The current carrying capability of superconductors is limited by the strength of the applied magnetic field. An intercomparison of critical current determinations in a Nb3Sn superconductor was undertaken between laboratories within the Community. Subsequently a second intercomparison involving laboratories in Europe, Japan and the USA was arranged as part of the Versailles Project on Advanced Materials and Standards (VAMAS). As the wires have to be heat treated to form the superconducting Nb3Sn phase, the intercomparisons were structured in order to identify differences arising from both the heat treatment and the critical current measurements.
Completed in 1986, this intercomparison showed an agreement on critical current measurements to within 6% irrespective of whether the specimens were heat treated individually by the participants or centrally in a single batch. The major source of differences was identified as induced strain effects, the magnitude of which depended upon the details of the mandrel upon which the specimen was wound and the method of fixing it in place.
22 laboratories from Europe, USA and Japan made measurements on 3 wires of different construction and hence current carrying capacity. The results reflected the same trends as found in the European intercomparison. The between laboratory standard deviation increased from 6% to 12% as the current carrying capacity of the wire decreased from approximately 550 to 400 A. The induced strain in the different experimental set-ups was again approximately equivalent to the observed scatter in the critical current data. A second round using a single wire on identical mandrels and more precisely defined test procedures reduced the scatter in the data from 8.3 to 2.2% at a field strength of 12 Tesla.
Funding SchemeCSC - Cost-sharing contracts
OX11 0QX Didcot
OX1 3PU Oxford