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

M. L. Reinke; I. H. Hutchinson; J. E. Rice; M. Greenwald; N. T. Howard; A. Hubbard; J. W. Hughes; J. L. Terry; S. M. Wolfe

doi:10.1063/1.4802197

journal article May 01, 2013

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

M. L. Reinke I. H. Hutchinson J. E. Rice M. Greenwald

N. T. Howard A. Hubbard

J. W. Hughes

J. L. Terry S. M. Wolfe

Physics of Plasmas Vol. 20 No. 5 · AIP Publishing

View at Publisher Save 10.1063/1.4802197

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&lt;nz,cos/⟨nz⟩&lt;0.3 are observed for r/a&lt;0.8, and accumulation on both the high-field side, nz,cos&lt;0, and low-field side, nz,cos&gt;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&lt;nz,sin/⟨nz⟩&lt;0.10, are observed over 0.5&lt;r/a&lt;0.9 with nz,sin&gt;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.

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Published: May 01, 2013
Vol/Issue: 20(5)

Authors

M

M. L. Reinke