Critical current measurement with spatial periodic bending imposed by electromagnetic force on a standard test barrel with slots

We have developed and validated a straightforward and fast method to investigate the response of technological superconducting strain sensitive wires (e.g., Nb3Sn) to a spatial periodic bending strain. In the present concept of cabled superconductors for application in nuclear fusion reactors the wires are twisted and cabled in several stages. When subjected to transverse electromagnetic forces after charging the magnet, the individual strands are subjected to spatial periodic bending with wavelengths in the order of 5-10 mm. Several apparatuses are presently under development to study the effect of bending on the transport properties, i.e., the voltage-current transition in terms of critical current (I-c) and n value. We propose a supplementary simple method to investigate the influence of bending strain by using a spatial periodic wire support on a broadly used standard I-c measurement barrel in combination with a Lorentz force. The bending force (BI) is varied by changing the applied field B. The peak bending strain in the Nb3Sn filaments is determined by the amplitude of the bending deflection, which is deduced from the mechanical axial tensile stress-strain properties of the wire. Three different spatial periodic wavelengths are applied and the results are in good agreement with the predictions. In addition we found a good agreement with results obtained by a more advanced experiment, named TARSIS, which is described briefly. The "barrel-with-slots" method can be applied easily and straightforward with minor effort and cost in laboratories having a standard I-c measurement facility for superconducting wire. (c) 2006 American Institute of Physics.