High-strength Composite HTS CORC® Cables for Plasma Fusion Confinement Systems
Advanced magnet systems for fusion would greatly benefit from the use of high-temperature superconductors (HTS). These materials allow fusion magnets to operate at higher magnetic fields, allowing for more compact machines, and operation at elevated temperatures, allowing for demountable magnets that provide easy access for maintenance. The brittle nature of HTS requires elaborate means to protect them against the high stresses associated with high-field magnet operation, which so far have prevented reliable high-field HTS magnets from becoming a reality. The goal of the Phase I SBIR program is to determine the feasibility of developing high-strength conductor-on-round-core (CORC®) cables and wires, made by winding several HTS REBCO tapes helically around a former. This would be achieved by designing and incorporating formers made of high-strength and composite materials that would make the CORC® conductor much more resilient to stresses during high-field magnet operation without compromising the high electrical stability of the conductor. During the Phase I program, we clearly demonstrated the feasibility of incorporating composite materials, designed by combining high-purity copper with high-strength alloy filaments such as TaW, into CORC® conductors that would significantly increase the critical stress under axial tension, while maintaining most of the electrical conductivity and flexibility under bending provided by pure copper formers. The composite materials have the ability to provide the CORC® conductor with similar strength, but much better conductivity, compared to pure off-the-shelf copper alloys, such as CuBe. The electrical stability of a high-strength composite CORC® wire at 4.2 K is comparable to that of CORC® wires with pure copper formers, which was demonstrated by comparing the voltage evolution across the conductor as a function of time due to a heater induced quench. High-strength CORC® wires with different layouts were then wound and tested in monotonic axial tension and transverse compression at 76 K. High-strength alloy and composite cores allowed the critical tensile stress of CORC® conductors to exceed our phase I goal of 300 MPa, making them some of the strongest superconductors available. The effect of axial tensile stress and strain, as well as transverse compressive load on the critical current of CORC® conductors is supported by analytical and finite element modeling that is a well-developed toolset to further optimize the conductors in the proposed Phase II program. We found that, in the case of axial tension, minimizing the tape winding pitch of the helically wound REBCO tapes allowed us to mechanically decouple the brittle REBCO film from the overall CORC® conductor. As a result, we were able to reach a tenfold increase in the irreversible strain limit under axial tension to over 7 % in optimized CORC® wires, compared to only 0.6 % in single REBCO tapes. The result presents a breakthrough for high-field magnet technology. The combination of high conductor strength and high strain tolerance allows a significant simplification of magnet design and construction that will aid the development of reliable high-field superconducting magnets for compact fusion machines. The successful Phase I program forms a solid base for the proposed Phase II program, in which high-strength CORC® conductors with composite cores will be developed into a commercial product. The results of the Phase I program provide us with high confidence of developing CORC® conductors with a critical tensile stress of 400 – 600 MPa and a strain tolerance of at least 3 % in combination with a critical load under transverse compression of more than 300 kN/m.
- Research Organization:
- Advanced Conductor Technologies
- Sponsoring Organization:
- USDOE Office of Science (SC), Fusion Energy Sciences (FES)
- DOE Contract Number:
- SC0020710
- OSTI ID:
- 1783421
- Type / Phase:
- SBIR (Phase I)
- Report Number(s):
- DOE-ACT-20710-1
- Country of Publication:
- United States
- Language:
- English
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