Cryogen—Free Setup for Local and Integral Magnetic Field Measurements of Superconducting Undulator Coils

Since in an undulator the photons add up coherently over the whole undulator length even small magnetic field errors can disturb the superposition of photons and reduce the intensity of the generated photon beam. Therefore, as in any other undulator, the magnetic field has to be measured with high accuracy. Precise measurements of the magnetic properties of conventional, i.e., permanent magnet based insertion devices (IDs) has undergone huge improvements over the past years and initiated a new era in synchrotron light sources worldwide. A similar breakthrough is now necessary in the field of superconducting insertion devices. Therefore, a part of our R&D program for superconducting insertion devices is to perform quality assessment of their magnetic field properties.

I. Introduction

Undulators are used in synchrotrons as sources for high brilliance X-ray beams [1]. For an undulator the important figures of merit are the phase error and the field integrals, where the field integrals along the beam axis should ideally be zero so that there is no effect on the electron beam orbit. The phase error is related to the quality of the emitted radiation. In order to obtain a photon beam with high brightness all photons emitted along the trajectory of one single electron must have a constant phase relation so that they interfere constructively. The phase and the wavelength are determined by the period length, field strength and the beam energy. If one of these parameters deviates in one or several periods the photons are not in phase.