Strand and Cable Development for a High Field ${\rm Nb}_{3}{\rm Al}$ Common Coil Magnet

Abstract

Research and development of copper stabilizedNb 3 Al strands and cables for a high field Nb 3 Al common coil magnet is ongoing with the cooperation of KEK-NIMS-Fermilab. In this work, newly designed K1 and K2 strands and cables were fabricated. The K1 strand used tantalum for the inter-filament matrix only and the K2 strand used it for the entire strand matrix. Wire breakage happened on the drawing of both precursors, and it may be caused by the de-bonding of the inter-filament as well as poorer cold-workability of the tantalum itself. Non Cu c 's at 12 T were 1,300 A mm 2 at 4.2 K and 1,600 A mm 2 at 1.9 K, an increase of about 20%. The low field magnetic instabilities at 4.2 K could be suppressed with K1 and K2 strands because of the tantalum inter-filament matrix. Similar superconducting characteristics were obtained on K1 and K2 strands. However, K2 strands showed excessive separation of the copper stabilizer when made into Rutherford cables, although K1 strands did not. The copper stabilizer prefers niobium as an interface material to obtain good bonding.

A new RHQT Nb3Al superconducting wire with a Ta/Cu/Ta three-layer filament-barrier structure

“…No flux jumps are observed at 4.2 K in the magnetization curve of a Ta filament-barrier RHQT Nb 3 Al conductor; replacing Nb with Ta is thus effective for suppressing flux jumps at 4.2 K. There are two other advantages of using Ta as a filament-barrier species: stiff Ta can better withstand electromagnetic forces at cryogenic temperatures and Ta has a higher melting point than Nb and is thus expected to provide better mechanical support for wires during the high-temperature reel-to-reel RHQ process [19]. However, replacing Nb with Ta certainly degrades the cold workability of the JR Nb/Al precursor and causes the precursor wire to break due to poor bonding at the Ta/Ta interface between neighboring Ta-coated JR filaments [22]. Microstructural observations of fractured cross-sections of precursor wires clearly reveal that wire breakage usually originates at the filament barrier of Ta, which has inferior cold workability to Nb.…”

R&D Efforts Towards High Field Accelerator Magnets at KEK

“…Fig. 2 shows magnetization curves of RHQwires with Nb-and Ta-matrix [5]. In comparison with Nb-matrix wire (F1), magnetization curves of Ta-matrix wires (K1 and K2) are very small and no flux jump can be seen.…”

Recent R&D on Superconducting Wires for High-Field Magnet

“…As far the critical current is concerned, REBCO and Bi-type HTS wires produce the largest critical current in field applications, and are currently available commercially. The critical current of Nb 3 Al wire fabricated by the RHQ method is largest at high-field in low-temperature superconducting (LTS) wire that is larger than that of the Nb 3 Sn wire in the A15-type material and NbTi [7][8][9][10][11][12]. More improvement in the critical current is the most important topic for the LTS wires.…”