|icc06_guo_presentation.pdf||2006-03-13 14:52:58||Houyang Guo|
Improved stability and confinement in a relaxed extremely high-beta plasma state
Author: Houyang Guo
Submitted: 2005-12-12 20:33:24
Co-authors: A.L.Hoffman, L.C.Steinhauer, K.E.Miller
University of Washington
14700 NE 95th Street, Suite 10
Redmond, WA 98052
Relaxation in low-beta plasma states has long been recognized and was usually described by the Taylor relaxation principle. However, the Taylor theory predicts force-free plasma states with zero beta. Recognizing that realistic plasma states have finite beta, some more general relaxation principles have been advanced, but none has been verified by experiment. Recently, strong evidence for relaxation in an extremely high-beta (over 85%) field reversed configuration (FRC) plasma state has been obtained from the Translation, Confinement and Sustainment (TCS) experiment. This high-beta plasma state was produced by highly super Alfvenic translation of a spheromak-like compact toroid (CT) . The initial translated CT has little poloidal flux, but has strong toroidal fluxes at the ends, in opposite directions. After extremely violent reflections at the end magnetic mirrors of the confinement chamber, the disorganized plasmoid settles into a near-FRC state with a spherical torus (ST)-like core . The toroidal field magnitude is much smaller than that of the poloidal field, but when combined with the high elongation and small aspect ratio, it results in a safety factor exceeding unity over much of the configuration with a significant shear at the edge. The electron helicity is roughly conserved during the highly dynamic relaxation process with substantial flux conversion from toroidal to poloidal. Significant toroidal rotation has been observed, but flow measurements to date are too incomplete to determine whether the ion helicity is conserved.
The final relaxed state exhibits significantly reduced transport with up to four times improvement in confinement . Modeling using the newly developed nearby-fluids theory  shows that a broad core of this relaxed high-beta plasma object is very close to a minimum energy state (MES), and suggests that the inner region of MES may be further broadened via active control of flows at the edge, so as to further improve the confinement. Such a relaxed extremely high-beta plasma state also exhibits a remarkable stabilizing property for global low-n modes such as the normally lifetime terminating n=2 centrifugally driven interchange modes present in theta-pinch formed FRCs. This is explained, for the first time, by a simple model taking into account magnetic shear and centrifugal effects. This work not only reveals an exciting new prospect for FRCs, but also has important impacts on the basic understanding of the formation and relaxation to natural plasma states with finite beta.
 H.Y. Guo, A.L. Hoffman, K.E. Miller, and L.C. Steinhauer, Phys. Rev. Lett. 92, 245001 (2004).
 H.Y. Guo, A.L. Hoffman, L.C. Steinhauer, and K.E. Miller, Phys. Rev. Lett. 95, 175001 (2005).
 L.C. Steinhauer, H.Y. Guo, "Nearby-fluids equilibria - II: zonal flows in a high-beta, self-organized plasma experiment", submitted to Phys. Plasmas.