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

The results of recent experimental and theoretical studies concerning the effects of plasma shape and current and pressure profiles on edge instabilities in DIII-D are presented. Magnetic oscillations with toroidal mode number n ≈ 2-9 and a fast growth time γ-1 = 20-150 μs are often observed prior to the first giant type I ELM in discharges with moderate squareness. High n ideal second ballooning stability access encourages edge 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. Calculations and experimental results show that ELM amplitude and frequency can be varied by controlling access to the second ballooning stability regime at the edge through variation of the squareness of the discharge shape. A new method is proposed to control edge instabilities by reducing access to the second ballooning stability regime at the edge using high order local perturbation of the plasma shape in the outboard bad curvature region.