Johannes Brötzner

We present a comprehensive study on the sputtering behavior of tungsten fuzz structures under deuterium and argon ion bombardment using three-dimensional simulations. We employed SDTrimSP-3D to investigate porosity as a predictor for the sputtering behavior, complementary to the well-established effects of surface roughness, with simulations conducted across multiple ion energies ( 500 eV – 2000 eV ) and incidence angles ( 0 ∘ − 85 ∘ ). Fuzz structures were algorithmically generated with varying porosities, and sputter yields were systematically calculated. The findings reveal a universal linear relationship between sputter yield and porosity/volume filling, demonstrating that porosity significantly suppresses sputtering. Importantly, the zero-porosity limit is given by the sputter yield of a rough, but solid surface rather than a flat target. This linear behavior holds consistently across the different ion types and energies for a wide range of incidence angles. When normalized to the rough-surface sputter yield, the slope of the porosity dependence is nearly constant, indicating a common underlying suppression mechanism. The study enhances fundamental understanding of sputtering in porous nanostructures and provides a simple, predictive framework with implications for material design, particularly in nuclear fusion research, where erosion of tungsten fuzz impacts reactor component lifetimes.

Angular distributions of sputtered ejecta are critical inputs for simulating regolith sputtering on airless bodies, where surface topography and porosity determine if particles escape or redeposit. We present new analysis and simulations demonstrating that, although current binary collision approximation (BCA) frameworks can reliably predict either total sputter yields or angular distributions for minerals, they cannot reproduce both simultaneously. This is a limitation with direct implications for exosphere and surface-evolution models that require realistic, three-dimensional ejecta plumes. Using SDTrimSP[1] for flat targets, SPRAY[2] for AFM-resolved rough surfaces, and SDTrimSP-3D[3] for porous regolith analogs, we quantify how morphology and binding energies control plume tilt and shape via a dedicated three-dimensional lobe-fitting approach designed for full-plume implementation in modeling tools. This framework addresses the need to move beyond simplified cosine laws and to incorporate azimuthal structure when propagating sputtered particles in regolith and exosphere simulations.Across surface morphologies, rough and porous targets deflect ejecta toward the surface normal, reducing the strongly forward-facing plumes typical of flat surfaces and introducing geometry-mediated effects such as shadowing and redeposition that reshape both yield and energy distributions. In this regime, we find a robust tradeoff between reproducing sputter yield and plume tilt: the “compound/hybrid” binding-energy model[4] capable of matching experimental mineral yields[5] fails to reproduce the experimentally observed forward tilt at oblique incidence on flat and, more significantly, on rough surfaces (Fig. 1). Conversely, adopting increased surface-binding energies (SBEs) informed by recent molecular dynamics work[6] strengthens the forward tilt and better matches rough-surface angular data but underestimates absolute mass yields by factors of 3–4.Figure 1: Model–data comparison of sputter yields and angular distributions for MgSiO3 under 4 keV He irradiation. Left: SDTrimSP model yields that result from the Jäggi et al. (2023) hybrid binding energy model (HB-C, blue dashed) with the increased surface-binding case (SB-C) based on Morrissey et al. (2024) surface binding energies, compared to the Biber et al. (2022) laboratory measurements (black, with 2 SD errors). Center: Normalized mass yield polar distributions from a flat surface at an incidence angle αin = 60° for said simulations. Right: Analogous to the center figure, but for a rough pellet surface. Methodologically, we map flat-surface SDTrimSP ejecta to measured roughness via ray tracing in SPRAY and construct porous regolith stacks in SDTrimSP-3D, then fit the resulting plumes with a single- or tri-lobe model that captures central and side “winglet” components associated with single-collision recoils (Fig. 2). This procedure allows for consistent comparison to laboratory angular data and direct use of fitted plumes in forward models. For rough surfaces, the simulations confirm a forward tilt relative to the sample surface normal but with magnitudes that depend sensitively on the binding-energy scheme. Only the higher-SBE case reproduces the observed forward-tilted distributions. For porous targets, preliminary comparisons and prior 3D simulations indicate stronger backscattering, further emphasizing the need for morphology-aware angular inputs in regolith sputtering models.Figure 2: Comparison of O sputter ejecta distributions sputtered from a rough surface (SPRAY) by 4 keV He at an incidence angle of αin = 75° from the surface normal (black arrow) resulting from (left) the Jäggi et al. (2023) hybrid binding energy (HB) model and from (right) a surface binding energy (SB) model with increased binding energies from Morrissey et al. (2024), fitted by a central plume (blue) and two forward-facing “winglet” plumes (red). These findings have two key implications. First, exosphere and regolith models should incorporate full 3D angular distributions rather than rely on simple cosine laws, especially for rough and porous substrates where plume tilt and azimuthal structure are non-negligible. Second, the inability to match yield and angle concurrently suggests missing physics in BCA implementations—most notably, yield-enhancing processes such as molecule and cluster sputtering that are not represented in single-atom BCA ejecta—and cautions against parameter tuning. In practice, our results recommend using binding-energy models that best reproduce angular distributions when plume directionality controls transport and redeposition, while acknowledging that total yields must then be corrected for non-BCA processes if absolute source strengths are required.In summary, angular distribution data are indispensable for realistic regolith sputtering simulations, yet existing BCA parameterizations cannot simultaneously reproduce experimental mass yields and angular distributions across flat, rough, and porous mineral surfaces. This demonstrates a fundamental limitation of current BCA-based models for planetary surfaces and motivates hybrid or extended frameworks that incorporate missing yield mechanisms while retaining accurate, morphology-dependent plume geometry.References[1] Mutzke, A., Toussaint, U. v, Eckstein, W., Dohmen, R. & Schmid, K. (2024). SDTrimSP Version 7.00. Technical Report[2] Cupak, C. et al. Appl. Surf. Sci. 570, 151204 (2021).[3] von Toussaint, U., Mutzke, A. & Manhard, A. Phys. Scr. T170, 014056 (2017).[4] Jäggi, N. et al. Planet. Sci. J. 4, 86 (2023).[5] Biber, H. et al. Planet. Sci. J. 3, 271 (2022).[6] Morrissey, L.S. et al. Planet. Sci. J. 5, 272 (2024).

