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Two-fluid mechanism of plasma rotation in field-reversed configuration

Author: Elena V Belova
Requested Type: Poster Only
Submitted: 2009-12-04 21:08:16

Co-authors: R. C. Davidson, C. Myers

Contact Info:
PPPL
P.O. Box 451
Princeton, NJ   08543-0
USA

Abstract Text:
End-shorting of the open magnetic field lines and particle loss are two mechanisms believed to contribute to plasma toroidal spin-up in field-reversed configurations (FRCs). End-shorting mechanism requires conducting boundaries, where tangential component of the electric field is zero, whereas the particle loss mechanism is related to loss of particles with preferential sign of toroidal velocity. Alternative spin up mechanism, which does not require conducting boundaries or particle losses, is presented. This mechanism relies on two-fluid description of plasma and finite parallel gradients of the plasma density. In particular, it is shown that the electron differential rotation with ω_e~p’/n results in twisted magnetic field lines and generation of antisymmetric toroidal field, which in turn leads to the ion toroidal spin-up. Time scale of the toroidal field generation is proportional to τ ~ S* L/Va, which is comparable to the Alfven time scale for reasonable values of L/Zs (where L is the scale length of the parallel density gradient, Zs is the separatrix half length, and S* is the FRC kinetic parameter). In experiments, regions of parallel density gradient can be formed, for example, near the ends, where the open-filed-line plasma comes into contact with the wall, or during the FRC formation. Results of two-fluid simulations using the HYM code are presented. Simulations demonstrate that plasma toroidal velocity can be comparable to the ion diamagnetic velocity even in the simulations with periodic boundary conditions (ie no end-shorting) and without the particle losses. Magnitudes of the generated toroidal field and rotation profiles are shown to depend strongly on the Lundquist number.

Characterization: A2,E5

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