Development of a photocathode electron gun for an ERL light-source

We have developed NEA-GaAs photocathode DC electron guns for energy-recovery linac (ERL) based next generation light source. One is a prototype which is operated at up to 250 kV and designed to deliver up to 50 mA beam current. A diagnostic beam line of the 250 kV gun has been constructed for evaluation of the NEA-GaAs photocathode. Another is a practical use machine which is operated at up to 500 kV and designed to deliver up to 10 mA beam current with emittance lower than 1 mm-mrad. The 500 kV gun has satisfyed performance required as an injector for the compact ERL, which is under construction at KEK (Tsukuba site) as a prototype of the ERL light source. Initial emittance of the NEA-GaAs has been measured using the 250 kV gun. The measurements have been performed by a single-slit-scan method with combination of a 50-μm slit and a YAG:Ce screen. The minimum emittance is measured to be 0.054 mm-mrad for the laser wavelength of 633 nm and spot size of 160 μm. Consequently, the effective thermal energy of 54 meV for the wavelength of 633 nm is obtained. In the development of the 500 kV gun, a ceramic insulator is the most challenging part. The ceramic insulator should have good insulation and appropriate resistivity to avoid local concentration of electron charge, which causes fatal damage on the ceramic due to cracking or punch-through. So far, ceramic insulators with surface resistivity by special coating and ceramic with bulk resistivity have been used for photocathode DC guns. Stable operation at a voltage of 500 kV, however, has never been achieved in such guns, yet. We adopted a segmented insulator for the 500 kV gun, where multiple ceramics are stacked in series. This design is similar to the JAEA thermionic cathode gun (250 kV) for a high-power FEL and the polarized electron gun (200 kV) at Nagoya University, both of which have been operated with a good robustness. The high-voltage test has been conditioned up to 500 kV. The conditioning up to 550 kV and the demonstration of 500 kV operation will be done soon. (author)