SUPERRADIANCE IN A SHORT-PULSE FREE-ELECTRON-LASER OSCILLATOR

We demonstrate that superradiance (SR) is a general feature of free-el ectron-laser (FEL) devices, incuding high gain amplifiers as well as l ow gain oscillators, and that SR pulses and spiking, due to synchrotro n instabilities, have the same origin. In particular, we present a dir ect observation of SR from an FEL oscillator. The ultrashort SR pulses are characterized by a peak intensity and pulse duration which depend , respectively, on the square and the inverse square root of the elect ron current. The spectral brightness, B, defined as the ratio between the efficiency and the relative rms spectral width, is constant and eq ual to 0.86.