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Stabilizing Effects of Rotating Magnetic Fields (RMF) on Field Reversed Configurations (FRC)
| Author: | Guo H.Y. |
| Coauthor: | G.R. Votroubek |
| Institution : | University of Washington |
| Abstract text: | Standard theta-pinch formed FRCs spin up in the diamagnetic direction and usually develop a severe n=2 rotational instability which destroys the configuration in about one-half its natural flux decay time. This instability has been stabilized by the application of static multipole fields with effective pressures on the order of the centrifugal pressure. In RMF formed and sustained FRCs the plasma spins up in the electron diamagnetic direction due to the strong RMF applied torque. Typical rotational velocities are equal to the ion thermal velocity, with ratios of w_i*r_s/v_ti much greater than for the theta-pinch formed FRCs. An n=2 distortion also develops for the RMF sustained FRCs, but can easily be kept small by keeping the separatrix radius r_s sufficiently close to the flux conserving coils and plasma tube walls. The distortion is localized to the region surrounding the field null, significantly reduced at the edge where the RMF is strong, as can be seen tomographically or on internal magnetic probe arrays. The FRC can be maintained for many tens of natural flux decay times, limited only by the RMF power supply. If a gap is left between two sections of RMF antennas, the n=2 distortion will develop there, while if an anti-parallel RMF drive is used, the stabilizing effect is even stronger. Contrary to expectation, a quadrapole RMF drive was less effective in stabilizing the n=2 rotational distortion than the normal dipole drive. There is no sign of any tilt instability for FRCs lasting up to 10 msec, but a distortion of the inner field lines is seen for some cases where the FRC is driven only at the edge, rotating at the electron drift velocity, which is several times higher than the ion rotation velocity. This is a Hall phenomenon that could affect tilt stability. |
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