Tests of niobium cathode for the superconducting radio frequency gun

For the superconducting all-niobium photocathode radio frequency gun project, we have studied the surface preparation techniques of the niobium cathode material. The quantum efficiency (QE) of high purity niobium (RRR=250) has been intensively measured at room temperature on a dedicated DC system. After buffer chemical polishing or electrolytic polishing, the initial QE is in the order of 10^-7 range tested by a 266 nm picosecond laser beam. However, the QE could be improved by more than two orders of magnitude after appropriate in situ laser cleaning. Both 266 nm ps-YAG and 248 nm ns-excimer laser beams have been used in the process of cleaning. The thresholds of the laser cleaning intensity were also experimentally determined to keep the morphology of the superconducting cavity surface.