Multivariable Multi-model-based magnetic control system for the current ramp-up phase in the National Spherical Torus Experiment (NSTX)

By leveraging the availability of real time EFIT, we propose a robust, multi-model-based, multi-input-multi-output (MIMO) magnetic controller to provide current regulation, position stabilization, and shape control of the plasma during the current ramp-up phase in the National Spherical Torus Experiment (NSTX). During the ramp-up phase of the tokamak discharge, the magnetohydrodynamic (MHD) equilibrium continually evolves. As a consequence, the plasma response model obtained via linearization around the changing MHD equilibrium evolves as well. A robust controller is designed to stabilize this family of plasma models, which are reformulated into a nominal model with uncertainty. 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 and X-point location. A singular value decomposition (SVD) of the nominal plasma model is carried out to decouple and identify the most relevant control channels. The DK-iteration method, combining H_∞ synthesis and μ analysis, is applied to synthesize a closed-loop controller that minimizes the tracking errors and optimizes input efforts. Computer simulations illustrate the performance of the robust, multi-model-based, shape controller, showing potential for improving the performance of present non-model-based controllers.