Stability of field-reversed ion rings

Abstract

Studies of the low-frequency stability of field-reversed ion rings in a dense uniform plasma background have been carried out using a linearized 3-D hybrid simulation model. For a moderately thick ring with aspect ratio on the order of 4:1, the azimuthal mode number l=1 radial mode (magnetohydrodynamic precession) is observed to be stable, because of the favorable field gradient arising from image currents in the outer wall. The l=1 axial (tilt) mode is unstable for the parameters chosen, as are the l=2 radial and axial (kink) modes, and the l=3 radial mode. All other modes are observed to be either stable or very weakly unstable; numerical difficulties arising from the local instability of individual orbits in the equilibrium field make it difficult to examine modes with small growth rates. The structures of the various unstable modes range from near rigid to quite complicated in displacement pattern. Some preliminary evidence for the existence of betatron resonance-driven instabilities is noted.