A method for measuring the FEL radiation power along an undulator

A highly desirable measurement in Free Electron Laser (FEL) experiments is the dependency of the radiation power along the undulator. Most designs of undulators prohibit detection of the radiation power within the undulator o r extracting the electron beam at arbitrary positions. Transport of both, t he radiation field and the electron beam, through the entire undulator, and thus an ongoing FEL interaction, is unavoidable. If there are many correct ion magnets distributed along the beam pipe, one can think of exciting a la rge orbit distortion downstream from any of these correctors. For a gain le ngth comparable to or larger than the beta function this excitation of a co herent betatron oscillation might degrade the FEL amplification to a level, for which the radiation power does not further grow over the remaining len gth of the undulator. This paper presents the efficiency of this method for the parameters of the VUV FEL at the TESLA Test Facility. (C) 2000 Publish ed by Elsevier Science B.V. All rights reserved.