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Start Over Domain Natural Sciences Community Princeton Plasma Physics Laboratory

61. Application of transient CHI plasma startup to future ST and AT devices

Author(s):
Hammond, K.C.; Raman, R.; Jardin, S.C.
Abstract:
Employment of non-inductive plasma start-up techniques would considerably simplify the design of a spherical tokamak fusion reactor. Transient coaxial helicity injection (CHI) is a promising method, expected to scale favorably to next-step reactors. However, the implications of reactor-relevant parameters on the initial breakdown phase for CHI have not yet been considered. Here, we evaluate CHI breakdown in reactor-like configurations using an extension of the Townsend avalanche theory. We find that a CHI electrode concept in which the outer vessel wall is biased to achieve breakdown, while previously successful on NSTX and HIT-II, may exhibit a severe weakness when scaled up to a reactor. On the other hand, concepts which employ localized biasing electrodes such as those used in QUEST would avoid this issue. Assuming that breakdown can be successfully attained, we then apply scaling relationships to predict plasma parameters attainable in the transient CHI discharge. Assuming the use of 1 Wb of injector flux, we find that plasma currents of 1 MA should be achievable. Furthermore, these plasmas are expected to Ohmically self-heat with more than 1 MW of power as they decay, facilitating efficient hand-off to steady-state heating sources. These optimistic scalings are supported by TSC simulations.
Type:
Dataset
Issue Date:
February 2019

62. Blob wakes in NSTX

Author(s):
Zweben SJ, Myra JR, Diallo A, Russell DA, Scotti F, Stotler DP
Abstract:
Transient small-scale structures were identified in the wake of blobs movingpoloidally through the SOL of high-powered H-mode plasmas in NSTX, using the gaspuff imaging (GPI) diagnostic. These blob wakes had a poloidal wavelength in therange 3.5 cm, which is significantly smaller than the average blob scale of~12 cm, and the wakes had a poloidal velocity of 1.5 km/sec in theelectron diamagnetic direction, which is opposite to the blob poloidal velocity inthese shots. These wakes were radially localized 0-4 cm outside the separatrix andoccurred within ~50 microsec after the passage of a blob through the GPI field of view.The clearest wakes were seen when the GPI viewing angle was well aligned with thelocal B field line, as expected for such small-scale structures given the diagnosticgeometry. A plausible theoretical interpretation of the wakes is discussed: theobserved wakes share some features of drift waves and/or drift-Alfven waves whichcould be excited
Type:
Dataset
Issue Date:
July 2019

63. Comment on ‘Numerical modeling of tokamak breakdown phase driven by pure Ohmic heating under ideal conditions’

Author(s):
Yoo, Min-Gu; Na, Yong-Su
Abstract:
In this comment, we point out possible critical numerical flaws of recent particle simulation studies (Jiang et al 2016 Nucl. Fusion 56 126017, Peng et al 2018 Nucl. Fusion 58 026007) on the electrical gas breakdown in a simple one-dimensional periodic slab geometry. We show that their observations on the effects of the ambipolar electric fields during the breakdown, such as the sudden reversal of the ion flow direction, could not be real physical phenomena but resulting from numerical artifacts violating the momentum conservation law. We show that an incomplete implementation of the direct-implicit scheme can cause the artificial electric fields and plasma transports resulting in fallacies in simulation results. We also discuss that their simple plasma model without considering poloidal magnetic fields seriously mislead the physical mechanism of the electrical gas breakdown because it cannot reflect important dominant plasma dynamics in the poloidal plane (Yoo et al 2018 Nat. Commun. 9 3523).
Type:
Dataset
Issue Date:
June 2019

