Improvement of the Trapped Field Performance of a Holed Superconducting Bulk Magnet

Abstract

Our goal is to generate a strong magnetic field using a REBCO bulk superconductor activated by pulsed field magnetization (PFM) for industrial application using the bulk magnet. As the diameter of the material increases and its critical current density rises, magnetic field trapping by PFM is difficult due to the strong magnetic shield. We have proposed a holed superconducting bulk magnet to improve the trapped field performance in large-size and high-J c bulk material. Magnetic flux can be supplied efficiently by reducing superconductivity in the portion with holes artificially. To confirm the validity of the proposed method, an experiment, in which a single pulsed field was applied with changing amplitude of the magnetic field and temperature, was carried out using a GdBCO bulk superconductor with four small holes. Although the magnetic flux penetrated at a lower applied field, there was a problem in that the maximum trapped field and the total magnetic flux decreased. In this paper, the size and number of holes were optimized to manage both the increase in the trapped field and easy flux penetration. A hole 1mm in diameter was drilled to half of depth of the thickness at the rim of a sample after investigating the magnetization characteristic of a GdBCO bulk 60 mm in diameter and 20 mm thick. A trapped field performance was evaluated in the sample, and afterwards, the hole was bored completely to the full depth. Then, the same experiment was carried out. The trapped field for a single hole was improved compared to the four holes, and it was confirmed that the 1-mm-diameter hole did not degrade the magnetization performance.Index Terms-Pulsed field magnetization, small hole, superconducting bulk magnet, trapped field performance. 1051-8223

Pulsed field magnetization of a rectangular Y–Ba–Cu–O bulk, single grain superconductor assembly

“…The external magnetic field B a (t) with a rise time of τ = 0.02 s was approximated using the following equation, where B max is the peak value of B a . This is applied as a boundary condition on the outer boundaries of the model Further increase in the applied field only reduced the peak trapped field, which is consistent with existing reports of other PFM studies [41][42][43]. The dramatic increase in trapped field is attributed to the flux jump phenomenon.…”

“…It was found, at 70 K, that the magnetic flux was trapped only at the edges of the sample when the applied field (B a ) was smaller than 2.06 T. Increasing the applied field slightly to B a = 2.23 T increased the peak trapped field abruptly to a maximum of 1.19 T, and was accompanied by a change in the geometry of the trapped field profile from M-shaped to cone-shaped. Further increase in the applied field only reduced the peak trapped field, which is consistent with existing reports of other PFM studies[41][42][43]. The dramatic increase in trapped field is attributed to the flux jump phenomenon.…”

Modelling and mitigating flux jumps in bulk high-temperature superconductors during quasi-static, high-field magnetisation

“…Experimentally however, researchers may wish to devise methods of improving thermal flow internally within the bulk (such as impregnation of a high-thermal conductivity fluid, e.g. an epoxy dispersed with copper powder, or by milling holes in the bulk [46,47], and subsequently filling them with a thermally conductive material, which has been found to improve heat dissipation and trapped field performance [48,49]). It should be noted here that increasing the ramp-time (t fc ) should reduce the heating rate, and thus also delay the onset of the flux jump.…”