The operation of a Free Electron Laser in the Self Amplified Spontaneous Emmission mode

The operation of a Free Electron Laser (FEL) at the TESLA Test Facilty (TTF) requires the electron trajectory to be aligned with precision of few micrometer in overlap with the photon beam. To achieve this goal precise position monitoring of the electron beam in the narrow undulator gap is mandatory. A beam position monitor based on a new microwave concept was designed, 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 the electron beam. The paper reports the concept, the design and tests of prototypes in a test bench in the laboratory and with beam. TTF FEL measurement results are also presented and possible improvements are discussed. 1