# Measurement of bipolar charge distribution of lunar dust simulant under VUV irradiation

Rémi Pacaud, Jean-Charles Matéo-Vélez, Sébastien Hess, Sylvain Ranvier

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Rémi Pacaud, Jean-Charles Matéo-Vélez, Sébastien Hess, Sylvain Ranvier. Measurement of bipolar charge distribution of lunar dust simulant under VUV irradiation. Planetary and Space Science, 2025, 263, 10.1016/j.pss.2025.106120. hal-05101514

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# Measurement of bipolar charge distribution of lunar dust simulant under VUV irradiation

Rémi Pacaud<sup>a,*</sup> <sup>●</sup>, Jean-Charles Matéo-Vélez<sup>a</sup>, Sébastien Hess<sup>a</sup>, Sylvain Ranvier<sup>b</sup>

## ARTICLE INFO

### Index Terms:

- Dust net charge

- Lunar exploration

- Sensor

- Space environment

- Photoemission

## ABSTRACT

Upcoming missions to the Moon represent new science opportunities and challenges. The electrostatic nature of the regolith combined with the solar wind makes it loft and adhere to almost any surface, which represents a threat for future manned and robotic missions. Understanding the charge state of the lunar soil under a representative environment is a key step towards ensuring safe lunar missions. While the global first order effect of exposure to the Sun's UV is to charge the soil positively, past experiments suggested that the transported dusts could be charged negatively. This counter-intuitive behavior was then supported by modeling, which explained the existence of negative charges but also predicted that of positively charged ones. To investigate the charging behavior of dust under a representative environment, we developed an experimental protocol based on a polarized sensitive sensor dedicated to the charge measurement of single dust grains with an accuracy of about 1 fC. The first set of measurements obtained with JSC-1A lunar dust simulants in high vacuum reveals the bipolar nature of lunar dust net charge in the regolith when exposed to UVs. Indeed, both positive and negative dusts were detected, supporting the complexity of the regolith charging processes suggested by the models.

## 1. Introduction

The lunar environment is surrounded by a plasma created by the solar wind, alongside with high energetic particles and micro-meteorites (Akisheva et al., 2021). Due to a quasi-absence of atmosphere around the Moon, the regolith is directly exposed to radiations. On the day side photoemission is the main phenomenon, leading to dust being positively charged to around +10 V (Colwell et al., 2007). On the night side it collects electrons from the plasma, leading to an estimated potential of -35 V to -100 V (Halekas et al., 2002). The boundary between the day and night sides is called the terminator and is subjected to electric fields (Mishra, 2020) that make dust levitate, loft and be carried away (Wang et al., 2011). Recent in-flight measurements show an annual dust deposition rate at a height of 190 cm above the lunar surface of about 21.4 µg/cm² (Li et al., 2019). Understanding the underlying physical phenomena of dust electrostatic transport is of paramount importance since it will be problematic (toxicity (Loftus et al., 2010), abrasiveness and adhesion (Lee, 1995)) for future manned and robotic missions. Experiments (Farr et al., 2021) showed that the topography of the dust layer at micron-scale (Poppe et al., 2012) needs to be considered to

better understand dust transport. The microcavities within the dust layer are random arrangements that play an important role in net and differential charging between neighboring dust grains, which influences the electrostatic transport of dust. Therefore, understanding the net charge value and sign of single dust grains on a regolith surface represents a key step towards ensuring safe future manned missions on airless bodies.

A dust layer exposed to VUV (Vacuum Ultra Violet) is generally expected to be positive (Criswell and De, 1977) but recent experiments indicate opposite conclusions. The patched charge model (Wang et al., 2016) suggests that dust mobilization after VUV irradiation is driven by negatively charged particles that repel each other because of different local charge distributions due to microcavities and photoemission within the dust layer. According to (Wang et al., 2016), the main reason for particles to acquire negative charges is hypothesized because of the presence of microcavities since dust layers are porous. The gaps between the dust grains make it possible for photoelectrons to be reabsorbed by other neighboring dust grains, which create local negative patches. On one dust grain, these patches then surpass the positive patches induced by direct photoemission on the illuminated side of the dust grain. One

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<footnote><sup>a</sup> DPHY, ONERA, Université de Toulouse, 31000, Toulouse, France</footnote>

