H-mode plasmas in the MAST spherical tokamak

Operating scenarios in the Mega-Ampere 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 β θ 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.