BEAM QUALITY AND STABILITY IMPROVEMENTS FOR A SINGLE-CELL PHOTOCATHODE RF GUN

Laser beam with high stability and quality to illuminate the photocathode is of great importance to generate an electron beam of high brightness from the photocathode RF gun system. In order to stabilize the electron beam, the laser system and the synchronization between the laser pulse and RF signal were improved. The synchronization with the time jitter of 1.2 ps was realized with a frequency feedback of the Ti:Sapphire oscillator and the RF generation from the laser pulses. Considering the emittance growth caused by the space charge effect, the ideal laser profile should be cylindrical with uniform density. The disturbed profile of the laser pulse coming from the third harmonics generator of Ti:Sapphire laser was homogenized as quasi-flattop with a microlens array. The horizontal normalized rms emittance of the electron beam was reduced to 2πmm-mrad for a charge per bunch of 0.1 nC due to the improvement of spatial profile. The laser injection angle of 66 degrees causes a laser spot image deformation to an ellipsoidal image on the cathode. This leads to a lag of arriving time on the cathode as well as the expansion in horizontal diameter of the laser beam. Deformation of laser profile to an ellipsoidal image was also carried out with a pair of cylindrical lenses to get a round image on the cathode. The transverse beam emittance was improved to 5.4 πmm-mrad at 0.2 nC with the deformation optics, while it was 6.6 πmm-mrad at 0.1 nC without them.