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101. Far-infrared tangential interferometer/polarimeter design and installation for NSTX-U

Author(s):
Scott, E.R.; Barchfeld, R.; Riemenschneider, P.; Domier, C.W.; Muscatello, C.M.; Sohrabi, M.; Kaita, R.; Ren, Y.; Luhmann Jr., N.C.
Abstract:
The Far-infrared Tangential Interferometer/Polarimeter (FIReTIP) system has been refurbished and is being reinstalled on the National Spherical Torus Experiment-Upgrade (NSTX-U) to supply real-time line-integrated core electron density measurements for use in the NSTX-U plasma control system (PCS) to facilitate real-time density feedback control of the NSTX-U plasma. Inclusion of a visible light heterodyne interferometer in the FIReTIP system allows for real-time vibration compensation due to movement of an internally mounted retroreflector and the FIReTIP front-end optics. Real-time signal correction is achieved through use of a National Instruments CompactRIO field-programmable gate array.
Type:
Dataset
Issue Date:
November 2016

102. Fast animal pose estimation using deep neural networks

Author(s):
Pereira, Talmo D.; Aldarondo, Diego E.; Willmore, Lindsay; Kislin, Mikhail; Wang, Samuel S.-H.; Murthy, Mala; Shaevitz, Joshua W.
Abstract:
Recent work quantifying postural dynamics has attempted to define the repertoire of behaviors performed by an animal. However, a major drawback to these techniques has been their reliance on dimensionality reduction of images which destroys information about which parts of the body are used in each behavior. To address this issue, we introduce a deep learning-based method for pose estimation, LEAP (LEAP Estimates Animal Pose). LEAP automatically predicts the positions of animal body parts using a deep convolutional neural network with as little as 10 frames of labeled data for training. This framework consists of a graphical interface for interactive labeling of body parts and software for training the network and fast prediction on new data (1 hr to train, 185 Hz predictions). We validate LEAP using videos of freely behaving fruit flies (Drosophila melanogaster) and track 32 distinct points on the body to fully describe the pose of the head, body, wings, and legs with an error rate of <3% of the animal's body length. We recapitulate a number of reported findings on insect gait dynamics and show LEAP's applicability as the first step in unsupervised behavioral classification. Finally, we extend the method to more challenging imaging situations (pairs of flies moving on a mesh-like background) and movies from freely moving mice (Mus musculus) where we track the full conformation of the head, body, and limbs.
Type:
Dataset
Issue Date:
30 May 2018

104. 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

105. Femtosecond X-ray Diffraction of Laser-shocked Forsterite (Mg2SiO4) to 122 GPa

Author(s):
Kim, Donghoon; Tracy, Sally J.; Smith, Raymond F.; Gleason, Arianna E.; Bolme, Cindy A.; Prakapenka, Vitali B.; Appel, Karen; Speziable, Sergio; Wicks, June K.; Berryman, Eleanor J.; Han, Sirus K.; Schoelmerich, Markus O.; Lee, Hae Ja; Nagler, Bob; Cunningham, Eric F.; Akin, Minta C.; Asimow, Paul D.; Eggert, Jon H.; Duffy, Thomas S.
Abstract:
The behavior of forsterite, Mg2SiO4, under dynamic compression is of fundamental importance for understanding its phase transformations and high-pressure behavior. Here, we have carried out an in situ X-ray diffraction study of laser-shocked poly- and single-crystal forsterite (a-, b-, and c- orientations) from 19 to 122 GPa using the Matter in Extreme Conditions end-station of the Linac Coherent Light Source. Under laser-based shock loading, forsterite does not transform to the high-pressure equilibrium assemblage of MgSiO3 bridgmanite and MgO periclase, as was suggested previously. Instead, we observe forsterite and forsterite III, a metastable polymorph of Mg2SiO4, coexisting in a mixed-phase region from 33 to 75 GPa for both polycrystalline and single-crystal samples. Densities inferred from X-ray diffraction data are consistent with earlier gas-gun shock data. At higher stress, the behavior observed is sample-dependent. Polycrystalline samples undergo amorphization above 79 GPa. For [010]- and [001]-oriented crystals, a mixture of crystalline and amorphous material is observed to 108 GPa, whereas the [100]-oriented crystal adopts an unknown crystal structure at 122 GPa. The Q values of the first two sharp diffraction peaks of amorphous Mg2SiO4 show a similar trend with compression as those observed for MgSiO3 glass in both recent static and laser-compression experiments. Upon release to ambient pressure, all samples retain or revert to forsterite with evidence for amorphous material also present in some cases. This study demonstrates the utility of femtosecond free-electron laser X-ray sources for probing the time evolution of high-pressure silicates through the nanosecond-scale events of shock compression and release.
Type:
Dataset
Issue Date:
November 2020

106. First impurity powder injection experiments in LHD

Author(s):
Nespoli F., Ashikawa N., Gilson E.P., Lunsford R., Masuzaki S., Shoji M., Oishi T., Suzuki C., Nagy A., Mollen A., Pablant N.A., Ida K., Yoshinuma M., Tamura N., Gates D.A., Morisaki T., and the LHD experiment group
Abstract:
Injection of impurities in the form of sub-millimeter powder grains is performed for the first time in the Large Helical Device (LHD) plasma, employing the Impurity Powder Dropper (IPD) [A. Nagy et al., RSI 2018], developed and built by PPPL. Controlled amounts of boron (B) and boron nitride (BN) powder are injected into the helical plasma. Visible camera imaging, UV and charge exchange spectroscopy measurements show that the injected impurities effectively penetrate into the plasma in two different magnetic configurations.The prompt effects of the impurities on the plasma are characterized as the injection rate is scanned. The injected impurities provide a supplemental electron source, causing the plasma density to increase, together with the radiated power. Beneficial effects on the confined plasma temperature are observed at low plasma densities, due to an increased efficiency in NBI power absorption. For $n_{e,av}<10^{19}m^{-3}$ the powder grains penetrate deeper into the plasma, as they can be less effectively deflected by the plasma flow in the divertor leg, which they have to cross first as they are injected from the top of the machine.In this case, the created B ions are observed to move outwards from UV spectroscopy and charge exchange measurements, due to the outwards direction of the radial electric field. This makes low density plasmas a better candidate for powder boronization techniques.
Type:
Dataset
Issue Date:
November 2020

