H-mode plasmas in the MAST spherical tokamak

Operating scenarios in the Mega-Ampère Spherical Tokamak (MAST) have been recently developed to access routinely H-mode plasmas over a wide range of plasma conditions. Gas refuelling at the inboard mid-plane has proved invaluable in facilitating L- to H-mode transitions. Already, a comprehensive range of H-mode behaviour has been observed providing similar and contrasting comparisons with conventional tokamaks. For example, similarities are found in H-mode edge and global evolutions, increases in confinement and ELM behaviour. In contrast, H-mode threshold powers are about an order of magnitude higher in MAST than those predicted by conventional aspect ratio tokamak scalings. At the highest βθ values obtained so far, transient confinement times of ~100 ms are achieved and calculations indicate that bootstrap currents may approach 50% of the plasma current.