The Moon, in its orbit around the Earth, is subjected to many environmental conditions. Lacking an atmosphere, the Moon is directly exposed to ultraviolet (UV) and X-rays from the Sun during periods of daylight. Additionally, it is exposed to solar wind emanating from the Sun, and when it is positioned away from the Earth, it encounters the Earth’s plasma, which is propelled by the solar wind. Due to these diverse electromagnetic radiation and plasma environments, the lunar surface undergoes electrical charging [1,2]. The primary factors contributing to the electrical charging of the lunar surface encompass the photoelectron current induced by solar light, ions or electrons originating from solar wind and Earth’s plasma, and secondary electrons emitted from the lunar surface due to the impact of high-energy electrons [3]. The magnitude of these four current sources fluctuates based on the Moon’s position. For instance, during lunar daytime, the prevailing factors are the photoelectron and plasma ion current. Conversely, during lunar nighttime, the plasma current predominantly influences surface charging. This is especially true in regions exposed to the Earth’s plasma, which has a high prevalence of high-energy electrons. In these specific areas, the current produced by secondary electrons is recognized as the primary contributor. These variations in current sources significantly impact the electrostatic phenomena observed on the lunar surface [4].

The trajectory of the Moon’s orbit exposes it to a spectrum of environmental factors, leading to the accumulation of electrical charges on its surface. These charges exhibit variability in response to local conditions. The significance of these charging phenomena for future manned lunar missions cannot be overstated, as the charging can induce dust adhesion or cause abrasion to spacesuits [5–7], material degradation [8,9], electromagnetic interference [10,11], etc. In anticipation of the diverse conditions on the lunar surface, it is crucial to replicate these environments within controlled laboratory settings to simulate the charging phenomena. Although many studies have utilized plasma to emulate the plasma environment proximate to the lunar surface, there has been a relative lack of research focused on investigating controllable plasma source positions, such as axial directions or designated locations [12–14]. Expanding these capabilities would enhance the flexibility of the experimental setup, enabling the simulation of various plasma drifting scenarios for more comprehensive research.

At the Korea Institute of Civil Engineering and Building Technology (KICT), a chamber that can simulate the charging environments of the lunar surface has been developed [15,16]. This chamber, equipped with UV light sources for positive charging and E-beam emitters for negative charging, provides a significant step towards emulating lunar surface charging conditions. However, the chamber cannot fully replicate the intricate charging environments encountered on the moon, particularly in diverse plasma environments.

In this study a plasma source to simulate the lunar surface charge environment is incorporated. A variety of plasma sources were assessed for their appropriateness. The selected plasma source assembly was introduced, with an emphasis on its capability to adjust the direction of plasma beams. Lastly, preliminary plasma diagnostics were conducted, measuring plasma potential, electron/ion density, and electron temperature, and the outcomes were compared with the actual plasma properties of the lunar surface.

Figure 1 illustrates the interior of the surface charging chamber at KICT, pictured before installation of the plasma chamber. A UV light source is positioned at the top of the chamber, emitting radiation with a wavelength of roughly 160 nm (equivalent to 7.75 eV). This energy level surpasses the work function of lunar simulants (~5 eV) [3], potentially inducing a photoemission effect from the simulant soil, which can result in a positively charged surface potential. Above the chamber, an electron beam is installed. This beam directly introduces electrons onto the simulated lunar soil surface, leading to a negative charge. The energy of the emitted electron ranges from 5 to 40 keV with a current of 1 mA. The effective area of this interaction has a diameter of 150 mm at a distance of 500 mm from the top of the chamber. The internal dimensions of the chamber are approximately 70 × 70 × 90 cm³. A vacuum environment ranging from 10^–6 mbar (under soil-free conditions) to 10^–4 mbar (under soil-present conditions) can be achieved by employing dry and turbo molecular pumps. A twelve-inch ConFlat flange-type vacuum pipe is located on the left side of the chamber’s body.