107. 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

109. 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

110. 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

111. 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

112. Gas Puff Imaging Diagnostics of Edge Plasma Turbulence in Magnetic Fusion Devices

Author(s):
Zweben, S.J.; Terry, J.L.; Stotler, D.P.; Maqueda, R.J.
Abstract:
Gas puff imaging (GPI) is a diagnostic of plasma turbulence which uses a puff of neutral gas at the plasma edge to increase the local visible light emission for improved space-time resolution of plasma fluctuations. This paper reviews gas puff imaging diagnostics of edge plasma turbulence in magnetic fusion research, with a focus on the instrumentation, diagnostic cross-checks, and interpretation issues. The gas puff imaging hardware, optics, and detectors are described for about 10 GPI systems implemented over the past ~15 years. Comparison of GPI results with other edge turbulence diagnostic results are described and many common features are observed. Several issues in the interpretation of GPI measurements are discussed, and potential improvements in hardware and modeling are suggested.
Type:
Dataset
Issue Date:
April 2017

113. 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

114. 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

115. 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

116. Gyrokinetic simulations of momentum flux parasitic to free-energy transfer

Author(s):
Stoltzfus-Dueck, T; Hornsby, W A; Grosshauser, S R
Abstract:
Ion Landau damping interacts with a portion of the E×B drift to cause a non-diffusive outward flux of co-current toroidal angular momentum. Quantitative evaluation of this momentum flux requires nonlinear simulations to determine fL, the fraction of fluctuation free energy that passes through ion Landau damping, in fully developed turbulence. Nonlinear gyrokinetic simulations with the GKW code confirm the presence of the systematic symmetry-breaking momentum flux. For simulations with adiabatic electrons, fL scales inversely with the ion temperature gradient, because only the ion curvature drift can transfer free energy to the electrostatic potential. Although kinetic electrons should in principle relax this restriction, the ion Landau damping measured in collisionless kinetic-electron simulations remained at low levels comparable with ion-curvature-drift transfer, except when magnetic shear was strong. A set of simulations scanning the electron pitch-angle scattering rate showed only a weak variation of fL with the electron collisionality. However, collisional-electron simulations with electron temperature greater than ion temperature unambiguously showed electron-curvature-drift transfer supporting ion Landau damping, leading to a corresponding enhancement of the symmetry-breaking momentum flux.
Type:
Dataset
Issue Date:
2022

117. 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

118. 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

119. 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

120. Hyperdiffusion of dust particles in a turbulent tokamak plasma

Author(s):
Nespoli, Federico; Kaganovich, Igor; Autricque, Adrien; Marandet, Yannick; Tamain, Patrick
Abstract:
The effect of plasma turbulence on the trajectories of dust particles is investigated for the first time. The dynamics of dust particles is computed using the ad-hoc developed Dust Injection Simulator code, using a 3D turbulent plasma background computed with the TOKAM3X code. As a result, the evolution of the particle trajectories is governed by the ion drag force, and the shape of the trajectory is set by the Stokes number $St\propto a_d/n_0$, with $a_d$ the dust radius and $n_0$ the density at the separatrix. The plasma turbulence is observed to scatter the dust particles, exhibiting a hyperdiffusive regime in all cases. The amplitude of the turbulent spread of the trajectories $\Delta r^2$ is shown to depend on the ratio $Ku/St$, with $Ku\propto u_{rms}$ the Kubo number and $u_{rms}$ the fluctuation level of the plasma flow. These results are compared with a simple analytical model, predicting $\Delta r^2\propto (Ku/St)^2t^3$, or $\Delta r^2\propto (u_{rms}n_0/a_d)^2t^3$. As the dust is heated by the plasma fluxes, thermionic emission sets the dust charge, originally negative, to slightly positive values. This results in a substantial reduction of the ion drag force through the suppression of its Coulomb scattering component. The dust grain inertia is then no longer negligible, and drives the transition from a hyperdiffusive regime towards a ballistic one.
Type:
Article
Issue Date:
July 2021

121. 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

122. 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

123. 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

124. 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

125. 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

126. 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

127. 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

128. Injected mass deposition thresholds for lithium granule instigated triggering of edge localized modes on EAST

Author(s):
Lunsford, R.; Sun, Z.; Maingi, R.; Hu, J.S.; Mansfield, D.; Xu, W.; Zuo, G.Z.; Diallo, A.; Osborne, T.; Tritz, K.; Canik, J.; Huang, M.; Meng, X.C.; Gong, X.Z.; Wan, B.N.; Li, J.G.
Abstract:
The ability of an injected lithium granule to promptly trigger an edge localized mode (ELM) has been established in multiple experiments. By horizontally injecting granules ranging in diameter from 200 microns to 1mm in diameter into the low field side of EAST H-mode discharges we have determined that granules with diameter > 600 microns are successful in triggering ELMs more than 95% of the time. It was also demonstrated that below 600 microns the triggering efficiency decreased roughly with granule size. Granules were radially injected from the outer midplane with velocities ~ 80 m/s into EAST upper single null discharges with an ITER like tungsten monoblock divertor. These granules were individually tracked throughout their injection cycle in order to determine their efficacy at triggering an ELM. For those granules of sufficient size, ELM triggering was a prompt response to granule injection. By simulating the granule injection with an experimentally benchmarked neutral gas shielding (NGS) model, the ablatant mass deposition required to promptly trigger an ELM is calculated and the fractional mass deposition is determined.
Type:
Dataset
Issue Date:
December 2017

