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

Abstract

Measurements of poloidal variation, [˜ over n][subscript z]/⟨n[subscript z]⟩ , 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 < n[subscript z,cos]/⟨n[subscript z]⟩ < 0.3 are observed for r/a < 0.8 , and accumulation on both the high-field side, n[subscript z,cos] < 0, and low-field side, n[subscript z,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 < n[subscript z,sin]/⟨n[subscript z]⟩ < 0.10, are observed over 0.5 < r/a < 0.9 with n[subscript z,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, n[subscript z]Z[superscript 2]/n[subscript i] ≪ 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, v[subscript θ,z] . Artificially modifying main-ion flow in parallel transport simulations is shown to impact both [˜ over n][subscript z]/⟨n[subscript z]⟩ and v[subscript θ,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.