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

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 İ_j, at which the current was changing at the time a given jump took place. İ´s in the range 10¹ <; İ <; 10⁷ A/sec were employed. Ij remained constant below some value of rate İ_jA 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. İ_jA represents the point where the power density, S_j, 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 I_j 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-current-induced flux jump may occur in a hard super conducting wire of radius R at a current I_fj = 5R[ ℒ π³CJ_c/ (∂J_c/∂T)]^1/2, where J_c is the critical current density; ∂J_c/∂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 T_c for many hard superconductors. Therefore, increased stability against flux jumping is predicted with rising temperature in the range 0 <; T <; 0.8 T_c. 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 T_c´s can be operated with more stability at temperatures greater than 4.2° K.