Self mode-locked oscillation and spectra of spontaneous emission anf small-signal gain of a raman type free-electron laser

Abstract

A low-energy waveguide-mode free-electron laser( FEL) has been constructed and its operation characteristics haves been investigated experimentally and theoretically. Self mode-locked oscillation and the spectra of spontaneous emission and small signal gain of the FEL system are discussed. Two kinds of low-energy-spread relativistic electron beams (Relativistic Photo-Electron Beam, Cold Relativistic Electron Beam) have been developed and their applicability to FEL research is verified. A simple analytical expression for the spectrum of spontaneous emission in a rectangular waveguide radiated by an electron beam passing through a linear wiggle magnetic field and an axial guiding magnetic field is derived by using the method of dyadic Green``s function. The measured spontaneous emission spectrum agrees well with the theoretical expectations and is found to be a homogeneously broadened spectrum. the central frequency of spontaneous emission is tunable in a wide range of 25~40 GHz by changing the axial guiding magnetic field. A general expression relating the small-signal gain coefficient with the spontaneous emission spectrum is derived by using the classical limit of the Einstein`` coefficient method and it is applied to the calculation of smallsignal gain coefficient of a waveguide-mode FEL. The experimentally measured spectrum of small-signal gain agrees well with theoretical expectation and is found to be that of homogeneously broadened spectrum. From the scaling behaviour of the small-signal gain with respect to RPE current and wiggle magnetic field it is clear that the FEL using the RPE works in low-gain Compton regime. FEL oscillation experiments using CREB have been performed and a mediumpower (~100kW) microwave 30 (GHz) has been generated. Periodical (25ns) short (5ns) pulse trains have been observed and the density of electrons at the center of the cross section of electron beam is modulated at the same period as that of the modulation of oscillation...