Zero-displacement end termination of undulators and wigglers

One of the more challenging aspects of hybrid permanent magnet undulator design is end-field termination. Klaus Halbach provided indication of correct excitation patterns for tapered wigglers, but implementation on hybrid undulators has proven more difficult. Initial results on APS undulators achieved complete passive gap dependence of both total residual angle and trajectory through the device, but with an entrance displacement. We have further refined the end field termination system by employing both partial volume and partial strength magnets. This achieves an entrance field configuration that maximizes the number of full strength pales without entrance or exit displacement, independent of the magnetic gap. This is all achieved with passive compensation techniques. End pole tuning is then used to correct small (0.2%) deviations in the measured versus calculated end field steering. We present the design approach and results of five devices that have used this configuration.