SDTrimSP Simulations Research Articles

Tritium-titanium target degradation due to deuterium irradiation for DT neutron production

In the present article, we have investigated tritium removal from tritium-titanium targets during fusion neutron production and the impact of tritium degradation on neutron production. The removal of tritium from the target is predicted for deuterium ion irradiation with the SDTrimSp code. We adopt the binary collision approximation method to simulate the recoils and projectile trajectories and concentration of constituents in the target. We have modelled four phenomena in our simulations; ion exchange, sputtering, outgassing of tritium, and thermal diffusion of hydrogen isotopes in the target caused by deuterium irradiation. Insignificant contributors such as burn-up of tritium in neutron production and loss of tritium due to radioactive decay are not included in our model. This tritium removal results in the nonuniform distribution of tritium in the target. A Python-based script is developed to investigate the effects of tritium removal on neutron production with these pristine and irradiated targets. This script uses the layered composition of the constituents’ atoms, DT reaction cross-section, and stopping power of deuterium ions in the target. The script is validated with the NeuSdesc code for the pristine target. Using the layered composition of tritium atoms in the target obtained from the SDTrimSp simulations, the script predicts the degradation in neutron production for different irradiation scenarios.

Establishing a Best Practice for SDTrimSP Simulations of Solar Wind Ion Sputtering

Solar wind (SW) ion irradiation on airless bodies can play an important role in altering their surface properties and surrounding exosphere. Much of the ion sputtering data needed for exosphere studies come from binary collision approximation sputtering models such as TRansport of Ions in Matter and its more recent extension, SDTrimSP. These models predict the yield and energy distribution of sputtered atoms, along with the depth of deposition and damage of the substrate, all as a function of the incoming ion type, impact energy, and impact angle. Within SDTrimSP there are several user-specific inputs that have been applied differently in previous SW ion sputtering simulations. These parameters can influence the simulated behavior of both the target and sputtered atoms. Here, we have conducted a sensitivity study into the SDTrimSP parameters in order to determine a best practice for simulating SW ion impacts onto planetary surfaces. We demonstrate that ion sputtering behavior is highly sensitive to several important input parameters including the ion impact angle and energy distribution and the ejected atom surface binding energy. Furthermore, different parameters can still result in similarities in the total sputtering yield, potentially masking large differences in other sputtering-induced behaviors such as the elemental yield, surface concentration, and damage production. Therefore, it is important to consider more than just the overall sputtering behavior when quantifying the importance of different parameters. This study serves to establish a more consistent methodology for simulations of SW-induced ion sputtering on bodies such as Mercury and the Moon, allowing for more accurate comparisons between studies.

Graphical user interface for SDTrimSP to simulate sputtering, ion implantation and the dynamic effects of ion irradiation

SDTrimSP is a popular simulation program to compute several effects of the interaction between an impinging ion and a solid, such as ion implantation ranges, damage formation or sputtering of surface atoms. We now introduce a graphical user interface for SDTrimSP to make its operation more accessible for a broad group of users. It is written as a separate Python program and is not restricted to any specific operating system. The interface allows a quick and easy start as well as the direct evaluation of SDTrimSP simulations. Its capabilities are demonstrated here in the form of several example cases, including the dynamic simulations with SDTrimSP, where ion-induced target changes are taken into account. The presented graphical user interface is made freely available to support a large number of users in performing simulations of ion–solid interaction.

Deuterium plasma sputtering of mixed Be-W layers

In ITER, the first mirrors would be vulnerable to deposition from the first wall materials, namely beryllium (Be) and tungsten (W) in mixed compositions. In this study we investigate the capactively coupled RF plasma sputtering of such mixed Be-W deposits on rhodium-coated substrates, using deuterium as the process gas, and track the enrichment of W during the sputtering process. Experiments were conducted on Be-W deposits with W concentration varying from 2 to 8.3 at.%, and maximal deuterium ion energies in the plasma of 70 and 220 eV. The evolution of the W concentration in the deposits during the plasma sputtering depended considerably on both deuterium ion energy as well as the initial concentration of W in the films. With 220 eV sputtering, a rapid enrichment of W was observed, with all the W changing from BexW before sputtering to metallic W and its oxides after sputtering. With 70 eV sputtering, there was no net change in the W concentration, as long as the initial concentration of W in the film was below 4 at.%. Moreover, the W remained considerably in the state of BexW after the sputtering as well. However, a W enrichment was also observed with 70 eV sputtering, when the W concentration in the films was higher (8.3 at.%). The rate of W enrichment was also observed to increase monotonically with the increase in the initial concentration of W in the deposits. SDTrimSP simulations performed with experimental parameters indicate that sputtering yield of Be increases with W concentrations in the Be-W layers due to backscattering of D projectiles off W atoms. These studies show that low energy deuterium sputtering of mixed Be-W layers with low W concentrations, allows for removal of Be without rapidly enriching W in the films, making it a promising option for ITER.

