a practical HTS rotor with high current. Using flux pumps enables the energizing of rotor coils inductively. Figure 8 shows the design and prototype of a recent flux pump developed by the University of Wellington in New Zealand. It demonstrated flux-pumping techniques capable of injecting kiloamps of current, with minimal heat load, while operating across large flux gaps (cryostat walls) of up to 14 mm. If this can be adapted for propulsion motors, it can enable a step change in the thermal budget. x Radiation and convection: Radiation losses are usually managed with a multilayer insulation covering the area with an SC coil or a silver coating on the surface. Convection losses are managed with vacuum insulation. Some of the existing HTS motors have already minimized these loss components. Open-Loop and Evaporative Cooling Open-loop cooling refers to the systems where a cryogenic liquid is boiled off to absorb the heat load generated in SC components to maintain the critical temperature. Figure 9 depicts the options of cryogenic fluids that can be used with existing superconductors, where the critical field in the SC coil is also a function of the temperature. As Figure 7. An HTS-ISM from Kyoto University, Japan. (Source: Sekiguchi et al. 2021.) Iron Stator Iron Rotor SC Stator Tape (a) SC Coil (a) Negative Copper Busbar HTS Stators Positive Copper Busbar Iron Yoke Rotor Magnet (b) Negative HTS Bus Ring Figure 6. (a) A HydroGenie HTS hydro generator (Source: Haran et al. 2017) and (b) an EcoSwing rotor covered with insulation. (Source: Bergen et al. 2019.) (b) Figure 8. A concept of (a) flux pump and (b) practical implementation. IEEE Electrification Magazine / JUNE 2022 49 Positive HTS Bus Ring