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
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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.