Novel correction procedure for compensating thermal contraction errors in the measurement of the magnetic field of superconducting undulator coils in a liquid helium cryostat.

Superconducting undulators (SCUs) can offer a much higher on-axis undulator field than state-of-the-art cryogenic permanent-magnet undulators with the same period and vacuum gap. The development of shorter-period and high-field SCUs would allow the free-electron laser and synchrotron radiation source community to reduce both the length of undulators and the dimensions of the accelerator. Magnetic measurements are essential for characterizing the magnetic field quality of undulators for operation in a modern light source. Hall probe scanning is so far the most mature technique for local field characterization of undulators. This article focuses on the systematic error caused by thermal contraction that influences Hall probe measurements carried out in a liquid helium cryostat. A novel procedure, based on the redundant measurement of the magnetic field using multiple Hall probes at known relative distance, is introduced for the correction of such systematic error.

Evidence for a percolation-driven transition to coherent surface superconductivity.

Above the upper critical field we have investigated the field dependences of the surface conductance, G'-iG" and the critical current J(c) of an electropolished pure niobium cylinder. The low frequency limits of G', G", and J(c) display power-law singularities, defining a transition to coherent surface superconductivity at H(c)(c3). The critical exponents as well as the dynamical scaling of G'-iG" are consistent with predictions for a two-dimensional percolation transition. Relating H(c)(c3) to the conventional onset field, we find H(c)(c3)/H(c3)=0.81, and, surprisingly, this ratio turns out to be independent of significant variations of H(c3) due to differently structured surfaces.