Measurement and calculation of the 'electron efficiency' on the 'CLIO' free-electron laser

This paper describe 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. A analytical expression of the 'Electron efficiency' is given here, and it is compared to the measurements.