Figure 1. Moon, Lunar surface charging, Vacuum chamber, Plasma Photograph of the inside of the surface charging chamber. Each area exposed to an electron beam and UV radiation was shaded in blue and violet, respectively.source, Langmuir diagnostics

The main factors considered for the integration of the plasma source into the chamber include the following: 1) non-interference with UV radiation and electron beam area: it was crucial to ensure that the plasma source did not interfere with the existing UV radiation or electron beam area. This separation allows each component to operate independently, maintaining effective operations. 2) minimal modification to chamber body: to preserve the integrity of the chamber and its original design, modifications were kept to a minimum. This ensures that the plasma source can be integrated seamlessly without affecting the overall structure or functionality of the chamber. 3) flexibility of plasma influence area: the plasma source was chosen based on its ability to influence specific chamber areas. This flexibility allows for precise targeting of regions for plasma treatment, facilitating detailed investigations while maintaining the chamber’s overall functionality.

In the process of integrating plasma sources into the chamber, two main prototypes were conceived [Figs. 2(a) and 2(b)]. In the prototypes either a capacitively coupled plasma (CCP) or a dielectric barrier discharge (DBD) plasma source is situated at the bottom of the chamber structure. These sources use two parallel planar electrodes, with alternating current generating plasma between them. The upper electrodes were designed to be a mesh-type material, allowing UV Figure 2. Various concepts for plasma source adoption: (a) CCP, (b) DBD, and (c) ICP type. The pink shaded areas depict plasma, and the dashed line represents the UV-exposed region. radiation to pass through the electrode holes. However, both prototypes faced challenges in accommodating regolith samples, as placing the sample on the lower electrode interfered with plasma generation. Additionally, difficulties arose with plasma generation using the mesh-type electrode, leading to the decision not to proceed with that prototype. An alternative prototype was designed to integrate an inductively coupled plasma (ICP) source positioned on the right side of the chamber, opposite the gas output pipeline located on the left [Fig. 2(c)]. This design effectively preserved the UV radiation zone and enabled sample placement by separating the plasma generation site from the sample. While this concept proved suitable, minor adjustments were necessary to prevent plasma loss due to the vacuum gas flow moving toward the direction of plasma diffusion.

Figure 2. Various concepts for plasma source adoption: (a) CCP, (b) DBD, and (c) ICP type. The pink shaded areas depict plasma, and the dashed line represents the UV-exposed region.

The finalized concept of a remote plasma source is shown in Fig. 3. In the source assembly head, as shown in Fig. 3(a), plasma is generated and then released from the source through a nozzle, which is also referred to as a ‘showerhead.’ The shaft’s length can be adjusted, allowing the plasma source to reach as far as the shaft extends [Fig. 3(b)]. This feature offers flexibility regarding the plasma diffusion area and minimizes the need to modify the chamber. Additionally, the flange elbow is interchangeable, enabling plasma sourcing at any desired angle [Figs. 3(c)-(e)].

Figure 3. Computer-aided design image illustrating the plasma source assembly: (a) head design and (b) head with flexible parts. (c), (d), and (e) show the assembly installed on the chamber from various angles.

The plasma source assembly adheres to the traditional structure of an ICP type source, which includes a coil, quartz tube, and other components. The head assembly unit is electrically connected to the chamber body, which grounds the unit. The density of electrons and ions can be controlled by adjusting the throughput of the input gas. The power generated by the radio frequency (RF) generator is controlled to adjust the temperature of the charged particles. The number of holes in the nozzle directly influences the plasma density - a greater number of holes results in a higher plasma density. Additionally, the nozzle can be positively or negatively biased to alter the properties of the positive or negative particles in the plasma, although this feature is not currently operational.

A single Langmuir probe diagnostics was performed to comprehend the characteristics of the plasma produced by the source [17–19]. This method entails inserting a probe into the plasma, applying a voltage, and measuring the resultant current. Subsequently, the current is analyzed to assess the properties of the plasma. This technique provides a straightforward measurement setup and the capability to extract diverse data from a single I–V curve. The plasma characteristics that can be measured include the plasma potential (V_p), electron density (n_e), ion density (n_i), and electron temperature (T_e). In this diagnostic setup, ion temperature measurements or vibrations due to RF signals were not considered [20].

