Transverse Coherence of a VUV Free Electron Laser

Rasmus Ischebeck



The transverse coherence is of paramount importance for many applications of a free electron laser (FEL). In this thesis, the first direct measurement of the transverse coherence of a free electron laser at vacuum ultraviolet wavelengths is presented. The diffraction pattern at a double slit was observed and the visibility of the interference fringes was measured. The experimental near field diffraction pattern is compared with simulations, taking into account the formation of the FEL radiation, the Fresnel diffraction in the near field zone and effects of the experimental set-up. Diffraction patterns have been recorded at various undulator lengths to measure the evolution of the transverse coherence along the undulator. The highest coherence is achieved at the end of the exponential growth regime, before the onset of saturation in the FEL process.