Confinement in START beam heated discharges

Plasma profile data, from a number of neutral beam heated discharges in the START spherical tokamak, have been exploited for transport analysis over a wide range in β (where β = 2μ0p/B2, μ0 is the permeability of free space, p is the plasma pressure, and B is the toroidal magnetic field strength). Magnetic equilibrium reconstructions, and Monte Carlo calculations of the neutral beam heat and particle sources, are incorporated in a particle and power balance code to extract the thermal diffusivities. Ion transport is found to be close to the level predicted by the Chang-Hinton formulation of neoclassical theory, with little head-room for significant anomalous ion heat transport, except close to the plasma edge. Off-diagonal corrections to the Chang-Hinton formula for the neoclassical ion thermal diffusivity do not significantly modify our conclusion. The electron thermal diffusivity exceeds the ion thermal diffusivity across the plasma cross section, and is comparable with the levels predicted by the Rebut-Lallia-Watkins and Lackner-Gottardi-Connor models of anomalous transport. The measured START ion temperature gradients lie below the critical gradients predicted by the IFS-PPPL model for the onset of the ion-temperature-gradient driven drift-wave turbulence. Data from the larger, higher current and lower collisionality devices, MAST and NSTX, will supplement START data and further improve our understanding of plasma transport in low aspect ratio tokamaks. In particular, these machines will be able to test whether ion transport remains neoclassical in larger spherical tokamaks, and will supplement data from conventional tokamaks to improve discrimination between theoretical models of anomalous transport. (author)