We perform Low-energy Ion Scattering with He ions ranging in energy from 1 keV to 3 keV on CaSiO 3 using a commercial electrostatic detector system and determine the charge fraction of scattered ions from comparison with binary collision approximation simulations. The simulations take composition changes due to surface cleaning Xe sputtering into account. Scattered He particles are separated in the simulation into single, dual, and multiple scattering trajectories. We find that the charge fraction of single and dual scattered He is about ten times higher than the one for multiple collisions. Our results show that the charge fraction of scattered He can be reliably extracted for different scattering regimes and that for a given He incidence energy, the entire scattered He spectrum can be modeled with only few constant parameters.

The interactions between micrometeoroid impacts and solar wind on the surface of the Moon remain a major area of investigation in the space weathering field. In this study, we first irradiated Fe-bearing enstatite [(Mg,Fe) ₂ Si ₂ O ₆ ] (1.50 at% Fe) with a nanosecond-pulse UV excimer laser ( λ = 248 nm) to simulate micrometeoroid impacts, using fluences above and below the ablation threshold to produce four different impact-processed regions. Next, we irradiated the surfaces with 4 keV He ⁺ to explore how prior impact processing changes the way solar wind ions interact with mineral surfaces. We performed comprehensive micro-scale to nano-scale characterization of the experimental products using scanning electron microscopy, scanning transmission electron microscopy, electron energy loss spectroscopy, and energy dispersive X-ray spectroscopy. In the samples irradiated above the ablation threshold, we observed the coexistence of nanophase Fe ( np Fe ⁰ ) and Fe ³⁺ -bearing glass, indicating that np Fe ⁰ formation was driven by charge disproportionation. In the sample irradiated at the highest laser fluence, we produced helium-bearing vesicular np Fe ⁰ , an enigmatic space weathering product known to occur widely on the Moon. Finally, we show that the temperature of impact melting and the phases produced by impact processing have a direct influence on the target’s sputtering behavior and overall helium capture capacity. Our findings demonstrate that micrometeoroid impacts play a critical role in modifying the crystal structure and chemistry of regolith grain surfaces and that variation in impact processing can fundamentally shift the subsequent interactions that solar wind ions have with affected surfaces.

