Design of Coupled or Uncoupled Multifilamentary SSC-Type Strands with Almost Zero Retained Magnetization at Fields Near 0.3 T

Abstract

Multifilamentary Cu-matrix strands with interfilamentary spacing as small as 0.2 μm can be almost fully decoupled by the addition of 0.5 wt.% Mn to the interfilamentary Cu. Decoupling in this way seems to be beneficial from a field-stability standpoint. On the other hand, the elimination of coupling does little to reduce residual strand-magnetization at the injection field of about 0.3 T when that field is approached, as usual, along the shielding branch of M(H). This residual diamagnetic magnetization (say M_R) of the winding material is responsible for unwanted distortion (multipole formation) of the dipolar field. It is demonstrated that M_R can be locally cancelled to zero by associating the strand with a small volume-fraction (less than 2%, depending on filament diameter) of pure Ni or any other low-field-saturable ferromagnetic material. The presence of the Ni has little effect on the shape of the M(H) hysteresis loop of the strand, other than to shift its wings uniformly in the + M (when H is positive) and -M directions, respectively. In practice, the Ni could be administered as: (a) additional filaments, (b) interfilamentary barriers, or (c) an electroplated layer on the outside of the strand.