Study of Giant Edge-Localized Modes in DIII-D and Comparison with Ballooning Theory

In DIII-D $H$-mode plasmas, large amplitude modulation of steep-edge electron-pressure gradients are observed during the occurrences of giant edge-localized modes (ELM's). For a considerable range of plasma currents (1 to 2 MA), the experimental pressure gradient before a giant ELM is close to the theoretically predicted marginal stability limit for the first regime of the ideal ballooning mode. These results, together with observations showing that ELM's originate in the region of bad curvature, strongly suggest that ELM's are triggered by ballooning mode instabilities.