H− and D− scaling laws for Penning surface-plasma sources

Abstract

The small-angle source (SAS), 4X source, and 8X source are Penning surface-plasma sources that produce high-current, high-brightness H− ion beams for accelerator applications. The scaling from the SAS (1X source) to the 4X source, and from the 4X source to the 8X source is at least as good as predicted by the scaling laws. In many instances, the scaling is better than predicted, particularly in the critical area of the efficiency with which H− ions are produced per unit of discharge power. Using ζ=jH−/FC, where FC is the cathode power density load, ζ4X≊2ζSAS and ζ8X≊1.5ζ4X. The jH− scaling is at least as good as predicted by the scaling laws—we have been able to produce the predicted H− current in both the 4X and the 8X sources. The SAS was scaled up in size to the 4X source, and the 4X source was scaled up in size to the 8X source, on the assumption that the effective kTH−=5 eV. We also assume that the effective kTH−=the effective kTD−. These temperature scalings appear to be obeyed. The D− current scaling appears to be even better than assumed, namely, jD−≊jH−.