Arc gradient coils design for unilateral magnetic resonance sensor based on Fourier series method

Traditional nuclear magnetic instruments are not convenient for measurement in situ due to the closed magnet structure and heavy weight. It is crucial to improve tree planting by exactly detecting moisture parameters of living trees. To investigate this problem, an open and portable magnetic resonance sensor was proposed. In this work, we introduced unilateral magnetic resonance sensor which can achieve onsite nondestructive measurement of large-volume samples due to its open structure. To fit the shape of living trees, the region of interest was set in an arc region. A magnet structure was optimized by ANSYS software. Because the magnet has a constant natural gradient along radial direction, we need gradient fields in the other two directions (Z and Y). Z/Y gradient coils for unilateral magnetic resonance sensor are studied based on the Fourier series method. Current density in gradient coils is assumed as Fourier series composed of trigonometric functions. We introduced stream function to establish relationship between the current density and the winding structure, then constructed an optimization model by combining power dissipation with the deviation between calculated field and target field. The coils are manufactured using a flexible printed circuit board and measured by gauss meter. The measurement and simulation results are consistent. Measurement results of the gradient coils showed that the magnetic fields have good linearity which is helpful to realize 3D measurement in the upcoming work.