Universal degradation of high-temperature superconductors due to impurity scattering: predicting the performance loss in fusion magnets

TL;DR

The paper tackles the problem of predicting high-temperature superconductor performance loss in fusion magnets due to neutron-induced impurity scattering. It derives a universal degradation function $F_D(D)$ by combining impurity-scattering–driven reductions of the depairing current density with enhanced flux creep, guided by Homes' law and the dirty-limit framework. The key contribution is showing that degradation follows a near-universal form across tapes and particle types, and providing an explicit expression for $F_D(D)$ dependent on a small set of parameters ($\alpha_p$, $\partial n/\partial D$, $K_\rho$), enabling predictions of $I_c$ changes using neutron-proxy irradiation data. This framework offers a practical route to forecast coated-conductor performance in fusion magnets without expensive neutron irradiation experiments, improving the reliability of designs for compact fusion devices.

Abstract

Predicting the change of performance of superconductors under neutron radiation is indispensable for designing compact fusion devices. The favorable enhancement of the critical current caused by flux pinning is separated from the degrading effect of increased scattering of the charge carriers to derive a degradation function from the expected change of the superfluid density (reducing to Homes law in the dirty limit) and the observed increase in flux creep. The degradation turned out to be a universal function of disorder, not depending on the particular tape nor the particle radiation: thermal and/or fast neutrons, as well as 1.2 MeV protons. The universal behavior enables the analysis of changes in flux pinning corrected by the adverse enhancement of scattering. A more reliable prediction of the performance change of coated conductors in a fusion reactor based on proxies for neutrons is anticipated.

Figures (4)

• Figure 1: Relative change of the critical current by particle radiation at 15 T, 30 K. Different tapes where exposed to fast neutron (S), thermal and fast neutrons (U) and 1.2 MeV protons (P). The line graphs refer to the modelling of the experimental data (symbols).
• Figure 2: Upper panel: The degradation function is nearly identically for all samples. $\Delta n = -\partial n / \partial D$. Lower panel: Change of pinning efficiency at 15, 30.
• Figure 3: Relative change of superfluid density in YBCO thin films. Experimental data were extracted from Ref. Franz1997. Theoretical prediction was calculated with $K_\rho=-16.5$ and $\alpha_\textrm{p}=3$, or assuming dirty limit
• Figure 4: Relative change of the exponent $n$ of the power law $I\propto E^n$ in coated conductors at 30 K, 15 T. Simple models for the energy barrier against flux creep (line graphs) form an envelop around the experimental data; suggesting the decreasing superfluid density as the main driver for the change in $n$.