Nuclear Magnetic Resonance Technology Under Flat-Top Pulsed High Magnetic Field

Pulsed-high-Magnetic Field Nuclear Magnetic Resonance（PMF-NMR）measurement method has great advantages in both the physical property manipulation ability of pulsed high magnetic field and the atomic-level microscopic detection ability of NMR.It can not only significantly improve the signal-to-noise ratio（SNR）,but also create more possibilities for for the discovery of high-field-induced exotic states of matter.However,in practical applications,there are still shortcomings such as low spectral line quality and low synchronization efficiency between the magnetic field and spectrometer.The fundamental reason is that the stability of the pulsed magnetic field is much lower than that of the steady-state magnetic field.At present,Wuhan National pulsed high Magnetic Field Center（WHMFC）has achieved a flat-top pulsed magnetic field（FTPMF）with the highest parameter of 64 T/10 ms,which has unique advantages for solving the problem of magnetic field fluctuation existing in PMF-NMR technology.Therefore,based on FTPMF condition,this paper has carried out in-depth research on PMF-NMR detection technology,independently developed a high-stability flat top PMF-NMR test system,and optimized PMF-NMR experimental environment by using the quasi-steady state characteristics of FTPMF.It provides a unique platform to carry out basic scientific research under extremely high magnetic field conditions.In terms of background magnetic field,a three-coil coupling type FTPMF model is proposed.Through the scheme of"large-scale clipping and small-scale filling",the original open-loop adjustment method is optimized into a closed-loop control system.The compensation process in the platform period is described,and the coupling model of electrical,magnetic and thermal multi-physics is established to compare and analyze the optimization effect before and after improvement.The 35 T pulse magnetic field experiment shows that the adjustment ability of the compensation coil is greater than 1 T and the adjustment accuracy can reach 0.01 T.Through equivalent experiments,it is verified that the model can generate FTPMF with the parameter of 62.3 T/9 ms/50 ppm,and the stability is 1～2 orders of magnitude higher than before.At the same stability and magnetic field intensity level,the duration is nearly 5 times higher than that of similar technologies,which provides better acquisition conditions and more sufficient polarization time for PMF-NMR experiment.As far as spectrometer,this thesis develops a high-frequency NMR spectrometer highly adapted to the pulse strong magnetic field environment.and deployed the PMF-NMR modular spectrometer model based on the software and hardware co-design.According to the task flow and asynchronous sequential logic,the distributed,multi-controller,cross-clock domain hierarchical control program and the aggregated human-computer interaction interface of PMF-NMR are written.The key devices such as RF phase shifter,RF modulation switch,broadband active time division duplexer and low noise pre-amplifier are homemade independently.Finally,the PMF-NMR spectrometer with the maximum transceiver frequency of 2.7 GHz,real-time bandwidth of 20 MHz,I/Q sampling rate of100 MS/s,RF phase shift angle of 0～2π,timing control accuracy of 0.01μs,and isolation between the receiving and transmitting ends higher than 100 d B is achieved,which lays a solid foundation for the independent development of PMF-NMR technology.In the construction of the experimental station,the cross-clock tree timing synchronization system between the magnetic field and the spectrometer is designed to realize the instantaneous linkage of PMF-NMR field and frequency,and the device coordination delay is less than 5μs.A long and narrow PMF-NMR probe equipped with dual-inductance tuning circuit is developed,which realized the functions of temperature control,magnetic field monitoring,signal detection and tuning and matching in the probe with a radial width of less than 10 mm.The PMF-NMR magnet,spectrometer,probe device and WHMFC supporting power supply,control and low-temperature facilities are integrated,and the first PMF-NMR test platform in China has been built.The platform is connected to the WHMFC main control system,realizing the resource sharing and orderly cooperation of the PMF-NMR experimental station and other scientific experimental stations.Based on the above work,a series of experiments are carried out.Firstly,the ⁶³Cu-NQR signal in Cu₂O sample is captured without magnetic field,proving that the homemade PMF-NMR spectrometer works well.Then,the ¹H-NMR signal in the YCu₃（OH）_6.5Br_2.5material is measured based on the self-made PMF-NMR spectrometer and probe in a 7 T steady-state magnetic field,which verified that the special probe can complete the functions of tuning,detection and temperature control.Furthermore,the Pick-up coil is pre-calibrated by the ESR phenomenon to obtain the experimental window.Then in the 22 T pulsed high magnetic field,the ⁹³Nb-NMR signal was successfully captured exciting a single spin-echo sequence.Through a series of fixed-frequency sweep field experiments,it is proved that the adjustment accuracy of the magnetic field can reach 0.01 T,which is the most accurate pulsed magnetic field used in NMR experiments at present.Furthermore,the thesis compares the signal acquisition of the multi-pulse sequence excitation under the normal pulsed high magnetic field and FTPMF environment.It is observed that in the quasi-steady environment provided by FTPMF,the resonance excitation efficiency can reach 100%and the reliability and controllability are much better than that of ordinary pulsed high magnetic fields.All the above work confirms the correctness and effectiveness of the technical route proposed in this thesis to carry out quasi-steady PMF-NMR experiments based on FTPMF.