Effect of pellet fuelling on energy transport in ohmically heated alcator C plasmas

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Abstract
Time-dependent transport analysis calculations have been carried out, using experimentally determined plasma parameters to obtain the variation of electron and ion thermal diffusivities following pellet injection into moderate-density Alcator C discharges. The ion thermal diffusivity, which is typically higher than neoclassical predictions by a factor of three to five in the gas-fuelled target plasma, is found to decrease after pellet injection to approximately the neoclassical value. The electron thermal conductivity is not reduced after pellet injection. The improvement in ion transport correlates with the peaking of the density profile and may be related to the reduction in the quantity ηi ≡ d ln Ti/d ln n, which is inferred to lie close to the critical value for stability of drift modes driven by the ion temperature gradient. Extrapolation of these results to higher-density plasmas, for which the electron and ion losses cannot be unambiguously measured, is consistent with previously reported increases in global energy confinement time accompanying pellet injection.