OBSERVATION OF SUPERRADIANCE IN A SHORT-PULSE FEL OSCILLATOR

Superradiance has been experimentally studied, in a short-pulse free-e lectron laser (FEL) oscillator. Superradiance is the optimal way of ex tracting optical radiation from an FEL and can be characterised by the following scale laws: peak optical power P, scales as the square of e lectron charge, Q, (P, proportional to Q(2)); the optical pulse durati on, 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 th e inverse of optical pulse length (eta proportional to 1/sigma(z) prop ortional to root Q), which also implies that the relative spectral bri ghtness defined by eta/(sigma(lambda)/lambda) remains constant and clo se to 0.86. To characterise the properties of the superradiant emissio n, we have measured the efficiency, optical pulse energy, pulse durati on 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 spectr ometer. We show that the superradiance in the oscillator has propertie s similar to that in a high-gain amplifier and discuss the links with spikes created by synchrotron instabilities.