Comparison of small edge-localized modes on MAST and ASDEX Upgrade

Abstract

Results from a set of ITPA-coordinated experiments on ASDEX Upgrade and MAST to compare the characteristics of small edge-localized modes (ELMs) are presented. In MAST a small ELM regime is established in connected double null discharges in a limited region of normalized pedestal pressure and collisionality. Type II ELMs on ASDEX Upgrade have high frequency and low energy loss and occur at high triangularity and close to double null. On both devices the transition from type I to small ELMs is connected with a similar threshold value of the pedestal collisionality. For the first time the temporal and spatial evolution of the filament structures observed during these small ELMs have been studied. The radial and toroidal velocities of the filaments in these small ELMs are slower compared with type I ELMs on both devices. The observations are compatible with the filaments in small ELMs originating closer to the last closed flux surface than is the case in type I ELMs. The toroidal mode number of the small ELMs, derived from the temporal evolution of the filaments, is typically a factor of two larger than for type I ELMs. The small ELMs on MAST have sufficient similarities to type II ELMs on ASDEX Upgrade that they should be classified as the same.