Ac Impendence Calculation Of Radio Frequency Coils In The Single-side Nuclear Magnetic Resonance Sensor

The single-side nuclear magnetic resonance(NMR)system is widely used in petroleum logging,soil moisture measurement,food quality detection,material characteristics analysis and other fields as a non-destructive testing method.Compared with traditional closed NMR system,it has the advantages of high openness and portable mobility.The radio frequency coil is the core structure in the single-side NMR sensor,which is responsible for the excitation and reception of RF signals.The signal-to-noise ratio(SNR)of the NMR sensor is an important index to measure its detection quality.The SNR is mainly affected by the main magnetic field,RF magnetic field and equivalent AC resistance of RF coils.Different with medical NMR system,the equivalent RF coil impedance is the main influencing factor influencing the SNR rather than resistance of the tested samples.The AC impedance of the RF coil is a crucial parameter in the optimization process of the RF coil.Single-sided NMR sensors generally work in the frequency range of hundreds of k Hz to several MHz.At high frequencies,the current on the cross-section of multi-turn coils is no longer uniformly distributed due to the effect of skin effect and proximity effect,and the current is concentrated in a narrow area near the coil surface.Because the single-side RF coils generally use PCB structure,coils are only tens to hundreds of micrometers in thickness dimension,and tens to hundreds of centimeters to meters in length dimension,and the model has no cylindrical symmetry(Analytic algorithm based on the separation variable method can't be used).This makes the calculation of the AC impedance of the multi-turn spiral RF coil a very complicated electromagnetic field problem and it's difficult to obtain accurate analytical solutions.The finite element method(FEM)which is commonly used at present occupies too much calculation resources and time due to the ultra-thin structure of the coil and the high operating frequency,which seriously affects the optimization efficiency of the RF coil.In this paper,we use a simplified partial equivalent element circuit method(PEEC)named Equivalent Filament Circuit Method to calculate the AC impedance of the RF coil in single-side NMR systems.Compared with the traditional partial equivalent element circuit method,the number of subdivision units is greatly reduced and the calculation efficiency is improved.The calculation takes into account the influence of skin depth and proximity effect at high frequency.The calculation process is simple and easy to operate,the speed is greatly improved compared with the finite element method,and the calculation time of a single coil is shortened to the second level.It is suitable for plane and arc RF coil structure,and can analyze the influence of near non-ferromagnetic infinite metal object on coil impedance.The calculation results of this method are about1% error compared with FEM and measurement,and the calculation time is greatly reduced compared with FEM.The main contents of this paper are as follows:(1)The classical PEEC method is simplified on the basis of the characteristics of the RF coils in single-side NMR systems,and the AC impedance of the RF coil is calculated by using the Equivalent Filament Circuit Method,which greatly reduces the subdivision units compared with the classical PEEC method.(2)Considering the influence of the presence of the near non-conductive infinite metal plate on the AC impedance of RF coil,the metal plane is subdivided according to the distribution characteristics of induced eddy current in the metal.The AC impedance of RF coil under the influence of eddy current is calculated by coupling the influence of eddy current to the equivalent circuit of RF coil by mutual inductance.(3)In order to accurately simulate the skin effect and proximity effect at high frequency,unequal subdivision strategy is applied to the RF coil and metal plane.This paper analyzes the influence of the division density on the calculation error,and provides experience for the operation of the Equivalent Filament Circuit Method.(4)The impedance calculation results of the Equivalent Filament Circuit Method are compared with the FEM and the measured results to verify the accuracy and speed of the method.The error is about 1% compared with FEM and measurement.It takes only 15 seconds while the two-dimensional FEM takes 240 seconds to calculate the same coil on the same computer,and the speed superiority is more obvious than the three-dimensional finite element.Finally,the structure of RF coil is optimized using the Equivalent Filament Circuit Method and the trend of SNR changing with coil pitch is analyzed.