Exhaust, ELM, and halo physics using the MAST tokamak

The scrape-off layer (SOL) and divertor target plasma of a large spherical tokamak (ST) are characterized in detail for the first time. Scalings for the SOL heat flux width in MAST are developed and compared with conventional tokamaks. Modelling reveals the significance of the mirror force to the ST SOL. Core energy losses, including during ELMs, in MAST arrive predominantly (>80%) at the outboard targets in all geometries. Convective transport dominates energy losses during ELMs, and MHD analysis suggests ELMs in MAST are Type III even at auxiliary heating powers well above the L–H threshold. ELMs are associated with a poloidally and/or toroidally localized radial efflux at ∼1 km s−1 well into the far SOL but not with any broadening of the target heat flux width. Toroidally asymmetric divertor biasing experiments have been conducted in an attempt to broaden the target heat flux width, with promising results. During vertical displacement events, the maximum product of the toroidal peaking factor and halo current fraction remains below 0.3, around half the ITER design limit. Evidence is presented to demonstrate that the resistance of the halo current path may have an impact on the distribution of the halo current.