An analytical approach towards passive ferromagnetic shimming design for a high-resolution NMR magnet

Abstract

This paper presents a warm-bore ferromagnetic shimming design for a high-resolution NMR magnet. The design is based on spherical harmonic coefficient reduction techniques. We have applied the technique to a 700 MHz magnet completed at the MIT Francis Bitter Magnet Laboratory. The 700 MHz magnet was composed of a low-temperature superconducting (LTS) 600 MHz NMR magnet (L600), purchased from a magnet vendor, and a 100 MHz high-temperature superconducting (HTS) insert (H100), designed and built at FBML. Based on the measured magnetic field data, we derived a total of 22 low-order spherical harmonic coefficients. Another set of spherical harmonic coefficients was calculated for iron pieces attached to the surface of a 50 mm wide and 72 mm high tube. To improve the magnet homogeneity, we applied a multiple objective linear programming method to minimize unwanted spherical harmonic coefficients. A result is a ferromagnetic shimming set of 74 iron pieces. Analysis shows that with this ferromagnetic shimming set, the magnet field homogeneity may be significantly improved, here by a factor of eight.