Rough-surface effect on sputtering of Cr bombarded by low-energy He plasma

• Microscopic cones grow on a Cr surface during He plasma exposure. • Angular distribution of sputtered atoms becomes more over-cosine-like (narrower). • Sputtering yield is significantly reduced. • GITR coupled with SDTrimSP simulations reproduce the experimental findings. The formation of microscopic cone-like structures on a Cr surface is observed under He plasma exposure at a low incident ion energy of ∼106 eV in the PISCES-A linear plasma device. 2D emission profiles of the Cr I line intensity, measured with a hyperspectral imaging camera, are compared with those modeled using the GITR Monte Carlo impurity transport code coupled with the SDTrimSP binary collision code. With the growth of cones, the experimental 2D emission profile becomes axially elongated and the emission intensity significantly drops, and these observations are successfully reproduced in the simulation. The simulation reveals that the deposition of sputtered Cr atoms onto the neighboring cones results in a more over-cosine-like angular distribution, i.e., a narrower distribution with more particles ejected in the surface normal direction, of sputtered Cr atoms with a significant reduction of the sputtering yield.

Experimental Insights Into Space Weathering of Phobos: Laboratory Investigation of Sputtering by Atomic and Molecular Planetary Ions

AbstractInvestigating the space weathering of the Martian moon Phobos represents an important step toward understanding the development from its origin to its present‐day appearance. Depending on Phobos’ orbital position, its surface is continuously sputtered by the solar wind and planetary ions that originate in the Martian atmosphere. Based on Mars Atmosphere and Volatile Evolution measurements, it has been proposed that sputtering by planetary O+ and O2+ ions dominates in the Martian tail region, where the planet mostly shadows Phobos from the solar wind. In these models, uncertainties for sputtering yield inputs still exist due to the lack of sufficient analog experiments. Therefore, sputtering measurements with O+, O2+, C+, and CO2+ ions between 1 and 5 keV were performed using augite samples as Phobos analogs. The experimental results for O+ irradiations show smaller mass changes than predicted by SDTrimSP simulations, which probably can be attributed to O implantation enabled by the Fe content of the target. Sputtering with O2+ and CO2+ in the low keV range shows no deviations in the sputtering yields attributable to molecular effects. Therefore, CO2+ ions will most likely be negligible for the sputtering of Phobos according to the current understanding of ion fluxes on the Martian moon. Ultimately, our experiments suggest that the sputtering contribution on Phobos by O ions is about 50% smaller than previously assumed. This does not change the qualitative outcome from previous modeling stating that planetary O ions are by far the dominant sputtering contribution on Phobos in the Martian tail region.

Solar wind Helium ion interaction with Mg and Fe rich pyroxene as Mercury surface analogue

The surface of Mercury is continuously exposed to impinging solar wind ions. To improve the understanding of space weathering and exosphere formation, a detailed investigation of the ion-surface interaction is necessary. Magnesium and iron rich pyroxene (Ca,Mg,Fe)2[Si2O6] samples were used as analogues for Mercury’s surface and irradiated with He+ ions at solar wind energies of 4 keV. Several regimes of implantation and sputtering were observed there. The total estimated mass of implanted He coincides with the mass decrease due to He outgassing during subsequent Thermal Desorption Spectroscopy measurements. Comparison to established modeling efforts and SDTrimSP simulations show that a He saturation concentration of 10 at.% has to be assumed. A complete removal of He is observed by heating to 530 K. On the surface of Mercury, temperatures between about 100 K and 700 K are expected. This temperature will therefore influence the implantation and release of He into Mercury’s exosphere.

