AM MODE-LOCKING OF A FREE-ELECTRON LASER-OSCILLATOR

This paper presents experimental and theoretical studies of a mode-loc ked free-electron laser (FEL) oscillator. In this experiment the FEL e mploys a continuous electron beam and it operates in the microwave reg ime. AM mode-locking is performed by modulating the attenuation of the FEL ring cavity by a PIN diode modulator. The modulation period is tu ned to match the RF roundtrip time in the ring cavity. The experimenta l results show the evolution of a single radiation macropulse. It cons ists of narrow micropulses in synchronism with the sinusoidal locking signal. The micro-pulse Period (similar to 37 ns) equals the roundtrip time and the modulation period. The micro-pulse width (similar to 5 n s) is limited by the FEL slippage time and by the dispersion in the wa veguide ring cavity. The effect of the mode locking to suppress asynch ronous oscillations is clearly observed in the experiment. A theoretic al model of the AM mode-locked FEL oscillator operating in the small s ignal regime is presented. This model includes the slow time variation of the e-beam energy and waveguide dispersion. The theoretical analys is agrees well with the experimental results.