Photoelectric Effect for Electrostatic Dust Mitigation in Lunar Exploration

Photoelectric Effect for Electrostatic Dust Mitigation in Lunar Exploration. M. Adachi1, K. Kohara1, and T. Morishita2, 1Department of Mechanical Engineering and Science, Kyoto University, 2Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency Introduction: Lunar regolith, a granular substance that blankets the Moon’s surface, poses obstacles in both human and robotic missions due to its tendency to adhere to exploration equipment and spacesuits, causing deterioration and malfunction. Therefore, cleaning technologies for the regolith are indispensable for long-term lunar exploration. While several cleaning technologies, primarily relying on mechanical or pneumatic methods, are available for terrestrial environments, the Electrodynamic Dust Shield (EDS) is an advanced cleaning technique that uses an electrostatic traveling wave to remove charged particles from target surfaces [1][2]. The cleaning mechanism does not require large power consumption or intermediate fluid materials, and the system can be designed to be compact and lightweight, making it an effective mitigation method during lunar exploration. Although the cleaning performance of an EDS system depends on the charge state of particles, this aspect has not been thoroughly studied. Moreover, developing additional charging methods for deposited particles is critical for practical applications of the EDS when the particles are not sufficiently charged. One such charging method involves UV irradiation in conjunction with EDS systems [3][4]. Demonstrations of this integration have shown that UV assistance improves cleaning efficiency. However, its effects have not been thoroughly and systematically investigated. Since lunar surfaces are naturally exposed to UV irradiation, understanding the mechanism of UV assistance in electrostatic dust mitigation is crucial. In addition, understanding this mechanism allows for the design of supplementary charging systems to enhance dust-cleaning efficiency for lunar applications. In this study, the impacts of UV irradiation and the parameter variations on the charging of particles, as well as the cleaning mechanism in the EDS system, were experimentally investigated. Experimental Setup: Fig. 1 shows the experimental setup, which consists of the EDS system and UV sources in a vacuum chamber. The EDS cleaning substrate was constructed by laminating a glass plate (thickness: 1.1 mm, dimension: 100 × 100 mm². TECHNO PRINT), on which ITO electrodes with a spiral shape were fabricated (thickness: 200 nm, width and pitch: 300 μm and 600 μm, respectively), a clear adhesive sheet (thickness: 50 μm. OCA81462, 3M), and a thin glass plate (thickness: 100 μm. D263Teco, SCHOTT). When high voltages of square waves with phase shifts were applied to the electrodes, the resultant electrostatic traveling wave moved outward, removing the charged particles deposited on the substrate. Lunar regolith simulant FJS-1 was distributed on the substrate surface as evenly as possible, and the substrate was placed in the vacuum chamber. Sizesorted particles with sizes of less than 25 μm, which were found to be difficult to remove in previous studies [5], were used in this study. To eliminatemoisture contained in the particles, they were heated in an oven at 120°C for more than 12 hours before being placed on the substrate. The particle loading was set at approximately 100 mg. After evacuating the air to below 8.0 × 10⁻³ Pa, cleaning experiments were conducted. A high-voltage power supply with four switching circuits for generating square waves was located outside the vacuum chamber. The generated 4-phase high voltages were applied through a feedthrough to the substrate electrodes. This process lasted for 60 seconds to remove particles under various UV conditions. Two types of UV sources were installed in the chamber: UV sources with wavelengths of 115–400 nm and 300–800 nm. The wavelength, irradiation distance from the UV source to the particles, and the irradiation protocol were varied as experimental parameters. One irradiation protocol involved simultaneous irradiation, where particles were irradiated during the EDS activation. The other protocol involved pre-irradiation, where particles were irradiated with UV for 30 seconds before activating the EDS. The cleaning efficiency was evaluated by measuring the weight ratio of the residual particles after cleaning to their initial amount. The experiments were repeated three times for each condition. Fig. 1 Experimental setup for regolith cleaning, employing the EDS system and UV sources. Cleaning Efficiency under UV Irradiation: Fig. 1 shows the cleaning efficiencies in cases with and without UV irradiation. These results demonstrated that most particles smaller than 25 μm, which were considered difficult to remove due to their relatively large adhesion force, were removed by the EDS system with the assistance of UV irradiation. In addition, the UV wavelength played an important role in the improvement. The cleaning efficiency with UV irradiation at a wavelength of 300–800 nm did not improve compared to the case without UV. However, more than 95% of the cohesive particles were removed when UV with a wavelength of 115–400 nm was utilized. This is because the wavelength is crucial for UV energy to exceed the work function of the particles, inducing the photoelectric effect and charging the particles, which significantly enhanced the cleaning efficiency. Fig. 2 shows the effect of the irradiation distance between the UV source and the particles. The distance range was limited by the chamber’s geometry and varied from 100 to 400 mm. Since the photoelectric effect depends on the wavelength but is independent of the irradiation distance, the distance did not significantly affect the cleaning efficiency as observed in these experiments. However, due to the limited range of irradiation distance imposed by the setup constraints, further experiments exploring a broader range of irradiation distances may yield different outcomes. Fig. 3 illustrates the impact of the UV irradiation protocol on the cleaning efficiency. This result indicates that the performance of the EDS system improved significantly under simultaneous irradiation. This notable difference suggests that the EDS system can achieve higher cleaning efficiency when cleaning targets are exposed to sunlight on the Moon's dayside. Fig. 2 Cleaning efficiencies of particles smaller than 25 μm under conditions with and without UV irradiation. The applied voltage was set to 2.0 kVp-p at 10 Hz. Two types of UV wavelength ranges were employed (irradiation distance: 100 mm; protocol: simultaneous irradiation). Fig. 3 Cleaning efficiencies of particles smaller than 25 μm under different UV irradiation distances (UV wavelength: 115-400 nm, simultaneous irradiation). The applied voltage for EDS was set to 2.0 kVp-p at 10 Hz. Fig. 4 Cleaning efficiencies of particles smaller than 25 μm in cases of simultaneous and pre irradiation (UV wavelength: 115-400 nm). The applied voltage for EDS was set to 2.0 kVp-p at 10 Hz. References: [1] Calle C. I. et al. (2011) Acta Astronaut., 69, 1082–1088, [2] Kawamoto H. and Hashime S. (2018) J. Electrost. 94, 38–43, [3] Schaible, M. J. et al. (2023) Acta Astronaut., 211, 674–683, [4] Wang, Y.-C. et al. (2024) Adv. Sapce Res., 74, 6194–6204, [5] Adachi, M. et al. (2024) Acta Astronaut., 228, 346–356