Observation of superradiance in a short-pulse fel oscillator

Superradiance has been experimentally studied, in a short-pulse free-electron laser (FEL) oscillator. Superradiance is the optimal way of extracting optical radiation from an FEL and can be characterised by the following scale laws: peak optical power P, scales as the square of electron charge, Q, (P, &unknown; Q2); the optical pulse duration, z, scales as the inverse of the square root of the charge, (z &unknown; 1/Q); the efficiency, , scales as the inverse of optical pulse length ( &unknown; 1/z &unknown; Q), which also implies that the relative spectral brightness defined by /(/) remains constant and close to 0.86. To characterise the properties of the superradiant emission, we have measured the efficiency, optical pulse energy, pulse duration and spectral width as functions of electron beam current and cavity loss for the optimum cavity length detuning. The efficiency has been deduced from measurements of electron beam energy spectra. The optical pulse duration has been determined from second-order autocorrelation measurements and the optical spectra determined using a grating spectrometer. We show that the superradiance in the oscillator has properties similar to that in a high-gain amplifier and discuss the links with spikes created by synchrotron instabilities

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. © 1997 The American Physical Societ