Temporal stability of the UVSOR-FEL micropulse

Optical cavity and control system of the UVSOR-FEL have been newly developed to improve performances and to obtain better availability for user application experiment. Lasings at visible and UV wavelength regions were successfully obtained in test experiments. By optimizing reflectivity and transmission of cavity mirrors, an average out-coupled power of more than 15 mW was achieved at a wavelength of 270 nm. Though a CW lasing zone at the top of a detuning curve was clearly obtained, an oscillation of FEL micropulse time jitter with 60 Hz appeared, which is probably due to a mechanical vibration of the cavity mirror caused by "sound noise" of the environment. Observed macrotemporal structure has been examined by a computer simulation including both effects of the mirror oscillation and the cavity length detuning. Results suggest that the mirror vibration should be reduced to less than 0.1 μm to suppress the amplitude of the time jitter comparable to an intrinsic temporal width of the FEL micropulse.