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

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.