A longitudinal feedback for the Super-ACO free electron laser stability in the UV

To take complete benefit of the FELʹs features, longitudinal feedback has been developed for operating the FEL at perfect tuning without jitter. The laser micropulse temporal position is detected and compared to a reference position corresponding to perfect tuning. The time difference is converted into a correcting voltage, which is applied to the RF frequency pilot to adjust the synchronism condition. When the feedback is established, the micropulse jitter is reduced, the intensity fluctuations on the ms scale are damped and wavelength drift is suppressed. It has thus significantly improved the FEL operation at perfect tuning, and provides a stable laser with all its parameters optimized for the users.