Annealing Cryogenically Irradiated High TemperatureSuperconductors with Current Pulses

Abstract

Tokamak fusion power plants rely on electrogmanets engineered from high temperature superconductors (HTS) made of Rare Earth Barium Copper Oxide (REBCO) to confine a thermonuclear grade plasma. The HTS performance must be predictable despite the radiation damage caused by fast neutrons from fusion reactions that damage to the REBCO microstructure, decreasing the magnet’s critical current. This lowers the reactor’s achievable magnetic field–and therefore performance. The damage, however, is not necessarily permanent. By applying a short current pulse above the critical current of the coated conductor, resistive heating briefly raises the REBCO’s temperature well above that of the surrounding cryogenic environment. This process, called annealing, heals defects and recovers some of the performance losses. Magnets are the limiting factor for tokamak lifetimes, therefore pulse annealing could dramatically increase the economical viability of fusion energy by reducing shutdown frequency and duration.

This experiment focuses on sending 400A pulses through an irradiated HTS tape to identify the optimal duration for critical current recovery. Using a cryogenic proton irradiation facility capable of applying current pulses as high as 2000A and as short as 100 ns, we found that a 400A pulse can display up to 400% critical current recovery with respect to the post-irradiation critical current value. The optimal length for this current pulse is 5.5 ms, which results in a maximum calculated temperature of 630K in the REBCO microstructure. Future works will pursue measuring (rather than calculating) the temperature in the REBCO microstructure and parameterizing the maximum critical current recovery at different pulse amplitudes.