Breakthrough in Generation of Polarized Electron Beams: Unveiling the World's First RF Electron Gun with GaAs Photocathode

Polarized electron beams play critical role in fundamental physics research by providing additional observables and opening new channels of discoveries. This discovery potential is well-known and is successfully used in high-energy and nuclear physics research. Recently, more conventional branches of science, such as ultra-fast electron microcopy, started exploitation unique features of polarized electrons. Currently GaAs crystals illuminated by circular polarized IR lasers remain the best choice for generating polarized electrons. All existing polarized sources are an electrostatic electron guns providing extremely high vacuum (XHV) conditions for survival of GaAs photo-emissivity. But limits in accelerating voltage and its gradient limits both the quality and quantity of available beams from these guns. These are the reasons why accelerator community was and is attempting to extend this technology to the radio-frequency electron guns, which are capable of accelerating beams with significantly higher accelerating gradients and total accelerating voltage. Unfortunately, all previous attempts of operating GaAs photocathodes in RF guns were unsuccessful. In this paper, we report on successful operation of GaAs photocathode in superconducting RF gun, describe in detail the accelerator system, used techniques, evolution of the GaAs quantum efficiency, and parameters of the generated electron beam.