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New microwave beam position monitors for the TESLA test facility—FEL

Beam-based alignment is essential for the operation of the SASE-FEL at the TESLA Test Facility Linac. In order to ensure the overlap of the photon beam and the electron beam, the position of the electron beam has to be measured along the undulator beamline with a high resolution. Due to the severe space limitations, a new microwave concept is being considered. It is based on special ridged waveguides coupling by small slots to the magnetic field of the electron beam. The four waveguides and slots of each monitor were split into two symmetric pairs separated in beam direction. All waveguides are about 35 degrees apart in azimuth from the horizontal axis and will be fabricated using electro-discharge machining (EDM). Waveguide-to-coax adaptors were designed to couple the signal of each waveguide into a coaxial cable. The goal is to measure the averaged position of a bunch train in a narrowband receiver with a center frequency of 12 GHz. A prototype of this monitor was built and tested on a testbench, as well as at the CLIC Test Facility at CERN. The paper summarizes the concept, the design, and further improvements of the waveguide monitor.