Research On Overhauser Effect Magnetic Sensors

As an effective method to detecting material properties and unknown world,weak magneticfield measurement has significant application in many areas such as geophysics magnetic measurement,marine magnetic measurement,space weakness magnetic field measurement and so on.The structure and properties of all weak magnetic field measurement sensors are differented with each other because of the different measurement principle.So their applications have difference to some extent.Overhauser magnetometer sensor is a kind of weak magnetic field measurement instrument which is based on radical dynamic nuclear polarization effect,and has the advantage of high accuracy,high sensitivity,low power consumption and continuous measurement.So it is widely used in geophysics,aeromagnetic survey,satellite magnetic field measurement,military,and various engineering fields,etc.The essence of dynamic nuclear polarization effect is double resonance phenomenon between electron system and proton system in free radical solution,which is Overhauser effect of electron and nucleus.The principle is to use Radio Frequency(RF)electromagnetic waves to excite the electron system in the solution.Due to the coupling,the electron system transfers energy to the proton system,so that the proton system is also in an excited state,thereby generating an electron-proton double resonance phenomenon.Compared with exciting proton system directly,the dynamic nuclear polarization effect can more greatly enhance proton polarization.This method can also increase the signal-to-noise ratio of output signal and improve the precision of the magnetic field measurement.The purpose of this dissertation is to study the principle of magnetic field measurement based on the dynamic nuclear polarization effect of radicals and the key technology of sensors,and to develop overhauser sensor prototypes and related test devices for the excitation of sensors in combination with modern electronic technologies.The paper introduces the classification and development trend of weak magnetic measurement sensors,analyzes the research status and inadequacies of proton sensor,and proposes advantages of the Overhauser sensor based on dynamic nuclear polarization method,and introduces the research status at home and abroad.At the same time,based on the theory of modern physics,the principle of using dynamic nuclear polarization to enhance the Free Induction Decay(FID)signal is further analyzed.Based on the theoretical analysis,the method of calculating the electromagnetic field in the sensor is studied and introduced.Based on the theory of dynamic nuclear polarization and the working mode of pulse-excited Overhauser sensor,the overall structure of the sensor is analyzed and introduced.The selection of free radicals and organic solvents in the sensor is studied and analyzed.The resonant frequency of the Free radicals solution is identified.Based on the principle of dynamic nuclear polarization effect magnetic field measurement and the selected free radical solution composition,a further study is performed on the resonant cavity structure suitable for Overhauser sensors.The advantages of capacitance-loaded coaxial resonant cavity in reducing the size of the sensor are studied and analyzed.The resonant cavity is modeled and simulated by software.According to the simulation results,the problem that the resonant frequency is difficult to match with the selected free radical species is analyzed based on the limitation of the capacitor-loaded coaxial resonant cavity.A cage based on the cage is proposed.The improved resonant cavity structure and the excitation method have been theoretically analyzed through the equivalent circuit method.The simulation and analysis of the resonant cavity has been performed through the COMSOL software.The simulation results show that the resonant frequency of the improved birdcage resonant cavity can be selected.The resonance frequency of the free radical solution is consistent with the resonant cavity,which verifies the feasibility of the resonant cavity design.The conductor material that composes the resonant cavity is analyzed and studied,and the silver foil is selected as the conductor of the resonant cavity.For the problem that the output signal of the sensor is weak and susceptible to noise interference,a balanced signal receiving coil and a silver shielding case are designed to reduce the effect of external electromagnetic interference on the output FID signal of the sensor.The coil parameters and the magnetic field characteristics of DC current are analyzed and simulated.Noise sources of the output signals of the receiver coil and the sensor are analyzed.The method of using the serial resonance to improve the signal-to-noise ratio of the FID signal is studied,and prototype of the sensor is designed and fabricated.In order to study the parameters of excitation time sequence,RF and DC excitation signals,a sensor excitation test platform is designed.The influence of the FID signal quality is tested and the excitation time sequence with 3s measurement interval is determined.It provides an experimental platform and reference for further selecting the time sequence of the shorter measurement interval and the parameters of the excitation signal.At the same time,the paper carried out corresponding magnetic field measurement experiments on the prototype of the sensor.According to the experimental data,the standard deviation of the sensor prototype and foreign commercial sensor products under artificial magnetic field is calculated.The experimental results are compared and analyzed,so that the uncertainty of the prototype and commercial sensor are obtained.Comparison test between the prototype and commercial sensor has been conducted in the field environment.It is verified that the prototype is close to mature commercial products in terms of output signal quality,measurement uncertainty and the ability to capture abnormal changes in magnetic field,which also validates the expected results in the sensor structure design and the study of key technologies.At the end of the thesis,the main contents and results of the research are summarized and the future research work is prospected,which will provide a certain reference for subsequent research.