129. Integrative Mechanisms of Social Attention

Author(s):
Bio, Branden; Graziano, Michael
Abstract:
Monitoring the attention of others is fundamental to social cognition. Most of the literature on the topic assumes that our social cognitive machinery is tuned specifically to the gaze direction of others as a proxy for attention. This standard assumption reduces attention to an externally visible parameter. Here we show that this assumption is wrong and a deeper, more meaningful representation is involved. We presented subjects with two cues about the attentional state of a face: direction of gaze and emotional expression. We tested whether people relied predominantly on one cue, the other, or both. If the traditional view is correct, then the gaze cue should dominate. Instead, people employed a variety of strategies, some relying on gaze, some on expression, and some on an integration of cues. We also assessed people’s social cognitive ability using two, independent, standard tests. If the traditional view is correct, then social cognitive ability, as assessed by the independent tests, should correlate with the degree to which people successfully use the gaze cue to judge the attention state of the face. Instead, social cognitive ability correlated best with the degree to which people successfully integrated the cues together, instead of with the use of any one specific cue. The results suggest a rethink of a fundamental component of social cognition: monitoring the attention of others involves constructing a deep model that is informed by a combination of cues. Attention is a rich process and monitoring the attention of others involves a similarly rich representation.
Type:
Dataset
Issue Date:
22 July 2021

130. Internal Rotation of ELM Filaments on NSTX

Author(s):
Lampert, Mate; Diallo, Ahmed; Zweben, Stewart; Myra, Jim
Abstract:
The data is formatted to text files. A corresponding file is provided for each figure.
Type:
Dataset
Issue Date:
September 2022

132. Intrinsic Rotation in Axisymmetric Devices

Author(s):
T Stoltzfus-Dueck
Abstract:
Toroidal rotation is critical for fusion in tokamaks, since it stabilizes instabilities that can otherwise cause disruptions or degrade confinement. Unlike present-day devices, ITER might not have enough neutral-beam torque to easily avoid these instabilities. We must therefore understand how the plasma rotates intrinsically, that is, without applied torque. Experimentally, torque-free plasmas indeed rotate, with profiles that are often non-flat and even non-monotonic. The rotation depends on many plasma parameters including collisionality and plasma current, and exhibits sudden bifurcations (rotation reversals) at critical parameter values.Since toroidal angular momentum is conserved in axisymmetric systems, and since experimentally inferred momentum transport is much too large to be neoclassical, theoretical work has focused on rotation drive by nondiffusive turbulent momentum fluxes. In the edge, intrinsic rotation relaxes to a steady state in which the total momentum outflux from the plasma vanishes. Ion drift orbits, scrape-off-layer flows, separatrix geometry, and turbulence intensity gradient all play a role. In the core, nondiffusive and viscous momentum fluxes balance to set the rotation gradient at each flux surface. Although many mechanisms have been proposed for the nondiffusive fluxes, most are treated in one of two distinct but related gyrokinetic formulations. In a radially local fluxtube, appropriate for rho star <<1, the lowest-order gyrokinetic formulations exhibit a symmetry that prohibits nondiffusive momentum flux for nonrotating plasmas in an up- down symmetric magnetic geometry with no ExB shear. Many symmetry-breaking mechanisms have been identified, but none have yet been conclusively demonstrated to drive a strong enough flux to explain commonly observed experimental rotation profiles. Radially global gyrokinetic simulations naturally include many symmetry-breaking mechanisms, and have shown cases with experimentally relevant levels of nondiffusive flux. These promising early results motivate further work to analyze, verify, and validate.This article provides a pedagogical introduction to intrinsic rotation in axisymmetric devices. Intended for both newcomers to the topic and experienced practitioners, the article reviews a broad range of topics including experimental and theoretical results for both edge and core rotation, while maintaining a focus on the underlying concepts.
Type:
Dataset
Issue Date:
November 2019

133. Inversion technique to obtain local ion temperature profiles for an axisymmetric plasma with toroidal and radial velocities

Author(s):
Bell, Ronald E.
Abstract:
A matrix inversion technique is derived to calculate local ion temperature from line-integrated measurements of an extended emission source in an axisymmetric plasma which exactly corrects for both toroidal velocity and radial velocity components. Local emissivity and toroidal velocity can be directly recovered from line-integrated spectroscopic measurements, but an independent measurement of the radial velocity is necessary to complete the temperature inversion. The extension of this technique to handle the radial velocity is relevant for magnetic reconnection and merging compression devices where temperature inversion from spectroscopic measurements is desired. A simulation demonstrates the effects of radial velocity on the determination of ion temperature.
Type:
Dataset
Issue Date:
February 2021

134. 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

135. 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

136. 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

137. 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

138. 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

140. 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

141. 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

142. MHD-blob correlations in NSTX

Author(s):
Zweben SJ; Fredrickson ED; Myra JR; Podesta M; Scotti F
Abstract:
This paper describes a study of the cross-correlations between edge fluctuations as seen in the gas puff imaging (GPI) diagnostic and low frequency coherent magnetic fluctuations (MHD) in H-mode plasmas in NSTX. The main new result was that large blobs in the SOL were significantly correlated with MHD activity the 3-6 kHz range in 21 of the 223 shots examined. There were also many other shots in which fluctuations in the GPI signal level and its peak radius Rpeak were correlated with MHD activity, but without any significant correlation of the MHD with large blobs. The structure and motion of the MHD is compared with that of the correlated blobs, and some possible theoretical mechanisms for the MHD-blob correlation are discussed.
Type:
Dataset
Issue Date:
May 2020

143. 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

144. 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

145. March Mathness: Effects of basketball on the brain

Author(s):
Antony, James; McDougle, Sam
Abstract:
Surprise signals a discrepancy between past and current beliefs. It is theorized to be linked to affective experiences, the creation of particularly resilient memories, and segmentation of the flow of experience into discrete perceived events. However, the ability to precisely measure naturalistic surprise has remained elusive. We used advanced basketball analytics to derive a quantitative measure of surprise and characterized its behavioral, physiological, and neural correlates in human subjects observing basketball games. We found that surprise was associated with segmentation of ongoing experiences, as reflected by subjectively perceived event boundaries and shifts in neocortical patterns underlying belief states. Interestingly, these effects differed by whether surprising moments contradicted or bolstered current predominant beliefs. Surprise also positively correlated with pupil dilation, activation in subcortical regions associated with dopamine, game enjoyment, and long-term memory. These investigations support key predictions from event segmentation theory and extend theoretical conceptualizations of surprise to real-world contexts.
Type:
Dataset
Issue Date:
2020