<footnote><sup>b</sup> BIRA-IASB, Avenue Circulaire 3, 1180, Brussels, Belgium</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail addresses: Remi.Pacaud@onera.fr (R. Pacaud), Jean-Charles.Mateo_Velez@onera.fr (J.-C. Matéo-Vélez), Sebastien.Hess@onera.fr (S. Hess), Sylvain.Ranvier@aeronomie.be (S. Ranvier).</footnote>
explanation provided in (Wang et al., 2016) is the fact that the electric field on the side of the dust grain that is negatively charged is much higher than the electric field on the positive side that is directly illuminated by photons. This implies that the charge of one dust grain is overall negative. Those experiments were carried out under VUV with irregular-shaped Mars simulants 38–45 µm in diameter and showed the ejection of negatively charged dust grains only (Schwan et al., 2017). Other experiments with JSC-1A simulants with high positive and negative voltages applied on surfaces located near the dust layer and VUV exposure showed evidence of the mobilization of positively charged dust grains (Champlain et al., 2016). More recent simulations done with SPIS-DUST (Oudayer et al., 2019) predict that both positive and negative net charges could be observed after VUV irradiation. Simulations were done on a multilayer pile of dust particles modelled as spheres and irradiated by VUV. It was found that the first two dust layers are charged more positively than the third layer. Those different results show the complexity of modeling and understanding the underlying phenomena of dust charge and electrostatic transport in a representative environment, both numerically and experimentally.

In this paper an experimental setup is described in section II to
investigate the net charge of a dust layer after VUV irradiation to deepen
our understanding of electrostatic transport. The charge state of indi-
vidual dust grains is analyzed after irradiation by a polarized dust de-
tector that attracts dusts of opposite sign. In order to be as representative
as possible to lunar conditions, irregular-shaped JSC-1A simulants with
a broad size distribution were selected. Their size ranges from the
micrometer scale to a few hundreds of microns. Section III presents the
results in terms of dust net charge polarity and magnitude.

## 2. Experimental setup and protocol

The objective of the present study is to measure the net charge sign

and value of dust grains for an environment representative of the
dayside of the Moon where the VUV is predominant. In order to mimic
lunar conditions, measurements are done under high vacuum (from
$10^{-3}$ mbar to $10^{-8}$ mbar (Dittmann, 1989)) since medium vacuum (from
$1$ mbar to $10^{-3}$ mbar) would probably reduce the charging effect of the
VUV. Our experimental setup reaches a vacuum level of at least $10^{-6}$
mbar. The surface potential of the dust layer is measured after the VUV
irradiation to understand the global charge sign and value of the dust
layer. A dedicated apparatus is used to attract charged dust grains and
measure individual charges of transported dust particles.

The experiments are performed in the DROP (Dust Regolith Or Particles) vacuum chamber, dedicated to the study of the electrostatic properties of lunar dust simulants. It is a cylinder of 35 cm diameter with a height of 30 cm. Fig. 1 is a schematic (not to scale) of a sectional view of the experimental setup inside the DROP vacuum chamber. The VUV irradiation is performed by the top flange. The measurement of the surface potential is done with a contactless probe connected to a linear feedthrough on the right flange. The left flange enables to push and pull the dust charge detector. Two other flanges (not represented in the schematic), are used as a high voltage feedthrough for the dust detector and for the pumping system.

2.1. *Charging*

Irradiation is carried out with a VUV lamp (from Hamamatsu, model L7293 L2D2) with peaks at 120 nm and 160 nm, generating a current density of photo emitted electrons of 5 nA/cm² on a copper plate. This is representative of the flux expected at the surface of the Moon on the day side at zenith around the equator. The lamp is located at the top of the chamber (Fig. 1) in order to illuminate the sample with a normal incidence angle. The lunar dust simulant holder is connected to the electrical ground. The experiments are done with a dust layer of 6 mm in

<figure><img src="image_4.png" /><figcaption>Fig. 1. Schematic of a sectional view of the detector within the DROP chamber.</figcaption></figure>
thickness and 40 mm in diameter. These charging conditions are similar to (Oudayer et al., 2019), except for the VUV flux, which is 8 times higher in (Oudayer et al., 2019). Another difference in the numerical approach of (Oudayer et al., 2019) is the modelling of 5 layers of 50 µm diameter spheres, leading to a 0.25 mm thick dust bed sample instead of 6 mm in the present paper.

To perform the experiments, the chamber has two linear feed-throughs (Fig. 1): the first one is used to move a contactless probe (Kelvin probe (Noras and Pandey, 2008) – KP) for macroscopic surface potential measurements of the dust layer and the second one is used to move the dust detector above the dust layer to measure the net charge of individual dust grains. Surface potential measurement is done with the KP before VUV irradiation at a distance of 5 mm above the dust layer. Once the surface potential is measured the probe is pulled away from the sample. The JSC-1A dust bed is irradiated during 5 min with VUV, after which the Kelvin probe is pushed and placed above the layer of JSC-1A to measure the surface potential. JSC-1A is a dielectric and its charge decay has already been studied under plasma (Wang et al., 2007; Ding et al., 2013), showing a characteristic time for the surface potential of about an hour. Even if the charging process used in the present paper is different, we assume that the charge level of JSC-1A does not significantly vary between the end of the VUV irradiation and the beginning of dust detection, since these two steps are separated by a few seconds.

