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

Abstract

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. Spinpolarized 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 10 5 C=cm 2 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.

Precise determination of the deuteron spin structure at low to moderateQ2with CLAS and extraction of the neutron contribution

“…Detailed descriptions of the accelerator and its strained GaAs polarized electron source can be found in Refs. [55][56][57][58].…”

The Qweak experimental apparatus

“…Conceptually, the system is rather simple [26]. Circularly polarized laser light is incident on a photocathode, producing electrons that are accelerated in an electrostatic field.…”

“…Several different spots could be utilized before the photocathode required a reactivation cycle. Ion back-bombardment is the predominant mechanism degrading the photocathode QE during electron emission [26]. This effect was mitigated by replacing the 1.5 m focal length lens with one of 2.0 m, thus increasing the FWHM 31 of the laser spot from 0.5 mm to 1.0 mm and distributing the ion damage over a bigger region [28].…”

“…It was alternately inserted and removed approximately every 8 hours. This slow helicity reversal was used to cancel 26 …”

“…Three lasers operating at a repetition rate of 499 MHz were used 21 to individually supply beam to each of the three experimental halls at JLab. The beams were combined [26] using a polarizing beam-splitter for the high-current halls and a partially transmissive mirror for the low-intensity hall. A consequence of this arrangement was that the Q weak Hall C beam had opposite polarization to the others.…”