PFC / JA - 89 - 4 Rev . Neoclassical Analysis of Impurity Transport Following Transition to Improved Particle Confinement

Strongly peaked impurity density profiles have been observed in Alcator C after frozen hydrogen pellet injection. More recent experiments in ASDEX, PBX, TEXT, JET, and TFTR have exhibited similar impurity accumulation during regimes of improved confinement. In this context, we present calculations of the neoclassically predicted equilibrium profiles of the intrinsic impurities in Alcator C. These theoretical calculations were performed for comparison with the experimentally determined peaked profiles observed after pellet fueling. Profiles of the main impurities in Alcator, carbon (C) and molybdenum (Mo), were measured using soft x-ray diagnostics. C exists in the plateau collisionality regime, and its transport is dominated by collisions with the hydrogen background ions and temperature gradient effects. Mo is in the Pfirsch-Schliiter regime, and it is driven mostly by collisions with C inside r/a ~ 0.25 and temperature gradients outside this radius. The rigorous multi-ion, mixed regime calculation necessary for the Mo transport is shown here. The predicted C profile is in excellent agreement with observation, and the profile predicted for Mo (which is not as close to equilibrium as C) is in fair agreement with observation.