Sustained High Beta Plasmas with Flat q-Profile in DIII-D

A first step to demonstration of ∼100% bootstrap Advanced Tokamak (AT) is achieving and sustaining high normalized beta (βn>5) for at least one current relaxation time, in plasma with a q-profile having high qmin (≥2) for high bootstrap current fraction, large radius of qmin (ρ(qmin)≥0.8) for good bootstrap alignment, and low q95 for high fusion gain. These q-profiles have been approached most closely in two types of discharges: a) current-hole discharges with q0>>qmin ∼ q95, b) flat q-profile discharges with q0∼qmin∼q95. In the current-hole plasmas the maximum the maximum value of βn has been limited to a low value (∼2) by internal MHD instabilities. On the other hand, flat q-profile plasma have achieved and sustained βN>4 for durations ≥600 ms in DIII-D. Detailed stability studies have shown the need to produce plasma discharges with a broad p-profile to raise ideal-wall instability thresholds and allow sustained operation at high βn. The flat-q approach is motivated by the hypothesis that a broad pressure profile may be obtained more easily by creating a broad q-profile. To this end, the toroidal field (BT) is ramped in DIII-D down while the plas is sufficiently hot to behave ideally. In ideal MHD, the flux is conserved as BT decreases and the flux surfaces expand. Both the current and the pressure profile are observed to follow the expansion of the core flux (see figure). With profile gradients closer to the plasma edge the calculated n=1βN limit with a conducting wall increases to 5.4,as needed for AT operation. Experimental results will be presented more fully. (Author)