ION DIVERGENCE IN MAGNETICALLY INSULATED DIODES

Magnetically insulated ion diodes are being developed to drive inertia l confinement fusion. Ion beam microdivergence must be reduced to achi eve the very high beam intensities required to achieve this goal. Thre e-dimensional particle-in-cell simulations [Phys. Rev. Lett. 67, 3094 (1991)] indicate that instability-induced fluctuations can produce sig nificant ion divergence during acceleration. These simulations exhibit a fast growing mode early in time, which has been identified as the d iocotron 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 with a frequency that is appr oximately the reciprocal of the ion transit time. This instability cou ples effectively to the ions, and can generate unacceptably large ion divergences (>30 mrad). Linear stability theory reveals that this mode has structure parallel to the applied magnetic field and is related t o the modified two-stream instability. Measurements of ion density flu ctuations and energy-momentum correlations have confirmed that instabi lities develop in ion diodes and contribute to the ion divergence. In addition, spectroscopic measurements indicate that lithium ions have a significant transverse temperature very close to the emission surface . Passive thin-film lithium fluoride (LiF) anodes have larger transver se beam temperatures than laser-irradiated active sources. Calculation s of the ion beam source divergence for the LiF film due to surface ro ughness and the possible loss of adhesion and fragmentation of this fi lm are presented. (C) 1996 American Institute of Physics.