Critical current density and stability of Tube Type Nb3Sn conductors

Abstract The critical current densities ( J c ) and stabilities of Tube Type Nb 3 Sn 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 J c , 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 J c s were seen for the lowest reaction temperatures, with 12 T transport J c values as high as 2450 A/mm 2 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, d eff , and RRR. The most stable strands showed stability currents, J s , of 8700 A/mm 2 and 15,300 A/mm 2 for 0.7 mm OD and 0.42 mm OD conductors, respectively.