Stable, lowloss joints for HTS fusion magnets

The feasibility of fusion as a practical energy source needs to be improved significantly by removing some of the restrictions that low temperature superconductors put on the fusion magnet systems. One method to simplify the magnet system is by using high temperature superconductors (HTS) that allow for a higher magnet performance and much larger temperature margins. Successful application of HTS in fusion magnets requires reliable, low resistance cable joints. There is currently no practical method of making reliable joints in high current HTS cables. This proposal seeks to develop reliable, low resistance joints between HTS magnet cables that would allow for practical fusion magnets to become a reality. Joints between magnet cables capable of carrying currents exceeding 60 kA will be developed into an industrial product with the goal of having a joint resistance of less than 10 nΩ at 4.220 K. During Phase I of the program, we demonstrated feasibility of our approach to make joints between HTS magnet cables and demonstrated a joint with resistance of 110 nΩ at 77 K in a subscale cable carrying 8 kA. We demonstrated that current was injected homogenously into the cable containing 111 superconducting tapes. During Phase II of the program, the joints will be further optimized to reduce the resistance to below 10 nΩ, while joints between cables containing as many as 400 superconducting tapes will be developed. Several full scale cables with joints will be constructed and their performance will be measured at 4.2 K while carrying currents in excess of 50 kA. Reliable, low resistance joints between high current HTS cables will enable compact fusion magnets, high field magnets for scientific experiments and the next generation of accelerator magnets for particle physics and proton cancer treatment facilities. They will also enable high density power transmission cables for Department of Defense applications. Key words Compact fusion magnets, high temperature superconducting cable joints, demountable fusion magnets. The development of practical and reliable joints between high temperature superconducting cables is necessary for the US to maintain their leadership position in fusion research, materials science, and high energy physics research.