STATUS OF SPRING-8 PHOTOCATHODE RF GUN FOR FUTURE LIGHT SOURCES

Since 1996, we have been researching a photocathode single-cell pillbox rf gun for future light sources, and to date have achieved a maximum field gradient of 187 MV/m with a chemical etching processed cavity. For the last three years, we have been focusing on the development of a stable and highly qualified UV laser source for the rf gun. The UV laser pulse (10 Hz) energy is 80 μJ/pulse at the entrance window. The energy stability (rms) of the laser has been reduced to 0.2~0.3% at the fundamental and 0.7~1.3 % at the third harmonic generations. This stability can be held continuously for two months, 24 hours a day. The improvement is the result of the stabilization of the laser system in a humidity-controlled cleanroom. In addition, the ideal spatial and temporal profiles of a shot-by-shot single laser pulse are essential to suppress the emittance growth of the electron beam from the rf gun. We prepared a deformable mirror for spatial shaping and a spatial light modulator based on fused silica plates for temporal shaping. With a computer-aided deformable mirror, we obtained a minimum horizontal normalized emittance of 1.74 mm mrad with a beam energy of 28.8 MeV, holding its net charge to 0.09 nC/bunch. We applied the variable quadrupole method to evaluate the emittance. The 3D shape of the laser was spatially flattop with a 1 mm diameter on the cathode and temporally a function of sech(t) with a pulse duration of 5 ps (FWHM). This laser pulse was illuminated on the copper cathode with normal incidence. We are currently preparing both adaptive optics to automatically optimize the electron beam parameters for lower emittance with a feedback routine