Helium-free field measurement system for superconducting undulator coils

Superconducting undulators generate, for a given period length and a given gap, higher fields than permanent magnet undulators. 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. In the presently existing magnetic measurement system for superconducting undulator mock-ups, the coils and the Hall probes are in liquid helium. In this paper a different technique is described. Coils and Hall probes are in vacuum and the coils are indirectly cooled by cryocoolers to 4 K. In order to speed up the cooling process the 4 K region is surrounded by a 80 K shield which is cooled by liquid nitrogen. The cryostat allows to measure coils up to a total length of 1.5 m.