LCLS Undulator Research Articles

The vibrating wire (VW) technique employs a stretched wire as a magnetic field sensor. Because of the wire’s small diameter (∼0.1 mm or smaller) and because the wire can be supported from outside the magnet, this technique is very appealing for field measurements in small gap/bore undulators with small good field regions and with limited access to the tested field. In addition, in the case of elliptical undulators, in which Hall probe (HP) measurements can be affected by the planar Hall Effect; VW technique can be used as an independent method to verify and supplement HP measurements. In this article, we studied the potential of the VW technique for measurement of magnetic field errors and for prediction of beam trajectories in undulator magnets, using a 3.8 m long LCLS undulator as a test bench. Introducing calibrated magnetic field distortion at various locations, we measured the sensitivity and spatial resolution of the method. The method demonstrated 0.9 mm precision in localizing of the field distortion at a distance up to a few meters as well as 0.37 G cm sensitivity to the variation of the local field and 2 G cm sensitivity to the total field integral. To compare Hall probe and Vibrating wire measurements side-by-side, we measured field errors in an LCLS undulator previously characterized by Hall probe measurements. The field errors found with the Vibrating Wire technique appeared to be in good agreement with errors measured with the Hall probe. Beam trajectory distortions calculated from both data sets are also in good agreement.