TURBULENT FEL THEORY AND EXPERIMENT ON ELSA AT BRUYERES-LE-CHATEL

We consider the asymptotic behaviour of long pulse high current Compto n free electron laser oscillators. It is known that if the current is high enough and the cavity losses low enough, sideband instabilities a nd non-linear mode couplings eventually lead to a strong broadening of the radiated spectrum, and to a strong efficiency enhancement. In thi s ''post-sideband'' regime, the electron dynamics along the wiggler is intrinsically stochastic, and the efficiency is due to chaotic diffus ion of the electrons toward lower energies, rather than to standard sy nchrotron oscillations. This results in new scaling laws for saturatio n properties. We have obtained simple analytical estimates for the ext racted efficiency and for the spectral width, in very good agreement w ith numerical simulations. The infrared ELSA free electron laser at Br uyeres-le-Chatel has been used to obtain experimental evidence for the se new scaling laws. In particular it has been verified that in the po st-sideband regime, the ratio of the extracted efficiency to the relat ive spectral width is independent of the operating parameters, and clo se to root 3/2 as predicted by theory.