The study utilized a Langmuir probe with a length of 25 mm and a diameter of 0.5 mm. To investigate the influence of the nozzleto-plasma distance on plasma characteristics, current measurements were taken at intervals ranging from 12.5 to 200.0 mm from the nozzle. The vacuum pressure was maintained below 1 × 10⁻⁵ mbar, and N₂ gas was introduced to stabilize the pressure at 5 × 10⁻⁴ mbar before generating the plasma. The RF power was set at 400 W for plasma generation. Current measurements were recorded while adjusting the biased voltage in 1 V increments from –100 to +100 V using a Keithley 6517B. Five identical conditions were tested to ensure the reliability of the results, and the values were averaged.

Figure 4 presents the results of I–V curve measurements, which were conducted based on the distance from the plasma source nozzle. In the graph, a positive current means electrons are entering the probe, while a negative current indicates ions entering the probe. The obtained I–V curve is representative of the typical results measured by this diagnostic method. Three distinct regions, the ion current saturation region, the electron current saturation region, and an intermediate transition region, were observed in the curve. As the probe gets closer to the plasma source, an increase in current magnitude was observed, likely due to an expected increase in plasma density. Differences in absolute current values between ions and electrons were noted at the saturation region. Specifically, ions tended to saturate in the lower current range below 0.2 mA, while electrons saturated at approximately 12 mA. This difference reflects the significantly higher speed of electrons moving toward the probe than ions. Regarding the saturation currents, apart from the electron saturation current at distances of 1.25 and 2.50 cm, a specific convergence to a constant value was not observed. This behavior can be attributed to an insufficient supply of electrons from the plasma, likely due to the relatively low plasma density.

Figure 4. I−V curve according to the distance from the plasma source nozzle.

From the I–V curve, the plasma potential V_p is calculated as follows:

Figure 5 illustrates the plasma potential according to the distance from the nozzle. The findings indicate that V_p progressively approaches zero as the distance from the nozzle expands. Based on these results, it is anticipated that V_p will reach zero at approximately 300 mm away from the nozzle.

Figure 5. Plasma potential according to the distance from the plasma source nozzle with its linear regression line (dashed).

The orbital motion-limited (OML) theory was employed to calculate the ion and electron densities. The OML theory is applied under conditions of low pressure, where ion-electron collisions are negligible [17,21]. Given that the experiment was conducted within a pressure range approximating 5 × 10⁻⁴ mbar, a level considered low relative to previous RF plasma diagnostic studies that utilized the OML theory, it can be inferred that the diagnostic approach based on the theory is applicable [21–23]. The theory hypothesizes that the gradient of the squared current I² with respect to the difference between plasma potential and bias potential |V_p − V_B| is proportional to the ion density n_i or electron density n_e. The slope of the tangent line can be determined by transforming the I–V graph from the saturation current region to the I²–V graph as follows:

which can be rewritten as follows:

where I is the total current, e is the elementary charge, n_s is the density of the particle, m_s is the mass of the particle, A is the area of the probe, d is the length of the probe, and V_B is the bias voltage. Using Eq. (3), the density of charged particles n_s can be derived.

The electron and ion densities according to the distance from the nozzle are shown in Fig. 6. The densities of ions and electrons, contrary to the linear diminution of V_p, exhibit an exponential decrease. This exponential decline in electron and ion densities is attributed to the expansion of the plasma in the vacuum chamber. As the plasma expands, the count of electrons and ions per unit volume decreases, reducing the densities of electrons and ions. Meanwhile, the densities of electrons and ions are nearly the same across all distances from the nozzle. Other observations are as follows: the electron density was slightly higher than the ion density at distances ranging from 12.5 to 75.0 mm from the nozzle. This is thought to be due to the nozzle being electrically grounded, which attracts the ions towards it. This tendency diminishes as the distance from the nozzle exceeds 75 mm. Moving further away from the nozzle, the ion density surpasses the electron density. This phenomenon warrants additional investigation to ascertain whether it is due to a measurement error or another factor.

