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) are observed. At higher input power, the H-modes become ELM-free. At high density and high input power (n-bare>2.2x1020m-3, Pin>2MW), the plasma often enters a new H-mode regime now called the enhanced Dα H-mode, which is a steady-state H-mode often without distinct ELMs but with high levels of Dα emission. Enhanced Dα H-modes are accompanied by persistent high-frequency broadband (50-400 kHz) density and magnetic fluctuations. Above about n-bare>3x1020m-3, the enhanced Dα H-modes may also exhibit bursting or continuous low m, n coherent magnetic fluctuations with frequencies from 60-90 kHz. In the H-mode, steep edge gradients are observed with a nine-channel electron cyclotron emission (ECE) polychromator 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 μs during an ELM. Calculations indicate that the plasma in the H-mode is often near ideal ballooning first stability limits, but no type I ELMs have yet been identified. Edge micro-tearing modes destabilized by a steep temperature gradient are also considered as a possible explanation of the observed fluctuations during the enhanced Dα H-mode. (author)