Progress toward fully noninductive discharge operation in DIII-D using off-axis neutral beam injection

Abstract

The initial experiments on off-axis neutral beam injection into high noninductive current fraction (f(NI)), high normalized pressure (beta(N)) discharges in DIII-D [J. L. Luxon, Fusion Sci. Technol. 48, 828 (2005)] have demonstrated changes in the plasma profiles that increase the limits to plasma pressure from ideal low-n instabilities. The current profile is broadened and the minimum value of the safety factor (q(min)) can be maintained above 2 where the profile of the thermal component of the plasma pressure is found to be broader. The off-axis neutral beam injection results in a broadening of the fast-ion pressure profile. Confinement of the thermal component of the plasma is consistent with the IPB98(y,2) scaling, but global confinement with q(min) > 2 is below the ITER-89P scaling, apparently as a result of enhanced transport of fast ions. A 0-D model is used to examine the parameter space for f(NI) = 1 operation and project the requirements for high performance steady-state discharges. Fully noninductive solutions are found with 4 < beta(N) < 5 and bootstrap current fraction near 0.5 for a weak shear safety factor profile. A 1-D model is used to show that a f(NI) = 1 discharge at the top of this range of beta(N) that is predicted stable to n = 1, 2, and 3 ideal MHD instabilities is accessible through further broadening of the current and pressure profiles with off-axis neutral beam injection and electron cyclotron current drive.