Light-Ion-beam transport research at NRL

Ion-driven inertial confinement fusion requires transport of intense beams over several meters to isolate the ion source from the target explosion and allow for focusing and time-of-flight bunching. Transport in a low pressure background gas is possible only if rapid beam-induced gas ionization leads to formation of a plasma with electrical conductivity sufficient to charge- and current-neutralize the beam. In order to minimize collisional energy loss and scattering of the beam, helium at 1-Torr pressure is proposed for the background gas. Transport in the 1-Torr regime is not well understood because this pressure falls between the high-density regime treated by resistive models, and the low-density regime treated by collisionless models. Experiments and theoretical analyses are being carried out to study beam-induced gas ionization in this pressure regime in order to evaluate its impact on the various transport schemes including ballistic transport with solenoidal lens focusing, self-pinched transport, z-discharge transport, and wire-guided transport. Work in this area is reviewed along with other transport considerations including beam transport efficiency, beam-driven instabilities, and beam energy losses.