Observation of a Bean Model Limit—A Large Decrease in Required Applied Activation Field for TFMs

Experiments are reported on pulsed activation of zero-field-cooled trapped field magnets (TFMs) containing improved broken-columnar pinning centers (PCs). The YBCO TFMs have J_C ~ 50 kA/cm² and maximum trapped field, BT,max ~ 2.2 T at 77 K. Several results are in disagreement with the well-established Bean model. Essentially full activation is obtained with an applied field at the surface of B_A ≈ 1.0 X BT,max. The Bean model predicts B_A ≥ 2 X BT,max. Low points in activation are observed at radial sample coordinates r ≈ 0.5 R, a result precluded by the model. Activation shows a discontinuous giant field leap, in disagreement with the smooth increase of B_T,max vs. B_A predicted by the model. Results are compared to TFMs containing Y211 PCs, which are well described by the Bean model. E.g., for Y211 PCs, field at inner r is the last to rise. For columnar PCs it is the first to rise, an additional conflict with the model. Also, the Bean model predictions are independent of J_C, B_T, and PCs, whereas major differences exist experimentally. We speculate that with high J_C and B_T, the very large, centrally-directed Lorentz force causes a discontinuous shift of the fluxoid mesh toward r = 0.