Strain dependence of critical currents in commercial high temperature superconductors

We describe two versions of a simple device that enable the J/sub c/ (B,T,/spl epsi/) characterisation of a <40 mm diameter single loop of HTS tape. The loop is mounted on a cylinder that is slit longitudinally at regular intervals around its circumference. Strain is applied by forcing a conical plug into the tapered bore of the cylinder which expands evenly as a consequence. The strain is measured by a strain gauge glued to the surface of the tape. The advantage of this configuration is that it enables insertion in the small bores of the highest field superconducting magnets and thus permits access to fields levels not easily available in more traditional linear pulling devices. Also the loop configuration is more representative of the coil geometry encountered in magnet applications. Representative data are presented and discussed critically.