The Influence of Laser Cleaning Treatment on the Quantum Efficiency of the Most Used Metallic Photocathodes: An Overview.

Abstract

This paper presents a comprehensive investigation into the quantum efficiency (QE) of metallic photocathodes used in modern high-performance radio frequency (RF) and superconducting radio frequency (SRF) guns. The study specifically examines how laser cleaning treatment impacts the QE of these photocathodes, providing detailed insights into their performance and potential improvements for accelerator applications, and assesses the chemical and environmental factors affecting the surface composition of metallic laser-photocathodes used in modern high-performance radio frequency (RF) and superconducting radio frequency (SRF) electron guns. This paper overviews the photocathode rejuvenation effects of laser cleaning treatment. Laser cleaning removes the oxides and hydrides responsible for the deterioration of photocathodes, increases the photoelectron emission quantum efficiency (QE) and extends the operational lifetime of high-brightness electron injectors. QE enhancement is analyzed with the aim of parametric cleaning process optimization. This study excludes semiconductor and thermionic cathodes, focusing solely on the widely used bulk and thin-film photocathodes of Cu, Mg, Y, Pb and Nb. Laser cleaning enhancement of QE in Cu from 5 × 10^-5 to 1.2 × 10^-4, in Mg from 5.0 × 10^-4 to 1.8 × 10^-3, in Y from 10^-5 to 3.3 × 10^-4, in Pb from 3 × 10^-5 to 8 × 10^-5, and in Nb from 2.1 × 10^-7 to 2.5 × 10^-5 is demonstrated. The analysis concludes with a specialized practical guide for improving photocathode efficacy and lifetime in RF and SRF guns.