Modal analysis of a seeded free-electron laser

Abstract

It has been shown that the direct seeding can enhance the performance of a free-electron laser (FEL) in terms of its spectral, temporal, and coherence properties and reduces fluctuations in FEL output energy and arrival-time jitter. The properties of the used seed photon pulse are of high importance. In this paper, we describe the influence of the M^{2} onto the achievable power contrast between the direct seeded and the unseeded FEL radiation. The results of these studies are compared with the data from the high harmonic generation direct seeding experiment “sFLASH” in Hamburg, Germany. A method to measure M^{2} from a single transverse intensity distribution of the high harmonics beam at waist is discussed.