ELECTRON SUPPRESSION IN THE H- BEAM FROM A PENNING SURFACE-PLASMA SOURCE

The ratio of electrons to negative ions extracted from Penning surface-plasma sources such as the 8X source is low even before any fur-ther 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.