A shimming technique for improvement of the spectral performance of APS Undulator A

The performance of insertion devices would achieve almost the ultimate level if a proper set of techniques could be developed to correct the magnetic field imperfections. It has been shown experimentally that the measured radiation characteristics of a magnetically fine-tuned insertion device are very close to those calculated for an ideal device. There are different techniques for correction of magnetic field errors. One used most often is a shimming technique capable of correcting both integrated and local field errors. In this note, some specific results of a shimming technique applied to APS insertion devices will be presented. This technique uses two types of shims: one for trajectory corrections and one for phase corrections. It has been demonstrated that trajectory shims could bring the rms phase errors to the level of 5 degrees, and the next shimming step when only phase shims are applied brings the rms phase errors as low as 1.5 degree