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					Particle flux reversal under strongly sheared flow

P.W. Terry
Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706

A.S. Ware
Department of Physics and Astronomy, University of Montana-Missoula, Missoula, MT 59812

D.E. Newman
Department of Physics, University of Alaska at Fairbanks, Fairbanks, Alaska 99775

Measurements of the particle flux in probe-induced shear layers often show a region of locally
inward transport for sufficiently strong shear.1 The inward flux region reproducibly occurs on the
inside edge of the shear layer, where the flow shear is positive. We examine here the physics and
conditions that can lead to localized flux reversals. We show that in a sheared magnetic field the
advection of electron density by turbulent E×B flow with linearly varying mean yields a particle
flux that reverses sign at certain locations along the direction of shear. The effect, calculated for
strong flow shear, resides in the cross phase between the scalar and flow fluctuations. As with a
simpler model,2 the cross-phase is strongly suppressed. The flux reversal is produced by the
interplay between the inhomogeneities of the magnetic shear and flow shear. Regions of inward
flux tend to wash out if the turbulence consists of closely-spaced modes of differing helicity, but
survive if modes of a single helicity are relatively isolated. We calculate the spatial profile of the
flux for a single helicity of ITG fluctuations with a collisional nonadiabatic electron density. The
possible relationship of these results to experimentally observed regions of locally inward flux is

1. J. Boedo, et al., Nucl. Fusion, in press.
2. P.W. Terry, D.E. Newman, and A.S. Ware, Phys. Rev. Lett. 87, 185001 (2001).

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