Measurement of the transverse coherence of a VUV Free Electron Laser

The transverse coherence is important for many applica-tions of a free electron laser (FEL). It depends on the inner structure of the electron bunch in the undulator, which is difficult to measure. It is therefore essential to determine the coherence properties of the FEL radiation directly. The coherence of the vacuum ultraviolet FEL at the TESLA Test Facility has been measured by recording the diffraction pattern of a double slit and measuring the visibility of the interference fringes. The experimental near field diffraction pattern is compared with a numeri-cal model, 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 for various ef-fective undulator lengths to measure the evolution of the transverse coherence along the undulator. This is compared to the expected evolution of the transverse radiation modes. DIFFRACTION AT A DOUBLE SLIT The intensity in the far field (or Fraunhofer) diffraction pattern of a double slit is described by the well-known equation I(x) = sin(πwx/(λL)) πwx/(λL