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Challenge hybrid FRC concept

Author: Loren C Steinhauer
Requested Type: Consider for Invited
Submitted: 2010-01-12 17:39:40

Co-authors: J.F. Santarius

Contact Info:
University of Washington
14700 N.E. 95th St., Ste 100
Redmond, WA   98052
USA

Abstract Text:
The idea of magnetic confinement in a field reversed configuration dates to about 1960, although the designation “FRC” didn’t emerged until about 1982. Despite questions about global stability, a small community of devotees has sustained the dream of an FRC-based ideal fusion system. Nearly two decades have passed since Artemis, the last comprehensive FRC reactor study. In the meantime a great deal of new physics has been uncovered. It is timely to consider how these affect the reactor embodiment.

The first “challenge” of the new perspective on FRC physics is the challenge to standard paradigms of a magnetic confinement system for fusion. In addition to the well-known unique properties of FRCs (inherently high-beta, natural divertor, simple magnetics), others have emerged: a strong tendency toward global relaxation, the presence of broken magnetic surfaces, a split separatrix, inherently high velocity shear in the edge region, the extreme isolation of the FRC core from material boundaries, the convective loss of electron energy, and the existence of two fundamental length scales. Some of these features have both an upside and a downside.

The second “challenge” of the new-perspective FRC toward a fusion system concerns how to make the difficult leap from current experiments to a fusion-grade plasma. The most challenging issues here are global stability and transport.

Global stability in the FRC context usually means tilt stability. The new perspective offers improvements here. The strong flow shear at the edge is a local stabilizing influence for low-frequency modes, tending to push the disturbance inward, away from the fragile separatrix region. In so doing it decreases the likelihood of a massive tearing-open of the closed field structure that might otherwise occur at the separatrix. The second improvement is the action of benign tearing activity in the core which regulates the current profile to maintain a marginally-stable state.

Transport, has emerged as the greatest challenge. Here, the isolation of the FRC, reduces the electron thermal loss to convective. On the other hand the transport rate at the edge is gyro-Bohm (or some reduced version of Bohm). Further, the surface-breaking associated with the relaxation in the core compromises the internal energy confinement and flattens the temperature profile. As a result a “leap” to a somewhat more favorable transport rate scaling is needed for fusion applications.

An interesting approach to improved transport is a hybrid concept in which an FRC “core” is imbedded in the central cell of a modified tandem mirror. In such a synergistic open-closed magnetic system, both mirror and toroidal physics contribute to confinement. The improved confinement on the open field lines by an electrostatic barrier, would reduce the drift parameter at the edge of the “FRC core” and improves the transport accordingly.

Characterization: D

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