Development of adaptive optical system of both spatial and temporal beam shaping for UV-laser pulse

We have been developing a stable and highly qualified UV-laser pulse as a light source of an rf gun for an injector candidate of future light sources. The CPA (chirped pulse amplification) Ti:Sapphire laser system is operated at a repetition rate of 10 Hz. At the third-harmonic generation (central wavelength: 263 nm), the laser pulse energy after a 45-cm silica rod is up to 850 mJ/pulse. In its present status, the laser's pulse energy stability has been improved down to 0.2-0.3% at the fundamental and 0.7-1.4% (rms; 10pps; 33,818 shots) at the third-harmonic generation, respectively. This stability has been held for one month continuously, 24 hours a day. The improvements we had passively implemented were to stabilize the laser system as well as the environmental conditions. We introduced a humidity-control system kept at 50-60% in a clean room to reduce damage to the optics. In addition, we prepared a deformable mirror for spatial shaping and a spatial light modulator based on fused-silica plates for temporal shaping. We are applying both of the adaptive optics to automatic optimization of the electron beam bunch to produce lower emittance with the feedback routine. After the improvements, we can now freely form any arbitrary laser intensity distribution on the surface of the cathode. A fiber bundle was used to passively shape the laser spatial and temporal profile for the backward cathode illumination. The profile was spatially shaped into a perfectly homogeneous one with a 90-cm long fiber bundle. This shaping technique is based in practice on pulse stacking with 1,300 different optical paths. The 80-fs laser is shaped as a quasi-Gaussian profile with a pulse duration of 16 ps (FWHM). Combining with the computer-aided deformable mirror, the fiber bundle laser homogenizer can realize the ideal ellipsoidal laser profile to suppress emittance growing due to the space charge effect. (author)