STUDIES OF EDGE LOCALIZED MODES ON ASDEX

The MHD behaviour connected with edge localized modes (ELMs) is studied using fast sample rate fluctuation diagnostics. It is found that ELMs on ASDEX are generally preceded by a coherent MHD mode of medium poloidal mode number (m = 10-15). At the onset of the ELM the mode behaviour changes to a turbulent one, leading to an outflux of heat and particles. These results are related to a model by Kerner and Jakoby, as described in Nucl. Fusion 29 (1989) 1959 (Section 7.2), which explains the ELM as a non-linear coupling of free-surface modes of resistive ballooning character. This model is in good agreement with the experimental findings.