Research On A Geomagnetic Measurement System Based On The Resonance Fluorescence Of Sodium Atoms

There are abundant water resources on the earth and most of them are saline water in the ocean.The oceans of the whole world occupy 71%of the earth's surface,and contain about 1.413×1018kg of medium concentration electrolyte aqueous solutions.The chemical composition of seawater is complex.There are 19g of chloride ion,11g of sodium ion,1.3g of magnesium ion and 0.9 g of sulfur(mainly SO42-)in 1kg seawater.The conductivities of these salt ions(anions and cations)are very high,reaching 2.5-6.0S/m.With the movement of salt ions in the ocean current,ocean magnetic fields will be generated.Monitoring the sea surface magnetic field is of great significance in geophysical mapping and underwater magnetic anomaly detection.At present,the conventional method of magnetic field measurement is to place the sensor at the position to be measured.This method has the disadvantages of high cost,obviously observed and limited by the interference from the carrier,and the locally stored data involves the problem of power consumption.This calls for a kind of remote sensing measurement method and equipment for ocean surface magnetic field.In order to solve this problem,we propose a remote measurement scheme of sea surface magnetic field,which is based on the resonance fluorescence of sodium atom.The remote detection system is a synchronous optically pumped magnetometer with Bell-Bloom configuration based on Na atoms.It is an all optically driven and measured scheme.No physical connection exists between the light emission end,the light measurement end and the Na atoms at the position to be measured.This structure is very suitable for the purpose of remote surveying.Magnetic resonance signals based on the remote sodium cell have been successfully detected.Because the Na optically pumped magnetometer is based on the polarization effect of Na atoms in the ground state,.and there are only abundant Na+ions and few Na atoms in the salt spray on the sea surface,it is impossible to arrange the Na optically pumped magnetometer directly.We use the laser-induced breakdown spectroscopy(LIBS)technique to break down the salt spray to generate laser-induced plasma,and the ground state Na atoms will be generated during the evolution of the plasma.In our laboratory,we simulated the propagation process of ultrafast laser pulses in the diffusion layer at the interface between seawater and atmosphere,which made the transparent optical medium salt fog break down.We invented a simple and reliable laser-induced plasma resonance absorption method,quantitatively analyzed the concentration and evolution process of Na atoms in laser-induced plasma,and established a mathematical model of evolution of single pulse-induced plasma,which was in good agreement with the experimental observation results.We have made and built a Na atom optically pumped magnetometer and a laser-induced plasma system.Our experiments have proved that it is feasible to use this system to measure the sea surface magnetic field by remote sensing.