Influence of n-value and rise rate on the Magnetic Field Penetration in HTS Bulks

The complete penetration magnetic field BP is a very important parameter of superconducting bulks submitted to an applied magnetic field. It is useful to know it for applications as superconducting permanent magnet synchronous motors. In this study, cylindrical HTS pellets were submitted to an axial applied magnetic field. Theoretical and experimental studies are presented. For numerical simulations, the E-J characteristic of HTS bulks was described by a power law relationship, i.e. E(J) = EC(J/JC)n. The influence of the n-value and of the rising rate Vb of the applied field on the BP of HTS cylindrical pellets was investigated, and a linear relationship between BP, Vb and n-value could be established. The experiment was based on direct measurements of the local magnetic field in the gap between two bulk HTS pellets. The measurements of the field penetration has been carried out on HTS pellets at 77 K by applying increasing axial magnetic fields with a quasi-constant sweep rate. The influence of the gap size between the two HTS pellets was studied with a particular attention. Two values of the complete penetration field BP have been measured at two different rising rates Vb, allowing n-value determination. We conclude that using a combination of measurements and numerical simulations allow determining in a quite simple way the n-value and the critical current density JC of real HTS pellets.