Uni HH, Hamburg, Germany
DESY, Hamburg, Germany
MAX-lab, Lund, Sweden
PSI, Villigen PSI, Switzerland
DELTA, Dortmund, Germany
University of Hamburg, Hamburg, Germany
The sFLASH project at DESY is an experiment to study direct seeding using a source based on the high-harmonic generation (HHG) process. In contrast to SASE, a seeded FEL exhibits greatly improved longitudinal coherence and higher shot-to-shot stability (both spectral and energetic). In addition, the output of the seeded FEL is intrinsically synchronized to the HHG drive laser, thus enabling pump-probe experiments with a resolution of the order of 10 fs. The installation and successful commissioning of the sFLASH components in 2010/2011 has been followed by a planned upgrade in autumn 2011. As a result of these improvements, in spring 2012 direct HHG seeding at 38 nm has been successfully demonstrated. In this contribution, we describe the experimental layout and announce the first seeding at 38 nm.
Uni HH, Hamburg, Germany
TU Darmstadt, Darmstadt, Germany
DESY, Hamburg, Germany
TEMF, TU Darmstadt, Darmstadt, Germany
In order to produce radiation pulses of a few femtoseconds at FELs like FLASH, different concepts have been proposed. Probably the most robust method is to create an electron bunch, which is in the most extreme case as short as one longitudinal optical mode. For FLASH this translates into a bunch length of a few micrometers only. In order to mitigate space charge effects, the bunch charge needs to be about 20 pC. The technical requirements to achieve this goal are discussed. This includes beam dynamics studies to optimize the injection and compression of small charge electron bunches. A reduced photo injector laser pulse duration helps to relax the RF tolerance which scales linear with the compression factor. A new photo injector laser with sub-picosecond pulse duration in combination with a stretcher is used to optimize the initial bunch length. The commissioning of the new laser system and first experiments are described. Limitations of the presently available electron beam diagnostics at FLASH for short, low charge bunches are analyzed. Improvements of the longitudinal phase space diagnostics and the commissioning of a more sensitive beam arrival time monitor are described.
Uni HH, Hamburg, Germany
CFEL, Hamburg, Germany
DESY, Hamburg, Germany
Many users at FLASH work on pump-probe experiments, where time resolution is determined by the duration of the SASE pulses. Therefore users have expressed the strong wish for shorter XUV pulses. The shortest possible pulse is a single longitudinal optical mode of the SASE radiation. The most direct way to realize this at FLASH would be to reduce the electron bunch length to only a few μm at the entrance of the undulator section. In the ideal case a bunch charge of 20pC is sufficient for the generation of such short bunches. A shorter bunch duration directly at the photo-cathode helps to overcome technical limitations of the bunch compression due to RF induced non-linearities and collective effects. Beam dynamic studies are being performed to optimize the parameters of the photo injector laser, of the accelerating modules, and of the bunch compression. This includes particle tracking starting from the cathode though the accelerating modules with the ASTRA code and through the dipole chicanes using CSRtrack. A comparison of the beam dynamics simulations with measurements is presented in this contribution. The expected SASE pulses are being simulated with the Genesis code.