THE VACUUM CHAMBERS FOR THE VUV SASE FEL AT THE TESLA TEST FACILITY (TTF FEL) AT DESY* U. Hahn, J. Pflüger, M. Rüter Hamburger Synchrotronstrahlungslabor HASYLAB at Deutsches Elektronen-Synchrotron DESY Notkestrasse 85, 22603 Hamburg, Germany P. K. Den Hartog, M. Erdmann, E. M. Trakhtenberg † , G. Wiemerslage, and S. Xu Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439 Abstract A vacuum chamber for the VUV SASE FEL undulators at the TESLA Test Facility (TTF) was designed, a prototype was built and tested, and seven complete chambers were manufactured. The chambers use the aluminum extrusion technology developed for the insertion device vacuum chambers of the Advanced Photon Source. Each chamber is 4.5 m long with a beam aperture of 9.5 mm and an ex- ternal thickness of 11.5 mm. Three of the chambers in- clude ports for integral beam position monitors (10 hori- zontal and vertical pairs) inserted into the chambers, and all of the chambers include grooves for mounting correc- tion coils. Bimetallic flanges (stainless steel to alumi- num) are welded to the ends of the chamber for connec- tion to the beamline. Special processing was performed to meet the stringent vacuum and particle-free require- ments of the TTF. 1 INTRODUCTION At DESY, a VUV free-electron laser (FEL) based on the principle of self-amplified spontaneous emission (SASE) is under construction [1] to make use of the unique elec- tron beam properties of the TESLA Test Facility (TTF). The FEL will be built in two phases [2]. Phase one, with a FEL operating down to 42 nm, is under construction. The major component for the generation of the FEL photon beam is the undulator, which will consist of three 4.5-m-long modules separated by 0.3-m-long beam diag- nostic sections. The undulators are permanent magnet structures with a fixed gap of 12 mm. The electron beam must be kept small over the entire undulator length by an added sequence of focusing and defocusing quadrupoles (FODO lattice) [3]. Additionally, electron beam position monitoring and steering in the undulator gap is needed to achieve a sufficient (< 12 μm) overlap [4] between the particle beam and the photon beam. Three 4.5-m-long vacuum chambers with an open aperture of 9.5 mm guide the electron beam through the undulator sections. The simple vacuum pipe becomes rather complicated by the addition of 40 electrodes for the beam position monitors required for each chamber and the related 36 correction coils. A special alignment system for the flat, flexible chamber is also needed. Because the chamber tube must reach a specific outgassing rate <1⋅10 -11 mbar⋅l/sec⋅cm 2 , the cleaning and assembly of the chamber was made inside a clean room better than class 100 [5]. 2 VACUUM CHAMBER DESIGN There are several design criteria for the undulator vacuum chamber: • The undulator gap size is 12 mm. • The chamber has to permit beam position measurement and steering in the gap. • The chamber has to be vertically and hori- zontally aligned within 0.1 mm. • Low electrical resistance and small micro- roughness of the inner beam pipe are needed to minimize resistive wall wake field effects on the beam. Therefore, the vacuum chamber for the DESY FEL un- ___________________________ *Supported in part by the U.S. Dept. of Energy, BES-Materials Sci- ences, under Contract No. W-31-109-Eng-38. † Email: emil@aps.anl.gov Figure 1: Cross section of the aluminum extrusion for the FEL vac- uum chamber. 0-7803-5573-3/99/$10.00@1999 IEEE. 1369 Proceedings of the 1999 Particle Accelerator Conference, New York, 1999