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ST394 - Observation of a Bean Model Limit – A Large Decrease in Required Applied Activation Field for TFMs

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

Roy Weinstein, Drew Parks, Ravi-Persad Sawh, Kent Davey, and Keith Carpenter

Texas Center for Superconductivity, and Physics Department,
University of Houston, Houston, TX 77204 USA

Email:  [email protected]

Abstract – 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 JC ~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 BA ≈ 1.0 x BT,max. The Bean model predicts BA ≥ 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 BT,max vs. BA 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 JC, BT, and PCs, whereas major differences exist experimentally. We speculate that with high JC and BT, the very large, centrally-directed Lorentz force causes a discontinuous shift of the fluxoid mesh toward r = 0. 

Keywords (Index Terms) — REBCO coated conductors, PLD and TFA-MOD processing, BHO, BZO and BSO additions as artificial pinning centers (APC), in-field critical currents, M-PACC project     

IEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), October 2014.
Received August 26, 2014; Selected September 29, 2014. Reference No. ST394; Category 2, 3. 
Preprint of ASC 2014 manuscript 2MOr2B-01 published online in IEEE Trans. Appl. Supercond.
(IEEE XPLORE) DOI: 10.1109/TASC.2014.2366722, on November 20, 2014.