PSFC / JA - 97 - 23 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 (Ie > 2.2 x 1020 m3, P," > 2 MW), the plasma often enters a new H-mode regime now called the Enhanced D, H-mode, which is a steady-state Hmode often without distinct ELMs but with high levels of D, emission. Enhanced Da H-modes are accompanied by persistent high frequency broadband (50 400 kHz) density and magnetic fluctuations. Above about i > 3 x 1020 m3, the Enhanced D, H-modes may also exhibit bursting or continuous low m, n coherent magnetic fluctuations with frequencies from 60 90 kHz. In H-mode, steep edge gradients are observed with a nine channel 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 ELM-free H-mode and between Type III ELMs. The pedestal collapses within 100 psec during an ELM. Calculations indicate that the plasma in 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 Enhanced D, H-mode. Submitted to Plasma Phys. Control. Fusion