Radiation exposure and magnetic performance of the undulator system for the VUV FEL at the TESLA Test Facility Phase-1 after 3 years of operation

Radiation damage to undulator systems made of permanent magnet materials like NdFeB or SmCo is a critical issue in SASE projects. It is of even more interest in high-duty cycle machines using superconducting accelerators such as the TESLA Test Facility (TTF), which is a prototype for the future X-FEL at TESLA. This paper reports on experience on the undulator system of the VUV-FEL of TTF-1, which ended on May 6, 2002 after nearly 3 years of operation. The radiation exposure of the undulator system was recorded continuously and meticulously over the whole installation period in the TTF linac. Results of these dosage measurements as well as magnetic measurements after de-installation are reported. A comparison with magnetic field data taken prior to installation is given.