## 2.2. Dust mobilization and detection

The detector is composed of a grid support, a grid, a Faraday cup (FC), a transimpedance amplifier and a data acquisition stage (DAQ). The system is biased with respect to the laboratory ground with a high voltage supply. The objective is to attract dust by electrostatic forces. Equipment that is electrically connected and biased at the same potential is represented within the red dotted line in Fig. 1. The grid's wires have a diameter of 0.40 mm and a spacing of 2 mm. This configuration makes the whole setup acts as a Faraday cage for more accurate measurements, in addition to our DROP. Mobilized charged dust grains enter vertically into the FC across an entrance of 10 mm diameter and an FC depth of 50 mm. A transimpedance amplifier from FEMTO® is directly connected to the output of the FC, which considerably lowers the electrical noise. The gain, bandwidth and maximal output voltage of the amplifier are $G = 10^{11}$ V/A, $f = 200$ Hz and $V_{out} = \pm 10$V, respectively. The output signal is transmitted with low noise coaxial cables to a digitization board made of a Picoscope 5442D with a bandwidth of 60 MHz. The digitized signal is transmitted to the computer acquisition program with an optical fiber and the data acquisition sample rate is 10 kS/s. One specificity of this setup is its capacity to bias the sensor with respect to the ground and dust: the grid, FC and digitalization stage are all biased at the same potential. This intends to avoid decelerating dust by electrostatic forces when they reach the FC. Would the FC be grounded to the laboratory reference ground, the dust particles would be significantly reflected back when entering the FC.

Once the surface potential has been measured, the Kelvin probe is pulled back and the detector is moved over the sample. The distance between the grid and the surface of the dust layer is 5 mm. Polarization is done gradually to reduce displacement current transitory effects that would cause perturbations on the Faraday cup measurements. The output signal is recorded over a duration of 8 min. The net charge Q of each detected dust grain is computed as follows:

$$Q = \frac{1}{G} \int_{t_0}^{t_1} U dt \qquad (1)$$

where G is the gain and U the output voltage of the transimpedance amplifier. $t_0$ and $t_1$ define the time interval of the detected dust.

## 3. Results and discussion

Two test sequences have been performed in this study. For each test sequence the surface potential was measured after the VUV irradiation between +3 V and +5 V, confirming the fact that the dust layer is globally positive. In the first test sequence two experiments were done: the detector was polarized at -2.5 kV for experiment 1A and +2.5 kV for experiment 1B. In the second test sequence the detector was polarized at -3.5 kV for experiment 2A and +3.5 kV for experiment 2B. The maximum electric field in the experiments is approximated to be around $7.10^5$ V/m, since the dust layer is at 5 mm from the FC. For any of the performed tests, dielectric breakdown of the dust layer is discarded since the threshold for JSC-1A is around $10^7$ V/m as shown in (Jordan et al., 2019). In addition, we have performed similar experiments without irradiating the sample to get a baseline of dust charges. These preliminary experiments showed that very few detections were observed, which indicates that the electric field alone is not enough to generate the detections. It was also observed that increasing the irradiation time did not affect the results for the two test sequences, which means that the sample's charge state has already reached saturation after irradiation. For this reason, all the experiments have been done with the same sample and irradiation was done every time before each test for reproducibility.

Figs. 2 and 3 show the output voltage of the transimpedance amplifier as a function of the detector polarization and time for tests 1A, 1B, 2A and 2B. The background noise is in black and the signal associated to dust particles counts is highlighted in red. The background noise's voltage standard deviation $\sigma_V$ has been computed and corresponds to a noise of 10 mV. When the polarity of the detector is negative, a rising positive peak triggers detection as seen in sub-plot b) of Fig. 2. When the polarity is positive, a falling negative peak triggers detection as seen in sub-plot d) of Fig. 2. The threshold to trigger detection was set to $\pm 60$ mV, which is $6 \times \sigma_V$ above the background noise, high enough to prevent false dust counts. The lower bound cut-off for charge detection is about $\pm 1$ fC and the standard deviation $\sigma_c$ of the measured net charge is lower than $10^{-3}$ fC. It means that the error due to the background noise on a measured net charge is at least three orders of magnitude lower than the measurement itself.