Important aspects concerning the origin and formation of the Moon’s exosphere, its tenuous gas envelope, remain puzzling with uncertainties regarding the importance of different effects. Two competing processes — micrometeoroid impact vaporization and solar wind ion sputtering — are considered key contributors to the ejection of particles into the exosphere. Here we present direct, high-precision yield measurements of solar wind ion sputtering using real lunar samples (Apollo 16 sample 68501), combined with advanced 3D simulations of regolith erosion. We find solar wind sputter yields up to an order of magnitude lower than previously used in exosphere models. The difference is primarily due to the suppressive effects of surface morphology, in particular the roughness and high porosity of the lunar regolith. Our results provide critical, experimentally validated sputter yield estimates and address long-standing modeling uncertainties. These results are particularly timely in light of upcoming and ongoing missions, such as the Artemis program at the Moon or BepiColombo at Mercury, contributing essentially to our understanding of how the surfaces of rocky bodies in the solar system are altered.

The solar wind, a stream of charged particles from the Sun, interacts with planetary bodies in a variety of ways. One such interaction is the reflection of solar wind ions from regolith surfaces where particles may be scattered as ions or undergo neutralisation processes, leading to the formation of energetic neutral atoms (ENAs). In either case, the scattered particles can in turn be observed by space missions providing valuable insight on the space weathering of rocky bodies and the interaction of their surfaces with the solar wind. Such information is available for, e.g., the reflection of protons at the Moon [1] or Phobos [2,3]. Similarly, the scattering of light ions off sample surfaces, referred to as low energy ion scattering (LEIS), is a commonly used ion beam analysis technique. In LEIS, the energies of ions scattered into a certain detector geometry are recorded. In these energy spectra, the positions and heights of peaks are indicative of the chemical surface composition. Consequently, LEIS is usually employed to monitor composition changes during processes like material deposition, erosion or chemical reactions [4]. Conventional LEIS setups commonly use electrostatic analysers, and thus the neutralised fraction of projectiles is not accessible. Quantifying the neutralisation probability is therefore tedious and also not available in binary collision approximation (BCA) simulations like SDTrimSP [5]. Because however the physics behind the ion scattering processes is the same as in the space environment, LEIS experiments are an ideal tool to study the reflection of solar wind ions in a simplified, controlled laboratory setting. In this study, we present measurements using He projectiles of energies ranging between 1 keV and 5 keV on samples prepared from the pyroxenoid wollastonite (CaSiO3). The samples were prepared as flat thin films on Si substrates such that in a first step, surface roughness or regolith-like porosity would not obfuscate our understanding of the underlying physical processes. While He is significantly less abundant in the solar wind than H, it is chemically inert and thus simplifies interpretation of the results. The Ca in the sample facilitates a greater relative mass difference to the projectile and therefore a better separation from the low-energy background compared to using silicates containing lighter elements, like Na or Mg. The experiments were carried out using a commercial LEIS setup (ionTOF Qtac) with an electrostatic analyser. Additionally, we performed BCA simulations with the codes SDTrimSP and IMINTDYN [6]. Using the former, we can model the surface composition changes during sputter cleaning of the sample, while the latter is capable of calculating the resulting LEIS spectra under consideration of the actual experimental geometry, ignoring however the charge state of the scattered ions. A key feature of IMINTDYN is further that it can separate the spectra into contributions from ions that scattered once, twice or multiple times, as well as by the sample species at which the scattering event took place.Using this combined experimental and numerical approach, we find that the multiple scattering contribution from deeper sample layers is suppressed by roughly an order of magnitude compared to single and double scattering events. Furthermore, since the simulations consider all scattered particles as neutral, while the analyzer detects only ions, comparison of the two allows for estimation of the charge fraction of scattered projectiles. This capability to assess the ion-to-neutral ratio is highly relevant for ENA studies and goes beyond what is accessible with conventional BCA simulations alone.[1] C. Lue et al., J. Geophys. Res. Space Phys. 123 (2018) 5289–5299.[2] Y. Futaana et al., J. Geophys. Res. Space Phys. 115 (2010), A10213.[3] Y. Futaana et al., J. Geophys. Res. Planets 126 (2021) e2021JE006969.[4] H.H. Brongersma et al., Surf. Sci. Rep. 62 (2007) 63–109.[5] A. Mutzke et al., IPP Report (2019).[6] H. Hofsäss, A. Stegmaier, Nucl. Instrum. Methods Phys. Res. B 517 (2022) 49–62.

