Quench Risk Increase With Radiation Damage

Superconducting magnets are often proposed to confine plasma in fusion reactors. Superconducting material enables the magnets to carry current densities that would melt materials with non-zero resistance. Quench occurs when superconductivity is lost and the current starts to generate heat. Unless prevented with a fast enough control system, the heat generated during a quench can cause catastrophic damage to the coils. This work describes a less-studied heating mechanism that increases the likelihood and aggressiveness of fusion magnet quenches. Defects accumulate in the magnet structural material under irradiation by the fusion process. The defects store energy in the material and change thermal and normal state electrical properties. Wigner energy is released when defects anneal. After a 0.9 mDPA neutron irradiation, a 10 K disturbance from 20 K is predicted to release enough energy to result in a final temperature of 40 K. Irradiation damage also reduces the quench time constant by increasing normal state resistivity and thus Ohmic heating. The continuous operation of a fusion reactor produces an increasingly unstable thermodynamic system in superconducting magnets by changing electrical and thermal properties with irradiation damage. The temperature margin between operation and quench runaway reduces with irradiation. The next steps are to include these observations in quench models and validate the predictions experimentally. Implications of this work is felt by all fusion powerplant projects planning to leverage superconducting magnets. Designs will recognize this risk with more stringent specifications on quench control systems and maximum duration of coil operation at cryogenic temperature between periodic releases of Wigner energy to avoid catastrophic quench failures.