved positioning of the divertor poloidal field coils, are projected to improve the CHI start-up phase in NSTX-U. Simulations in the NSTX-U configuration with constant in time coil currents show that with strong flux shaping the injected open field lines (injector flux) rapidly reconnect and form large volume of closed flux surfaces. This is achieved by driving parallel current in the injector flux coil and oppositely directed currents in the flux shaping coils to form a narrow injector flux footprint and push the injector flux into the vessel. As the helicity and plasma are injected into the device, the oppositely directed field lines in the injector region are forced to reconnect through a local Sweet–Parker type reconnection, or to spontaneously reconnect when the elongated current sheet becomes MHD unstable to form plasmoids. In these simulations for the first time, it is found that the closed flux is over 70% of the initial injector flux used to initiate the discharge. These results could work well for the application of transient CHI in devices that employ super conducting coils to generate and sustain the plasma equilibrium.
Type:
Dataset
Issue Date:
April 2016

94. Physics design for a lithium vapor box divertor experiment on Magnum-PSI

Author(s):
Schwartz, Jacob; Emdee, Eric; Goldston, Robert; Jaworski, Michael
Abstract:
The lithium vapor box divertor is a potential solution for power exhaust in toroidal confinement devices. The divertor plasma interacts with a localized, dense cloud of lithium vapor, leading to volumetric radiation, cooling, recombination, and detachment. To minimize contamination of the core plasma, lithium vapor is condensed on cool (300°C to 400°C) baffles upstream of the detachment point. Before implementing this in a toroidal plasma device with a slot divertor geometry, we consider an experiment with a scaled baffled-pipe geometry in the high-power linear plasma device Magnum-PSI. Three 15 cm-scale open cylinders joined by 6 cm diameter ‘nozzles’ are positioned on the plasma beam axis upstream of a target. The central box may be loaded with several tens of grams of lithium, which can be evaporated at 650°C to produce a vapor predicted, using a simple plasma-neutral interaction model, to be dense enough to cause volumetric detachment in the plasma. The power delivered to the target and box walls as measured by increases in their temperatures after a 10 s plasma pulse can be compared to determine the effectiveness of the vapor in detaching the plasma. Direct Simulation Monte Carlo simulations are performed to estimate the flow rates of lithium vapor between the boxes and to estimate the trapping of H2 delivered by the plasma in the boxes, which could inadvertently lead to detachment. Details of the geometry, simulations, and possible diagnostic techniques are presented.
Type:
Dataset
Issue Date:
30 January 2019

95. A scalable real-time framework for Thomson scattering analysis: Application to NSTX-U

Author(s):
F. M. Laggner, A. Diallo, B. P. LeBlanc, R. Rozenblat, G. Tchilinguirian, E.Kolemen, the NSTX-U team
Abstract:
A detailed description of a prototype setup for real-time (rt) Thomson scattering (TS) analysis is presented and implemented in the multi-point Thomson scattering (MPTS) diagnostic system at the National Spherical Torus Experiment Upgrade(NSTX-U). The data acquisition hardware was upgraded with rt capable electronics (rt-analog digital converters (ADCs) and a rt server) that allow for fast digitization of the laser pulse signal of eight radial MPTS channels. In addition, a new TS spectrum analysis software for a rapid calculation of electron temperature (Te) and electron density (ne) was developed. Testing of the rt hardware and data analysis soft-ware was successfully completed and benchmarked against the standard, post-shot evaluation. Timing tests were performed showing that the end-to-end processing time was reproducibly below 17 ms for the duration of at least 5 s, meeting a 60 Hz deadline by the laser pulse repetition rate over the length of a NSTX-U discharge. The presented rt framework is designed to be scalable in system size, i.e. incorporation of additional radial channels by solely adding additional rt capable hardware. Furthermore, it is scalable in its operation duration and was continuously run for up to 30 min, making it an attractive solution for machines with long discharge duration such as advanced, non-inductive tokamaks or stellarators.
Type:
Dataset
Issue Date:
March 2019

96. Application of IR imaging for free-surface velocity measurement in liquid-metal systems

Author(s):
Hvasta, M. G.; Kolemen, E.; Fisher, A.
Abstract:
Measuring free-surface, liquid-metal flow velocity is challenging to do in a reliable and accurate manner. This paper presents a non-invasive, easily-calibrated method of measuring the surface velocities of open-channel liquid-metal flows using an IR camera. Unlike other spatially-limited methods, this IR camera particle tracking technique provides full field-of-view data that can be used to better understand open-channel flows and determine surface boundary conditions. This method could be implemented and automated for a wide range of liquid-metal experiments, even if they operate at high-temperatures or within strong magnetic fields.
Type:
Dataset
Issue Date:
January 2017

97. Experimental calibration procedures for rotating Lorentz-force flowmeters

Author(s):
Hvasta, M. G.; Slighton, N. T.; Kolemen, E.; Fisher, A. E.
Abstract:
Rotating Lorentz-force flowmeters are a novel and useful technology with a range of applications in a variety of different industries. However, calibrating these flowmeters can be challenging, time-consuming, and expensive. In this paper, simple calibration procedures for rotating Lorentz-force flowmeters are presented. These procedures eliminate the need for expensive equipment, numerical modeling, redundant flowmeters, and system down-time. The calibration processes are explained in a step-by-step manner and compared to experimental results.
Type:
Dataset
Issue Date:
2017