The Moon is subjected to a variety of influences in the space environment. One of these is the solar wind, a plasma stream consisting of mostly H+ and He2+ ions, that impinges on the lunar surface. As a consequence, material is released through the process of ion sputtering, mostly on an atomic level. These ejecta subsequently take part in the formation of the lunar exosphere [1]. Constraining their physical properties, most notably the parameters sputtering yield, ejecta angular distribution and their energy distribution, is thus crucial to properly model the exosphere creation [2]. Such investigations have been of interest for decades and have recently been carried out with samples representative for the lunar mineralogy [3–6].In this contribution, we present our current investigations on the aforementioned parameters using samples prepared from material collected during the Apollo 16 mission. Using a quartz crystal microbalance (QCM), we are able to measure mass changes due to sputtering caused by H and He ions and therefore also the sputtering yield. Additionally, we place another QCM in the experimentation chamber in a rotatable manner that collects the ejecta. Doing so enables us to probe the angular distribution of the ejecta. For these experiments, we use two types of samples: flat vitreous films as well as pellets pressed from lunar regolith and prepared according to [7]. Along with numerical simulations considering the sample morphology, this allows us to untangle intrinsic material properties from modifications thereof due to surface roughness. Lastly, we will present plans for future measurements to experimentally resolve the ejecta energy distribution. These energy distributions of particles sputtered from compound materials (rather than monatomic ones) are an actively researched area, especially from a numerical standpoint [8–11] – experimental data are scarce, however. This study combining the three physical quantities describing the sputtering process will therefore close a knowledge gap and be applicable not only to the Moon, but also to the sputtering of other planetary bodies.[1] B. Hapke, J. Geophys. Res. Planets 106 (2001) 10039–10073[2] P. Wurz, et al., Icarus 191 (2007) 486–496[3] P.S. Szabo, et al., Icarus 314 (2018) 98–105[4] H. Biber, et al., Nucl. Instrum. Methods. Phys. Res. B 480 (2020) 10–15[5] H. Biber, et al., Planet. Sci. J. 3 (2022) 271[6] M.J. Schaible, et al., J. Geophys. Res. Planets 122 (2017) 1968–1983[7] N. Jäggi, et al., Icarus 365 (2021) 114492[8] L.S. Morrissey, et al., J. Appl. Phys. 130 (2021) 013302[9] H. Hofsäss, A. Stegmaier, Nucl. Instrum. Methods. Phys. Res. B 517 (2022) 49–62[10] L.S. Morrissey, et al., ApJL 925 (2022) L6[11] R.M. Killen, et al., Planet. Sci. J. 3 (2022) 139

Sputtering by solar wind ions is a key process driving the ejection of high-energy particles into the exospheres of airless bodies like asteroids, Mercury and the Moon. In view of upcoming missions which will deliver new in-situ data on these exospheres like the Artemis program at the Moon and BepiColombo at Mercury, a deeper understanding of sputtering effects is crucial. In this work, we combine sensitive quartz crystal microbalance measurements and numerical simulations to quantify sputter yields of Apollo soil 68501 under solar wind relevant conditions. We find that none of the commonly used simulation codes can reliably predict laboratory sputter yields without experimental benchmarks. All of the employed packages significantly overestimate the sputter yields of flat samples by at least a factor of 2 for the case of hydrogen. When accounting for surface roughness and regolith-like porosity, sputter yields are decreased even further to 7.3E-3 atoms\ion and 7.6E-2 atoms\ion for H and He at solar wind energies of 1 keV\amu, respectively. The reduced yields of porous regolith structures are largely independent of the ion incidence angle, making them applicable across a wide range of lunar latitudes. This study highlights the need for experimental validation of sputtering models to ensure accurate predictions for space weathering and lunar exosphere composition.

