Neoclassical analysis of impurity transport following transition to improved particle confinement

Abstract

Strongly peaked impurity density profiles have been observed in Alcator C after injection of frozen hydrogen pellets. Recent experiments in TEXT, ASDEX, PBX, JET and TFTR have exhibited similar impurity accumulation during regimes of improved confinement. The paper presents calculations of the neoclassically predicted equilibrium profiles of intrinsic light and heavy impurities in Alcator C and light impurities in TEXT. These calculations were performed for comparison with experimentally determined peaked profiles observed after pellet fuelling. In both machines, carbon exists in the plateau collisionality regime and its transport is dominated by collisions with the hydrogen background ions and by temperature gradient effects. In Alcator C, molybdenum is in the Pfirsch-Schliiter regime and is driven mostly by collisions with carbon inside r/a ≃ 0.25 and by temperature gradients outside this radius. The full neoclassical multi-ion, mixed regime calculation required for the transport of carbon and molybdenum is presented. The predicted carbon profile in TEXT is in good agreement with observation; in Alcator C, less outward diffusion or an additional inward drift is required for the predicted carbon profile to agree with observation. The profile predicted for molybdenum (which may not be as close to equilibrium as carbon) is in fair agreement with observation. While the results of these studies do not support a rigorous claim of agreement with neoclassical impurity transport, the observed profiles for different regimes and experiments are consistently close to neoclassical-like peaking predictions.