Some rate dependent aspects of flux jumping in wires of type II superconductors

Abstract

Time varying currents have been applied to wire samples of Nb25%Zr, Nb33%Zr, and Nb. Flux jumping effects were observed by monitoring the flux motion voltage Vf developed along a given length, l, of sample. The currents Ij at which flux jumps occurerd were studied as a function of the rate Ij, at which the current was changing at the time a given jump took place. I's in the range 101 < I < 107 A/sec were employed. Ij remained constant below some value of rate IjA characteristic of a given sample. Above this rate Ij begins to increase monotonically from its constant value, i.e., the sample becomes more stable against flux jumping. IjA represents the point where the power density, Sj, crossing the wire surface begins to exceed S = 0.8 W /cm2, the value which characterizes the onset of film boiling in the helium bath. Sj is calculated from the current Ij and the flux motion voltage Vf present just prior to a given flux jump. A recently proposed criterion for the onset of magnetic instabilities predicts that a transport-cur rent-induced flux jump may occur in a hard super conducting wire of radius R at a current Ifj = 5R[ ℒ π3CJc/ (∂Jc/∂T)]1 over 2, where Jc is the critical current density; ∂Jc/∂T, the derivative of the critical current density with temperature; and C, the volume specific heat of the superconductor. This function increases from zero at T = a to a maximum at T ≃ 0.8 Tc for many hard superconductors. Therefore, increased stability against flux jumping is predicted with rising temperature in the range 0 < T < 0.8 Tc. The increased stability at higher rates found in the present experiments reflects a heating effect due to the poor heat transfer to the helium bath once the power density at the surface of a sample exceeds the 0.8 W /cm2 required to initiate film boiling. Thus, the results indicate that superconducting devices made with certain type II materials having high Tc's can be operated with more stability at temperatures greater than 4.2° K.