146. Massive Gas Injection Valve Development for NSTX-U

Author(s):
Raman, R.; Plunkett, G.J.; Way, W.-S.
Abstract:
NSTX-U research will offer new insight by studying gas assimilation efficiencies for MGI injection from different poloidal locations using identical gas injection systems. In support of this activity, an electromagnetic MGI valve has been built and tested. The valve operates by repelling two conductive disks due to eddy currents induced on them by a rapidly changing magnetic field created by a pancake disk solenoid positioned beneath the circular disk attached to a piston. The current is driven in opposite directions in the two solenoids, which creates a cancelling torque when the valve is operated in an ambient magnetic field, as would be required in a tokamak installation. The valve does not use ferromagnetic materials. Results from the operation of the valve, including tests conducted in 1 T external magnetic fields, are described. The pressure rise in the test chamber is measured directly using a fast time response baratron gauge. At a plenum pressure of just 1.38 MPa (~200 psig), the valve injects 27 Pa.m^3 (~200 Torr.L) of nitrogen with a pressure rise time of 3 ms.
Type:
Dataset
Issue Date:
May 2016

148. 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

149. 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

150. Mitigation of Alfven activity by 3D magnetic perturbations on NSTX

Author(s):
Kramer, G.J; Bortolon, A.; Ferraro, N.M.; Spong, D.A.; Crocker, N.A.; Darrow, D.S.; Fredrickson, E.D.; Kubota, S.; Park, J.-K.; Podesta, M.; Heidbrink, W.W.
Abstract:
Observations on the National Spherical Torus eXperiment (NSTX) indicate that externally applied non-axisymmetric magnetic perturbations (MP) can reduce the amplitude of Toroidal Alfven Eigenmodes (TAE) and Global Alfven Eigenmodes (GAE) in response to pulsed n=3 non-resonant fields. From full-orbit following Monte Carlo simulations with the 1- and 2-fluid resistive MHD plasma response to the magnetic perturbation included, it was found that in response to MP pulses the fast-ion losses increased and the fast-ion drive for the GAEs was reduced. The MP did not affect the fast-ion drive for the TAEs significantly but the Alfven continuum at the plasma edge was found to be altered due to the toroidal symmetry breaking which leads to coupling of different toroidal harmonics. The TAE gap was reduced at the edge creating enhanced continuum damping of the global TAEs, which is consistent with the observations. The results suggest that optimized non-axisymmetric MP might be exploited to control and mitigate Alfven instabilities by tailoring the fast-ion distribution function and/or continuum structure.
Type:
Dataset
Issue Date:
August 2016

151. 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

152. 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

153. 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

154. 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

155. Multi-Fluid and Kinetic Models of Partially Ionized Magnetic Reconnection

Author(s):
Jara-Almonte, J.; Murphy, N.A.; Ji, H.
Abstract:
Magnetic reconnection in partially ionized plasmas is a ubiquitous and important phenomena in both laboratory and astrophysical systems. Here, simulations of partially ionized magnetic reconnection with well-matched initial conditions are performed using both multi-fluid and fully-kinetic approaches. Despite similar initial conditions, the time-dependent evolution differs between the two models. In multi-fluid models, the reconnection rate locally obeys either a decoupled Sweet-Parker scaling, where neutrals are unimportant, or a fully coupled Sweet-Parker scaling, where neutrals and ions are strongly coupled, depending on the resistivity. In contrast, kinetic models show a faster reconnection rate that is proportional to the fully-coupled, bulk Alfv\'en speed, $v_A^\star$. These differences are interpreted as the result of operating in different collisional regimes. Multi-fluid simulations are found to maintain $\nu_{ni}L/v_A^\star \gtrsim 1$, where $\nu_{ni}$ is the neutral-ion collision frequency and $L$ is the time-dependent current sheet half-length. This strongly couples neutrals to the reconnection outflow, while kinetic simulations evolve to allow $\nu_{ni}L/v_A^\star < 1$, decoupling neutrals from the reconnection outflow. Differences in the way reconnection is triggered may explain these discrepancies.
Type:
Dataset
Issue Date:
2021

156. 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

160. Neoclassical transport in strong gradient regions of large aspect ratio tokamaks

Author(s):
Trinczek, Silvia; Parra, Felix I.; Catto, Peter J.; Calvo, Iván; Landreman, Matt
Abstract:
We present a new neoclassical transport model for large aspect ratio tokamaks where the gradient scale lengths are of the size of the ion poloidal gyroradius. Previous work on neoclassical transport across transport barriers assumed large density and potential gradients but a small temperature gradient, or neglected the gradient of the mean parallel flow. Using large aspect ratio and low collisionality expansions, we relax these restrictive assumptions. We define a new set of variables based on conserved quantities, which simplifies the drift kinetic equation whilst keeping strong gradients, and derive equations describing the transport of particles, parallel momentum and energy by ions in the banana regime. The poloidally varying parts of density and electric potential are included. Studying contributions from both passing and trapped particles, we show that the resulting transport is dominated by trapped particles. We find that a non-zero neoclassical particle flux requires parallel momentum input which could be provided through interaction with turbulence or impurities. We derive upper and lower bounds for the energy flux across a transport barrier in both temperature and density and present example profiles and fluxes.
Type:
Dataset
Issue Date:
2023

