PTRANSP Simulation and Experimental Test of a Robust Current Profile and beta_N Controller for Off-axis Current Drive Scenarios in the DIII-D Tokamak
W. Shi, W. Wehner, J. Barton, M.D. Boyer, E. Schuster, A. Kritz, T.C. Luce, J.R. Ferron, M.L. Walker, D.A. Humphreys, B.G. Penaflor and R. Johnson
American Control Conference
Washington, DC, USA, June 17-19, 2013
Abstract
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During the tokamak discharge, especially the ramp-up phase, the plasma
state equilibrium continually evolves. As a consequence, the plasma
response model should evolve as well. We first identified a linear
plasma response model of the rotational transform iota profile and
beta_N around a desired equilibrium. Then, an uncertainty is
introduced to the identified model to partially account for the
dynamic character of the plasma state equilibrium evolution. A robust
controller is designed to stabilize this family of plasma models,
which are reformulated into a nominal model with uncertainty. A
singular value decomposition (SVD) of the nominal identified model is
carried out to decouple and identify the most relevant control
channels in steady-state. The DK-iteration method, combining
H_infinity synthesis and mu analysis, is applied to synthesize a
closed-loop controller that minimizes the tracking error and input
effort. The feedback controller is then augmented with an anti-windup
compensator, which keeps the given profile controller well-behaved in
the presence of magnitude constraints in the actuators and leaves the
nominal closed-loop unmodified when no saturation is present. PTRANSP
simulations and experimental results in DIII-D illustrate the
performance of the model-based controller.