QE and life time of Cs2Te photocathodes on copper for SRF gun-II at HZDR

The SRF gun-II at HZDR has been stably applied as the electron source for high power THz radiation since 2018, generating CW beams with bunch charges up to 300 pC at 100 kHz. It is an excellent demonstration that SRF guns can work reliably in a high power user facility. In order to generate higher current beam with MHz repetition rate, Cs2Te photocathodes are required. However, in last two experiments with Cs2Te, the Mo substrate plugs were overheated in superconducting rf cavity. The reason is that different thermal expansion coefficient of Mo and Cu led to a bad thermal contact between the Mo plug and Cu holder. Thus we decided to use Cu as new substratum of Cs2Te cathodes. In last year we prepared several Cs2Te photocathodes on Cu plugs and improved the vacuum of cathode transfer system in order to achieve satisfied lifetime. In this contribution, we will present the study result of QE and life time of Cs2Te photocathodes with different thickness.