161. Neural pattern change during encoding of a narrative predicts retrospective duration estimates

Author(s):
Lositsky, Olga; Chen, Janice; Toker, Daniel; Honey, Christopher; Hasson, Uri; Norman, Kenneth
Abstract:
What mechanisms support our ability to estimate durations on the order of minutes? Behavioral studies in humans have shown that changes in contextual features lead to overestimation of past durations. Based on evidence that the medial temporal lobes and prefrontal cortex represent contextual features, we related the degree of fMRI pattern change in these regions with people's subsequent duration estimates. After listening to a radio story in the scanner, participants were asked how much time had elapsed between pairs of clips from the story. Our ROI analysis found that the neural pattern distance between two clips at encoding was correlated with duration estimates in the right entorhinal cortex and right pars orbitalis. Moreover, a whole-brain searchlight analysis revealed a cluster spanning the right anterior temporal lobe. Our findings provide convergent support for the hypothesis that retrospective time judgments are driven by 'drift' in contextual representations supported by these regions.
Type:
Dataset
Issue Date:
12 March 2016

162. 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

163. Noise correlations in the human brain and their impact on pattern classification

Author(s):
Bejjanki, Vikranth R.; da Silveira, Rava Azeredo; Cohen, Jonathan D.; Turk-Browne, Nicholas B.
Abstract:
Multivariate decoding methods, such as multivoxel pattern analysis (MVPA), are highly effective at extracting information from brain imaging data. Yet, the precise nature of the information that MVPA draws upon remains controversial. Most current theories emphasize the enhanced sensitivity imparted by aggregating across voxels that have mixed and weak selectivity. However, beyond the selectivity of individual voxels, neural variability is correlated across voxels, and such noise correlations may contribute importantly to accurate decoding. Indeed, a recent computational theory proposed that noise correlations enhance multivariate decoding from heterogeneous neural populations. Here we extend this theory from the scale of neurons to functional magnetic resonance imaging (fMRI) and show that noise correlations between heterogeneous populations of voxels (i.e., voxels selective for different stimulus variables) contribute to the success of MVPA. Specifically, decoding performance is enhanced when voxels with high vs. low noise correlations (measured during rest or in the background of the task) are selected during classifier training. Conversely, voxels that are strongly selective for one class in a GLM or that receive high classification weights in MVPA tend to exhibit high noise correlations with voxels selective for the other class being discriminated against. Furthermore, we use simulations to show that this is a general property of fMRI data and that selectivity and noise correlations can have distinguishable influences on decoding. Taken together, our findings demonstrate that if there is signal in the data, the resulting above-chance classification accuracy is modulated by the magnitude of noise correlations.
Type:
Dataset
Issue Date:
August 2017

164. Nonlinear fishbone dynamics in spherical tokamaks

Author(s):
Wang, F.; Fu, G.Y.; Shen, W.
Abstract:
Linear and nonlinear kinetic-MHD hybrid simulations have been carried out to investigate linear stability and nonlinear dynamics of beam-driven fishbone instability in spherical tokamak plasmas. Realistic NSTX parameters with finite toroidal rotation were used. The results show that the fishbone is driven by both trapped and passing particles. The instability drive of passing particles is comparable to that of trapped particles in the linear regime. The effects of rotation are destabilizing and a new region of instability appears at higher qmin (>1.5) values, qmin being the minimum of safety factor profile. In the nonlinear regime, the mode saturates due to flattening of beam ion distribution, and this persists after initial saturation while mode frequency chirps down in such a way that the resonant trapped particles move out radially and keep in resonance with the mode. Correspondingly, the flattening region of beam ion distribution expands radially outward. A substantial fraction of initially non- resonant trapped particles become resonant around the time of mode saturation and keep in resonance with the mode as frequency chirps down. On the other hand, the fraction of resonant passing particles is significantly smaller than that of trapped particles. Our analysis shows that trapped particles provide the main drive to the mode in the nonlinear regime.
Type:
Dataset
Issue Date:
January 2017

165. Nonlinear growth of magnetic islands by passing fast ions in NSTX

Author(s):
Yang, James; Fredrickson, Eric; Podestà, Mario; Poli, Francesca
Abstract:
The growth of magnetic islands in NSTX is modeled successfully, with the consideration of passing fast ions. It is shown that a good quantitative agreement between simulation and experimental measurement can be achieved when the uncompensated cross-field current induced by passing fast ions is included in the island growth model. The fast ion parameters, along with other equilibrium parameters, are obtained self-consistently using the TRANSP code with the assumptions of the ‘kick’ model (Podestà et al 2017 Plasma Phys. Control. Fusion 59 095008). The results show that fast ions can contribute to overcoming the stabilizing effect of polarization current for magnetic island growth.
Type:
Dataset
Issue Date:
2022

166. 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

167. Nonlinear simulations of beam-driven Compressional Alfvén Eigenmodes in NSTX

Author(s):
Belova, E.V.; Gorelenkov, N.N.; Crocker, N.A.; Lestz, J.B.; Fredrickson, E.D.; Tang, S.; Tritz, K.
Abstract:
Results of 3D nonlinear simulations of neutral-beam-driven compressional Alfven eigenmodes (CAEs) in the National Spherical Torus Experiment (NSTX) are presented. Hybrid MHD-particle simulations for the H-mode NSTX discharge (shot 141398) using the HYM code show unstable CAE modes for a range of toroidal mode numbers, n=4-9, and frequencies below the ion cyclotron frequency. It is found that the essential feature of CAEs is their coupling to kinetic Alfven wave (KAW) that occurs on the high-field side at the Alfven resonance location. High-frequency Alfven eigenmodes are frequently observed in beam-heated NSTX plasmas, and have been linked to flattening of the electron temperature profiles at high beam power. Coupling between CAE and KAW suggests an energy channeling mechanism to explain these observations, in which beam-driven CAEs dissipate their energy at the resonance location, therefore significantly modifying the energy deposition profile. Nonlinear simulations demonstrate that CAEs can channel the energy of the beam ions from the injection region near the magnetic axis to the location of the resonant mode conversion at the edge of the beam density profile. A set of nonlinear simulations show that the CAE instability saturates due to nonlinear particle trapping, and a large fraction of beam energy can be transferred to several unstable CAEs of relatively large amplitudes and absorbed at the resonant location. Absorption rate shows a strong scaling with the beam power.
Type:
Dataset
Issue Date:
April 2017

