Development and realization of a 40 GHz bunch arrival time monitor for FLASH and European XFEL

This thesis documents the development of an arrival time monitor system for the Free-Electron-Laser in Hamburg (FLASH) at Deutschen Elektronen-Synchrotron (DESY) with improved resolution. The necessity for the development of this system arose from the requirements of a time resolution of 10 fs for experiments with electron bunch charges of 20 pC instead of 500 to 3000 pC. This demands a bandwidth of about 40 GHz for the entire system. Initially a study of new pickup electrode candidates was carried out by means of computer simulations. Considering the electrical and mechanical properties as well as manufacturability a cone-shaped pickup was selected. Limits for the production tolerances were determined through a detailed series of simulations. The measurement results of the electrical behavior of a prototype showed good agreement with simulation results. The electrical signal of the pickup is conducted onto electro-optical modulators (EOM). The increased bandwidth requirements demand for a replacement of the existing EOMs. There are only a few EOMs, which nominally offer the required bandwidth. Those were investigated in the laboratory for their properties such as the optical modulation depth and the insertion loss. Based on these results the most suitable type was selected. Finally, cabling concepts are presented. The extended range of bunch charges demands to two operational modes, one for low and one for high charges. Each mode has a fine and a coarse channel, where the coarse channel determines the operating window for the fine channel. The choice of the RF cables is important to guarantee the high accuracy of the entire system. The RF cables requires a high bandwidth and also a low attenuation. A prototype of the new arrival time monitor system was implemented and measured. Due to availability issues, initially an EOM with lower bandwidth was installed in order to demonstrate functionality of the system. According to computer simulations the new arrival time monitor system will achieve the required accuracy of 10 fs above bunch charge of about 55 pC once the preliminary EOM is replaced with the final one. The unexpectedly high attenuation of the RF signal from the pickup to the EOM above 20 GHz, does not allow to achieve this accuracy at 20 pC. At this bunch charge an accuracy of about 27 fs should be expected.