Ultra-High Brightness Electron Beams from Very-High Field Cryogenic Radio-frequency Photocathode Sources

TL;DR

This paper explores the use of cryogenic RF copper structures at ultra-high fields to develop high-brightness electron sources for compact X-ray free-electron lasers, demonstrating significant potential improvements in beam quality and FEL performance.

Contribution

It introduces the concept of cryogenic, ultra-high field RF photoinjectors operating at 500 MV/m, showing their advantages for high-brightness electron beams and potential applications in compact FELs.

Findings

Achieved surface electric fields of 500 MV/m in cryogenic RF copper structures.

Simulations indicate the potential for high-brightness, low-emittance electron beams suitable for compact X-ray FELs.

Discussed the benefits of C-band operation and experimental considerations for future implementation.

Abstract

Recent investigations of RF copper structures operated at cryogenic temperatures performed by a SLAC-UCLA collaboration have shown a dramatic increase in the maximum surface electric field, to 500 MV/m. We examine use of these fields to enable very high field cryogenic photoinjectors that can attain over an order of magnitude increase in peak electron beam brightness. We present beam dynamics studies relevant to X-ray FEL injectors, using start-to-end simulations that show the high brightness and low emittance of this source enables operation of a compact FEL reaching a photon energy of 80 keV. The preservation of beam brightness in compression, exploiting micro-bunching techniques is discussed. While the gain in brightness at high field is due to increase of the emission current density, further increases in brightness due to lowering of the intrinsic cathode emittance in cryogenic operation are also enabled. While the original proposal for this type of cryogenic, ultra-high field photoinjector has emphasized S-band designs, there are numerous potential advantages that may be conferred by operation in C-band. We examine issues related to experimental implementation in C-band, and expected performance of this type of device in a future hard X-ray FEL such as MaRIE.