171. 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

172. 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

173. On the scattering correction of fast-ion D-alpha signal on NSTX-U

Author(s):
Hao, G.Z; Heidbrink, W.W.; Liu, D.; Stagner, L.; Podesta, M.; Bortolon, A.
Abstract:
Analysis of fast-ion D-alpha (FIDA) data on National Spherical Torus Experiment-Upgrade (NSTX-U) shows that the cold Dα line contaminates the FIDA baseline. The scattered light is comparable to the FIDA emission. A scattering correction is required to extract the FIDA signal. Two methods that relate the scattered light contamination to the intensity of the cold Dα line are employed. One method uses laboratory measurements with a calibration lamp; the other method uses data acquired during plasma operation and singular value decomposition analysis. After correction, both the FIDA spectra and spatial profile are in better agreement with theoretical predictions.
Type:
Dataset
Issue Date:
June 2018

174. Operational Space and Performance Limiting Events in the First Physics Campaign of MAST-U

Author(s):
Berkery, John
Abstract:
The MAST-U fusion plasma research device, the upgrade to the Mega Amp Spherical Tokamak, has recently completed its first campaign of physics operation. MAST-U operated with Ohmic, or one or two neutral beams for heating, at 400-800 kA plasma current, in conventional or “SuperX” divertor configurations. Equilibrium reconstructions provide key plasma physics parameters vs. time for each discharge, and diagrams are produced which show where the prevalence of operation occurred as well as the limits in various operational spaces. When compared to stability limits, the operation of MAST-U so far has generally stayed out of the low q, low density instability region, and below the high density Greenwald limit, high beta global stability limits, and high elongation vertical stability limit. MAST-U still has the potential to reach higher elongation, which could benefit the plasma performance. Despite the majority of operation happening below established stability limits, disruptions did occur in the flat-top phase of MAST-U plasmas. The reasons for these disruptions are highlighted, and possible strategies to avoid them and to extend the operational space of MAST-U in future campaigns are discussed.
Type:
Dataset
Issue Date:
18 January 2023

175. Optimization of the angular orientation for a fast ion loss detector in a tokamak

Author(s):
Darrow, D.
Abstract:
A scintillator type fast ion loss detector measures the gyroradius and pitch angle distribution of superthermal ions escaping from a magnetically confined fusion plasma at a single location. Described here is a technique for optimizing the angular orientation of such a detector in an axisymmetric tokamak geometry in order to intercept losses over a useful and interesting ranges of pitch angle. The method consists of evaluating the detector acceptance as a function of the fast ion constants of motion, i.e. energy, canonical toroidal momentum, and magnetic moment. The detector acceptance can then be plotted in a plane of constant energy and compared with the relevant orbit class boundaries and fast ion source distributions. Knowledge of expected or interesting mechanisms of loss can further guide selection of the detector orientation. The example of a fast ion loss detector for the National Spherical Torus Experiment-Upgrade (NSTX-U) is considered.
Type:
Dataset
Issue Date:
January 2017

176. 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

177. Parallel electron force balance and the L-H transition

Author(s):
Stoltzfus-Dueck, T.
Abstract:
In one popular description of the L-H transition, energy transfer to the mean flows directly depletes turbulence fluctuation energy, resulting in suppression of the turbulence and a corresponding transport bifurcation. However, electron parallel force balance couples nonzonal velocity fluctuations with electron pressure fluctuations on rapid timescales, comparable with the electron transit time. For this reason, energy in the nonzonal velocity stays in a fairly fixed ratio to the free energy in electron density fluctuations, at least for frequency scales much slower than electron transit. In order for direct depletion of the energy in turbulent fluctuations to cause the L-H transition, energy transfer via Reynolds stress must therefore drain enough energy to significantly reduce the sum of the free energy in nonzonal velocities and electron pressure fluctuations. At low k, the electron thermal free energy is much larger than the energy in nonzonal velocities, posing a stark challenge for this model of the L-H transition.
Type:
Dataset
Issue Date:
May 2016

178. Parametric dependencies of resonant layer responses across linear, two-fluid, drift-MHD regimes

Author(s):
Park, Jong-Kyu
Abstract:
Non-axisymmetric magnetic fields arising in a tokamak either by external or internal perturbations can induce complex non-ideal MHD responses in their resonant surfaces while remaining ideally evolved elsewhere. This layer response can be characterized in a linear regime by a single parameter called the inner-layer Delta, which enables outer-layer matching and the prediction of torque balance to non-linear island regimes. Here, we follow strictly one of the most comprehensive analytic treatments including two-fluid and drift MHD effects and keep the fidelity of the formulation by incorporating the numerical method based on the Riccati transformation when quantifying the inner-layer Delta. The proposed scheme reproduces not only the predicted responses in essentially all asymptotic regimes but also with continuous transitions as well as improved accuracies. In particular, the Delta variations across the inertial regimes with viscous or semi-collisional effects have been further resolved, in comparison with additional analytic solutions. The results imply greater shielding of the electromagnetic torque at the layer than what would be expected by earlier work when the viscous or semi-collisional effects can compete against the inertial effects, and also due to the intermediate regulation by kinetic Alfven wave resonances as rotation slows down. These are important features that can alter the nonaxisymmetric plasma responses including the field penetration by external fields or island seeding process in rotating tokamak plasmas.
Type:
Dataset
Issue Date:
26 July 2022

