All superconductors in high field magnets operating above 12 T are brittle and subjected to large strains because of the differential thermal contraction between component parts on cool-down and the large Lorentz forces produced in operation. The continuous scientific requirement for higher magnetic fields in superconducting energy-efficient magnets, in applications such as fusion, means we must understand and control the high sensitivity of critical current density Jc to strain ε. This thesis presents detailed Jc(B,T,ε,θ) transport measurements as a function of field B, temperature T, strain ε and angle θ with respect to the applied magnetic field, on high field superconductors that include the very widely observed inverted parabolic dependence for Jc(ε).
It is usually assumed that a coincidence occurs between the Fermi energy and a peak in the density of states in the unstrained state which leads to a peak in the superconducting properties in the unstrained state. The long-standing interpretation of Jc(ε) data attributes the inverted parabolic strain behaviour to the averaged response of the underlying material. Features of the data in this work are identified in both HTS REBCO tape and LTS A15 wires which show that both of these assumptions are incorrect.
A new analysis is presented which shows the inverted parabolic nature of Jc(ε) is the result of competition between domains with opposing strain dependencies and successfully accounts for the features of the data incompatible with standard assumptions. It is concluded that the origin of the competing domains in REBCO tape lies in twinned domains and in A15 wires is caused by percolative current flow across grain boundaries orientated in different directions which respond to an applied strain in different manners due to the Poisson effect. This work provides fresh insight into how improvements might be made to high field superconductors under strain
