Development of an improved multilayer Halbach assembly magnet for PMR relaxometry based on optimization of magnet parameters

Relevance . A magnet is an important block in the design of measuring and analytical equipment based on nuclear (proton) magnetic resonance methods. A large volume of interpolar space and homogeneity of the magnetic field are desirable qualities of a magnet, directly affecting the accuracy of measurements. The Halbach magnetic assembly (HMA) is a promising type of magnet and is a priority for research for use in new generations of portable PMR relaxometers. They have smaller dimensions and weight but create a significantly greater magnetic field in the gap compared to dipole magnets of the same mass and distance between the poles. Therefore, the study and search for solutions to optimize the HMA parameters is an urgent task in the development of improved portable PMR relaxometers. Objective . The aim of the work is to study and determine the factors influencing such important magnet parameters as: mass, dimensions, magnetic flux density and homogeneity of the magnetic field in the magnet gap and to develop mathematical equations to describe the influence of these factors on the magnet parameters. Based on the results of calculations and software modeling, the practical goal was to create an experimental prototype of HMA for evaluating and calibrating the quantities in mathematical equations. Method . The applied method was the theory of magnetic combinations composed of identical magnets proposed by Klaus Halbach and other researchers in the field of magnets, based on which the corresponding mathematical equations are developed. ANSYS Maxwell magnetic field modeling software was used for preliminary evaluation of the theoretical calculations. An experimental study method was used in combination with statistical analysis to collect and process data during tests on the HMA prototype. Results . The result is the development of a method for optimizing the HMA parameters by determining the influence of factors on the HMA parameters based on calculations and software modeling of the HMA prototype to test the optimization. As part of the study, a device for measuring magnetic field values in the range of 0–1.25 T with a resolution of 4 mV/mT based on a Hall sensor and an Arduino board was also developed, which complemented the research. Conclusion . Based on the calculations of optimized parameters using a mathematical model, a prototype of the HMA was developed. It weighs 4.5 kg, dimensions 140×140×120 mm, and consists of 10 layers of cubic magnets stacked on top of each other. The developed HMA has a low price, is easy to manufacture and assemble, and has the flexibility of changing the design to adjust the magnetic field values in the gap. With a design of 10 layers, the magnetic flux density in the gap reaches B 0 = 0.344 T with inhomogeneity ΔB/B 0 = 2000 ppm in a volume of 6 cm 3 in the center of the gap with a diameter of d = 30 mm. The value of the magnetic flux density in the HMA gap is like the value in the dipole magnets used in the PMR-NP1 and PMR-NP2 relaxometers, while the mass and dimensions are significantly less.