A full mode set electromagnetic stability analysis of magnetically insulated ion diodes

Abstract

An analysis of the stability of magnetically insulated ion diodes is presented that includes electromagnetic perturbations both parallel and perpendicular to the applied magnetic field, and is fully relativistic. The theory represents a generalization of previous work, which is either electrostatic or excludes wave motion parallel to the applied magnetic field. The analysis reveals a fast growing, low-phase velocity mode that is identified as a modified two-stream instability. This mode is similar to the low frequency mode observed in three-dimensional (3-D) particle simulations, and may be a major cause of ion divergence. It is shown that allowing diode electrons to respond to perturbations in the direction of the applied magnetic field introduces a new set of electron space-charge waves that cause ion modes to become unstable at low frequency and phase velocity. Furthermore, it is shown that these electron space-charge waves are significantly influenced by electromagnetic effects, which therefore cannot be ignored.