Plasma Rotation and Transport in MAST Spherical Tokamak

Full text: The role of the toroidal rotation driven by the NBI heating in determining the confinement properties of MAST spherical tokamak plasmas is investigated. The resulting E x B flow shear can be strong enough to suppress low-k ion scale transport close to the neo-classical level in the plasma core, although the electron transport remains anomalous. High resolution kinetic-and q-profile diagnostics on MAST allow the conditions favoring the formation of internal transport barriers to be investigated, including the role of ExB flow and magnetic shear, MHD and rational surfaces. Poloidal rotation measured using CXRS is found to be small (a few km/s) and comparable in magnitude to the predictions of neo-classical theory, hence making a small contribution to flow shear in the plasma core. Formation of ITBs in L-mode plasmas, in which the ion transport is also highly anomalous in the outer regions, is favoured by conditions of high toroidal flow and reversed magnetic shear. With co-current directed NBI heating, ITBs in the momentum and ion thermal channels tend to form in the vicinity of qmin, particularly when this passes through a rational value. In contrast, with counter-NBI ITBs of greater radial extent form outside qmin due to the broader profile of E x B flow shear produced by the greater prompt fast-ion loss torque. A number of gyrokinetic or MHD fluid turbulence codes are now available which incorporate the physics of flow shear. A beam emission spectroscopy (BES) system for low-k turbulence measurements is to be installed in 2010 and a synthetic BES diagnostic has been developed which will allow direct comparison of the turbulence characteristics with results of the simulations. (author)