The structure of ELMs and the distribution of transient power loads in MAST

Full text: The spatial distribution of energy released from the core plasma during edge localised modes (ELMs) and disruptions is a key area of study for ITER, where the resultant power loadings, both inside and outside the divertor region, have an important impact on the choice of plasma facing materials. The excellent diagnostic visibility afforded by the spherical geometry of the MAST tokamak, remote first wall and extensive divertor and edge diagnostics make it an ideal device for the study of transient power loads. An important aspect of this work is the poloidal and toroidal localization of the energy deposited during the transient event and in particular the fraction that reaches the divertor compared to the first wall. The observations presented in this paper provide strong evidence for the ELMs examined on MAST being a filament like structure, which is extended along a field line, is generated on a 100 μ s timescale, erupts from the outboard side and connects back into the plasma. This filament is observed on several diagnostics to extend beyond the separatrix and ELM effluxes are observed radially up to 20 cm from the plasma edge. For the first time a high-speed picture has been obtained of an ELM showing this filament like structure. Divertor target power loadings during locked mode disruptions have been derived from IR camera measurements. The loss of thermal energy from the core plasma was derived from Thomson scattering temperature and density profiles. In the disruptions analysed, three well defined phases are observed. At the beginning of the thermal quench around 50 % of the thermal energy is released from the core. Target power loads rise but there is no significant broadening of the heat flux width. Around the time of the current redistribution, the remaining thermal energy is released in a rather short period of a few hundred μ s. The heat flux width in this phase broadens by a factor 10 but the peak heat flux still rises. In the third phase, the plasma current begins to decay and magnetic energy is released from the core. Although the heat flux width for most of the disruption period does appear to broaden, there can be phases of the disruption during which significant energy is released without broadening. (author)