An x-ray autocorrelator and delay line for the VUV-FEL at TTF/DESY

Abstract

In order to do jitter-free X-ray pump and probe experiments at the VUV-FEL at DESY / Hamburg (TTF2) as well as to characterize the temporal structure of its high power pulses an X-ray autocorrelator has been designed and is being engineered for photon energies up to 200 eV. The optomechanical design is based on geometrical beam splitting of the incomming FEL beam by a sharp mirror edge. Due to the limited reflection and the strong absorption of soft X-ray radiation an all-reflective geometry with grazing incidence angles at the mirrors has been chosen. The actual design represents a compromise between size and total delay range, on the one hand, and efficiency on the other hand. Thus the optomechanical device allows to handle high power X-ray pulses with high efficiency (50%). The total delay is about 25 ps with a femtosecond resolution. A further advantage of the special autocorrelator design is the lack of any angle deviation of the outgoing beam direction. Thus the autocorrelator can be integrated permanently into one of the FEL beamlines and measurements can be done with or without the beam splitter by slighly moving the whole chamber without breaking the vacuum. First experiments are planned in 2006 utilizing two-photon photoemission from noble gases in order to measure the temporal width of the FEL pulses at 40 eV.