Progress in ion beam power coupling and ion-driven hohlraums on PBFA II

Light ions could provide an efficient driver for high-yield and energy applications of inertial confinement fusion. Working towards this goal, the authors have demonstrated well-diagnosed, ion-driven hohlraum targets at {approximately} 60 eV radiation temperature on the PBFA II accelerator, with specific power deposition of 800-1400 TW/gm. This parameter regime is of interest to both light- and heavy-ion drivers, and corresponds to the early phase of a high-yield target drive. Further increases in target temperature require increasing the ion beam driver intensity. Direct measurements of ion diode dynamics are providing insights to improve driver capability. Maximum beam intensity is obtained by both increasing the coupled lithium beam power and decreasing the beam microdivergence. The microdivergence of the beam near the ion source appears to be a major contributor early in the beam pulse. Experiments have shown that a parasitic non-lithium ion beam consumes a large fraction of the available power. Calculations indicate the coupled beam power can be increased by up to a factor of 3 by eliminating this parasitic loss. Preliminary experiments to clean the ion diode surfaces are under way.