Superradiance in a short pulse FEL oscillator and its relevance to the X-ray FEL

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Abstract
Superradiance (SR) in the short pulse free-electron laser (FEL oscillator is the optimal way of extracting optical radiation from an electron beam. SR is characterized by a peak intracavity optical power, P, scaling as the square of the electron charge, Q, ( P ∝Q 2 ), an optical pulse duration, σ z , scaling inversely with the square root of the charge, (σ 2 ∝Q −1/2 ) and an efficiency, η, scaling with the inverse of optical pulse length (η∝1/σ z ∝Q 1/2 ). The latter scaling also implies that the relative spectral brightness, η/(σ λ /λ), is constant and close to ∛/2. Similar scaling of the peak power, temporal width, energy and efficiency with the cavity quality factor also exist. The efficiency in SR emission is enhanced above the usual natural efficiency, η=1/2N u , for the weakly saturated CW FEL, where N u is the number of undulator periods. We show that SR in the oscillator has analogous properties to that in a high gain amplifier and discuss the relevance of these analogies to the X-ray FEL starting from self-amplified spontaneous emission (SASE).

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