179. 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

180. 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

181. 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

182. Plasma Facing Components with Capillary Porous System and Liquid Metal Coolant Flow

Author(s):
Khodak, Andrei; Maingi, Rajesh
Abstract:
Liquid metal can create a renewable protective surface on plasma facing components (PFC), with an additional advantage of deuterium pumping and the prospect of tritium extraction if liquid lithium (LL) is used and maintained below 450 C, the temperature above which LL vapor pressure begins to contaminate the plasma. LM can also be utilized as an efficient coolant, driven by the Lorentz force created with the help of the magnetic field in fusion devices. Capillary porous systems can serve as a conduit of LM and simultaneously provide stabilization of the LM flow, protecting against spills into the plasma. Recently a combination of a fast-flowing LM cooling system with a porous plasma facing wall (CPSF) was investigated [Khodak and Maingi (2021)]. The system takes an advantage of a magnetohydrodynamics velocity profile, as well as attractive LM properties to promote efficient heat transfer from the plasma to the LL at low pumping energy cost, relative to the incident heat flux on the PFC. In case of a disruption leading to excessive heat flux from the plasma to the LM PFCs, LL evaporation can stabilize the PFC surface temperature, due to high evaporation heat and apparent vapor shielding. The proposed CPSF was optimized analytically for the conditions of a Fusion Nuclear Science Facility [Kessel et al. (2019)]: 10T toroidal field and 10 MW/m2 peak incident heat flux. Computational fluid dynamics analysis confirmed that a CPSF system with 2.5 mm square channels can pump enough LL so that no additional coolant is needed.
Type:
Dataset and Image
Issue Date:
2021

183. 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

184. 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

186. Prediction of electron density and pressure profile shapes on NSTX-U using neural networks

Author(s):
Boyer, Mark; Chadwick, Jason
Abstract:
A new model for prediction of electron density and pressure profile shapes on NSTX and NSTX-U has been developed using neural networks. The model has been trained and tested on measured profiles from experimental discharges during the first operational campaign of NSTX-U. By projecting profiles onto empirically derived basis functions, the model is able to efficiently and accurately reproduce profile shapes. In order to project the performance of the model to upcoming NSTX-U operations, a large database of profiles from the operation of NSTX is used to test performance as a function of available data. The rapid execution time of the model is well suited to the planned applications, including optimization during scenario development activities, and real-time plasma control. A potential application of the model to real-time profile estimation is demonstrated.
Type:
Dataset
Issue Date:
February 2021

187. Princeton Open Ventilation Monitor

Author(s):
Bourrianne, Philippe; Chidzik, Stanley; Cohen, Daniel; Elmer, Peter; Hallowell, Thomas; Kilbaugh, Todd J.; Lange, David; Leifer, Andrew M.; Marlow, Daniel R.; Meyers, Peter D.; Normand, Edna; Nunes, Janine; Oh, Myungchul; Page, Lyman; Periera, Talmo; Pivarski, Jim; Schreiner, Henry; Stone, Howard A.; Tank, David W.; Thiberge, Stephan; Tully, Christopher
Abstract:
The detailed information on the design and construction of the Princeton Open Ventilation Monitor device and software are contained in this data repository. This information consists of the electrical design files, mechanical design files, bill of materials, human subject recording and analysis code, and a copy of the code repository for operating the patient monitors and central station.
Type:
Dataset, Software, and Image
Issue Date:
22 November 2021

190. Quantitative imaging of carbon dimer precursor for nanomaterial synthesis in the carbon arc

Author(s):
Vekselman, V.; Khrabry, A.; Kaganovich, I.; Stratton, B.; Selinsky, R. S.; Raitses, Y.
Abstract:
Delineating the dominant processes responsible for nanomaterial synthesis in a plasma environment requires measurements of the precursor species contributing to the growth of nanostructures. We performed comprehensive measurements of spatial and temporal profiles of carbon dimers in sub-atmospheric-pressure carbon arc by laser-induced fluorescence. Measured spatial profiles of carbon dimers coincide with the growth region of carbon nanotubes (Fang et al 2016 Carbon 107 273-80) and vary depending on the arc operation mode, which is determined by the discharge current and the ablation rate of the graphite anode. The carbon dimer density profile exhibits large spatial and time variations due to motion of the arc core. A comparison of the experimental data with the 2D simulation results of self-consistent arc modeling shows a good agreement. The model predicts well the main processes determining spatial profiles of carbon dimers.
Type:
Dataset
Issue Date:
January 2018

191. 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

192. Quasi-static and dynamic magnetic tension forces in arched, line-tied magnetic flux ropes

Author(s):
Myers, Clayton; Yamada, Masaaki; Ji, Hantao; Yoo, Jongsoo; Jara-Almonte, Jonathan; Fox, William
Abstract:
Solar eruptions are often driven by magnetohydrodynamic instabilities such as the torus and kink instabilities that act on line-tied magnetic flux ropes. Recent laboratory experiments designed to study these eruptive instabilities have demonstrated the key role of both dynamic (Myers et al 2015 Nature 528, 526) and quasi-static (Myers et al 2016 Phys. Plasmas, in press) magnetic tension forces in contributing to the equilibrium and stability of line-tied magnetic flux ropes. In this paper, we synthesize these laboratory results and explore the relationship between the dynamic and quasi-static tension forces. While the quasi-static tension force is found to contribute to the flux rope equilibrium in a number of regimes, the dynamic tension force is substantial mostly in the so-called failed torus regime where magnetic self-organization events prevent the flux rope from erupting.
Type:
Dataset
Issue Date:
December 2016

193. Quasioptical modeling of wave beams with and without mode conversion: II. Numerical simulations of single-mode beams

Author(s):
K. Yanagihara, I. Y. Dodin, and S. Kubo
Abstract:
This work continues a series of papers where we propose an algorithm for quasioptical modeling of electromagnetic beams with and without mode conversion. The general theory was reported in the first paper of this series, where a parabolic partial differential equation was derived for the field envelope that may contain one or multiple modes with close group velocities. Here, we present a corresponding code PARADE (PAraxial RAy DEscription) and its test applications to single-mode beams in vacuum and also in inhomogeneous magnetized plasma. The numerical results are compared, respectively, with analytic formulas from Gaussian-beam optics and also with cold-plasma ray tracing. Quasioptical simulations of mode-converting beams are reported in the next, third paper of this series.
Type:
Dataset
Issue Date:
August 2019

194. Quasioptical modeling of wave beams with and without mode conversion: III. Numerical simulations of mode-converting beams

