Vibrating wire apparatus for periodic magnetic structure measurement

Abstract

Devices with periodic magnetic structures such as wigglers and undulators are often key elements in synchrotron radiation sources. In applications where the coherence of the emitted radiation is important, magnetic field errors distorting the periodicity of the field can significantly reduce the performance of the devices. Thus, the measurement, localization, and correction of the field errors can be a critical issue.This article presents a new method for magnetic field measurements in periodic magnetic structures. The method uses a vibrating taut wire passing through the magnetic structure, and it involves measurements of the amplitudes and phases of the standing waves excited on the wire by the Lorentz force between an AC current in the wire and the surrounding magnetic field. For certain arrangements of the wire, vibrations in the wire will be excited by only non-periodic magnetic field component, i.e., by the error field. By measuring the phase and amplitude of these waves, one can reconstruct the error field distribution and then correct it.The method was tested on a permanent magnet wiggler with 19.8cm period and a peak field of ∼7000G. It demonstrated ∼0.6G RMS sensitivity, δBrms/Brms∼1.2×10−4 and spatial resolution sufficient to identify poles generating the field error. Good agreement was found between field error measurements obtained with the vibrating wire method and with traditional Hall probe field mapping.