ELMS AND FAST EDGE FLUCTUATIONS IN ALCATOR C-MOD

Just above the H-mode threshold power at low density, small amplitude high repetition rate (1-5 kHz) type III edge-localized modes (ELMs) ar e observed. At higher input power, the H-modes become ELM-free. At hig h density and high input power ((n) over bar(e) > 2.2 x 10(20) m(-3), P-in > 2 MW), the plasma often enters a new II-mode regime now called the enhanced D-alpha H-mode, which is a steady-state H-mode often with out distinct ELMs but with high levels of D-alpha emission. Enhanced D -alpha H-modes are accompanied by persistent high-frequency broadband (50-400 kHz) density and magnetic fluctuations. Above about (n) over b ar(e) > 3 x 10(20) m(-3) the enhanced D-alpha H-modes may also exhibit bursting or continuous low m, n coherent magnetic fluctuations with f requencies from 60-90 kHz. In the H-mode, steep edge gradients are obs erved with a nine-channel electron cyclotron emission (ECE) polychroma tor and a 38-channel, high-resolution, edge soft-x-ray array. The soft -x-ray pedestal width remains around 3 to 4 mm in the ELM-free H-mode and between type III ELMs. The pedestal collapses within 100 mu s duri ng an ELM. Calculations indicate that the plasma in the H-mode is ofte n near ideal ballooning first stability limits, but no type I ELMs hav e yet been identified. Edge micro-tearing modes destabilized by a stee p temperature gradient are also considered as a possible explanation o f the observed fluctuations during the enhanced D-alpha H-mode.