Beam position monitors inside the FEL-undulator at the TESLA Test Facility Linac

Beam-based alignment is essential for the operation of a SASE-FEL at the TESLA Test Facility Linac. It requires the transverse beam position to be measured at several points inside and between the undulator modules with a resolution of better than 5 μm. Between the undulator modules cylindrical cavities will be used, excited in the TM ₁₁₀-mode by an off-center beam. The amplitude of this mode is detected in a homodyne receiver by mixing the cavity output and a 12 GHz reference signal. For monitors inside the undulator modules the realization of two different concepts is under way. The first one is a button-type monitor using very small feedthroughs. The second one is a structure consisting of four ridged waveguides oppositely arranged around the chamber, where the averaged position of a single bunch train will be measured in a narrowband X-band receiver. The fabrication and test of prototypes is under way. This paper summarizes the designs and some preliminary results