“Flux Jumps” in Cable-in-Conduit Conductors Induced by Transient Magnetic Field

A jump-like temperature increase that can not be explained only by AC losses was observed during the tests of Nb-Ti CICC model samples in transient magnetic fields. The samples were placed between two racetracks that generate background transient magnetic field (TMF). The flux jumps occurred in CICC samples even during the slow discharge of racetrack coils. A physical model was developed to explain this phenomenon. According to this model, transient magnet field generates the screening currents in the loops formed by the superconducting strands and normal "matrix". In a real winding the additional currents can be either added to or subtracted from the transport current in superconducting strands depending on a mutual orientation of the varying magnet flux density and the transport current vectors. The critical current of the strands can be exceeded as result of a simultaneous increase of the strands temperature due to AC losses and the screening current. After the sample reaches its critical state the energy stored in the loops quickly dissipates in the normal matrix of the strands. This results in a jump-like increase of the conductor temperature