Optimizing a rapid shutdown module for pre-polarization current in surface nuclear magnetic resonance

Surface nuclear magnetic resonance (SNMR) technology is widely used for hydrological investigations. At high ambient noise levels, the use of a pre-polarization field can enhance the amplitude of the free induced decay (FID) signal. However, the spatial distribution of the pre-polarization field is inhomogeneous, and so it is vital to optimize the shutdown ramp and time for all combinations of the pre-polarization field amplitude and relative orientation. For this purpose, we designed a rapid and adiabatic shutdown circuit for pre-polarization surface nuclear magnetic resonance (PSNMR) systems. In the initial stage of the shutdown process, the magnetic field energy stored in the direct current transmitting coil is transferred to the capacitor by a semi-controlled full bridge circuit. At the end of the shutdown period, the pre-polarization current decays exponentially toward zero. The shutdown process can be controlled by adjusting the circuit parameters. The shutdown ramp and time of the designed circuit are verified through a series of simulations and experiments. As the initial voltage of the capacitor increases from 65 to 300 V, the initial amplitude of the exponential decay of the pre-polarization current increases from 15 to 48 A and the shutdown time decreases from 1.4 to 1 ms. Our design scheme provides a more reliable concept for the shutdown process in the case of multiple transmitting coils. This allows a larger gain of the FID signal to be obtained in specific PSNMR applications when pre-polarization currents with slightly different channel parameters need to be shutdown uniformly.