Waveguide Monitors - a New Type of Beam PositionMonitors for the TTF FEL

The operation of a Free Electron Laser (FEL) at the TESLA Test Facilty (TTF) requires theelectron trajectory to be aligned with precision of few micrometer in overlap with the photonbeam. To achieve this goal precise position monitoring of the electron beam in the narrowundulator gap is mandatory. A beam position monitor based on a new microwave concept wasdesigned, built and tested in order to avoid possible sparking problems of pick-up monitors.It consists of four ridged waveguides which couple by small slots to the magnetic field of theelectron beam. The paper reports the concept, the design and tests of prototypes in a test benchin the laboratory and with beam. TTF FEL measurement results are also presented and possibleimprovements are discussed