Multivariable Model-based Shape Control for the National Spherical Torus Experiment (NSTX)
W. Shi, M. Alsarheed, E. Schuster, M.L. Walker, J. Leuer, D.A. Humphreys and D.A. Gates
Fusion Engineering and Design 86 (2011) 1107-1111
Abstract
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Plasma current, position and shape control is a challenging problem due to the strong coupling between the different
parameters describing the shape of the plasma. By leveraging the availability of rtEFIT, this paper proposes a
robust model-based multi-input–multi-output (MIMO) controller to provide real-time shaping, position stabilization
and current regulation in NSTX. The proposed controller is composed of three loops: the first loop is devoted to
plasma current regulation, the second loop is dedicated to plasma radial and vertical position stabilization, and
the third loop is used to control the plasma shape. This control approach transforms the shape control problem into
an output tracking problem. The goal is the minimization of a quadratic cost function that describes the tracking
error in steady state. A singular value decomposition (SVD) of the nominal plasma model is carried out to decouple
and identify the most relevant control channels. The H∞ technique is used to minimize the tracking errors and
optimize input efforts. Computer simulation results illustrate the performance of the robust model-based shape
controller, showing potential for improving the performance of present non-model-based controllers.