  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.
