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

Abstract

On-going, round robin tests of the trapped field at the top and bottom surfaces of twocarbon fibre reinforced polymer (CFRP)-clad, epoxy resin-reinforced large grain samples ofSm–Ba–Cu–O fabricated at ISTEC, Japan, by top-seeded melt growth (TSMG) havebeen performed by four European laboratories over a period of 16 months. Fluxprofiles were measured by scanning Hall probe by the participating groups andwere found to agree to within approximately 7% over the measurement periodwhen the effects of incomplete flux penetration in some of the tests were takeninto account. Overall the measurements suggest that the trapped flux profile ismost sensitive to changes in magnetizing field, experimental geometry and thesample–Hall probe separation. As a result, variations in these parameters shouldbe minimized during flux mapping. In general a magnetizing field of times the maximum trapped field for field cooled (FC) samples is required to ensurecomplete magnetization of a homogeneous cylindrical sample with an aspect ratio of, corresponding to the geometry of the samples measured here. Finally, 20 min relaxationtime following magnetization was observed to be sufficient to yield consistent measurementof the peak trapped field within the error of the measurement, which was typically lessthan 4%. Other than the recommended magnetizing field of 3 T, which should bedetermined by aspect ratio, temperature and maximum trapped field rather than bepredefined, the experimental conditions were confirmed by this interim report generallyto lie within the guidelines described in a draft International ElectrotechnicalCommission (IEC) Technical Committee 90 (TC90) standardization document on fluxmapping. On the basis of the present preliminary results the proposed 5% spreadin measured trapped field and a 20 min relaxation period do not appear to beunreasonable criteria for trapped field measurements performed at different laboratories.