ELM activity during limiter H-modes on TFTR

MHD activity at the plasma edge is studied during limiter H-modes. The ELMs are observed as large drops in the edge density and temperature, and are also clearly observed on the Mirnov coils and soft X-ray signals, during the H-mode phase. The ELMs are found to have a precursor instability dominated by an m/n=1/0 mode at ∝100-500 kHz. Simultaneously, a burst of hot electrons is observed to originate about 20 cm inside the plasma and propagate to the plasma edge. The burst of Dα emission may be triggered by the arrival of the hot electrons at the limiter surface. (orig.)