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Experimental investigation into ELM filament formation on MAST

Radial profiles of electron temperature and density through type I ELM filaments have been obtained from a new edge Thomson scattering diagnostic at MAST. The lasers were fired in burst mode, 5 µs apart, to study profile evolution of a single filament as it moves toroidally past the laser beams. The plasma particle and energy loss due to each filament can be deduced from these profiles. As the filaments move out of the plasma, the ne pedestal is seen to collapse locally inwards by as much as 7.5% of the plasma minor radius. Insight into the toroidal structure of the perturbation has been obtained from high time resolution interferometry data. The interferometer data show excursions in line integral density through the midplane of the plasma occurring from 150 µs before the onset of the ELM particle loss. These excursions are due to the evolution of the spatial structure of the plasma during the ELM and indicate that the filaments may develop from broader structures. By combining the toroidal structure information from the interferometer and the radial structure information from the TS system with other diagnostic data on MAST, a two dimensional picture of the ELM phenomenon is obtained.