Parallel transport studies of high-Z impurities in the core of Alcator C-Mod plasmas

Abstract

Measurements of poloidal variation, ñz/⟨nz⟩, in high-Z impurity density have been made using photodiode arrays sensitive to vacuum ultraviolet and soft x-ray emission in Alcator C-Mod plasmas. In/out asymmetries in the range of −0.2<nz,cos/⟨nz⟩<0.3 are observed for r/a<0.8, and accumulation on both the high-field side, nz,cos<0, and low-field side, nz,cos>0, of a flux surface is found to be well described by a combination of centrifugal, poloidal electric field, and ion-impurity friction effects. Up/down asymmetries, −0.05<nz,sin/⟨nz⟩<0.10, are observed over 0.5<r/a<0.9 with nz,sin>0 corresponding to accumulation opposite the ion ∇B drift direction. Measurements of the up/down asymmetry of molybdenum are found to disagree with predictions from recent neoclassical theory in the trace limit, nzZ2/ni≪1. Non-trace levels of impurities are expected to modify the main-ion poloidal flow and thus change friction-driven impurity density asymmetries and impurity poloidal rotation, vθ,z. Artificially modifying main-ion flow in parallel transport simulations is shown to impact both ñz/⟨nz⟩ and vθ,z, but simultaneous agreement between measured and predicted up/down and in/out asymmetry as well as impurity poloidal rotation is not possible for these C-Mod data. This link between poloidal flow and poloidal impurity density variation outlines a more stringent test for parallel neoclassical transport theory than has previously been performed. Measurement and computational techniques specific to the study of poloidal impurity asymmetry physics are discussed as well.