Author(s):
K. Yanagihara, I. Y. Dodin, and S. Kubo
Abstract:
This work continues a series of papers where we propose an algorithm for quasioptical modeling of electromagnetic beams with and without mode conversion. The general theory was reported in the first paper of this series, where a parabolic partial differential equation was derived for the field envelope that may contain one or multiple modes with close group velocities. In the second paper, we presented a corresponding code PARADE (PAraxial RAy DEscription) and its test applications to single-mode beams. Here, we report quasioptical simulations of mode-converting beams for the first time. We also demonstrate that PARADE can model splitting of two-mode beams. The numerical results produced by PARADE show good agreement with those of one-dimensional full-wave simulations and also with conventional ray tracing (to the extent that one-dimensional and ray-tracing simulations are applicable).
Type:
Dataset
Issue Date:
August 2019

195. Real-time Radiative Divertor Feedback Control Development for the NSTX-U Tokamak using a Vacuum Ultraviolet Spectrometer

Author(s):
Soukhanovskii, V.A.; Kaita, R.; Stratton, B.
Abstract:
A radiative divertor technique is planned for the NSTX-U tokamak to prevent excessive erosion and thermal damage of divertor plasma-facing components in H-mode plasma discharges with auxiliary heating up to 12 MW. In the radiative (partially detached) divertor, extrinsically seeded deuterium or impurity gases are used to increase plasma volumetric power and momentum losses. A real-time feedback control of the gas seeding rate is planned for discharges of up to 5 s duration. The outer divertor leg plasma electron temperature Te estimated spectroscopically in real time will be used as a control parameter. A vacuum ultraviolet spectrometer McPherson Model 251 with a fast charged-coupled device detector is developed for temperature monitoring between 5 and 30 eV, based on the delta n=0;1 line intensity ratios of carbon, nitrogen or neon ions lines in the spectral range 300 to 1600 A. A collisional-radiative model-based line intensity ratio will be used for relative calibration. A real-time Te-dependent signal within a characteristic divertor detachment equilibration time of ~ 10-15 ms is expected.
Type:
Dataset
Issue Date:
November 2016

196. 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

197. 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

198. Reduced Physics Model of the Tokamak Scrape-off-Layer for Pulse Design

Author(s):
Zhang, Xin
Abstract:
The dynamic interplay between the core and the edge plasma has important consequences in the confinement and heating of fusion plasma. The transport of the Scrape-Off-Layer (SOL) plasma imposes boundary conditions on the core plasma, and neutral transport through the SOL influences the core plasma sourcing. In order to better study these effects in a self-consistent, time-dependent fashion with reasonable turn-around time, a reduced model is needed. In this paper we introduce the SOL Box Model, a reduced SOL model that calculates the plasma temperature and density in the SOL given the core-to-edge particle and power fluxes and recycling coefficients. The analytic nature of the Box Model allows one to readily incorporate SOL physics in time-dependent transport solvers for pulse design applications in the control room. Here we demonstrate such a coupling with the core transport solver TRANSP and compare the results with density and temperature measurements, obtained through Thomson scattering and Langmuir probes, of an NSTX discharge. Implications for future interpretive and predictive simulations are discussed.
Type:
Dataset
Issue Date:
January 2023

199. Reductions in Retrieval Competition Predict the Benefit of Repeated Testing

Author(s):
Rafidi, Nicole S; Hulbert, Justin C; Brooks, Paula P; Norman, Kenneth A
Abstract:
Repeated testing (as opposed to repeated study) leads to improved long-term memory retention, but the mechanism underlying this improvement remains controversial. In this work, we test the hypothesis that retrieval practice benefits subsequent recall by reducing competition from related memories. This hypothesis implies that the degree of reduction in competition between retrieval practice attempts should predict subsequent memory for the practiced items. To test this prediction, we collected electroencephalography (EEG) data across two sessions. In the first session, participants practiced selectively retrieving exemplars from superordinate semantic categories (high competition), as well as retrieving the names of the superordinate categories from exemplars (low competition). In the second session, participants repeatedly studied and were then tested on Swahili-English vocabulary. One week after session two, participants were again tested on the vocabulary. We trained a within-subject classifier on the data from session one to distinguish high and low competition states. We then used this classifier to measure competition across multiple retrieval practice attempts in the second session. The degree to which competition decreased for a given vocabulary word predicted whether that item was subsequently remembered in the third session. These results are consistent with the hypothesis that repeated testing improves retention by reducing competition.
Type:
Dataset
Issue Date:
April 2018

200. Regarding the optimization of O1-mode ECRH and the feasibility of EBW startup on NSTX-U

Author(s):
Lopez, N; Poli, F
Abstract:
Recently published scenarios for fully non-inductive startup and operation on the National Spherical Torus eXperiment Upgrade (NSTX-U) (Menard et al 2012 Nucl. Fusion 52 083015) show Electron Cyclotron Resonance Heating (ECRH) as an important component in preparing a target plasma for efficient High Harmonic Fast Wave and Neutral Beam heating. The modeling of the propagation and absorption of EC waves in the evolving plasma is required to define the most effective window of operation, and to optimize the launcher geometry for maximal heating and current drive during this window. Here, we extend a previous optimization of O1-mode ECRH on NSTX-U to account for the full time-dependent performance of the ECRH using simulations performed with TRANSP. We find that the evolution of the density profile has a prominent role in the optimization by defining the time window of operation, which in certain cases may be a more important metric to compare launcher performance than the average power absorption. This feature cannot be captured by analysis on static profiles, and should be accounted for when optimizing ECRH on any device that operates near the cutoff density. Additionally, the utility of the electron Bernstein wave (EBW) in driving current and generating closed flux surfaces in the early startup phase has been demonstrated on a number of devices. Using standalone GENRAY simulations, we find that efficient EBW current drive is possible on NSTX-U if the injection angle is shifted below the midplane and aimed towards the top half of the vacuum vessel. However, collisional damping of the EBW is projected to be significant, in some cases accounting for up to 97% of the absorbed EBW power
Type:
Dataset
Issue Date:
June 2018