COMMISSIONING OF AN ELECTRO-OPTIC ELECTRON BUNCH LENGTH MONITOR AT FLASH

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 electrooptic (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.