Light impurity density profiles following hydrogen gas puff and hydrogen pellet fuelling have been obtained via charge exchange recombination spectroscopy on TEXT. After hydrogen pellet fuelling, C+6 and O+8 profiles are more strongly peaked on axis than the electron density profile. In contrast, light impurity profiles measured during sawtoothing high density gas puff fuelled discharges peak to a lesser degree relative to the electrons, with lower central densities than in the pellet case. The shapes of the peaked pellet impurity profiles are well described by neoclassical theory. The measured transport coefficients are larger than neoclassical values. The inferred inward convective velocity is consistent with values measured by impurity injection during sawtoothing discharges, but the diffusion coefficient is an order of magnitude lower than that obtained with impurity injection.
Microstability analysis of pellet fuelled discharges in MAST