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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