Abstract
The dependence of edge stability on plasma shape and local pressure gradients, P, in the DIII-D and JT-60U tokamaks is studied. The stronger plasma shaping in DIII-D allows the edge region of DIII-D discharges with Type I ("giant") ELMs to have access to the second region of stability for ideal ballooning modes and larger edge P′than JT-60U Type I ELM discharges. These JT-60U discharges are near the ballooning mode first regime stability limit. DIII-D results support an ideal stability based working model of Type I ELMs as low to intermediate toroidal mode number, n, MHD modes. Results from stability analysis of JT-60U Type I ELM discharges indicate that predictions from this model are also consistent with JT-60U edge stability observations.
