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Evidence of relaxation and spontaneous transition to a high-confinement state in high-beta steady-state plasmas sustained by rotating magnetic fields

Author: Houyang Y. Guo
Requested Type: Consider for Invited
Submitted: 2006-12-14 18:36:44

Co-authors: A.L.Hoffman, K.E.Miller, R.D.Milroy, L.C.Steinhauer

Contact Info:
University of Washington
14700 NE 95th Street, Suite 10
Redmond, WA   98052

Abstract Text:
Relaxation in low-beta plasma states, such as solar and space plasmas, as well as laboratory plasmas including spheromaks and reversed field pinches, has long been recognized and is usually described in terms of the Taylor relaxation principle. However, the Taylor theory predicts force-free plasma states which have intrinsically zero beta. Recently, strong evidence for relaxation in an extremely high-beta field-reversed configuration (FRC) plasma state has been obtained from the translation, confinement and sustainment (TCS) experiment [1]. This high-beta plasma state is produced by highly super-Alfvenic translation, expansion, and capture of a spheromak-like plasmoid produced by conventional theta-pinch technology. The initial translating plasmoid has little poloidal field, but strong, oppositely-directed toroidal fields at its ends. After extremely violent reflections from the magnetic mirrors at the ends of a confinement chamber, the plasmoid quickly relaxes into a spherical-torus (ST)-like FRC state with a broad core conforming to a two-fluid minimum energy state [2,3].

The theta-pinch formed/translated FRCs are, however, limited to relatively low magnetic flux and sub-ms lifetimes. TCS has demonstrated, for the first time, formation and steady-state sustainment of flux-confined, elongated FRCs for up to 100 resistive decay times, using rotating magnetic fields (RMF) [4]. RMF also provides stability to the potentially disrupting n=2 rotating interchange mode [5]. New evidence of relaxation has also appeared in this long pulse operation, as signified by the self-generation of a significant toroidal field, with spontaneous transition to a higher confinement state. Temporally and spatially resolved magnetic measurements reveal that the transition occurs at the onset of the low-frequency drift (LFD) modes. The basic RMF structure changes dramatically during the transition; a new axial RMF B_z field appears. This, along with the normal radial and azimuthal RMF components, B_r and B_theta, provides current drive in both toroidal and poloidal directions. The latter would serve to sustain the magnetic helicity. The final relaxed state exhibits the following key properties [6]: (1) a near-force-free state in the core region, (2) a large q~1 along with significant magnetic shear near the magnetic axis, and (3) significantly improved confinement. These results will be presented.

[1] H.Y. Guo et al., Phys. Rev. Lett. 92, 245001 (2004).
[2] H.Y. Guo et al., Phys. Rev. Lett. 95, 175001 (2005).
[3] L.C. Steinhauer, H.Y. Guo, Phys. Plasmas 13, 052514 (2006).
[4] A.L. Hoffman et al., Nucl. Fusion 45, 176 (2005).
[5] H.Y. Guo et al., Phys. Rev. Lett. 94, 185001 (2005).
[6] H.Y. Guo et al., Phys. Rev. Lett. 97, 235002 (2006).

Characterization: A2,E3


University of Maryland

Innovative Confinement Concepts Workshop
February 12-14, 2007
College Park, Maryland

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