Towards model-based current profile control at DIII-D

A key goal in control of an advanced tokamak (AT) discharge is to maintain safety factor (q) and pressure profiles that are compatible with both MHD stability at high toroidal beta and a high fraction of the self-generated bootstrap current. This will enable high fusion gain and non-inductive sustainment of the plasma current for steady-state operation. In this work we report progress towards enabling model-based active control of the current profile at DIII-D. Initial results on modeling-for-control and simulation of the dynamic evolution of the poloidal flux profile during and just following the ramp-up of the plasma current are presented. The magnetic diffusion equation is combined with empirical correlations obtained at DIII-D for the density, temperature and non-inductive current to introduce a simplified dynamic model describing the evolution of the poloidal flux, and therefore the q profile, during the inductive phase of the discharge. The physical model is rewritten in a control-oriented formulation and the control challenges asocciated with the problem are discussed. (C) 2007 Elsevier B.V. All rights reserved.