Tearing-mode instability in a field-reversed ion layer

Tearing-mode stability properties are investigated for azimuthally symmetric perturbations about an intense proton layer (P layer) immersed in a background plasma. The analysis is carried out within the framework of a hybrid-kinetic model. It is assumed that the layer is thin, with the radial thickness (2a) much smaller than the mean radius (R0). Tearing-mode stability properties are investigated for the choice of ion layer distribution function in which all ions have the same perpendicular energy in a frame rotating with angular velocity ωϑ and a step-function distribution in axial velocity vz. Moreover, the stability analysis is carried out for low frequency perturbations with ‖ω‖≲ω̂ci where ω is the complex eigenfrequency and ω̂ci is the ion cyclotron frequency. The dispersion relation is derived including the important dielectric influence of the background plasma and the background layer electrons. It is shown that an axial velocity spread stabilizes perturbations with sufficiently short axial wavelength. Moreover, the tearing-mode growth rate is significantly reduced whenever np/nb≳≳1, where np is the background plasma density and nb is the density of layer ions.