Field uniformity enhancement in a prototype high-temperature superconducting dipole magnet

초록

High-temperature superconducting (HTS) magnets are applied across MRI, NMR, and fusion, and particle accelerators are likewise advancing toward higher magnetic fields, creating a need for high-field magnets. In synchrotrons, high-field dipole magnets of saddle geometry are required, and REBCO has drawn attention as a promising material to realize them. For these applications, the magnetic field must be spatially homogeneous along the beam line and temporally stable. However, for REBCO dipole magnets, there are few studies that rigorously report field uniformity and temporal stability on the beam line, and efforts to improve the uniformity itself remain scarce. Here we demonstrate improved field uniformity in a conduction-cooled, no-insulation (NI) REBCO saddle dipole magnet engineered to produce a 0.5 T central field. Two strategies to mitigate magnetic field uniformity due to screening-current-induced field (SCIF) or fabrication errors were assessed: current sweep reversal (CSR) and passive shimming using an AISI 1008 ferroshim. CSR produced $\sim$0.4$\times$$10{<}^{>}-4$ of improvement in spatial uniformity, while the ferroshim improved uniformity 4.5$\times$$10{<}^{>}-4$. Harmonics are sextupole-dominant with small skew residuals, and temporal drift is $\le$0.38$\times$$10{<}^{>}-4$$\text{h}{<}^{>}-1$. Our results demonstrate the impact of each magnetic-field improvement technique on field uniformity using numerical simulations and experimental validations. These results outline a practical route to low-temperature-superconducting(LTS)-class field uniformity in a compact and cryogen-free dipole magnet with the NI HTS technique.

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