Effects of hot electrons on the stability of a closed field line plasma

Motivated by the electron cyclotron heating being employed on dipole experiments, the effects of a hot species on stability in closed magnetic field line geometry are investigated by considering a Z-pinch plasma. The interchange stability of a plasma of background electrons and ions with a small fraction of hot electrons is considered. The species diamagnetic drift and magnetic drift frequencies are assumed to be of the same order, and the wave frequency is assumed to be much larger than the background, but much less than the hot drift frequencies. An arbitrary total pressure dispersion relation is obtained, with the background plasma treated as a single fluid, while a fully kinetic description is employed for the hot species. The analysis of the dispersion relation shows that two different kinds of resonant hot electron effects modify the simple magnetohydrodynamic interchange stability condition. When the azimuthal magnetic field increases with radius, there is a critical pitch angle above which the magnetic drift of the hot electrons reverses. The interaction of the wave with the hot electrons with pitch angles near this critical value always results in instability. When the magnetic field decreases with radius, magnetic drift reversal is not possible and only low speed hot electrons will interact with the wave. Destabilization by this weaker resonance effect can be avoided by carefully controlling the hot electron density and temperature profiles