CERN Accelerating science

Article
Title	Undulators for the BESSY Soft-X-Ray FEL
Author(s)	Bahrdt, Johannes ; Frentrup, Winfried ; Gaupp, Andreas ; Kuske, Bettina ; Meseck, Atoosa ; Scheer, Michael
Affiliation	(BESSY GmbH, Berlin)
Publication	2004
In:	26th International Free Electron Laser Conference and 11th FEL User Workshop 2004, Trieste, Italy, 29 Aug - 3 Sep 2004, pp.e-proc. THPOS03
Subject category	Accelerators and Storage Rings
Abstract	BESSY plans a linac based high gain harmonic generation FEL user facility with three FEL lines [1]. The modulators and most of the radiators are planar pure permanent magnet undulators. The last radiator and the final amplifier produce radiation of linear polarization with arbitrary orientation as well as elliptically or helically polarized light. They will have a modified APPLE II design which provides higher fields compared to a conventional APPLE II [2]. Detailed calculations for this design will be presented. FEL 3D-calculations provide information about the radiation field distribution at the end of the undulator. A beamline designer needs the information about the effective source size at the waist and the location of the waist. The electric fields calculated by GENESIS have been propagated and the source characteristics have been derived for various FEL parameters. The FEL process requires tight gap tolerances within and between modules. We present a new gap measurement system with a gap positioning accuracy of 1.5 microns.