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

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

65. Electron inertial effects on linearly polarized electromagnetic ion cyclotron waves at Earth's magnetosphere

Author(s):
Kim, Eun-Hwa; Johnson, Jay; Lee, Dong-Hun
Abstract:
We discuss a role of the electron inertial effect on linearly polarized electromagnetic ion cyclotron (EMIC) waves at Earth. The linearly polarized EMIC waves have been previously suggested to be generated via mode conversion from the fast compressional wave at the ion-ion hybrid (IIH) resonance. When the electron inertial effects are neglected, the wave normal angle of the mode-converted IIH waves is 90 degrees because the wavevector perpendicular to the magnetic field becomes infinite at the IIH resonance. When the electron inertial effect is considered, the mode-converted IIH waves can propagate across the magnetic field lines and the wavelength perpendicular to the magnetic field approaches the electron inertial length scale near the Buchsbaum resonance. These waves are referred to as electron inertial waves. Due to the electron inertial effect, the perpendicular wavenumber to the ambient magnetic field near the IIH resonance remains finite and the wave normal angle is less than 90 degrees. The wave normal angle where the maximum absorption occurs in a dipole magnetic field is 30-80 degrees, which is consistent with the observed values near the magnetic equator. Therefore, the numerical results suggest that the linearly polarized EMIC wave generated via mode conversion near the IIH resonance can be detected in between the Buchsbaum and the IIH resonance frequencies, and these waves can have normal angle less than 90 degrees.
Type:
Dataset
Issue Date:
April 2019

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

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

67. 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

68. 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

69. 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

70. 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

71. 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

72. 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

73. 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

74. 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

75. 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

76. 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

77. 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

78. Spontaneous multi-keV electron generation in a low-RF-power axisymmetric mirror machine

Author(s):
Swanson, C.;Cohen, S.A.
Abstract:
Title: Spontaneous multi-keV electron generation in a low-RF-power axisymmetric mirror machine Abstract: X-ray emission shows the existence of multi-keV electrons in low-temperature, low-power, capacitively-coupled RF-heated magnetic-mirror plasmas that also contain a warm (300 eV) minority electron population. Though these warm electrons are initially passing particles, we suggest that collisionless scattering -- mu non-conservation in the static vacuum field -- is responsible for a minority of them to persist in the mirror cell for thousands of transits during which time a fraction are energized to a characteristic temperature of 3 keV, with some electrons reaching energies above 30 keV. A heuristic model of the heating by a Fermi-acceleration-like mechanism is presented, with mu non-conservation in the static vacuum field as an essential feature.
Type:
Dataset
Issue Date:
May 2019

79. Study of the impact of pre- and real-time deposition of lithium on plasma performance on NSTX

Author(s):
Canal, G.P.; Maingi, R.; Evans, T.E.; Kaye, S.M.; Mansfield, D.K.
Abstract:
The efficiency of two lithium (Li) injection methods used on the National Spherical Torus Experiment (NSTX) are compared in terms of the amount of Li used to produce equivalent plasma performance improvements, namely Li evaporation over the divertor plates, prior to the initiation of the discharge, and real-time Li injection directly into the plasma scrape-off layer during the discharge. The measurements show that the real-time method can affect the energy confinement and edge stability of NSTX plasmas in a more efficient way than the Li evaporation method as it requires only a fraction of the amount of Li used by the evaporation method to produce similar improvements.
Type:
Dataset
Issue Date:
January 2019

80. TRANSP-based Optimization Towards Tokamak Scenario Development

Author(s):
W.P. Wehner, E. Schuster, M.D. Boyer, F. Poli
Abstract:
An optimization approach that incorporates the predictive transport code TRANSP is proposed for tokamak scenario development. Optimization methods are often employed to develop open-loop control strategies to aid access to high performance tokamak scenarios. In general, the optimization approaches use control-oriented models, i.e. models that are reduced in complexity and prediction accuracy as compared to physics-oriented transport codes such as TRANSP. In the presented approach, an optimization procedure using the TRANSP code to simulate the tokamak plasma is considered for improved predictive capabilities. As a test case, the neutral beam injection (NBI) power is optimized to develop a control strategy that maximizes the non-inductive current fraction during the ramp-up phase for NSTX-U. Simulation studies towards the achievement of non-inductive ramp up in NSTX-U have already been carried out with the TRANSP code. The optimization-based approach proposed in this work is used to maximize the non-inductive current fraction during ramp-up in NSTX-U, demonstrating that the scenario development task can be automated. An additional test case considers optimization of the current ramp rate in DIII-D for obtaining a stationary plasma characterized by a flat loop voltage profile in the flattop phase.
Type:
Dataset
Issue Date:
April 2019

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