Some Rate-Dependent Aspects of Flux Jumping in Nb-25% Zr Tubes

A series of experiments have been performed on rate dependence of flux jumping in tubular samples of Nb‐25% Zr using pulsed magnetic fields. The field H₁ for which the first flux jump occurred was studied as a function of the rate of change of field Ḣ₁ at the time of the flux jump. The range of Ḣ₁ was 10¹<Ḣ₁<10⁵ kOe/sec. The data were taken using a specially designed inductance‐resistance‐capacitance circuit for producing pulsed magnetic fields.¹ Silicon‐controlled rectifiers were used as switching elements in the circuit. The dependence of H₁ on Ḣ₁ was studied for a series of tubular samples of varying wall thickness (10, 20, 40, and 100 mil). These samples had a 0.36 cm i.d. and were 3.2 cm in length. The results are shown in Fig. 1. It was found that H₁ is constant for 10¹<Ḣ₁<10³ kOe/sec. The level of H₁ in this region increases as the tube outer diameter increases. At some value of rate Ḣ_1A, H₁ begins to increase monotonically from its constant value and continues to do so up to the highest rates attainable with the present apparatus. Ḣ_1A increases for thinner‐walled samples and represents the point where the power density S at the surface of the tube begins to exceed S=0.8 W/cm², the value which characterizes the onset of film boiling² in the helium bath. The magnitude of the departure of H₁ from its constant value is greater for the thicker‐walled samples. At Ḣ₁≃10⁵ kOe/sec, H₁ has increased by a factor of two to three over its value in the plateau region. These results can be interpreted in terms of a recently proposed criterion^3–7 for the onset of magnetic instabilities in hard superconductors at a field $$H_{\mathrm{fj}}{\mathrm{=[-\pi }}^3{\mathrm{CJ}}_c{\mathrm{/(\partial J}}_c{\mathrm{/\partial T)]}}^{\text{1/2}}$$ , 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=0 and rises to a maximum at T≃0.7 T_c for Nb‐26% Zr. Thus, increased stability is predicted with rising temperature in the range 0<T<0.7 T_c. It is felt that the present results reflect a heating effect due to the poor heat transfer to the helium bath once the surface power density exceeds 0.8 W/cm². This result points to the possible stability advantage to be had by operating superconducting devices made with type II materials at a somewhat elevated temperature.