Rotational stability of plasma blobs

Abstract

The stability of plasma blobs which have both density and temperature higher than the surrounding plasma, and can transport heat as well as particles, is considered. It is shown that the internal blob temperature profile Te(r) can drive azimuthal rotation or spin vθ(r) about the blob axis, which produces a robust m=2 rotational instability in the interchange limit (k∥=0). The theory includes the effects of the centrifugal and Coriolis forces, the sheared velocity vθ(r), and the axial sheath boundary condition. Estimates show that finite-Larmor-radius stabilization is ineffective, but the sheath conductivity can be strongly stabilizing. The blob rotational instability has only a small direct impact on the particle and energy transport, but it serves as a useful diagnostic for the underlying blob spin, which is an important variable in determining the blob’s radial velocity. A separate branch of temperature-gradient-driven sheath instabilities, predicted in the eikonal limit, is not observed for low mode numbers.