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, proportional to Q(2)); the optical pulse duration, sigma(z), scales as the inverse of the square root of the charge, (sigma(z) proportional to 1/root Q); the efficiency, eta, scales as the inverse of optical pulse length (eta proportional to 1/sigma(z) proportional to root Q), which also implies that the relative spectral brightness defined by eta/(sigma(lambda)/lambda) 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.