Development of a high average current polarized electron source with long cathode operational lifetime

• Published in: Physical review special topics. PRST-AB. Accelerators and beams Vol. 10; no. 2; p. 023501
• Main Authors: Sinclair, C. K., Adderley, P. A., Dunham, B. M., Hansknecht, J. C., Hartmann, P., Poelker, M., Price, J. S., Rutt, P. M., Schneider, W. J., Steigerwald, M.
• Format: Journal Article
• Language: English
• Published: College Park American Physical Society 01.02.2007
• Subjects: ACCELERATORS; AFFINITY; BRIGHTNESS; CATHODES; Cathodic polarization; CEBAF ACCELERATOR; CESIUM; CHARGE DENSITY; Circular polarization; Electron affinity; ELECTRON BEAMS; Electron bombardment; ELECTRON EMISSION; ELECTRON GUNS; ELECTRON SOURCES; Electron spin; ELECTRONS; Ion bombardment; Laboratories; LASERS; LIFETIME; Negative electron affinity; NUCLEAR PHYSICS; OXIDATION; PARTICLE ACCELERATORS; PHOTOCATHODES; Photoelectric emission; PHOTOEMISSION; Photonic band gaps; PHOTONS; POLARIZED BEAMS; Ultrahigh vacuum
• ISSN: 1098-4402; 1098-4402; 2469-9888
• DOI: 10.1103/PhysRevSTAB.10.023501

Substantially more than half of the electromagnetic nuclear physics experiments conducted at the Continuous Electron Beam Accelerator Facility of the Thomas Jefferson National Accelerator Facility (Jefferson Laboratory) require highly polarized electron beams, often at high average current. Spin-polarized electrons are produced by photoemission from various GaAs-based semiconductor photocathodes, using circularly polarized laser light with photon energy slightly larger than the semiconductor band gap. The photocathodes are prepared by activation of the clean semiconductor surface to negative electron affinity using cesium and oxidation. Historically, in many laboratories worldwide, these photocathodes have had short operational lifetimes at high average current, and have often deteriorated fairly quickly in ultrahigh vacuum even without electron beam delivery. At Jefferson Lab, we have developed a polarized electron source in which the photocathodes degrade exceptionally slowly without electron emission, and in which ion back bombardment is the predominant mechanism limiting the operational lifetime of the cathodes during electron emission. We have reproducibly obtained cathode 1/e dark lifetimes over two years, and 1/e charge density and charge lifetimes during electron beam delivery of over 2×105C/cm2 and 200 C, respectively. This source is able to support uninterrupted high average current polarized beam delivery to three experimental halls simultaneously for many months at a time. Many of the techniques we report here are directly applicable to the development of GaAs photoemission electron guns to deliver high average current, high brightness unpolarized beams.