Instabilities in magnetically insulated ion diodes

Summary form only given, as follows. Magnetically insulated ion diodes are being developed to drive inertial confinement fusion (ICF). To achieve this goal, very high beam intensities are required. Thus reducing the ion beam divergence is of considerable importance. Three dimensional particle-in-cell simulations have demonstrated that instability induced fluctuations cause significant ion beam divergence. These simulations exhibit a fast growing mode early in time, which has been identified as the diocotron instability. The divergence generated by this mode is modest due to the relatively high frequency (>1 GHz). Later a low-frequency low phase-velocity instability develops. This instability couples effectively to the ions and can generate unacceptably large ion divergences, since the frequency is approximately the reciprocal of the ion transit time. A review of the linear stability theory of magnetically insulated ion diodes, indicates that the results depend rather sensitively on the details of the diode equilibrium model and the completeness of the stability analysis. We present new results based on a realistic family of ion diode equilibrium models using a fully relativistic electromagnetic stability analyses, which includes electron and ion perturbations both parallel and perpendicular to the applied magnetic field. This family of equilibria allows us to study the transition from the early diocotron growth to the low frequency ion mode.