Determination of the complete penetration magnetic field of a HTS pellet from the measurements of the magnetic field at its top-center surface
Résumé
Our basic method to characterize HTS pellets, which has been presented in earlier publications, is based on the measurement of their complete penetration magnetic flux density BP. The method allows assessing the critical current density JC and the n-value through the classical power law model modeling the magnetic behavior of pellets and the direct measurement of BP. A first method used two slightly axially separated HTS disks to determine BP in the gap between the disks. However, this first method exhibits two main disadvantages. The first disadvantage is that the magnitude of the derived values depends on the spacing e between the two pellets. The second one is that the two pellets ought to be strictly identical for the method to lead to representative values of a given pellet. To overcome those constrains and limitations, we have been looking for a new improved method that relies only on the measurement involving a single pellet. The following work introduces the principle of this new method. The main idea is thus to deduce BP from the magnetic flux density measured at the center of the top surface of the pellet referred to as BT when the applied magnetic flux density Ba equals BP. The influence of the applied magnetic flux density ramping rate and the geometric dimensions of the pellet on the ratio BP /BT is studied and used to assess BP from the measured BT. It is shown that the proposed method allows achieving a substantial improvement on the accuracy of the Jc and n-value measurements of a given superconducting disk over the past method based on two HTS pellets.