Effects of plasma shape and profiles on edge stability in DIII-D

The results of recent experimental and theoretical studies concerning the e ffects of plasma shape and current and pressure profiles on edge instabilit ies in DIII-D are presented. Magnetic oscillations with toroidal mode numbe r n approximate to 2-9 and a fast growth time gamma(-1) = 20-150 mu s are o ften observed prior to the first giant type I ELM in discharges with modera te squareness. High n ideal second ballooning stability access encourages e dge instabilities by facilitating the buildup of the edge pressure gradient and bootstrap current density, which destabilize the intermediate to low n modes. Analysis suggests that discharges with large edge pressure gradient and bootstrap current density are more unstable to n > 1 modes. Calculatio ns and experimental results show that ELM amplitude and frequency can be va ried by controlling access to the second ballooning stability regime at the edge through variation of the squareness of the discharge shape. A new met hod is proposed to control edge instabilities by reducing access to the sec ond ballooning stability regime at the edge using high order local perturba tion of the plasma shape in the outboard bad curvature region.