A review of plasma boundary phenomena in the mega ampere spherical tokamak

Enhanced diagnostics and an expanded operational space in Mega Ampere Spherical Tokamak (MAST) has lead to a wealth of data on the plasma boundary becoming available over the last year of operations and has significantly advanced the understanding of this region in the spherical tokamak and, more generally, is contributing to an improved understanding of the underlying plasma phenomena for this important region of the tokamak. Amongst the observations reported in this paper are that MAST is able to achieve simultaneous high confinement and high density with Type III ELMs for connected double-null discharges in which most (>93%) of the power efflux from the core observed at the targets (including during ELMs) arrives at the large wetted area outboard targets and is evenly distributed between the upper and lower divertor regions. Significant particle fluxes are reported well beyond the outboard separatrix during ELMs (up to 20 cm distant from the plasma) and appear to be associated with the radial expansion of a localised structure at around 1 km s-1. The result of preliminary experiments to broaden the SOL by asymmetric divertor biasing are also presented which show an encouraging, qualitative agreement with theory and demonstrate clear evidence for both SOL broadening and target power reduction