Final focusing of Z-discharge transported ion beams

An experimental program is underway at NRL to demonstrate final focusing of transported ion beams. Z-discharge channels are used to transport ion beams efficiently from the ion source to an ICF target, providing isolation and distance for time-of-flight power compression. Channel stability and beam energy loss constraints force a minimum channel diameter for a given ion beam power. Final focusing uses an additional short Z-discharge with larger current to reduce the spot size of the ion beam from the channel diameter to the target diameter. The Gamble II accelerator is used to produce a proton beam from a pinched-reflex diode. The effect of the final focus is measured with a shadowbox at the exit of the final focus cell, and the transport is measured by the /sup 19/F(p, alpha gamma ) reaction from 50% transparent Teflon screens placed in the channel. Analysis of the shadowbox damage patterns indicates that the final focus cell plasma is taken into account. The 2.54-cm-diameter ion beam from the channel is reduced in diameter to less than 1.4 cm.<>