Non-Steady State NMR Effect and Application on Time-Varying Magnetic Field Measurement

Abstract

The measurement of a time-varying magnetic field is different from a constant magnetic field, due to its field intensity variation with time. Usually, the time-varying magnetic field measurement converts the solution of the magnetic induction intensity into the calculation of the induced electromotive force (EMF); then, the magnetic induction intensity is obtained by the time integration of the EMF, but the process is vulnerable to external interference. In this paper, a non-steady state nuclear magnetic resonance (NSS-NMR) scheme for the measurement of a time-varying magnetic field is proposed. In a time-varying magnetic field environment, an RF excitation signal with a certain frequency bandwidth is applied to excite the nuclear spin system. The NSS-NMR signal, which varies with time in the frequency range corresponding to the frequency bandwidth of the RF excitation, could finally be obtained after a series of processing of the probe output signal. During the NSS-NMR experiment, an orthogonal dual-coil probe is adopted to synchronously generate the RF excitation and induce the probe output signal. Moreover, a directional coupler that utilized in the experiment outputs a reference signal from the coupling port for the subsequent signal processing. The experimental results show that the weak NSS-NMR signal is indeed observed. The longitudinal time-varying magnetic field ranges from 0.576 T to 0.582 T, which is inverted by the Larmor precession relationship, have been successfully detected based on the so-called NSS-NMR effect.

A Non-Steady State NMR Effect Based Time-Varying Magnetic Field Measurement Method and Experimental Apparatus

“…H. Ma et al have proposed another longitudinal time-varying magnetic field measurement scheme which is based on the so called nonsteady state nuclear magnetic resonance (NSS-NMR) effect [13], [14], [15]. In 2022, H. Ma and X. Zeng had performed the NSS-NMR experiment by exciting the spin nuclei with a band-limited white Gauss signal and acquired the weak NSS-NMR signal [16]. In this paper, a set of TVMF measuring apparatus is developed.…”

“…The above equivalent circuit parameters could be extracted as following [16]. Firstly, applying constant current or static charge on the turn loops of the two coils, calculating the spatial electric field distribution and magnetic field distribution by the finite element method (FEM) [24], then three-dimensional numerical integrating the electric field energy and magnetic field energy distributed in the region of the whole orthogonal dual-coil structure, finally establishing the linear equations among the field energies and the circuit parameters and then solving the equations to obtain the parameter extraction results which are shown in Table Ⅰ.…”