Experiments on stability of Nb3Sn ribbons

The major instability problem in Nb3Sn ribbon superconductors is known to be due to flux jumps caused by magnetic field components perpendicular to the face of the ribbons (H$$_\perp$$). Likewise, techniques which are intended to prevent this instability are also well known and include (1) reducing the width of the ribbon, (2) using low current rated material in regions of large H$$_\perp$$), (3) paralleling the ribbon with good electrical conducting material, and (4) providing as much cooling as possible. The decision, in 1968, to employ Nb3Sn ribbon in IMP, a large plasma physics research facility, was made without full knowledge of how bad the instability really was. While difficult force problems and machining and assembly problems were being solved, experiments were performed to determine the best way to reach the IMP quadrupole design point. The stability question was answered by performing experiments with relatively large test coils called ''cusp coils'' which showed that stabilization could be achieved by cowinding 0.006 in. aluminum ribbon along with the Nb3Sn ribbon