用于行星原位分析的微型微焦斑X射线激发源

Miniature Micro-Focus X-Ray Source for In-Situ Elemental Analysis in Planetary Exploration

基于微型X射线闭管的X射线荧光谱仪是深空探测新一代元素就位分析技术,研制了一款微型微焦斑X射线闭管作为X射线荧光分析的激发源。设计了一种新型的单极静电聚焦透镜,仿真模拟了电子聚焦光学结构尺寸对电子束运动轨迹的影响,并对静电聚焦结构的形状和尺寸进行了优化。完成了微型一体化X射线源的加工和装调,搭建了X射线源性能测试装置。微型一体化X射线源工作电压2~50 kV、X射线强度不稳定性为0.3%、高压不稳定性为0.21%。在高压50 kV、电子电流50μA的情况下,微型一体化X射线源的总功耗5 W,焦斑尺寸177μm×451μm,可以满足深空探测行星表面物质成分原位分析需求。

Objective The measurement of the chemical composition of rock and soil on planetary surfaces is of great significance to the study of the composition,geological history,and life information of planets,and it is a basic requirement for planetary exploration.In the deep space exploration missions carried out by humans in the past,chemical composition measurement mainly relies on the Alpha particle Xray spectrometer(APXS)based on radioisotope activation technology.Although the APXS has been successfully applied,the technology also has shortcomings such as limited types of measurement elements,long spectrum measurement time,and high radiation leakage risks.In recent years,with the rapid development of miniature Xray source technology,Xray fluorescence analysis instruments using miniature Xray tubes as excitation sources are becoming a new generation of insitu elemental analysis technology in deep space exploration.NASA has successfully used a miniature microfocus Xray tube in the Xray fluorescence spectrometer on the Curiosity Mars rover launched in 2020.In contrast,the research on miniature Xray sources in China starts relatively late.Limited by the research progress of miniature Xray sources,there is currently no space Xray fluorescence spectrometer based on miniature Xray tubes in China.In order to provide technical support for China′s future deep space exploration program,an integrated miniature Xray source is developed as the excitation source for Xray fluorescence analysis.Methods The integrated miniature Xray source includes a miniature microfocus Xray tube and a miniature highvoltage power supply(HVPS).In view of the requirements of resources and mechanical properties in the aerospace environment,the miniature microfocus Xray tube is designed as an endwindow transmissiontarget metalceramic Xray tube using heated tungsten filament.The miniature HVPS is a negative HVPS using a Cockcroft Walton voltage multiplier.Due to the demand for Xray tubes,isolation transformers for filament power supply and highvoltage feedback circuits have been added.A new type of electrostatic focusing lens used for unipolar Xray tubes is designed.The passive electron optics design is achieved by shaping of metal components of the tube.The structure of the electrostatic focusing lens includes an electron suppression groove and two orthogonal stacked focusing grooves.The electron suppression groove located under the tungsten wire can absorb the electrons emitted by the tungsten wire towards the bottom,so as to prevent these electrons that are difficult to focus from reaching the anode.The two focusing grooves are located between the filament and the anode.The focal spot size of the electron beam can be adjusted by changing the length and width of the two focusing grooves.Results and Discussions The optimum shape of the metal components of the electrostatic focusing lens is simulated by using charged particle optical simulation software.Finally,when the high voltage is 50 kV,the simulated focal spot size,or the full width at half maximum(FWHM),of 60μm×227μm is obtained.The sealed Xray tube is made after structural processing and vacuum sealing.After that,the Xray tube and the HVPS are potted in a highly insulating material to prevent arcing(Fig.5).The size of the developed miniature integrated Xray source is 118 mm×76 mm×42 mm.An Xray source performance testing platform is set up.The Xray energy spectrum test results show that the working voltage of the miniature integrated Xray source is adjustable between 2-50 kV(Fig.6).The intensity stability test results show that the output Xray intensity increases with time,which is mainly caused by the increase in temperature of the miniature Xray source after working for a long time.During a test lasting for 45 min,the output Xray intensity instability is 0.30%(Fig.7).The high voltage instability is 0.21%(Fig.8).In order to verify the performance of the new electrostatic focusing lens designed in this paper,the focal spot size of the miniature Xray source is tested by using the pinhole imaging method.The variation of Xray source spot size with high voltage is similar to the simulation results.When the high voltage increases,the focal spot size decreases.When the high voltage of the Xray source is 50 kV,the focal spot size(or FWHM)is 177μm×451μm(Fig.9).When the high voltage is 50 kV and the tube current is 50μA,and the total power consumption of the Xray source is 5 W.Conclusions In this paper,a new type of electrostatic focusing optical structure is proposed.On this basis,a miniature integrated microfocus Xray source is developed.The test results verify the performance of the new electrostatic focusing structure and the miniature integrated microfocus Xray source.The developed miniature integrated Xray source can provide load technical support for China′s future planetary exploration program.The development of the miniature integrated microfocus Xray source is of great significance for China to carry out insitu analysis of planetary surface material composition.In the future,we will further reduce the size of the miniature integrated microfocus Xray source through structural optimization design to meet the needs of different space applications.