Long sample high sensitivity critical current measurements under strain

Abstract

The extreme strain sensitivity of the critical current in Nb 3Sn superconductors at high fields is well known and must be taken into account in the design of high field magnets where strains up to 0.5% may be encountered. Current density/field measurements are relatively easy to perform but current density/strain measurements require fairly elaborate equipment and are time consuming in their execution. As a result commercial data is readily available for predicting current density at a particular field for a particular conductor but its measurement at a particular strain is less available. In this Paper an apparatus is described in which the sample is attached on the outside of the turns of a very thick spiral spring such that when one end of the spring is rotated with respect to the other the outer surface and, therefore, the sample is either stretched or even compressed. The sample geometry of six turns of 40 mm diameter over a length of 70 mm, is very similar to that typically used in current density/field measurements and using it strain tests can similarly be easily performed. Long samples can be tested in small bore ( ≈ 55 mm) solenoids where > 1–2% field variation can be achieved over the measured length and current transfer effects can be avoided so that unambiguous measurements can be obtained at a sensitivity of > 1.0 μV m −1. The strain sensitivity of the conductors used in this work increased between 3.6 and 5.4% per decade increase in electric field sensitivity at 11 T.