Quench Risk Increase With Irradiation Damage

Abstract

Superconducting material enables fusion reactor magnet concepts to operate with current densities that would melt materials with non-zero resistance. The application of superconducting material is considered essential for net-positive power machines. Catastrophic damage can occur when superconductivity is lost and the current generates heat. This scenario is called a quench. Stabilizer material carries the magnet current (typically copper) during a quench and is the focus of this work. Irradiation-induced defects store energy in the Cu crystalline lattice. The release of stored energy in the magnet materials, combined with the associated magnet material property changes, can cause extreme off-normal events in superconducting magnets that worsen with fluence at an increasing rate. Stored energy can be released causing local heating and increasing the risk of a quench. For example, following irradiation at 4.6K and an estimated fluence of 0.45*10^18 n/cm^2, an energy release of 0.023 J/g was measured from Cu when increased in temperature from 10K to 18K, which would have been enough energy to create the same temperature increase spontaneously. Extrapolations of experimental data are used to estimate when spontaneous heating can occur due to the release of energy stored in irradiation-induced defects. Critical fluence values are estimated between 1.74*10^18 n/cm^2 and 2.85*10^19 n/cm^2 for neutron irradiation of Cu at a temperature of 20K. High-temperature ramp rate in-situ cryogenic calorimetry experiments of magnet materials following irradiation would provide more clarity to designers of fusion magnet systems. Due to the increased quench risk with superconducting magnet dose, magnetic confinement reactor designers should consider the frequency of maintenance temperature cycles to maintain an appropriate level of risk during operation.