Probing and improving coherence in Free-electron lasers

Sammanfattning: Free electron lasers or FELs, have transformed the way science has been done in the last decades by providing coherent, highly brilliant radiation at wavelengths not achievable with conventional lasers. Their most important characteristic, along side brightness, is their coherence, which allows users to perform experiments that rely on precise phase characteristics of the probing radiation.The FEL generates intense radiation by transferring energy from a relativistic electron beam to the EM field. Both the electron beam properties and the FEL-process itself determine the final characteristics of the FEL radiation. Through specific numerical and experimental tools it is possible to probe the coherence properties of FELs and gain insight into the fundamental processes influencing them. By understanding these processes, we can improve the coherence of the resulting FEL radiation using optical lasers or magnetic elements to act on specific features of the electron beam.Investigations of transverse coherence by a Young-like experimental setup revealed differences between SASE and seeded FELs in the way that coherence is built up during the amplification process. Using the existing formalism, we used a double slit experiment to determine the transverse coherence of an FEL at various stages in the amplification process in both seeded and SASE operation. The results revealed significant differences in the way coherence is built up in the two FEL operation modes.Longitudinal coherence improvement by EEHG seeding requires a strong magnetic chicane, which can makes it vulnerable to collective instabilities in the electron beam. This work shows how one can mitigate the effect of these instabilities by a creating a wavelength dependence with time of the seed laser. The EEHG technique is also explored here as a seeding option for SXL, the soft X-ray FEL at MAX IV.