Dynamic behavior of the NIJI-IV storage ring free electron laser

Temporal and spectral characteristics of the NIJI-IV FEL were investigated around 300 nm using a dual-sweep streak camera and a monochromator. It was found that a fine cavity tuning within ∼3.6 μm resulted in an obvious change of the lasing mode from quasi-cw to cw through a pulsed region, though the average laser power is almost the same. By measuring the dynamic behavior of both temporal pulse width and laser line width, FEL brilliance was found to be enhanced by a factor of ∼7.4 in the cw mode, compared with the quasi-cw mode. Phenomena relating to the laser startup were observed in detail with periodic gain switching operation. Laser dynamics of the NIJI-IV FEL will be totally discussed.