Analysis and design of the transmitting mode on the pre-polarization surface nuclear magnetic resonance system

Surface nuclear magnetic resonance (SNMR) technology is widely used in the detection of groundwater due to its non-invasive, qualitative, and quantitative advantages. Nonetheless, SNMR is difficult to employ in a high ambient noise level because of the weak level of nanovolt signals (10^-9 V). To solve this problem, pre-polarization (PP) technology is utilized for SNMR detection. That is, the combination of direct current, i.e., PP pulse, with alternating current (AC) pulses is utilized to increase the signal amplitude of shallow hydrogen protons. However, the PP and AC pulses on the same transmitting coil should be output independently when using the PP SNMR system. Meanwhile, to avoid magnetization loss, the process of shutting down the PP field must be both rapid and adiabatic. To solve the above problems, we improved the transmitting part of the PP SNMR system and designed a discharge circuit for PP pulses. The feasibility of the design was demonstrated through both software simulation and actual testing. When the PP current is 91 A, it can be turned off within 3 ms. Via further water measurements in an electromagnetically shielded room, we demonstrated that a PP system with a PP pulse discharge circuit can effectively increase the initial amplitude of the signal.