| Sensitivity and accuracy mark the minimum resolution of precision measurement devices and the absolute accuracy of measurement results.In some applications,such as medical diagnosis of human magnetic feld,sensitivity is more critical than accuracy,however in metrology,accuracy is extremely important.Cesium-helium magnetometer combines the advantages of high sensitivity of optical pump magnetometer and high accuracy of proton magnetometer.This is because helium atom has zero nuclear magnetic moment,so that there is no nuclear slowing down efect and high-order Zeeman efect,which would guarantee a better accuracy.There were not enough studies of experimental settings,nor suffcient discussions about the theory behind,therefore the performance of cesiumhelium magnetometer are not optimized in many crucial aspects.In this paper,we report our study of laser-pumped Cs-4He magnetometer performance referring to the magnetic resonance sensitivity by controlling the density of each reacting particles in cesium-helium plasma and its physics behind.1.We placed our Cs-4He magnetometer in a uniform magnetic feld enviroment,where the geomagnetic feld was adjusted by the Helmholtz coils,which keeps uniformity better than 0.02 n T within 10 cm of the working area.Therefore,we can ignore the efect of magnetic feld gradient on Larmor precession of metastable helium atom.2.We test the magnetic resonance performance under diferent temperature,high level voltage,low level voltage and pulse duration.Our results show that,(1)the increase of high level voltage can be used to signifcantly improve the signal strength without afecting the linewidth too much.(2)The increase of the low level voltage will allow the amplitude of signal to have an optimal value without afecting the linewidth.We also demonstrate that the conclusion found by previous study that the linewidth of metastable helium atoms decreases with low level voltage,only when metastable helium atomic particle population density is low.The increase of low-level voltage brings in more free electrons that can help for polarization transform,but after some point,ambipolar difusion efect gets too big to push free electrons hitting on the glass wall.(3)The pulse duration would give an optimal resonance amplitude,but the linewidth increases with the pulse duration since the metastable helium atomic density is increased.This means that the product of the maximum resonance signal and the pulse repetition frequency changes with temperature,and the higher the temperature,the greater the product value is.Our study provides experimental foundation for improving the sensitivity and accuracy of cesium-helium magnetometer.3.The increase of temperature can efectively improve the sensitivity of cesium-helium magnetometer by increasing atomic density without afecting the coherent time.4.We also optimized the modulation signal of RF coils and found the minimum linewidth is 550 Hz with 230 Hz modulatioin frequency and 0.005 m V modulation amplitude.5.In the end we did calculations to fnd the intrinsic linewidth caused by Penning ionization(117 Hz)and spin-exchange collision(328 Hz).The intrinsic linewidth is 445 Hz.6.To eliminate the linewidth broadening due to RF coils,we designed an Bell-Bloom cesium-helium atomic magnetometer.The optimal experimental conditions for Bell-Bloom cesium-helium magnetometer are: temperature 302 K,modulation frequency 28 Hz,magnetic feld intensity 0.1 n T.The results provide a reference for further improving the performance of Cs-4He magnetometer and ofer possibilities for more accurate physical constants measurements,such as the gyromagnetic ratio. |
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