【作者】 杨鹏；

【导师】 马洪；

【作者基本信息】 华中科技大学 ， 信息与通信工程， 2020， 硕士

【摘要】 脉冲强磁场与磁体装置为物质特性研究提供了新的技术手段,对脉冲磁场强度的精确测量可以为磁体设计与优化提供有力支撑。因为脉冲强磁场持续时间短、场强变化速率快,基于常规脉冲核磁共振的磁场测量技术无法获得场强的连续时变特性。此时可采用非稳态核磁共振测量技术。该技术是在纵向时变磁场环境下,持续对特定的原子系综施加宽带射频连续激励从而激发其非稳态进动和章动过程,由此产生非稳态核磁共振信号。通过对非稳态核磁共振信号进行采集和处理计算其时频图,然后根据拉莫尔进动关系最终获得纵向磁场时变特性。为了实时采集与分析非稳态核磁共振信号,本文对非稳态核磁共振实验系统中若干关键技术展开研究。本文首先分析并研制一套宽带正交双线圈核磁共振探头,并提出通过三阶Π型匹配网络扩展其阻抗匹配带宽和隔离度带宽。然后对非稳态核磁共振实验中的信号处理方法进行设计和优化,提出了变步长维纳自适应干扰抵消算法来大幅度抑制发射泄漏信号以提高所接收信号的信干比,并借鉴短时傅里叶变换思想,设计了基于子空间分解的短时MUSIC谱估计方法和平滑伪魏格纳-威尔变换法对非稳态核磁共振信号进行时频分析,从而改善时变磁场的场强反演分辨率和精度。最后,本文使用LabVIEW和Matlab混合编程技术研制了一套非稳态核磁共振信号实时采集与信号处理实验系统。该系统将射频发射/接收、双路信号同步高速采集、常规脉冲核磁共振信号处理、非稳态核磁共振信号处理等功能集为一体。本文利用该系统进行非稳态核磁共振实验,成功检测到纵向时变磁场下氢核的非稳态自由感应衰减信号,并由此反演出纵向磁场的时变律。更多还原

【Abstract】 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 Π-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.更多还原