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 X-ray 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 X-ray source technology, X-ray fluorescence analysis instruments using miniature X-ray tubes as excitation sources are becoming a new generation of in-situ elemental analysis technology in deep space exploration. NASA has successfully used a miniature micro-focus X-ray tube in the X-ray fluorescence spectrometer on the Curiosity Mars rover launched in 2020. In contrast, the research on miniature X-ray sources in China starts relatively late. Limited by the research progress of miniature X-ray sources, there is currently no space X-ray fluorescence spectrometer based on miniature X-ray tubes in China. In order to provide technical support for China's future deep space exploration program, an integrated miniature X-ray source is developed as the excitation source for X-ray fluorescence analysis.

The integrated miniature X-ray source includes a miniature micro-focus X-ray tube and a miniature high-voltage power supply (HVPS). In view of the requirements of resources and mechanical properties in the aerospace environment, the miniature micro-focus X-ray tube is designed as an end-window transmission-target metal-ceramic X-ray tube using heated tungsten filament. The miniature HVPS is a negative HVPS using a Cockcroft Walton voltage multiplier. Due to the demand for X-ray tubes, isolation transformers for filament power supply and high-voltage feedback circuits have been added. A new type of electrostatic focusing lens used for unipolar X-ray 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.

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 60 μm×227 μm is obtained. The sealed X-ray tube is made after structural processing and vacuum sealing. After that, the X-ray tube and the HVPS are potted in a highly insulating material to prevent arcing (Fig. 5). The size of the developed miniature integrated X-ray source is 118 mm×76 mm×42 mm. An X-ray source performance testing platform is set up. The X-ray energy spectrum test results show that the working voltage of the miniature integrated X-ray source is adjustable between 2-50 kV (Fig. 6). The intensity stability test results show that the output X-ray intensity increases with time, which is mainly caused by the increase in temperature of the miniature X-ray source after working for a long time. During a test lasting for 45 min, the output X-ray 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 X-ray source is tested by using the pinhole imaging method. The variation of X-ray 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 X-ray source is 50 kV, the focal spot size (or FWHM) is 177 μm×451 μm (Fig. 9). When the high voltage is 50 kV and the tube current is 50 μA, and the total power consumption of the X-ray source is 5 W.

In this paper, a new type of electrostatic focusing optical structure is proposed. On this basis, a miniature integrated micro-focus X-ray source is developed. The test results verify the performance of the new electrostatic focusing structure and the miniature integrated micro-focus X-ray source. The developed miniature integrated X-ray source can provide load technical support for China's future planetary exploration program. The development of the miniature integrated micro-focus X-ray source is of great significance for China to carry out in-situ analysis of planetary surface material composition. In the future, we will further reduce the size of the miniature integrated micro-focus X-ray source through structural optimization design to meet the needs of different space applications.