Joint Research Activity 2
Instrumentation for Femtosecond Pulses:
Synchronization and Diagnostics

The objectives of JRA2 are to develop diagnostics and instrumentation required for high-resolution time resolved applications of the new accelerator based femtosecond (fs) VUV and x-ray sources. One of the key interests in fs x-ray sources is the opportunity to investigate ultrafast processes that happen on time scales ranging from several fs to several picoseconds (ps). On a fs time scale atomic nuclei are practically “standing still” whereas electrons can still move. Consequently the evolution of the electronic structure can be probed in detail as the atoms start moving apart if, for instance, a chemical bond is broken. Also the atomic motion itself can be probed stroboscopically using fs x-ray pulses with wavelengths in the Ångstrom region. The availability of fs x-ray sources can thus be expected to revolutionize the scientific capabilities in many different areas ranging from biology and life sciences, environmental science and chemistry, to magnetism and complex materials.
Exploitation of the unique properties of these new sources for a broad range of applications requires the development of suitable diagnostic tools. They can, in general, not simply be transferred from synchrotron radiation or optical laser diagnostics due to the very special short pulse radiation characteristics. For an FEL, for instance, the lasing process starts from noise and amplifies this noise to saturation, individual radiation pulses differ in their intensity, temporal structure, and spectral distribution. In addition, the arrival time of the FEL pulses at the sample is expected to jitter from pulse to pulse. Therefore, it is necessary to measure the relevant parameters of every single pulse in a non-destructive way, i.e. to use a small fraction of the FEL radiation to measure its intensity, coherence, spectral and temporal structure and its exact arrival time. In addition, many of the proposed diagnostic tools rely on physical processes that are to a large extent unexplored today. For example, non-linear effects are normally used to determine the temporal overlap between pump and probe pulses. Almost no experience exists about such processes in the soft and hard x-ray range.
These challenges are addressed in JRA2 by a collaboration of groups from BESSY, CNRS, DESY, ELETTRA, MAXLAB, SLS and SOLEIL. Utilizing the existing storage ring based FemtoSlicing sources at (BESSY) and (SLS) as well as the linac based FEL source at DESY fs x-ray and electron pulse diagnostics are developed.

Contact: Hermann Dürr, BESSY, Tel. +49 30 6392 3443

Fig. 1: Photograph (top) and schematics (bottom) of the angular separation scheme (see Khan et al Phys. Rev. ST Accel. Beams 8, 040704 (2005)) of the BESSY FemtoSlicing source designed to generated fs soft x-ray pulses.

Fig. 2: Schematic overfiew of the SLS FemtoSlicing source designed to generated fs x-ray pulses in the hard x-ray region.