Confinement and exhaust in the Mega Ampère Spherical Tokamak

The Mega Ampère Spherical Tokamak (MAST) is now accessing regimes with high normalized confinement relative to international scalings, <iopmath latex="$H_{\rm H}$">H_H</iopmath>(IPB98(y, <iopmath latex="$2))\gt {\sim}1$">2))>~1</iopmath> at high normalized density, <iopmath latex="$\bar n_{\rme} \gt 60$">&barn;_e>60</iopmath>&percent; of the Greenwald density. Data from MAST H-modes suggest that the aspect ratio dependency of international confinement and L–H threshold scalings may need to be modified to improve predictions for ITER. Access to H-mode on MAST is strongly affected by both the divertor magnetic geometry and fuelling location, with the formation of an edge transport barrier being facilitated by operation near the symmetric, connected double-null configuration and with poloidally localized inboard gas puffing. The ELMs on MAST appear to be Type III in nature, even in the highest performance plasmas and with the maximum available auxiliary heating power. ELM energy losses are less than 4&percent; of stored energy in all regimes so far explored. These Type III ELMs are associated with a reduction in the pedestal density but no significant change in the pedestal temperature or temperature profile, indicating that energy is convected from the pedestal region into the scrape-off layer. Analysis of the energy observed to arrive at the divertor targets indicates that ELM losses are predominantly on the low field side. ELM effluxes are observed up to 20 cm from the plasma edge at the outboard mid-plane and are associated with the radial motion of a feature at an average velocity of 1.2 km s<iopmath latex="$^{-1}$">⁻¹</iopmath>.