Continuously Wound High-temperature Superconducting Cable for the Central Solenoid of DEMO Fusion Reactor

Fusion reactors are a promising economic source of clean energy. In order to achieve the break-even operation, the future reactors, such as DEMO, will feature very high, > 16 Tesla, magnetic fields, which can be only created by magnet coils wound with the second generation (2G) superconducting wire. The 2G wires are currently manufactured as thin (1-2 micron) YBCO layer deposited on a metal substrate. The substrate takes a considerable portion of the cross-section, thus reducing the engineering current density, preventing effective inter-filament current sharing and transposition. The geometry is also responsible for high magnetization loss. Reducing the magnetization loss is especially critical for the central solenoid which operates under pulsed load. Brookhaven Technology Group proposes development of a new type of high-temperature superconducting cable for the central solenoid of a high-field fusion reactor. The product combines two recent innovations: (i) Reduction of AC loss by bundling narrow, 1-2 mm, exfoliated YBCO filaments into a transposed cable, (ii) continuous winding technology which eliminates labor-intensive and expensive handling of narrow filaments. The engineering critical current density of the wire will thus increase to over 2,000 A/mm^2, in 10 T, HIIc, 4.2 K; the projected cost of the filament is expected to remain below $20/KA*m, for the same operating conditions. The cable geometry enables x5 magnetization loss reduction compared to the state of the art 2G cables. Phase I activities will include the following tasks: (i) design and assembly of a continuous cabling system prototype. The system would allow for reel-to-reel manufacturing of a cable from a wide exfoliated tape; (ii) Manufacturing of a cable prototype of the spiral stack geometry; (iii) Low-temperature transport measurements and high field, 0.6 Tesla, measurement of magnetization loss. At the end of Phase I, BTG will characterize a 1 m coupon of superconducting cable comprised of four layers of the exfoliated filament. The cable will contain only copper, silver and pinning-enhanced YBCO. Low-temperature measurements will be carried out at National High Magnetic Field Laboratory and Wright Patterson Air Base. The Phase II goal is development of a scaled-up cabling system capable of producing > 1 km of the cable.