Optimization of DIII-D advanced tokamak discharges with respect to the β limit

Results are presented from comparisons of modeling and experiment in studies to assess the best choices of safety factor q profile, pressure profile, and discharge shape for high Β, steady-state, noninductive advanced tokamak operation in the DIII-D device [J. L. Luxon, Nucl. Fusion 42, 614 (2002)]. These studies are motivated by the need for high qmin ΒN to maximize the self-driven bootstrap current while maintaining high toroidal Β to increase fusion gain. Modeling shows that increases in the normalized beta ΒN stable to ideal, low toroidal mode number (n=1,2), instabilities can be obtained through broadening of the pressure profile and use of a symmetric double-null divertor shape. Experimental results are in agreement with this prediction. The general trend is for qmin ΒN to increase with the minimum q value (qmin) although ΒN decreases as qmin increases. By broadening the pressure profile, ΒN ≈4 is obtained with qmin ≈2. Modeling of equilibria with near 100% bootstrap current indicates that operation with ΒN ≈5 should be possible with a sufficiently broad pressure profile.