Trapped field properties of a Y-Ba-Cu-O bulk by pulsed field magnetization using a split coil inserted by iron yokes with various geometries and electromagnetic properties

We have investigated, both experimentally and numerically, the trapped field characteristics of a standard Y-Ba-Cu-O bulk of 30 mm in diameter and 14 mm in thickness magnetized by pulsed field magnetization (PFM) using a split coil, in which three kinds of iron yoke are inserted in the bore of the coil: soft iron with a flat surface, soft iron with a taper, and permendur (50Fe + 50Co alloy) with a flat surface. The highest trapped field, BTmax, of 2.93 T was achieved at 40 K in the case of the permendur yoke, which was slightly higher than that obtained for the flat soft iron or the tapered soft iron yokes, and was much higher than 2.20 T in the case without the yoke. The insertion effect of the yoke on the trapped field characteristics was also investigated using numerical simulations. The results suggest that the saturation magnetic flux density, Bsat, of the yoke acts to reduce the flux flow due to its hysteretic magnetization curve and the higher electrical conductivity, σ, of the yoke material also acts to suppress the flux increase rate. A flux jump (or flux leap) can be reproduced in the ascending stage of PFM using numerical simulation, using an assumption of relatively high Jc(T, B) characteristics. The insertion effect of the yoke with high Bsat and σ is discussed to enhance the final trapped field from both electromagnetic and thermal points of view.