Directional asymmetries in the Alcator C edge plasma

Visual inspection of the full poloidal ring limiters of Alcator C identifies two directionally asymmetric damage regions at the high field side of the plasma: At the top location on the electron side (as defined by plasma current) and at the bottom location on the ion side. We report direct measurements of edge plasma parameters in the SOL plasma on the top location using Janus, a two-sided, multi-diagnostic edge probe. Higher ion and electron temperatures and densities at the probe location occur on the electron side under standard operating conditions. The degree of asymmetry cannot be explained by limiter configuration alone, but the direction of the toroidal field and variation of the plasma horizontal position, change the magnitude of asymmetry. Mechanisms that may cause the directional asymmetries include prallel plasma flow and asymmetric perpendicular transport into the collecting flux tubes. Measurements made by DENSEPACK, a full poloidal array of Langmuir probes, show that strong poloidal asymmetries in both plasma density and electron temperature in the scrape-off region of Alcator C exist. Such asymmetries may arise from asymmetric perpendicular transport and/or act to drive plasma flows along field lines. (orig.)