Feedback Control of the Safety Factor Profile in DIII-D Advanced Tokamak Discharges

J.R. Ferron, V. Basiuk, T.A. Casper, E.J. Doyle, Q. Gao, P. Gohil, C.M. Greenfield, F. Imbeaux, J. Lohr, T.C. Luce, M.A. Makowski, D. Mazon, M. Murakami, Y. Ou, J.-M. Park, C.C. Petty, P.A. Politzer, T.L. Rhodes, M. Schneider, E. Schuster, M.A. Van Zeeland, M.R. Wade, and A. Wang,

IAEA Fusion Energy Conference

Chengdu, China, October 16-21, 2006

Abstract

Active feedback control for regulation of the safety factor (q) profile at the start of the high stored energy phase of an Advanced Tokamak discharge has been demonstrated in the DIII-D tokamak. The time evolution of the on-axis or minimum value of q is controlled during and just following the period of ramp-up of the plasma current using electron heating to modify the rate of relaxation of the current profile. In L-mode and H-mode discharges, feedback control of q is effective with the appropriate choice of either off-axis electron cyclotron heating or neutral beam heating as the actuator. The q profile is calculated in real time from a complete equilibrium reconstruction fitted to external magnetic field and flux measurements and internal poloidal field measurements from the motional Stark effect diagnostic. This is the first use of this accurate calculation method for real time q profile identification and control. Comparisons of experimental measurements and transport code predictions of the time evolution of the current profile are used to validate transport codes for use in testing of real time feedback control algorithms. In some cases, the modeled noninductive current must be located farther from the axis than is predicted by theory in order to obtain agreement between the simulations and the experiment.