Dependence of the DIII-D beta limit on the current profile

The maximum beta values achieved in DIII-D are not fully described by the simple scaling law βmax∝I/aB. There is, in addition, a dependence on the form of the current profile as parameterized by the safety factor q and internal inductance li. The maximum experimentally achieved value of normalized beta βN = β/(I/aB) varies from 3.5 at low safety factor q (q95<3) to 5 at higher values of q. At low q, discharges are terminated by disruptions at high βN and at both the low and high li boundaries of the stable range. These disruptions are attributed to external and global kink modes. At higher q, such disruptions are much less frequent, and beta is limited by slowly growing resistive modes, fishbones, and possibly by ballooning modes. At each value of q, the maximum beta tends to increase with internal inductance li. A numerical study of kink mode stability has shown a similar trend for optimized pressure profiles. These observations have suggested a new scaling law for the operational beta limit: βmax=4li(I/aB), which fits the DIII-D data well. (author) 13 refs., 4 figs