Plasma Rotation and Transport in the MAST Spherical Tokamak

The formation of internal transport barriers (ITBs) is investigated in MAST spherical tokamak (ST) plasmas. The roles of E×B flow shear, q-profile (magnetic shear) and MHD activity in their formation and evolution are studied using data from high-resolution kineticand q-profile diagnostics. In L-mode plasmas, with co-current directed NBI heating, ITBs in the momentum and ion thermal channels form in the negative shear region just inside qmin. In the ITB region the anomalous ion thermal transport is suppressed, with ion thermal transport close to the neo-classical level, although the electron transport remains anomalous. Linear stability analysis with the gyro-kinetic code GS2 shows that all electrostatic micro-instabilities are stable in the negative magnetic shear region in the core, both with and without flow shear. Outside the ITB, in the region of positive magnetic shear and relatively weak flow shear, electrostatic micro-instabilities become unstable over a wide range of wave-numbers. At ITG length scales, flow shear reduces linear growth rates and narrows the spectrum of unstable modes, but flow shear suppression of ITG modes is incomplete. Flow shear has little impact on growth rates at ETG scales. This is consistent with the observed anomalous electron and ion transport in this region. With counter-NBI ITBs of greater radial extent form outside qmin due to the broader profile of E×B flow shear produced by the greater prompt fast-ion loss torque.