Determination of temporal FEL pulse properties: challenging concepts and experiments

One of the most challenging tasks for the FEL photon diagnostics is the precise determination of the FEL pulse duration - maybe even getting information on the substructure of the SASE pulses. The knowledge of the temporal pulse characteristics is not only important for nonlinear interactions which rely purely on the correct determination of the intensity, but also to gain insight on the dynamics of the investigated processes. Here, the resolution of pump-probe experiments relies heavily on knowledge of the pulse duration for one-color pump-probe and in addition on the precise arrival time difference for two-color pump-probe experiments. Due to the wide range of available parameters at the existing and planed FELs the photon energies are ranging from VUV to X-rays with pulse durations of few fs or even sub fs range up to pulses with several 100 fs pulse duration. Thus, a variety of methods has to be investigated and utilized in order to characterize the temporal structure of these pulses. Moreover due to the statistical nature of the SASE process the pulse shape (consisting of few up to hundreds of sub-pulses) varies from pulse to pulse. Here, techniques to characterize the pulse shot by shot are needed - increasing the level of complexity in contrast to averaging techniques by far. An overview of the different techniques is given in this text.