Simulation and optimisation of homogeneous permanent magnet for portable NMR applications

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 B₀ 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 B₀ 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.