High‐purity ion beam production at high current densities with a liquid‐helium‐cooled series‐field‐coil extraction ion diode

Experiments are described in which a high‐purity, high‐power (0.15 TW, 1 MeV) proton beam is generated from an ion source consisting of H2 gas frozen onto a liquid‐helium‐cooled copper anode at 4.2 K in a series‐field‐coil extraction diode on the 0.7 TW HydraMITE‐II accelerator. Peak anode proton current densities of 2 kA/cm2 were measured. This current density is a factor of 100 higher than those obtained in previous liquid‐helium‐cooled cryogenic diode experiments on small accelerators and is in the range required for high‐power ion beam applications. Thomson parabola, Faraday cup, and carbon activation measurements indicate an ion beam proton fraction close to 100% for the cryogenic source, compared to 50–70% for the standard hydrocarbon anode tested. The cryogenic proton source is believed to consist of no more than a few monolayers of molecular hydrogen. The hydrogen‐coated cryogenic anode shows a faster initial anode turn‐on than other materials. However, source‐limited emission from the thin hydrog...