Figure 6. Electron and ion density according to the distance from the nozzle. The dashed lines are visual guides and do not fit any specific models.

OML theory is known to provide a more accurate fit for ion dynamics than for electron dynamics, and thus the electron current I_e was determined by subtracting the ion current I_i, which is acquired from Eqs. (2) and (3), from the total current; that is, I_e = I–I_i. The second derivative of the obtained electron current I_e with respect to the voltage difference between the plasma and the probe (V = V_p–V_B) allows us to derive the energy distribution function as follows [24–26]:

where ε is the electron energy and f_e(ε) is the electron energy distribution function. Integrating the product of the energy ε and the derived distribution function f_e (ε), followed by normalization by the density n_e, yields the electron temperature T_e according to the following expression.

where 〈ε〉 is the average electron energy and k is the Boltzmann constant.

The electron temperature acquired from Eq. (5) according to the distance from the nozzle is shown in Fig. 7. It was observed that the electron temperature decreases linearly with increasing distance from the nozzle, similar to the behavior of the plasma potential. The linear decrease in electron temperature with distance from the nozzle can be attributed to plasma expansion. As the plasma diffuses from its source, it expands into a larger volume, and this expansion leads to a decrease in the density of the plasma, which reduces the frequency of collisions between particles. Since these collisions are the primary mechanism for transferring thermal energy among particles, a decrease in collision frequency leads to a reduction in the overall thermal energy of the plasma, resulting in a decrease in electron temperature [27].

Figure 7. Electron temperature according to the distance from the nozzle with its linear regression line (dashed).

Figure 8 presents the temperatures and densities of electrons/ions of the lunar surface plasma in different environments [28]. Plasma generated by the plasma source developed in this research, particularly its temperature, can suitably simulate daytime plasma in the solar wind region. For broader applicability, it is suggested that the electron temperature of the plasma source should exceed approximately 100 eV to represent the plasma in the plasma sheet accurately. It was also shown that ion temperatures exceed electron temperatures in regions such as the magnetosheath, magnetotail, and plasma sheet, deviating from the ordinary cases observed in experimental plasma. Regarding density, the plasma on the lunar surface is significantly less dense, with a density around 8 to 10 orders of magnitude lower than that of the plasma from the source developed here. Accordingly, it is necessary to enhance the plasma source to generate plasma with lower densities and more varied temperatures, which could significantly improve the accuracy of the simulations. This would enable a more accurate simulation of the conditions of the lunar surface plasma, particularly in terms of density and temperature. Through this approach, the range of lunar surface conditions the plasma source can simulate could be broadened, enhancing its research applicability and utility.

Figure 8. Parameters of lunar surface plasma in different environments: (a) temperature and (b) density (modified from the source. Adapted with permission from [28], Copyright 2021, National Aeronautics and Space Administration).

Simulating lunar surface charging environments is essential for the success of manned lunar missions. The moon is subject to dynamic charging conditions due to solar radiation, solar wind, and the Earth’s plasma. Understanding and replicating these charging effects is crucial to adequately prepare astronauts and equipment for the diverse conditions they will encounter on the lunar surface. This study introduced a plasma source with a controllable position and beaming angle, and its plasma characteristics were thoroughly investigated. The adjustable direction and location of the plasma source simplify the experimental setup and allow simulation of a broader range of lunar environments. This is achieved by controlling the drifting angle, which replicates the direction of the solar wind and, potentially, the wake boundary or plasma wake region. By incorporating UV light into the plasma source setup, the experimental conditions more closely resemble the lunar surface environment during daytime and nighttime. Future work should focus on increasing the versatility of particle temperature control and generating low-density plasma to simulate a broader range of plasma environments.

This research was funded by the Department of Future & Smart Construction Research of the Korea Institute of Civil Engineering and Building Technology, grant number 20240184-001.

The author declares no conflicts of interest.