Superradiance in a Short-Pulse Free-Electron-Laser Oscillator

We demonstrate that superradiance (SR) is a general feature of free-electron-laser (FEL) devices, incuding high gain amplifiers as well as low gain oscillators, and that SR pulses and spiking, due to synchrotron instabilities, have the same origin. In particular, we present a direct 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 electron current. The spectral brightness, $B$, defined as the ratio between the efficiency and the relative rms spectral width, is constant and equal to $0.86$.