Ion beam transport in laser-initiated discharge channels

Abstract

Inertial-confinement fusion reactors with light ion-beam drivers will require that several intense ion beams propagate a few meters from separate diodes to a single target. Each beam would be confined and guided by a laser-initiated z discharge in a background gas. In this experiment we have studied the transport of a proton beam in such a laser channel. A pulsed, line-tuned, CO2 laser triggers the discharge in 4–20 Torr of ammonia gas. When the discharge current reaches peak value, an ion beam is injected from a pinch-reflex ion diode on the Hydra accelerator. The transported proton beam is detected by nuclear activation of carbon targets. Total current transport efficiencies of up to 50% were achieved in 13–45 kA, one-meter-long discharges. The most reproducible results were found for discharge currents of less than 30 kA; framing photography shows that the channel is unstable for kink modes at higher currents. Conditions in the discharge which led to this kink instability are examined, and the implications for reactor studies are considered. Operational data for the pinch-reflex diode operating in a high-impedance mode are also presented.