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Disruptivity Reduction Plan for NSTX-U, Including Characterization of Causes and Implementation of Kinetic Stability Theory Models

Author: John W. Berkery
Requested Type: Poster Only
Submitted: 2015-01-16 14:14:10

Co-authors: S.A. Sabbagh, Y.S. Park, S.P. Gerhardt, J.E. Menard

Contact Info:
Columbia University
Department of Applied Physics
New York, New York   10027
USA

Abstract Text:
The first step of a plan to reduce plasma disruptivity at high plasma beta in the National Spherical Torus Experiment-Upgrade (NSTX-U) is to make a detailed characterization of disruption causes in the device with quantitative metrics. Disruption categories (such as locked modes, vertical instability, resistive wall modes (RWM), etc…) and their sequential connections analogous to those used on JET [P.C. de Vries et al., Nucl. Fusion 51, 053018 (2011)] will be adopted, with specific additions for NSTX-U. Next, theory will be applied to understand and improve disruption prediction and avoidance, including real-time reduced marginal stability models and application of models for linear or non-linear stability criteria. One specific example is the use of kinetic RWM stabilization physics theory. First, ideal MHD limits have been explored in detail for the NSTX device. Calculations with the DCON code for a large number of experimental equilibria show that previous calculations made for a more limited dataset of the no-wall limit on the plasma beta were relatively accurate [S.A. Sabbagh, et al. Nucl. Fusion 53, 104007 (2013)]. The no-wall limit dependence on pressure peaking was in line with theoretical expectations, and the decrease in the no-wall beta limit with increasing aspect ratio was found to be slightly larger than previously estimated, which has implications for the somewhat larger aspect ratio of NSTX-U. Ideal MHD is insufficient to explain experimental stability, however, and knowledge of stabilizing kinetic resonances between plasma rotation and particle motions will also be implemented. A real-time estimate of ExB frequency will be monitored to determine if the plasma rotation is in a favorable or unfavorable range and then various actuators such as non-resonant magnetic braking or changing neutral beam injection sources for rotation control will be used to return the plasma to a more stable state [J.W. Berkery et al., Phys. Plasmas 21, 056112 (2014)].

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