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
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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.