98. Study of liquid metal surface wave damping in the presence of magnetic fields and electrical currents

Author(s):
Fisher, A.E.; Hvasta, M.G.; Kolemen, E.
Abstract:
Experiments and predictions of surface wave damping in liquid metal due to a surface aligned magnetic field and externally regulated j × B force are presented. Fast-flowing, liquid-metal plasma facing components (LM-PFCs) are a proposed alternative to solid PFCs that are unable to handle the high heat flux, thermal stresses, and radiation damage in a tokamak. The significant technical challenges associated with LM-PFCs compared to solid PFCs are justified by greater heat flux management, self-healing properties, and reduced particle recycling. However, undesirable engineering challenges such as evaporation and splashing of the liquid metal introduce excessive impurities into the plasma and degrade plasma performance. Evaporation may be avoided through high-speed flow that limits temperature rise of the liquid metal by reducing heat flux exposure time, but as flow speed increases the surface may become more turbulent and prone to splashing and uneven surfaces. Wave damping is one mechanism that reduces surface disturbance and thus the chances of liquid metal impurity introduction into the plasma. Experiments on the Liquid Metal eXperiment Upgrade (LMX-U) examined damping under the influence of transverse magnetic fields and vertically directed Lorentz force.
Type:
Dataset
Issue Date:
March 2019

99. Development of miniaturized, spectroscopically assisted Penning gauges for fractional helium and hydrogen neutral pressure measurements

Author(s):
Flesch, K.; Kremeyer, T.; Schmitz, O.; Soukhanovskii, V.; Wenzel, U.
Abstract:
Direct measurements of the helium (He) fractional neutral pressure in the neutral gas around fusion devices is challenging because of the small mass difference between the abundant D2 molecules and the He atoms which will be produced by D-T fusion. To study He exhaust, Penning gauges were used to measure total neutral pressure assisted by spectroscopy to resolve the D and He partial pressures. In this contribution, initial results are shown from developing this technique into a miniaturized configuration for direct in-situ measurements in the divertor of fusion devices. The configuration is based off a gauge originally designed for the National Spherical Tokamak Experiment-Upgrade (NSTX-U). The goal of this new miniaturized design it to reduce the space required by the gauge on the device and use of the inherent magnetic field of the machine rather than permanent magnets inside the gauge, enabling it to be adapted into a system that can be extended directly into the divertor region. The feasibility test of the method for NSTX-U and the Wendelstein 7-X (W7-X) stellarator are surveyed. For W7-X, a commercial Penning Gauge has been installed on an outboard vacuum flange as a generic feasibility test in the neutral gas environment of a stellarator. At an integration time of 25s, helium lines can be seen down to 10^-5 mbar and H-alpha lines down to 10^-6 mbar. Successful measurement of the total as well as the fractional neutral pressures of He and H has been shown. A first prototype of the miniature Penning gauge has been tested in Madison and shows a near linear power law scaling between current and pressure: I = C*P^n with n = 1.0 - 1.2. Pressure measurements were achieved starting at 10^-3 mbar and down to 10^-6 mbar. A modular gauge is being assembled, which allows easy interchangeability of the anode to test new anode geometries, in order to improve optical access and increase spectroscopic sensitivity. This shall enable an increase of the time resolution of the spectroscopically assisted fractional neutral pressure measurements to up to 1kHz.
Type:
Dataset
Issue Date:
November 2016

100. Laboratory study of low-beta forces in arched, line-tied magnetic flux ropes

Author(s):
Myers, Clayton; Yamada, Masaaki; Ji, Hantao; Yoo, Jongsoo; Jara-Almonte, Jonathan; Fox, William
Abstract:
The loss-of-equilibrium is a solar eruption mechanism whereby a sudden breakdown of the magnetohydrodynamic force balance in the Sun's corona ejects a massive burst of particles and energy into the heliosphere. Predicting a loss-of-equilibrium, which has more recently been formulated as the torus instability, relies on a detailed understanding of the various forces that hold the pre-eruption magnetic flux rope in equilibrium. Traditionally, idealized analytical force expressions are used to derive simplified eruption criteria that can be compared to solar observations and modeling. What is missing, however, is a validation that these idealized analytical force expressions can be applied to the line-tied, low-aspect-ratio conditions of the corona. In this paper, we address this shortcoming by using a laboratory experiment to study the forces that act on long-lived, arched, line-tied magnetic flux ropes. Three key force terms are evaluated over a wide range of experimental conditions: (1) the upward hoop force; (2) the downward strapping force; and (3) the downward toroidal field tension force. First, the laboratory force measurements show that, on average, the three aforementioned force terms cancel to produce a balanced line-tied equilibrium. This finding validates the laboratory force measurement techniques developed here, which were recently used to identify a dynamic toroidal field tension force that can prevent flux rope eruptions [Myers et al., Nature 528, 526 (2015)]. The verification of magnetic force balance also confirms the low-beta assumption that the plasma thermal pressure is negligible in these experiments. Next, the measured force terms are directly compared to their corresponding analytical expressions. While the measured and analytical forces are found to be well correlated, the low-aspect-ratio, line-tied conditions in the experiment are found to both reduce the measured hoop force and increase the measured tension force with respect to analytical expectations. These two co-directed effects combine to generate laboratory flux rope equilibria at lower altitudes than are predicted analytically. Such considerations are expected to modify the loss-of-equilibrium eruption criteria for analogous flux ropes in the solar corona.
Type:
Dataset
Issue Date:
November 2016

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