PPPL-5180

Poloidal asymmetries in edge transport barriers

Authors:  R. M. Churchill, C. Theiler, B. Lipschultz, I. H. Hutchinson, M. L. Reinke, 
D. Whyte, J. W. Hughes, P. Catto, M. Landreman, D. Ernst, C. S. Chang, R.
 Hager, A. Hubbard, P. Ennever, J. R. Walk, and the Alcator C-Mod Team

Abstract:  Measurements of impurities in Alcator C-Mod indicate that in the pedestal region signi ficant poloidal asymmetries can exist in the impurity density, ion temperature, and main ion density. In light of the observation that ion temperature and electrostatic potential are not constant on a flux surface [Theiler, Nucl. Fus., 2014], a technique based on total pressure conservation to align pro files measured at separate poloidal locations is presented and applied. Gyrokinetic neoclassical simulations with XGCa support the observed large poloidal variations in ion temperature and density, and that the total pressure is approximately constant on a flux surface. With the updated alignment technique, the observed in-out asymmetry in impurity density is reduced from previous publishings [Churchill, Nucl. Fus., 2013], but remains substantial (nz,H/nz,L ~ 6). Candidate asymmetry drivers are explored, showing that neither non-uniform impurity sources nor localized fluctuation-driven transport are able to explain satisfactorily the impurity density asymmetry. Since impurity density asymmetries are only present in plasmas with strong electron density gradients, and radial transport timescales become comparable to parallel transport timescales in the pedestal region, it is suggested that global transport e ffects relating to the strong electron density gradients in the pedestal are the main driver for the pedestal in-out impurity density asymmetry.
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Presented at:  APS-DDP 2014 New Orleans, LA, November 2015

Submitted to:  Physics of Plasmas
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Download PPPL-5180 (pdf 2.5 MB 41 pp)
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