Long lifetime of bialkali photocathodes operating in high gradient superconducting radio frequency gun

Abstract

High brightness, high charge electron beams are critical for a number of advanced accelerator applications. The initial emittance of the electron beam, which is determined by the mean transverse energy (MTE) and laser spot size, is one of the most important parameters determining the beam quality. The bialkali photocathodes illuminated by a visible laser have the advantages of high quantum efficiency (QE) and low MTE. Furthermore, Superconducting Radio Frequency (SRF) guns can operate in the continuous wave (CW) mode at high accelerating gradients, e.g. with significant reduction of the laser spot size at the photocathode. Combining the bialkali photocathode with the SRF gun enables generation of high charge, high brightness, and possibly high average current electron beams. However, integrating the high QE semiconductor photocathode into the SRF guns has been challenging. In this article, we report on the development of bialkali photocathodes for successful operation in the SRF gun with months-long lifetime while delivering CW beams with nano-coulomb charge per bunch. This achievement opens a new era for high charge, high brightness CW electron beams.

Overview of the Semiconductor Photocathode Research in China

“…The first K 2 CsSb photocathode system [53,54] in China was designed by the Shanghai Institute of Applied Physics (SINAP, now SARI) in 2012, where the first K 2 CsSb photocathode was grown with this system. The vacuum is as low as 10 −9 Pa, which is similar to that of bialkali deposition systems at other laboratories around the world [10,12,18,36,55,56].…”

“…Long dark/charge lifetime [12]. The charge lifetime, determined by the photocathode and drive laser together, should be large enough at an acceptable length of time without interrupting the machine operation to change or rejuvenate the photocathode [13,14].…”

Electron-Hadron Colliders: EIC, LHeC and FCC-eh

“…Most recently an SRF gun, continuous-wave (CW) operated and equipped with an alkali CsK 2 Sb photocathode, provided high-charge electron beams with small transverse emittance and remarkably long lifetimes of up to a month in stable, continued operation [30,31]. The availability of efficient spinpolarized electron sources is, on the other hand, relevant for electron-hadron colliders like the EIC.…”

First experience with He conditioning of an SRF photoinjector

“…0.15 mm-mrad slice emittance for 100 pC, 400 ps bunches), and has also generated record high charges per bunch (up to 20 nC) [2]. The cavity employs high quantum efficiency (QE) room temperature CsK 2 Sb photocathodes, and has demonstrated the ability to maintain the cathode's QE of 3-4% for several months of continuous operation [13].…”