A new field correction scheme for superconducting undulators by modification the iron pole geometry

Permanent magnet material undulators are used in many research facilities worldwide to produce high brightness synchrotron radiation for basic and applied research. Their effectiveness is limited in low energy storage rings because of a lack of sufficient magnetic field intensity. Superconducting undulators can produce higher fields and therefore higher photon energies, especially at lower electron beam energies. Undulator radiation is emitted in a line spectrum where the fundamental wavelength is determined by the undulator period and strength, beam energy and harmonic number. For a given beam energy, use of these higher harmonics is desirable, because they allow the provision of higher photon energies as high as soft or hard x-rays from 1 to 10 keV. The photon flux in such harmonics is however strongly dependent on the integrity of the periodic properties of the magnetic field. Small field and phase errors will reduce the photon intensity dramatically. Correction methods as employed for permanent magnet material undulators are not applicable in superconducting undulators. In this paper, we discuss a new approach for field corrections based on a variation of the magnetic field saturation properties of individual poles. We demonstrate its efficiency in ensuring photon fluxes which are close to theoretical expectations.