Design of a multilayer Halbach permanent magnet for human finger NMR detection

Permanent magnet is the key component of nuclear magnetic resonance (NMR) instrument. Based on the principle of Halbach array, this study designs and optimizes a multilayer Halbach permanent magnet for human finger NMR detection. The quality factor (QF) is introduced to represent the performance of magnetic field, firstly. The impact on magnetic performance, caused by various magnet heights, is analyzed using Finite Element Method (FEM). In order to compensate the end effect, the radial adjustment method is proposed, combining the adjustments of the radial size and the radial position of the magnet blocks. Comparing the effect on magnetic field of three different plans, the results show that combining the adjustments of inner radius and outer radius shims the field dramatically. Finally the designed construction is stacked by 11 layers of magic rings and each layer is composed of 8 trapezoid-shaped magnets. The magnetic field of the center cylindrical region (f20 x 20 mm) of the structure is calculated, whose final strength achieves up to 447 mT and homogeneity is within 500 ppm. The designed magnet structure is manufactured and the measurement is finished in the end. The proposed construction has the advantages of small size and high uniformity, which has an extensive application prospect in portable NMR system.