MotivationSolar wind ion precipitation on airless rocky bodies like the Moon has been linked to a variety of processes summarised by the term space weathering, including darkening and a red shift in optical reflectance spectra, the formation of vesicles and implantation of ions in mineral grains and the formation of an amorphised rim therein [1]. Solar wind irradiation additionally leads to the release of surface material via the kinetic sputtering process. These ejecta undergo ballistic trajectories and take part in the formation of the Lunar exosphere, which is well documented by space missions and ground-based observations. To properly gauge the importance of the sputtering contribution, however, a more detailed physical implementation of the sputtering process is necessary than used so far in, e.g., [2]. Prior, this has typically been approximated by simulation codes using the binary collision approximation (BCA) like SRIM [3], see also the review in [4]. On the other hand, more recent literature suggests significant limitations of this software [5,6] – especially related to real-life surfaces like lunar regolith. We present a combined experimental and numerical work on the sputtering yields of lunar regolith and discuss the validity of popular BCA codes.MethodsExperimental data on sputtering yields, i.e., the amount of material released normalised per number of incidence ions, were obtained using a quarz crystal microblance (QCM) setup [7]. To overcome limitations on the sample configuration that come with this approach, an additional QCM served as a catcher for the sputtered material [8]. This allowed us to conduct experiments on rough bulk samples and additionally probe the ejecta angular distribution. We perform such experiments with lunar regolith (from Apollo 16 sample #68501) and helium and hydrogen ions at solar wind energies of 1 keV/nucleon. Numerical investigations were carried out both using SRIM and variants of SDTrimSP in its 1D and 3D version [9]. The latter in particular enables systematic investigations with varying surface roughness comparable to the experiments, as well as surface porosity resembling the regolith structures observed on the Moon [10].ResultsIn line with previous studies, a comparison for flat samples reveals that the BCA approach consistently overestimates sputtering yields, with SRIM deviating the most [6]. By introducing surface roughness through consideration of regolith grains, the sputtering yield as a function of incidence angle is both flattened and reduced. Both effects become even more pronounced when surface porosity is included in the description. Three-dimensional simulations match these results qualitatively and, when the initial offset is factored out, also quantitatively. We will present these findings in greater detail and discuss possible implications in the context of the lunar exosphere.[1] B. Hapke, J. Geophys. Res. Planets 106 (2001) 10039–10073.[2] P. Wurz, U. Rohner, J.A. Whitby, C. Kolb, et al., Icarus 191 (2007) 486–496.[3] J.F. Ziegler, M.D. Ziegler, J.P. Biersack, Nucl. Instrum. Methods. Phys. Res. B 268 (2010) 1818–1823.[4] P. Wurz, S. Fatemi, A. Galli, J. Halekas, et al., Space. Sci. Rev. 218 (2022) 10.[5] K. Wittmaack, Nucl. Instrum. Methods Phys. Res. B 380 (2016) 57–70.[6] P.S. Szabo, H. Biber, N. Jäggi, M. Brenner, et al., ApJ 891 (2020) 100.[7] G. Hayderer, M. Schmid, P. Varga, HP. Winter, et al., Rev. Sci. Instrum. 70 (1999) 3696–3700.[8] H. Biber, J. Brötzner, N. Jäggi, P.S. Szabo, et al., Planet. Sci. J. 3 (2022) 271.[9] U. von Toussaint, A. Mutzke, A. Manhard, Phys. Scr. 2017 (2017) 014056.[10] P.S. Szabo, A.R. Poppe, H. Biber, A. Mutzke, et al., Geophys. Res. Lett. 49 (2022) e2022GL101232