A Prototype Phase Shifter for the Undulator Systems at the TESLA X-ray FEL

Five SASE FEL’s and five spontaneous undulator systems are proposed for the FEL lab at TESLA. The SASE systems are the longest. Their total length can be in excess of 300m. In order to keep the beam size small over the whole system a strong focusing FODO lattice is needed. The parameters for the X-ray FEL’s allow to separate between the undulators and the strong focusing. This has the advantage that the undulator system may be segmented into 5m long undulator segments and 1.1m long intersections, which include the FODO quadrupoles. For most of the undulator systems the wavelength will be tunable. This is done by tuning the gap at a fixed energy. As a consequence the proper phasing of radiation emitted by individual segments becomes gap dependent too. In order to get the proper phase match, the electron beam has to be slightly delayed so that radiation emitted by the poles of one segment has a phase delay of multiples of 2π with respect to radiation emitted by the poles of any other segment. For TESLA a phase shifter for the SASE5 undulator system has the worst case requirements since this device has the highest period length, K parameter, operates at the longest wavelength and its wavelength is tunable by a factor of about 6.3. In this report a prototype design is proposed, which can be used at the SASE5 system and allows tuning over the full wavelength range. Properties are derived using analytical formulae. A design for a small electromagnet which can be used for the phase shifter and does not need water cooling is presented as well.