Flux Flow and a New Critical-Current Formula

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 Tc 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 Jc = αc[(μ0Hc2−B) / (B0+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.