Dark Current Research
During the operation of an RF photo injector with high gradient, not only the photoelectron beam, but also the dark current is produced. The dark current is a part of the field emission from the cavity and photocathodes. Field emission electrons are further accelerated through the gun together with the photocurrent beam. It belongs to the unwanted beam, which may leads to beam loss and facility damage, meanwhile creates additional background for primary or radiation beam users.
In the present SRF Gun-II wth Cs2Te, the detected dark current in Faraday cup is about 110 nA with Gun gradient 8 MV/m. The dark current from cathode area can be rebuilt on the downstream YAG screen with gun solenoid. The studies on the dark current sources and the reduction of its effect are going on at ELBE. For example, a fast kicker has been developed to kick only photocurrent beam to the neutron target in order to clean the measurement background.
R. Xiang, et al., Dark Current Measurements at the Rossendorf SRF Gun, FEL 2013, August 26-30, 2013, New York, USA
R. Xiang, et al., Operation of Cs2Te in SRF gun for THz user shifts, EWPAA2022, 20-22.Sep.2022, Milano, Italy