Multivariable model-based shape control for the National Spherical Torus Experiment (NSTX)

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.