Dissipative instabilities at the edge of reversed-field pinches driven by a combination of plasma gradients and a parallel current

Abstract

Stability in the edge region of reversed-field pinches is analyzed within the context of a two-fluid model. Two major sources of instability are identified: in combination with a parallel electric field, either an electron temperature gradient and/or a density gradient leads to rapid growth (of several to many Ohmic heating rates) over a region of several millimeters around the mode-rational surfaces in the edge region. The basic signature of both instabilities is electrostatic. In the case of the density gradient mode, it relies on the effects of electron compressibility, whereas the temperature gradient mode can be identified as the current-convective instability by taking the limit of zero density gradient, thermal force, and electron compressibility. The possibility of enhanced particle loss and thermal convection as a nonlinear consequence of the instability is indicated. A criterion for the overlap of the linearly unstable regions in the edge of reversed-field pinches is obtained.