Trapped Fields >1 T in a Bulk Superconducting Ring by Pulsed Field Magnetization

Abstract

One potential application of magnetized RE-Ba-Cu-O (where RE = rare earth or Y) bulk superconductors is as a highfield alternative to conventional permanent magnets in desktop NMR and MRI systems. Pulsed field magnetization (PFM) is one of the most promising practical methods of magnetizing such bulks. However, the trapped fields obtained by PFM are much lower than those obtained using quasi-static methods like field-cooling magnetization (FCM) due to heating during PFM. Furthermore, bulk superconducting rings have proved more difficult to magnetize via PFM than discs. The reported trapped fields in single bulk superconducting rings magnetized by PFM are less than 0.35 T at the centre of the bore due to thermomagnetic instabilities. In this work, systematic PFM measurements on a bulk Gd-Ba-Cu-O ring were carried out and a trapped field of 1.3 T at 55 K was achieved using a multi-pulse, stepwise cooling (MPSC) method. In the MPSC method, a sequence of pulsed fields is used to magnetize the ring bulk. The pulsed field is increased in small increments and the sample temperature is decreased sequentially. Consequently, as some field is already trapped after the first pulse, the motion of the flux for subsequent pulses will be reduced, leading to less heat generated in the bulk sample. This greatly improves the thermomagnetic stability of the PFM process, enabling larger trapped fields.

Dynamics of magnetic flux propagation in bulk, single grain superconducting rings during pulsed field magnetisation

“…Bulk superconducting rings have proved extremely difficult to magnetize by PFM when compared with discs, with maximum trapped fields less than 0.60 T obtained by typical singlepulse PFM [28,29]. Although, it should be noted that higher trapped fields have recently been reported with more advanced PFM techniques that attempt to overcome this issue: 0.76 T at 73 K using waveform control to elongate the pulse waveform [24] and 1.3 T at 55 K using multi-pulse stepwise-cooling PFM [44]. Here, the propagation of magnetic flux during PFM across the mid-plane of a ring stack is measured and compared with numerical predictions.…”

“…Furthermore, the additional flux motion due to this heating following field reversal is negligible for the disk when compared with the ring, which will lead to further heating within the ring geometry. Consequently, extending the duration over which the field reversal near the inner edge of the ring occurs, and subsequent flux motion-either through control of the magnetising waveform shape [24], or use of a multi-pulse magnetisation process, such as multipulse, stepwise-cooling (MPSC) [44], would be beneficial for the magnetisation of bulk superconducting rings for use as TFMs.…”

Direct fabrication of high-quality ring-shaped REBa2Cu3O y bulk magnets by the single-direction melt growth (SDMG) method

“…16) Given the promising applications of ring-bulks such as magnetic resonance imaging (MRI), high gradient trapped field magnets (HG-TFM), and magnetic shielding, in addition to desktop NMR, there is an increasing amount of research aiming to better understand their theoretical magnetization processes and mechanical characteristics. [17][18][19][20][21][22][23][24][25][26][27] In recent years, we have developed and reported on a novel bulk growth technique, the single-direction melt growth (SDMG) method, to address the challenges of scalability, homogeneity, and flexibility in shapes in the conventional top-seeded method. 28,29) In this SDMG approach, large bulk plates cut from commercially available TSMG-processed REBCO bulks, which exhibit relatively high peritectic temperature (T p ) such as EuBCO, are utilized as seed plates.…”