EFFECTS OF PLASMA SHAPE AND PROFILES ON EDGE STABILITY IN DIII-D

摘要

The results of recent experimental and theoretical studies concerning the effects of plasma shapernand current and pressure profiles on edge instabilities in DIII-D are presented. Magnetic oscillationsrnwith toroidal mode number n ≈ 2-9 and a fast growth time γ~(-1) = 20-150 μs are often observed prior tornthe first giant type I ELM in discharges with moderate squareness. High n ideal ballooning secondrnstability access encourages edge instabilities by facilitating the buildup of the edge pressure gradientrnand bootstrap current density which destabilize the intermediate to low n modes. Analysis suggestsrnthat discharges with large edge pressure gradient and bootstrap current density are more unstable to nrn> 1 modes. Calculations and experimental results show that ELM amplitude and frequency can bernvaried by controlling access to the second ballooning stability regime at the edge through variation ofrnthe squareness of the discharge shape. A new method is proposed to control edge instabilities byrnreducing access to the second ballooning stability regime at the edge using high order localrnperturbation of the plasma shape in the outboard bad curvature region.