Refurbishment of Radiation-Damaged Undulators

Partial radiation damage has been observed over the previous five years of operation for a few of thePETRA III permanent magnet undulators at DESY. The degradation of the affected devices has been observed by achange in the energy tuning curves and the spectral properties of the undulator harmonics, and also by direct measurementsof the peak field distribution of the magnet structure in the storage ring tunnel [1]. During the recent shutdown ofthe machine, two undulators were removed from the tunnel for repair. One of the devices (U29) has been retuned afterflipping of all magnets of the hybrid structure. In addition to this temporary repair, one further magnet structure (U23)has been completely disassembled and refurbished by application of a rare earth diffusion process and a coating to thepermanent magnets. We will report on the details of the refurbishment of these undulators which were both fully broughtback to specifications and are meanwhile back in operation