Research On Key Technologies In Non-steady State Nuclear Magnetic Resonance Experimental System

The pulsed strong magnetic field and magnet device provide new technical means for the study of material properties,and the accurate measurement of the pulsed magnetic field strength can provide strong support for magnet design and optimization.Because of the short duration of the pulsed magnetic field and the fast rate of field strength change,the magnetic field measurement technology based on conventional pulsed NMR cannot obtain the continuous time-varying characteristics of the field strength.At this time,non-steady-state NMR measurement technology can be used.This technology is to continuously apply broadband radio frequency continuous excitation to a specific atomic ensemble under a longitudinal time-varying magnetic field environment to stimulate the non-steady state precession and nutation process of the atomic ensemble,thereby generating a non-steady state NMR signal.The time-frequency diagram of the non-steady state NMR signal is collected and processed,and then the time-varying characteristics of the longitudinal magnetic field are finally obtained according to the Larmor precession relationship.In order to collect and analyze the non-steady state NMR signal in real time,this paper studies several key technologies in the non-steady state NMR experimental system.This paper first analyzes and develops a set of broadband orthogonal dual-coil NMR probes,and proposes to expand its impedance matching bandwidth and isolation bandwidth through a third-order ?-type matching network.Then,the data processing method in the non-steady state NMR experiment was designed and optimized,and the variable step size Wiener adaptive interference cancellation algorithm was proposed to greatly suppress the transmitted leakage signal to improve the signal to interference ratio(SIR)of the received signal.Based on the ideas of short-time Fourier transform,a short-time MUSIC spectrum estimation method based on subspace decomposition and a smooth pseudo-Wigner-Wille transform method are proposed for time-frequency analysis of non-steady state NMR signals.,Thereby improving the resolution and accuracy of the field strength inversion of the time-varying magnetic field.Finally,this paper uses LabVIEW and Matlab mixed programming technology to develop a set of non-steady state NMR signal real-time acquisition and data processing experimental system.The system integrates functions such as radio frequency transmitting/receiving,dual-channel signal synchronous high-speed acquisition,conventional pulse NMR data processing,and non-steady state NMR data processing.This paper uses this system to perform non-steady state NMR experiments,and successfully detects the non-steady state free induction decay signal of the hydrogen under a longitudinal time-varying magnetic field,and inverts the time-varying law of the longitudinal magnetic field.