Round robin tests on large grain melt processed Sm?Ba?Cu?O bulk superconductors

On-going, round robin tests of the trapped field at the top and bottom surfaces of two carbon fibre reinforced polymer (CFRP)-clad, epoxy resin-reinforced large grain samples of Sm–Ba–Cu–O fabricated at ISTEC, Japan, by top-seeded melt growth (TSMG) have been performed by four European laboratories over a period of 16 months. Flux profiles were measured by scanning Hall probe by the participating groups and were found to agree to within approximately 7% over the measurement period when the effects of incomplete flux penetration in some of the tests were taken into account. Overall the measurements suggest that the trapped flux profile is most sensitive to changes in magnetizing field, experimental geometry and the sample–Hall probe separation. As a result, variations in these parameters should be minimized during flux mapping. In general a magnetizing field of times the maximum trapped field for field cooled (FC) samples is required to ensure complete magnetization of a homogeneous cylindrical sample with an aspect ratio of , corresponding to the geometry of the samples measured here. Finally, 20 min relaxation time following magnetization was observed to be sufficient to yield consistent measurement of the peak trapped field within the error of the measurement, which was typically less than 4%. Other than the recommended magnetizing field of 3 T, which should be determined by aspect ratio, temperature and maximum trapped field rather than be predefined, the experimental conditions were confirmed by this interim report generally to lie within the guidelines described in a draft International Electrotechnical Commission (IEC) Technical Committee 90 (TC90) standardization document on flux mapping. On the basis of the present preliminary results the proposed 5% spread in measured trapped field and a 20 min relaxation period do not appear to be unreasonable criteria for trapped field measurements performed at different laboratories.