RF FRONT END FOR HIGH BANDWIDTH BUNCH ARRIVAL TIME MONITORS IN FREE-ELECTRON LASERS AT DESY

High gain free-electron lasers (FELs) can generate ultra short X-ray pulses in the femtosecond range. For a stable operation of the FEL, the precise knowledge of the bunch arrival time is crucial. The bunch arrival time monitors (BAMs) at FLASH achieve a resolution less than 10 fs for bunch charges higher than 500 pC which is sufficient for the beam based stabilization system of the Free-Electron Laser in Hamburg FLASH [1]. Increased demands for low bunch charge operation mode down to 20 pC at FLASH II and the European X-ray free-electron laser XFEL require an upgrade of the existing beam diagnostic equipment. A new high bandwidth BAM with new developed cone shaped pickups [2] promises sub-10 fs resolution for both, the high and low bunch charge operation mode. This paper adresses the RF signal path of the high bandwidth BAMs for FLASH II and XFEL. It comprises radiation resistant coaxial cables, combiners and limiters up to a frequency of 40 GHz from the pickup electrodes to the Mach Zehnder Modulator (EOM). Detailed investigations of the signal path using measurements and simulations with AWR Microwave Office allows for a good prediction of the signal quality and shape at the EOM.