Multivariable Multi-Model-based Magnetic Control System for the Current Ramp-up Phase in the National Spherical Torus Experiment (NSTX)
W. Shi, J. Barton, M. Alsarheed and E. Schuster
50th IEEE Conference on Decision and Control
Orlando, Florida, December 12-15, 2011
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Abstract
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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_\inf synthesis
and \mu 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.
