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

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 beta, 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 q(min)beta(N) to maximize the self-driven bootstrap current while maintaining high toroidal beta to increase fusion gain. Modeling shows that increases in the normalized beta beta(N) 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 q(min)beta(N) to increase with the minimum q value (q(min)) although beta(N) decreases as q(min) increases. By broadening the pressure profile, beta(N)approximate to 4 is obtained with q(min)approximate to 2. Modeling of equilibria with near 100% bootstrap current indicates that operation with beta(N)approximate to 5 should be possible with a sufficiently broad pressure profile. (c) 2005 American Institute of Physics.