Fig. 2 shows 4 plots. Sub-plot a) shows multiple dust detections for a negative polarity of -2.5 kV and sub-plot c) for a positive polarity of 2.5 kV, confirming SPIS predictions that both positive and negative charges are observed after the VUV irradiation (Oudayer et al., 2019). For the clarity of explanations only one peak is presented for each experiment. In sub-plot b) two peaks are observed: peak 1a and peak 1b. Peak 1a is positive with a computed net charge of $171$ fC (~ $1.1 \times 10^6$ e), and corresponds to the particle entering the FC. Peak 1b is negative with a computed net charge of $158$ fC (~ $9.7 \times 10^5$ e) and corresponds to the same particle falling back onto the dust layer. Most of the counts follow this pattern, i.e. a positive current followed by a negative current, whose positive and negative charge values usually differ with less than 10 %. The slight discrepancy between the computed charges can be explained by the choice of the time intervals for charge integration (in equation (1): $t_1-t_0$) and by the electrical background noise. It could also be induced by particles hitting the FC's walls and transmitting some of their net charge at impact before falling back. This two peaks pattern is consistent with two detections of the same particle, first when it enters the sensor and second when it gets out from it. This is explained by the fact that particles enter at high velocity, which means that they bounce off the inside walls of the FC and then fall back. If a charged particle enters the FC, image charges within the FC are displaced because of Coulomb forces, which generates a signal of a given sign. As the particle falls back, image charges go back to their initial place, which generates the same signal, but of opposite sign. In sub-plot d) peaks 2a and 2b have nearly the same net charge (~-41 fC, ~ $2.5 \times 10^5$ e and ~-42 fC, ~ $2.6 \times 10^5$ e) but a different dynamics: the full width at half maximum of peak
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Output voltage of the transimpedance amplifier (V) as a function of the polarization time for a bias voltage of -2.5 kV and +2.5 kV (sub-plots a) and c). The background noise is in black and detected peaks are highlighted in red. Sub-plots b) and d) are examples of detections. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Output voltage of the transimpedance amplifier (V) as a function of the polarization time for a bias voltage of -3.5 kV and +3.5 kV.</figcaption></figure>

2b is 4 times longer than peak 2a, which suggests for example that the particle hit the FC's wall and that a mechanical energy transfer slowed down the particle after impact. A more detailed analysis of the induced current in the FC is necessary to solve this kind of uncertainty. Nevertheless, the cumulated net charge is consistently computed and is a valuable output of the detector whatever the dust dynamics inside the detector. The total number of detections is compared between the two experiments. 12 particles were detected in experiment 1A and 17 in experiment 1B. Out of a total of 29 counts, 41 % of dust grains are positive and 59 % negative. However, the number of detections is not high enough to draw robust conclusions on the net charge sign distribution. The number of detections is much higher for experiments 2A and 2B, thus more reliable. In the absence of the dust layer no charge signal spike is measured.

Fig. 3 shows the experimental results for an applied voltage of -3.5

kV (sub-plot a)) and +3.5 kV (sub-plot b)). The total number of detections has been computed and a histogram of the total number of detections as a function of the particle's net charge for experiments 2A and 2B is plotted in Fig. 4. Each bin of the graphic has a size of 1 fC. Only 4 particles with a net charge close to $|±1~fC|$ are detected, which can be explained by the background noise that prevents the detection of dust grains with a lower net charge. 498 particles were detected in experiment 2A and 396 in experiment 2B. Out of a total of 894 counts and regardless of the particle's size, 56 % are positive and 44 % negative. The net charge sign distribution is close to being equally distributed between positive and negative dust grains, confirming the results of experiments 1A and 1B. However, when added together, the total amount of positive charges is ~11.7 pC when it is ~ -10.6 pC for negative charges. It represents a difference of +1.1 pC, confirming the Kelvin probe measurements that the surface potential is positive. The
<figure><img src="image_7.png" /><figcaption>Fig. 4. Number of detected particles as a function of their net charge for experiments 2A and 2B. A zoom-in is shown for lower charges.</figcaption></figure>

highest net charges measured for experiments 2A and 2B are respec-
tively 423 fC and –691 fC. Based on the number of counts in experiment
2A, 285 counts out of 498 (57 %) have a net charge lower than 10 fC. For
experiment 2B, 234 counts out of 396 (59 %) have a net charge lower
than –10 fC in absolute value. It shows that a large number of dusts have
a net charge lower than 10 fC when submitted to the VUV only.

