Radiofrequency Coil With Low Electric Field Design For NMR Spectrometer At High Fields

In NMR experiments,the radiofrequency(RF)excitation electromagnetic field to which samples are subjected includes a magnetic field B1 and an electric field E1.The resonance effect usually refers to the disturbance generated by the oscillating B1 field,while the electric field E1 component in the RF field does not contribute to the excitation of the sample.The electric field is one of the main factors that generate heat by RF excitation.In NMR experiments at high fields,the frequency of RF is high and the power of RF is high.In some experiments,complex pulse sequences of long pulses and multiple pulses are used.Other experiments often require thousands of pulse sampling.Continuous excitation of the RF pulse will generate a large amount of heat on the sample,which cause the actual temperature of the sample to rise.These will produce errors in the detection,affect the reaction mechanism analysis.The temperature of the experimental sample rises sharply,which also destroys the structure of the experimental sample,affects the accuracy of the experimental results and shortens the life of the sample.In in-situ electrochemical-nuclear magnetic resonance experiments,the electric field of the RF pulse in accumulation experiments may have an effect on some transients and results of in-situ detection.Therefore,the heat generated by the RF pulse in the sample is one of important problems that NMR accumulation experiments at high fields needs to be faced.Minimizing the E/B ratio in the sample area is one of important goals of the design of NMR coils.Based on the theory of magnetic resonance,electromagnetic field shielding and RF coil theory,we propose and develop a structure of the low electric field of the saddle coil which is suitable for NMR spectrometers at high fields using CST,a finite element analysis electromagnetic simulation software modeling and simulation aided design.Firstly,we simulate the rectangular strip-shaped shielding layer of the solenoid coil,the hollowed-out annular shielding layer of the double-layered saddle coil,the hollowing guard ring combined with annular shielding layer of the AG coil.Secondly,on the basis of this,the variation law of the internal electromagnetic field of the coil caused by the change of the shielding parameters of the axial coil and the transverse coil are explored and the optimal shielding parameters are selected for the three coils.Finally,a low electric field of the double-layer saddle coil of 500MHz NMR spectrometer is developed for 195Pt in the low temperature of solid-phase EC-NMR experiments.The research results show that the proposed coil structure can effectively reduce the electric field component of the RF field and improve the quality of EC-NMR experiments.The average electromagnetic ratio of the solenoid coil after the addition of the rectangular strip-shaped shielding layer is reduced by 85.2%;the average electromagnetic ratio of the double-layer saddle coil is reduced by 92.9%after adding the hollow-out annular shielding layer;the average electromagnetic ratio of the AG coil added with hollow guard ring combined with annular shielding layer is reduced by 99.7%.