Microwave Characterization of the Waveguide Bpm

For electron beam orbit observation at the TESLA Test Facility FEL one undulator module will be equipped with ten waveguide-type beam position monitors having a design frequency of 12 GHz. Since the operation of this waveguide BPM is related to microwave concepts, a precise characterization of its microwave behaviour is necessary to describe and optimize the system performance. The purpose of this paper is to summarize the theory, different simulations and measurements of the device. Calculations and MAFIA simulations are presented for the coupling into the ridged waveguide as well as for the design of the waveguide-to-coax transition. Beside that the resonant frequencies of trapped modes inside a waveguide are estimated. Finally, back reactions on the electron beam in terms of wakefields are estimated by numerical simulations and by a simple analytical model.