Flux Flow and a New Critical-Current Formula

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Abstract
An experimental study has been conducted to measure the flow of magnetic flux through the wall of a specimen of Nb‐25% Zr, and thereby to infer the magnetic induction and currents in the material. It was found that if at some magnetic field the specimen were heated to above T c and recooled, then if the applied field were subsequently changed at a constant rate, the rate of flux flow into or out of the sample would increase linearly with time. If measured at a series of magnetic fields, the rates of these increases, or the flow rate slopes were found to be a smoothly increasing function of average applied field. Calculations based on eight published critical‐current density models did not satisfactorily explain these observations. It was found, however, that the empirical current‐density model J c = α c [(μ0 H c2−B) / (B 0+B)] did, in fact, give excellent agreement with the measured flow‐rate behavior. In this paper the experimental and analytical results are described and the new critical‐current density model is compared with earlier ones.