Recently, the space science creative team "Polar Ionosphere-Magnetosphere Coupling" research group has achieved a series of research results in plasma diagnosis and Langmuir probe device development using the independently developed vacuum chamber platform, which is of great significance and value for the application of plasma electrostatic probe diagnosis technology and the research and development of related loads for space plasma detections.

Fig. 1. (a) The 1-D schematic diagram of virtual cathode potential structure near a thermal cathode in a vacuum. (b) An ideal I–V curve of emissive probe in a vacuum.

Fig. 2. (a) The relationship between the potential well depth of the virtual cathode near the emissive probe filament and the filament heating current as well as its 1-D PIC simulation results. (b) The relationship between the position of the virtual cathode near the emissive probe filament and the filament heating current as well as its 1-D PIC simulation results.

Research on plasma emissive probe diagnostic technology. The emissive probe is usually considered to be superior to the traditional Langmuir probe in the accurate measurement of the plasma potential. However, during the emissive probe diagnostic, the spatial distribution of the thermoelectron is formed near the filament surface due to the difference in the initial energy of the thermoelectron generated by the heated filament, which is also called "virtual cathode" (as shown in Fig. 1). The "virtual cathode" is one of the most important measurement errors in the emissive probe diagnostic of the plasma potential. Recently, the research group has developed a 1-D model of thermionic emission and derived the analytical expressions of the potential barrier and the spatial size of the virtual cathode with the assumption that the initial velocity of the thermoelectron is Maxwell distribution. The theoretical model has been verified experimentally in the vacuum chamber platform independently developed by the team. It is first measured that the depth of the virtual cathode potential well generated by the tungsten filament is about the magnitude of volts and the spatial size is about the magnitude of millimetres. The experimental results are also consistent with the numerical results obtained by a 1-D PIC simulation, as shown in Fig. 2. This part of the research work was jointly completed by Associate Researcher Li Jianquan and Postdoctoral candidate Li Shuhan, and the results were published on VACUUM.

Development of miniaturized Langmuir probe. In view of the large dynamic range and small amplitude of the current collected by the Langmuir probe in space plasma detections, the team members designed and developed a set of nonlinear micro-current acquisition devices. The current collected by the Langmuir probe can be nonlinearly amplified, which greatly improves the dynamic range of the acquisition on the premise of ensuring the accuracy of weak current acquisition. The independently developed device adopts a miniaturization design, has a small volume and low power consumption, and can be used to measure plasmas with electron density ranging from 10⁸-10¹³ m^-3. Most importantly, it can also be installed on micro-nano satellites to achieve a refined detection of the ionosphere. The related result has been published on Sensors and Materials. Wang Jin, PhD student at Shandong University, is the first author, and the corresponding author is Professor Zhang Qinghe, his supervisor.

Experimental validation of Langmuir probe. The research group has carried out plasma diagnostic experiments using Langmuir probe with cold and hot conditions in the vacuum chamber platform, and the results show that the adsorption of atoms and molecules on the surface of Langmuir probe can significantly affect the plasma diagnostic results. The plasma potential, electron temperature and electron density obtained by the unheated Langmuir probe can be underestimated by about 10%, 15% and 30%, respectively, compared with the plasma diagnostic results obtained by the clean thermal Langmuir probe. This part of the research work was published in the Journal of Vacuum Science & Technology A. Li Jianquan, Associate Researcher of Shandong University, is the first author, and the corresponding author is Lu Wenqi, Associate Professor of Dalian University of Technology. The publication of the results also attracted the attention and further guidance of V. A. Godyak, an internationally renowned expert in Langmuir probe diagnostic.

In recent years, based on the multidisciplinary advantage of Shandong University, the research group has independently developed a vacuum chamber experimental platform and established the Space Plasma Detection Laboratory through cooperation with the School of Mechanical, Electrical & Information Engineering. With Professor Zhang Qinghe as the scientific PI and the technical PI Du Qingfu, Professor of the School of Mechanical, Electrical & Information Engineering, the research team has jointly developed a prototype of needle-type Langmuir probe which is suitable for tiny satellites, published 4 SCI papers and applied for 9 national invention patents. At present, the group has signed an agreement with Minospace Technology Company to carry a satellite and plans to conduct the satellite experiment this summer. It is expected to obtain independent observation data in the second half of this year. After the success of the onboard experiment, the group will strive to carry satellites with more Langmuir probe loads to support independent observation data for China’s space physics and space weather research, modelling and prediction.

These studies were supported by the Innovation Team Construction Fund of Shandong University’s PanDeng Program, as well as the major international (regional) joint research project, General Project and Youth Project of the National Natural Science Foundation of China.

Related papers and the links are as follows (* means corresponding author):

[1] Li Shuhan and Li Jianquan*. Studies of virtual cathode characteristics near thermionic emission cathodes in a vacuum. VACUUM, 2021, 192: 110496. https://doi.org/10.1016/j.vacuum.2021.110496

[2] Li Jianquan, Xie Xinyao, Li Shuhan* and Zhang Qinghe. Reliable potential and spatial size of virtual cathode obtained by an emissive probe with accurate filament temperature in a vacuum. VACUUM, 2022, 200: 111013. https://doi.org/10.1016/j.vacuum.2022.111013

[3] Li Jianquan, Zhang Qinghe, Xing Zanyang and Lu Wenqi*. Comparative studies of cold/hot probe techniques for accurate plasma measurements. Journal of Vacuum Science & Technology A, 2022, 40(3): 033001. https://doi.org/10.1116/6.0001461

[4] Wang Jin, Zhang Qinghe*, Du Qingfu, et al. Nonlinear micro-current acquisition device applied to onboard Langmuir probe instrument. Sensors and Materials, 2021, 33(12): 4157-72. https://doi.org/10.18494/sam.2022.3626