Development in DIII-D of High Beta Discharges Appropriate for Steady-state Tokamak Operation With Burning Plasmas

J.R. Ferron, V. Basiuk, T.A. Casper, J.C. DeBoo, E.J. Doyle, Q. Gao, A.M. Garofalo, C.M. Greenfield, C.T. Holcomb, T.C. Luce, M. Murakami, Y. Ou, C.C. Petty, P.A. Politzer, H. Reimerdes, E. Schuster, M. Schneider, and A. Wang

IAEA Fusion Energy Conference

Geneva, Switzerland, 13-18 October 2008

Abstract

Ideally, tokamak power plants will operate in steady-state at high fusion gain. Recent work at DIII-D on the development of suitable high beta discharges with 100% of the plasma current generated noninductively (fNI = 1) is described. In a discharge with 1.5 < qmin < 2, a scan of the discharge shape squareness was used to find the value that maximizes confinement and achievable beta_N. A small bias of the up/down balance of the double-null divertor shape away from the ion Bx\Nabla 􏰔B drift direction optimizes pumping for minimum density. Electron cyclotron current drive with a broad deposition profile was found to be effective at avoidance of a 2/1 NTM allowing long duration at 􏰀beta_N = 3.7. With these improvements, surface voltage 􏰔0–10 mV, indicating fNI=􏰔1, was obtained for 0.7 \tau_􏰔R (resistive time). Stationary discharges with 􏰀 beta_N = 3.4 and fNI=0.9 that project to Q = 5 in ITER have been demonstrated for \tau_􏰔R. For use in development of model based controllers for the q profile, transport code models of the current profile evolution during discharge formation have been validated against the experiment. Tests of available actuators confirm that electron heating during the plasma current ramp up to modify the conductivity is by far the most effective. The empirically designed controller has been improved by use of proportional/integral gain and built-in limits to beta_􏰀N to avoid instabilities. Two alternate steady-state compatible scenarios predicted to be capable of reaching \beta_􏰀N = 5 have been tested experimentally, motivated by future machines that require high power density and neutron fluence. In a wall stabilized scenario with qmin > 2, beta_N = 4 has been achieved for 2 s = \tau_􏰔R. In a high internal inductance scenario, which maximizes the ideal no-wall stability limit, beta_􏰀N=4.8 has been reached with fNI > 1.