Hot Electron Instability in a Dipole Confined Plasma

In plasma containing energetic electrons, two interacting collective modes, an MHD-like mode and a hot electron interchange (HEI) mode\footnote{N. A. Krall, Phys. Fluids, {\bf 9}, 820 (1966).}, may be present. The linear stability of interchange modes in a z-pinch at arbitrary beta, including a bulk and hot electron species was recently studied\footnote{N. Krasheninnikova, P. J. Catto, Phys. Plasmas, {\bf 12}, 32101 (2005).}. Using the dispersion relation derived in this reference we show that when necessary conditions are satisfied the two modes may be present or absent in a closed-field line magnetic confinement geometry such as a hard core z-pinch or a dipole. The HEI instability and the MHD-like centrifugally-driven mode have been studied previously\footnote{B. Levitt, \textit{et al.}, Phys. Plasmas, {\bf 9}, 2507 (2002), and {\bf 12}, 055703 (2005).}, including a comparison between the measured mode structure and the predictions of a global low-beta simulation. The radial eigenmode is seen to effect the saturation level of the mode. In the \urllink{Levitated Dipole Experiment}{http://psfcwww2.psfc.mit.edu/ldx/} electron cyclotron resonance heating produces high beta plasmas containing hot electrons, and instability observations will be discussed and compared with theoretical predictions.