Development of a bulk superconducting magnet for benchtop nuclear magnetic resonance

• Main Author: Beck, Michael
• Format: Dissertation
• Language: English
• Published: University of Cambridge 2022
• Subjects: Bulk Superconductors; High Temperature Superconductors; Nuclear Magnetic Resonance; Numerical Modelling; Pulsed Field Magnetisation
• DOI: 10.17863/CAM.96595

Stacks of bulk high temperature superconducting (HTS) rings are promising candidates for the generation of the strong polarising fields required for nuclear magnetic resonance (NMR) when utilised as trapped field magnets (TFMs). To date, these stacks have been magnetised by the quasi-static field cooled magnetisation (FCM) technique within NMR-grade fields, requiring access to large and expensive magnetising fixtures - limiting the accessibility and reach of such systems. Portable, low-cost magnetisation techniques such as pulsed field magnetisation (PFM) have been shown to readily trap fields up to 5 T within disc shaped samples. This work investigates the viability of magnetising stacks of ring-shaped bulk HTS by PFM to generate magnetic fields suitable for NMR. First a two-dimensional (2D) axi-symmetric model of a single ring, based on the wellestablished H-formulation, was developed and iteratively refined to remove numerical errors from the solution. This model was then validated against analytical solutions for the quasi-static problem, before being expanded to account for thermal effects during rapid field application. The penetration of magnetic flux into ring-shaped bulk HTS was found to occur from both the inner and outer faces of the ring, degrading the stability of the magnetisation process during PFM. These results were verified experimentally. The influences of ring geometry and critical current density characteristic (Jc) on the trapped field were investigated - along with the use of inserts to improve the trapped field strength and mitigate the instabilities. The use of inserts gave mixed results but open several avenues for further investigation. The model - without inserts - was then expanded to predict the behaviour of stacks identical rings when axially spaced with variable separation. Increasing the number of rings within the stack is an effective way of improving the magnitude of the trapped field but does not necessarily improve the homogeneity of the trapped field sufficiently for NMR. Constructing the stack with variable separation between the rings can result in a highly uniform field with only a small effect on the peak strength. Finally, as practical samples do not exhibit perfect axi-symmetry, a three-dimensional (3D) model of a ring stack is implemented with spatially varying Jc. The validity of the spatial Jc distribution was validated by comparison with a real sample. The influence of this non-uniformity on the trapped field under both FCM and PFM was investigated - for which it was found that axial non-uniformity may be exploited to improve the homogeneity of the trapped field. Any circumferential variation significantly degrades the trapped field properties but can be mitigated by use of multiple samples with weaker regions deliberately misaligned. These results provide novel insight into the rapid magnetisation of ring-shaped bulk HTS, and methods through which the inherent instabilities may be mitigated - opening new pathways to high-field, low-cost, portable NMR systems.