Electron suppression in the H− beam from a Penning surface-plasma source

Abstract

The ratio of electrons to negative ions extracted from Penning surface-plasma sources such as the 8X source is low even before any further steps are taken to suppress the electrons. For the 8X source the e−/H− ratio is typically four or five to one for H− operation and nine to one for D− operation. Because the coextracted e− present a power-loading problem to the 8X-source extraction system, methods to dissipate and/or reduce the power in the e− beam must be developed before extracting a dc H− or D− beam. Thus, we systematically varied the geometry of a cylindrical collar installed in the near-extraction region of the 8X source. The observed dependence of the extracted e− and H− currents on the collar radii and lengths suggests that a conical collar would provide superior electron suppression. The conical collar that we tested lowered the e−/H− ratio to 0.9/1 without reducing the extracted H− current.