Core heat transport in the MAST Spherical Tokamak

High-β spherical tokamak (ST) plasmas have intrinsic properties which favour the suppression of anomalous transport. Transport has been studied in NBI heated plasmas in the MAST ST device, where it is found that ion thermal transport is typically close to the neo-classical level. Calculations of the ITG microstability with the GS2 gyro-kinetic code suggest that this form of turbulence may be suppressed by the high ExB shearing rates in these plasmas. Electron transport is somewhat higher and cannot be explained from mixing length estimates of ETG turbulence. This is perhaps due instead either to micro-tearing modes in the core plasma or extended radial structures in the saturated turbulence. Micro-stability is also favoured by low magnetic shear and this has been used to produce high-performance L- and H-mode plasmas with improved core confinement as well as plasmas exhibiting ITBs in both the ion and electron channels. Broad electron ITBs have been produced with counter-NBI heating in which anomalous electron transport apparently has been reduced by the very high ExB shearing rates prevailing in these plasmas. Such studies also contribute towards testing the transport and ITB physics basis for the ITER device. (author)