Critical current density and stability of Tube Type Nb3Sn conductors

The critical current densities (Jc) and stabilities of Tube Type Nb3Sn conductors have been measured. The strands had superconducting subelement counts ranging from 192 to 744, and flat-to-flat filament sizes (for 0.7 mm OD wire) of from 35 μm down to 15 μm. These Tube Type conductors had a very simple structure: prior to heat treatment the filaments consist of a Sn core surrounded by a thin Cu tube, itself surrounded by a Nb or Nb alloy tube. Eight different strand types were investigated using various techniques including SEM, residual resistance ratio (RRR), transport Jc, and stability measurement. Most strands were studied at 0.7 mm OD, with one representative at 0.42 mm. The transport measurements were made at 4.2 K in fields up to 14 T. Numerous heat treatment schedules were investigated, with reaction temperatures ranging from 615 °C to 650 °C, and times ranging from 36–500 h. The highest Jcs were seen for the lowest reaction temperatures, with 12 T transport Jc values as high as 2450 A/mm2 observed. The RRRs were lower for longer time and higher temperature reactions and ranged from 4 to 180. Strand stability was a strong function of the effective filament diameter, deff, and RRR. The most stable strands showed stability currents, Js, of 8700 A/mm2 and 15,300 A/mm2 for 0.7 mm OD and 0.42 mm OD conductors, respectively.

► Transport and stability properties of Tube Type Nb3Sn with up to 744 subelements have been studied.
► Transport Jcs as high as 2450 A/mm2 were seen at 12 T.
► Strand heat treatment controlled transport Jc, RRR, and stability.
► Stability currents as high as 8700 A/mm2 and 15,300 A/mm2 were measured.