A Variable Temperature Walters Spiral Probe for the Critical Current Measurement of Superconducting Strands

Abstract

We have developed a probe for the critical current measurements of low temperature superconducting strands at various field, temperature and strain. A 30 cm-long strand sample is soldered on a Walters spiral made of beryllium copper alloy and compressive or tensile axial strain can be applied up to 0.7%. Temperature control ability was tested using a MgB 2 wire up to 30 K. Three cernox sensors are attached on a spiral adjacent to the strand and the temperature of the spiral is controlled within 50 mK of target temperature during the critical current measurement up to 80 A. Extensive critical current measurements have been carried out for an internal-tin processed Nb 3 Sn strand. It was found that the measured field, temperature and strain dependence of the critical current for the internal-tin Nb 3 Sn strand is in agreement with the recent scaling law based on strong coupling theory of superconductivity.Index Terms-Eliashberg theory, scaling law for flux pinning, strain dependence of the critical current, Walters spiral probe.

Toward a standard for critical current versus axial strain measurements of Nb3Sn

“…The driving system should also allow axial movement of the spring. Details can be found in the literature [10][11][12][13][14]. As an example, at the University of Geneva the Walters spring is driven by a DC-servomotor with a gearbox at room temperature and the angular position of the motor can be measured by an optoencoder.…”

“…The principle of the Walters spring is now well established worldwide, and many institutions have developed probes [10][11][12][13][14]. The scope of this work is to review the use of a Walters spring for the measurement of critical currents versus axial strain and magnetic field.…”