Magnet Design of a Prototype Structure for the X-ray FELs at TESLA

XFEL undulators for the TESLA project require high quality in order to drive the SASE process into saturation. On the other hand, as large quantities are needed for the five SASE undulators, it is rather important not to oversize the undulator design. The magnet structure is the most delicate and most expensive part of an insertion device. This report proposes a magnet design for a SASE undulator with tunable wavelength in the 1 Å regime. The layout of the magnet structure is optimized for a 60 mm undulator period which requires a magnet volume of 400 cm per period to gain a peak field of 1.33 T. An end pole configuration of the gap variable device is suggested so that both field integrals are trimmed close to zero for all gaps.