Nevertheless, it is important to highlight that those measurements
might not be fully representative of the effect of the VUV. It can be
difficult to distinguish the pure effect of the VUV to side-effects such as
induced conduction or tribocharging that can partly cloud the effect of
the VUV on the net charge build-up of lunar dust. The application of high
voltages over a short distance can lead to strong local electric fields that
may induce electrical conduction responsible of a net charge exchange
within the dust layer. In addition, one aspect not considered initially
when designing the sensor are dust particles falling back onto the dust
layer and producing an excess of net charge by tribocharging.

The bias voltage has increased by a factor of 1.4 between experi-
ments 1 and 2 but the total number of counts has increased by a factor of
42 between experiments 1A and 2A and by a factor of 23 between ex-
periments 1B and 2B. There is a seemingly non-linear effect when the
bias voltage is increased, which could be due to different reasons.

1) A threshold effect at around ±2.5 kV for a distance of 5 mm between the grid and the dust layer. It could be near to an equilibrium where the electric force needed to lift the particles is close to the forces keeping the particles on the regolith surface (gravity and cohesion). In addition, the electric field must be high enough to move the detached particles close enough to the sensor head.

2) Dust grains falling back onto the sample may generate tribocharging and both charge and momentum transfer could occur when dust particles collide with each other. So after a time period difficult to evaluate, it may be difficult to distinguish VUV initial charging from tribocharging and mechanical impacts.

4. Conclusions and prospects

The net charge measurement of single dust grains under a repre-
sentative environment is a challenging but important matter for future
exploration missions. A dedicated protocol involving VUV irradiation,
surface potential acquisition and lunar dust net charge measurements
with a detector has been defined to measure the net charge of single JSC-
1A dust particles. Charges ranging from about ±1 fC up to several
hundreds of fC have been measured.

The experiments showed that dust grains can be positively and
negatively charged after the VUV and then detached by applying a

voltage of opposite sign. Our measurements show that at least 50 % of
detected dust have a net charge lower than 10 fC in magnitude in both
cases, with some dust grains with a net charge of a few hundreds of fC.

The minimum voltage required to loft dust particles in the lab is approximately ±2.5 kV applied at a distance of 5 mm from the dust layer, assumed to be flat. The rate of dust grains detection did not increase proportionally to the applied voltage. It is much higher at ±3.5 kV than ±2.5 kV which suggests a threshold effect due to gravity and cohesion forces.

In future experiments we intend to improve the setup and track
particles in real time with a high speed camera. The electrical signal
could be matched with visual information, enabling to draw a link be-
tween the net charge and mass (or effective radius) of the dust particle.
The ratio of dust net charge over mass is indeed a key factor of dust
dynamics and trajectories. The design of the Faraday cup could be
slightly modified to prevent particles from falling back onto the sample,
which would help distinguish the effect of the VUV from self tri-
bocharging, when dust collide with each other. Other types of dust
simulants could be tested in order to compare with JSC-1A and assess
the effect of dust composition and of their electrical properties.

With an increasing number of mission projects to the Moon, there is
an urgent need to anticipate lunar dust related issues. Predicting dust
mobilization induced by the natural environment or by human or ro-
botic activities and assessing their effects on materials and equipment is
a complex task requiring combined numerical modeling and ground
testing. The test conditions are a sensitive issue since lunar dust simu-
lants sourced from Earth suffer from a lack of representativity regarding
electrostatic and surface energy properties. Contamination of dust par-
ticles by air volatiles put another level of uncertainties on the in-
teractions between grains and their environment. To cover such gaps, it
is necessary to fly instruments to the Moon such as the dust charge
sensor presented in this paper. To this end, in the frame of the DUSTER
project, a breadboard instrument comprising dedicated electronics
front-end, high voltage power supply and data processing unit is under
development and will be tested in dusty environment under vacuum.

CRediT authorship contribution statement

**Rémi Pacaud:** Writing – review & editing, Writing – original draft, Validation. **Jean-Charles Matéo-Vélez:** Writing – review & editing, Validation. **Sébastien Hess:** Writing – review & editing, Validation. **Sylvain Ranvier:** Writing – review & editing, Validation.
Declaration of competing interest

The authors declare the following financial interests/personal re-
lationships which may be considered as potential competing interests:
Rémi PACAUD reports financial support was provided by European
Union. If there are other authors, they declare that they have no known
competing financial interests or personal relationships that could have
appeared to influence the work reported in this paper.

Acknowledgment

The work done in this paper has been possible thanks to the *Dust Study, Transport and Electrostatic Removal for Exploration Missions (DUSTER)* project financed by the European Union's HORIZON research and Innovation program under grant agreement n° 101082466.

Data availability

Data will be made available on request.

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