Uni HH, Hamburg
DESY, Hamburg
The Free Electron Laser FLASH has been upgraded during winter 2009/10. Amongst other components, a third harmonic module operating at 3.9 GHz (ACC39) has been installed. Together with the energy chirp induced by off-crest operation, it allows for a linearisation of the longitudinal phase space, leading to a uniform compression of the electron bunch with final bunch lengths of 150 μm rms. In contrast to the old non-linear compression scheme, peak current and bunch length are extremely sensitive to the phases of ACC39 and ACC1 and have to be monitored continuously. The foreseen bunch length is within the resolution of electro-optic spectral decoding methods. An ytterbium fibre laser system in combination with a 175 μm thick GaP crystal is used to achieve a good match between the electric field phase velocity and the laser pulse group velocity in the electro-optic crystal. This ensures a large modulation of the polarisation of the chirped laser pulse in the EO crystal. The information on the electron bunch length carried by the laser pulse is decoded in a spectrometer and read out with an InGaAs line scan camera.
Uni HH, Hamburg
DESY, Hamburg
The free electron laser in Hamburg (FLASH) underwent major modifications during a 6 months shutdown like the installation of a 3rd harmonic module, a seeding experiment (sFLASH) and a 7th accelerating module. Also instrumentation has been improved. A new compact electro-optic (EO) bunch length monitor has been installed downstream the first bunch compressor. At this position, the bunches are expected to have a length of about 1 ps, well suited for the resolution of an EO bunch length monitor with spectral decoding of the time (EO-SD). The setup uses a commercial ytterbium fiber laser, a compact optics inside the beam pipe designed at PSI (Switzerland) and a spectrometer with fast InGaAs line scan camera. These components, together with RF synchronisation unit and readout electronics, will be installed in the accelerator tunnel. Reliability, robustness and high uptime are key features as the EO monitor is meant to serve as permanent beam diagnostics. Here we report on the commissioning of the components and first experiments with the complete system.
DESY, Hamburg
Uni HH, Hamburg
At FLASH, UV and soft X-Ray pulses with durations in the order of 10 fs are generated. To fully exploit the opportunities provided by these short laser pulses, an optical synchronization system provides the possibility to synchronize external lasers and stabilize the electron bunch arrival time with 10 fs precision. A seeded free-electron-laser (FEL) section, called sFLASH, is installed upstream of the existing SASE undulators. After higher-harmonic-generation, the femtosecond seed laser pulse needs to be temporarily and spatially overlapped with the electron bunch. Furthermore, for time-resolved pump-probe experiments, using an experimental laser and the FEL pulse, either of sFLASH or of the ordinary SASE process, the synchronization between pump and probe laser pulses is crucial. While the best performance for synchronizing these lasers within 10 fs will be achieved by using an optical cross-correlator, in this paper we present a precursor that relies on an RF-based locking mechanism. The setup includes a coarse and a fine phase measurement between the laser pulses of the reference and the synchronized system after their conversion to an RF signal.
