Measurement and calculation of the `Electron efficiency' on the `CLIO' Free-Electron Laser

Abstract

This paper describes the recent measurements of ‘Electron efficiency’ which has been done with the ‘CLIO’ Free-electron laser (FEL). The ‘Electron efficiency’ is the relative energy loss of the electron beam in the undulator during the FEL interaction. It gives an absolute measurement of the optical power produced by the FEL, which is then compared to the direct measurement using a power meter at the exit of the FEL. An analytical expression of the ‘Electron efficiency’ is given here, and it is compared to the measurements. THE CLIO FREE ELECTRON LASER The ‘CLIO’ Free Electron Laser (FEL) is an infrared tuneable laser source [1]. It uses a linear accelerator, which produces an electron beam of adjustable energy from 13MeV to 50MeV. The laser spectral range is tuneable from 5µm to about 120µm. Such a wide broadband is achieved by using, for λ >100µm, a special set of toroidal mirrors. Also, in order to reduce the cavity losses at large wavelength, a waveguide is installed in place of the vacuum chamber in the undulator section. The laser extraction of the cavity is achieved by hole coupling in the front mirror. The best configuration of radius of curvature for the cavity mirrors has been obtained with a numerical code [2] [3]. This code calculates the propagation of the laser wave front A(x,y) in the optical cavity. It uses an iterative process of wave propagation in the cavity, which converges to a steady state laser mode As(x,y) corresponding to the FEL saturation regime. It takes into account the design of the cavity (mirrors, vacuum chamber,…) including the waveguide effect in the undulator section, and the hole coupling in the front mirror. This code gives the amplitude distribution As(x,y) of the laser mode at saturation, in any point of the cavity, and it gives all related parameters : the cavity losses L, the ‘extraction rate’ Tx of hole coupling, the optical mode cross-section Σo,…