Demonstrating H− beam focusing using an elliptical einzel lens

H− ion sourceresearch is being performed at the ISIS spallationneutron and muon facility on a dedicated Vessel for Extraction and Source Plasma Analyses (VESPA). The ion extraction and optics system presently being used on ISIS is centered on a combined-function sector dipole magnet. This traps cesium vapor escaping the ion source; mass-separates co-extracted electrons and stripped neutrals, and weak-focusses the highly asymmetric slit-shaped ion beam. Unfortunately the added drift length through the magnet under strong space-charge forces means up to 50% of the beam is collimatedon the magnet. The VESPA has shown that the ISIS ion sourceactually produces 80 mA of beam current at standard settings, but because of magnet collimationonly 55 mA is injected into the solenoid Low Energy Beam Transport (LEBT). A new purely electrostatic post-extraction system incorporating an einzel lenswith an elliptical aperture is currently under test. This allows much greater flexibility of perveanceand phase space m...