Magnetic measurements and tuning of the LCLS prototype undulator

A full-length 30-mm-period prototype undulator segment has been constructed for the Linac Coherent Light Source (LCLS) project. It is a 3.42-m-long planar permanent magnet hybrid device with a fixed gap near 6.35 mm. The magnetic structure was assembled from a set of 500 magnets that were purchased from Shin-Etsu. A subset of the magnets was measured in a Helmholtz coil system to check the accuracy of the vendorʹs measurements of the magnetic moments. A specially designed half-period fixture was used for Hall probe measurements of the magnetic field of all the magnet blocks. The magnets were sorted based on the vendorʹs data of the total magnetic moment and our measurements of the field integrals. The trajectory calculated from Hall probe measurements of the assembled device was within the specified tolerance (deviation <2 μm in both planes) but showed a trajectory offset and kick at the entrance. A small number of shims was applied to minimize the trajectory offset and kick and to increase the amplitude of the spontaneous radiation. The measured maximum gap taper at the ends of the device and the temperature dependence of the effective magnetic field are also reported. The tuned device met all specified tolerances.