A light weight, simple design NMR apparatus consists of 24 identical magnets arranged in Halbach array was designed and built. The homogeneity of the magnetic field B0 can be improved by dividing a long magnets into several rings. The size of the useful volume depends on both the gap between each ring and some others shim magnets. Our aim is to enhance the sensitive volume and to maintain the highest magnetic static field (B0). This apparatus generates a B0 field strength of about 0.1 T. This work focuses on the magneto-static simulation of NdFeB magnets arrangement and on the comparison with the measurement of the magnetic field strength and homogeneity in three dimensions (3D). The homogeneity of the magnetic field B0 is optimized with the help of CAD and mathematical software. Our results were also validated with a Finite Element Method (FEM). The simulation results of the strength and of the homogeneity of B0 field were compared to those obtained with a digital gaussmeter. The homogeneity in the magnet longitudinal axis and the field B0 strength are similar. However, the homogeneity in transverse plane differs from simulation and measurement because of the quality of the magnets. In order to improve the homogeneity, we propose a new shim method.

This paper focuses on the magneto static simulation of magnet arrangement used for portable NMR (Nuclear Magnetic Resonance) apparatus. In NMR experiments, a very homogeneous magnetic field is required in a large sample volume. In our case, NdFeB magnets are used to generate a homogeneous field, with a Halbach arrangement. The homogeneity of the magnetic field B 0 can be improved by dividing a long configuration of magnets into several rings. The size of useful volume is dependent on both the gap between each ring and some others shim magnets. Our aim is to enhance the sensitive volume size while maintain the highest magnetic field. In this goal, we optimized the gap between the two rings and put two shim magnets rings in the bore of the structure. Optimizing the position of the shim magnets allow drastic improvement. The homogeneity of the magnetic field B 0 is optimized with the help of CAD and mathematical software. Our results were confirmed with a Finite Element Method. As a result the method described here achieves a significant improvement of the homogeneity in the particular case of Halbach arrangement.