What´s the time?: Clocking 4th generation X-ray light sources
Two collaborations with contributions from the Helmholtz-Center Berlin recently performed the world wide first experiments that directly determined the temporal structure of fs – X-ray pulses from a 4th generation light source. The impact of the two related publications is immense as they show, that the experimentally determined values can differ significantly from the theoretical predictions and that online measurements of the pulse duration and pulse shape may be compulsory in experiments critically depending on knowledge of e.g. peak flux.
Both measurements were performed at the soft X-ray free electron laser FLASH in Hamburg applying an autocorrelation and a cross correlation approach respectively. The autocorrelation experiment was based on beam split and delay unit developed and constructed at the Helmholtz-Center Berlin in collaboration with the University of Münster .This apparatus provides jitter-free pulse pairs for time-resolved x-ray pump/x-ray probe experiments over a photon energy range of 30 eV to 250 eV. The nonlinear autocorrelation of soft X-ray pulses from FLASH utilizing two-photon double -ionization of He atoms yielded an average pulse duration of 29 +/- 5 fs for an FEL tuned to a wavelength of 24 nm .
The second, cross correlation approach, is based on the combination of fully synchronized phase stable THz pulses with the soft X-ray pulses (for the design of the instrumentation see ). The measurements, carried out within a collaboration led by the group of Prof. M. Drescher of the University of Hamburg, allowed to determine the soft X-ray pulse duration and to some extent even the temporal structure of individual X-ray pulses. These properties of the X-ray pulse are measured by analyzing the photoelectron packets generated by photoionization of rare gas atoms in presence of the synchronized strong THz pulse. The electric field of the THz pulse acts as a fast streaking field, and allows to map the temporal structure of the soft X-ray pulse into electron momentum distribution . The measurements, performed at a photon energy of 92 eV, show an unexpected large scatter of the X-ray pulse durations between few fs to 50 fs and therefore underline the importance of appropriate, preferably inline diagnostic for the temporal properties of the X-ray pulses at 4th generation facilities like FLASH, FLASH II or the proposed planned energy recovery linac in Berlin. Instrumentation development towards this goal is pursued within the frame of a large THz photondiagnostic collaboration involving HZB, UHH, DESY, X-FEL, FZ-D and the PTB.
 R. Mitzner et. al., Spatio - temporal coherence of free electron laser pulses in the soft x-ray regime, Optics Express 16 (2008), 19909.
 R. Mitzner et. al., Direct autocorrelation of soft-x-ray free-electron-laser pulses by time-resolved two-photon double ionization of He, Phys. Rev. A 80 (2009), 025402.
 M. Gensch et. al., New infrared undulator beamline at FLASH, Infrared Phys. Technol. 51 (2008), 423.
 U. Fruehling, M. Wieland, M. Gensch et. al., Single-shot terahertz-field driven X-ray streak camera, Nature Photonics 3 (2009), 523.
Dr. Michael Gensch and Dr. Rolf Mitzner