Photon diagnostics on the VUV FEL at DESY: First lasing around 100 nm

A single-pass free electron laser (FEL) based on the principle of self-amplified spontaneous emission (SASE) is currently under test at the TESLA Test Facility at DESY. So far lasing has been observed in the vacuum ultraviolet (VUV) region between 80 and 180 nm. The photon beam has been characterized by different techniques in order to provide experimental evidence for SASE and to compare with numerical simulations. This paper presents some of the first results and summarizes the techniques used for the measurement of the photon beam intensity, its angular and spectral distribution, and its statistical properties.