Demonstrating H− beam focusing using an elliptical einzel lens

• Main Authors: Lawrie, S. R., Faircloth, D. C., Letchford, A. P., Whitehead, M. O., Wood, T.
• Format: Conference Proceeding
• Language: English
• Subjects: Cesium; Cesium vapor; Collimation; Emittance; High voltages; Ion extraction; Perveance; Phase matching; Quadrupoles; Spallation; Transport
• ISSN: 0094-243X; 1551-7616
• DOI: 10.1063/1.4995782

H− ion source research is being performed at the ISIS spallation neutron 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 collimated on the magnet. The VESPA has shown that the ISIS ion source actually produces 80 mA of beam current at standard settings, but because of magnet collimation only 55 mA is injected into the solenoid Low Energy Beam Transport (LEBT). A new purely electrostatic post-extraction system incorporating an einzel lens with an elliptical aperture is currently under test. This allows much greater flexibility of perveance and phase space matching for injection into the LEBT and Radio Frequency Quadrupole (RFQ). This paper discusses high voltage breakdown mitigation strategies and presents the first results of the novel elliptical transport system. So far, 70 mA of beam has been transported through the new system with a normalized transverse RMS emittance of 0.2 π mm mrad.