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icc_2011_poster.pdf2011-08-14 23:03:08Andrey Kabantsev
icc2011_kabantsev.pdf2011-08-14 23:00:00Andrey Kabantsev

MITIGATION OF COLLISIONAL NEOCLASSICAL TRANSPORT BY FLUSHING PARTICLES THROUGH A “RUFFLED” SEPARATRIX

Author: Andrey A Kabantsev
Requested Type: Consider for Invited
Submitted: 2011-06-10 12:57:00

Co-authors: D.H.E.Dubin, C.F.Driscoll

Contact Info:
UCSD
9500 Gilman Drive
La Jolla, CA   92093
USA

Abstract Text:
Recent experiments [1] and theory [2] have characterized a novel form of "chaotic" neoclassical transport, and here we show that this chaotic transport may actually REDUCE the total neoclassical transport.

We focus on separatrix-trapped particles, and on their bounce-averaged radial excursions due to bulk asymmetries such as toroidal curvature; collisional separatrix crossings then cause standard neoclassical transport.

However, the separatrices themselves are never perfectly symmetric, nor perfectly aligned with the transport-inducing asymmetries. When the separatrix has variations ("ruffles") in the plasma drift direction, then marginally trapped particles chaotically shift from trapped to passing and back, leading to transport which is essentially independent of collisionality.

This new chaotic transport becomes significant when the ruffle width is equal to the collisional separatrix width; and the chaotic separatrix crossings may either ENHANCE or SUPPRESS the net transport, depending on the phase angle between the ruffle and the transport-driving asymmetry. Suppression of net transport occurs when the ruffle is phased such that there is no net radial step from the chaotic transitions between trapped populations, but there is a reduced collisional separatrix layer and reduced collisional transport.

This mitigation of neoclassical transport has been verified in our experiments with applied electrostatic separatrices, where the transport-driving asymmetry and the separatrix ruffles are also fully controlled. When the phase angle of the ruffle is aligned "against" the transport-driving asymmetry, a factor-of-three suppression of neoclassical transport is observed.

1. A.A.Kabantsev, D.H.E.Dubin, C.F.Driscoll, Yu.A.Tsidulko. Phys. Rev. Lett. 105, 205001 (2010).
2. D.H.E.Dubin, C.F.Driscoll, Yu.A.Tsidulko, Phys. Rev. Lett. 105, 185003 (2010).

Characterization: A5,D1

Comments:

University of Washington

Workshop on Innovation in Fusion Science (ICC2011) and
US-Japan Workshop on Compact Torus Plasma
August 16-19, 2011
Seattle, Washington

ICC 2011