Development of a 500-KV photo-cathodedc gun for the erl light sources in Japan

Energy recovery linac (ERL) based next generation light source such as X-ray FEL requires high brightness electron gun. We have developed a 500-kV, 10-mA photocath-ode DC gun. A segmented ceramic insulator with guard rings is employed to improve stability and robustness at high voltage operation by keeping field emission electrons away from the ceramic surface. A Cockcroft-Walton power supply and the insulator are installed in a SF6 tank and high voltage test up to 550kV was successfully done without a metal rod which supports a cathode electrode. All the vacuum chambers are made of chemically polished titanium alloy with very low out-gassing. A GaAs photocathode is activated in a separate preparation system. Up-to-date status of the gun development will be presented in detail.