Degradation of superconductors due to impurity scattering: predicting the performance loss in fusion magnets

Abstract

Predicting the performance change of superconductors under radiation is indispensable for designing compact fusion devices. Various commercial coated conductors were exposed to thermal and/or fast neutrons, as well as 1.2 MeV protons to explore their radiation tolerance. The critical current changes non-monotonically with particle fluence due to a competition of enhanced flux pinning by the defects and a reduction of superfluid density predicted for increased scattering of the charge carriers. A method was developed to separate the favorable enhancement of pinning from the adverse effect of increased scattering. The degradation of the critical current turned out to be universal if quantified by the decrease in transition temperature. It does not depend on a particular tape nor the particle radiation and is modeled quantitatively using the expected change of the superfluid density and the observed increase in flux creep. A degradation function is introduced that enables an analysis of changes in flux pinning corrected by the adverse effects that always occurs simultaneously with the introduction of defects. Homes’ law, a relation between superfluid density and transition temperature, is obtained as a by-product in the dirty limit and generalized to arbitrarily clean samples. In the supplementary data, a table listing all parameters is provided.