ark current investigation of a direct-current and superconducting radio-frequency combined photocathode gun

Photocathode guns with a low dark current are highly desired, especially in the case of high-brightness continuous-wave (CW) operation. The direct current - superconducting radiofrequency (DC-SRF) gun, a hybrid photocathode gun combining a direct-current gap and a superconducting radiofrequency (SRF) cavity, effectively isolates the photocathode from the SRF cavity and offers significant advantages in minimizing dark current levels. This paper presents an in-depth study on the dark current of the newly developed high-brightness DC-SRF photocathode gun (DC-SRF-II gun). Especially, a systematic experimental investigation of the dark current has been conducted and a comprehensive understanding of its formation has been achieved through compliant simulation and measurement. Besides, measures for attaining sub-nanoampere dark current in the DC-SRF-II gun are presented, including design considerations, cavity processing, assembly, and conditioning. Our work establishes a strong foundation for achieving high-performance operation of the DC-SRF-II gun and also provides valuable reference for other photocathode guns.