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

Fusion reactors are a promising economic source of clean energy.In order to achieve 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 ?m) YBCO layer deposited on a ~ 100 ?m thick metal substrate.The substrate takes a considerable portion of the cross-section, thus reducing the engineering current density it also prevents effective inter-filament current sharing and transposition.In addition, the wide tape geometry is responsible for high magnetization loss.Reducing the magnetization (i.e.power) loss is especially critical for the central solenoid, which operates under pulsed load.Brookhaven Technology Group (BTG) proposes development of a new type of high-temperature superconducting cable architecture for the central solenoid of a high-field fusion reactor.The new architecture 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 cable geometry enables x5 magnetization loss reduction compared to commercially available state of the art 2G cables.In the Phase I effort, the BTG team explored concurrent filament slicing and transposition of exfoliated filaments.In-field AC loss of the cable strands was measured at Wright-Patterson Air Base.The transposition tolerance and effect of transposition on AC loss was established.The 1.5 meter long cable strand demonstrated critical current of 520 A at 77 K for an 8-filament partially transposed cable.In the proposed Phase II project, the BTG team will design, build and test a novel type of cabling machine that combines slicing and cabling in one step.The technology forgoes the labor-intensive and risky handling of individual filaments.The slicing will be performed by parallel scanned beam, which greatly simplifies the machine design.The novel laser technology will minimize the edge damage of the filament, thus enabling narrow, < 1 mm wide, cables.The cable will be used to wind several 2” test coils, which will be tested at 4.2 K, 14 tesla external field.We will also design and build a prototype central solenoid insert which is expected to demonstrate 1.2 tesla field and a novel quench detection method.