Dynamic Potential Sputtering of Lunar Analog Material by Solar Wind Ions

Abstract Pyroxenes ((Ca, Mg, Fe, Mn)2Si2O6) belong to the most abundant rock forming minerals that make up the surface of rocky planets and moons. Therefore, sputtering of pyroxenes by solar wind ions has to be considered as a very important process for modifying the surface of planetary bodies. This is increased due to potential sputtering by multiply charged ions; to quantify this effect, sputtering of wollastonite (CaSiO3) by He2+ ions was investigated. Thin films of CaSiO3 deposited on a quartz crystal microbalance were irradiated, allowing precise, in situ, real time sputtering yield measurements. Experimental results were compared with SDTrimSP simulations, which were improved by adapting the used input parameters. On freshly prepared surfaces, He2+ ions show a significant increase in sputtering, as compared to equally fast He+ ions. However, the yield decreases exponentially with fluence, reaching a lower steady state after sputtering of the first few monolayers. Experiments using Ar8+ ions show a similar behavior, which is qualitatively explained by a preferential depletion of surface oxygen due to potential sputtering. A corresponding quantitative model is applied, and the observed potential sputtering behaviors of both He and Ar are reproduced very well. The results of these calculations support the assumption that mainly O atoms are affected by potential sputtering. Based on our findings, we discuss the importance of potential sputtering for the solar wind eroding the lunar surface. Estimated concentration changes and sputtering yields are both in line with previous modeling for other materials, allowing a consistent perspective on the effects of solar wind potential sputtering.

Angle-dependent sputter yield measurements of keV D ions on W and Fe and comparison with SDTrimSP and SDTrimSP-3D

The influence of surface roughness on the sputter yield was investigated as a function of the angle of incidence. In this work, nm-smooth and rough samples with roughnesses on the µm length scale were produced by depositing thin Fe and W films on smooth and rough Si substrates via magnetron sputtering. The surface morphology of the samples was determined by atomic force microscopy. The samples were exposed to a 6 keV D3+ ion beam (2 keV/D) under various angles of incidence ranging from 0° to 75° with respect to the surface normal to fluences of the order of 1022 D/m2. The layer thickness was measured by Rutherford backscattering spectrometry (RBS) before and after erosion. The resulting sputter yields were compared to simulations performed with SDTrimSP (static and dynamic) and SDTrimSP-3D (static), showing good qualitative agreement in all cases, as well as agreement with literature data at normal incidence. This constitutes the first experimental benchmark of SDTrimSP-3D. A discrepancy in the value of the sputter yield for smooth W at normal incidence was observed between the SDTrimSP simulations and the experimental values obtained in this work and found in literature. Analogous experiments were performed to study the sputter yield at normal incidence of 2 keV/D on smooth Au and 6 keV He on smooth W. These sputter yields were also compared to SDTrimSP simulations and literature, showing good agreement in all cases.

Tungsten surface enrichment in EUROFER and Fe-W model systems studied by high-resolution time-of-flight rutherford backscattering spectroscopy

Time-of-flight Rutherford backscattering spectroscopy (ToF-RBS) with incident heavy ions offers a substantially improved depth resolution compared to conventional RBS with incident He ions. The new Garching ToF-RBS detector is located at a scattering angle of 150° with a free flight path of 1.313 m and a time resolution of 600 ps. The experimentally achieved depth resolution at the surface of W-containing Fe samples is 2–3 nm using incident Si ions. Model systems consisting of Fe layers with 0.7, 1.5 and 4.2 at% W and EUROFER steel (containing 0.34 at% W + Ta) were eroded by 200 eV D ions to a fluence of 1023 D/m2 at 310 K and 900 K. W depth profiles were measured using ToF-RBS, light impurities at the surface were detected using time-of-flight elastic recoil detection analysis (ToF-ERDA). The data sets from the two techniques were analyzed self-consistently. In all cases W enrichment at the surface was observed. The enrichment at 310 K is in good agreement with SDTrimSP simulations if the experimentally achieved depth resolution is taken into account. Annealing of the model systems with 0.7 and 1.5 at% W to 900 K for 10 h in vacuum resulted also in a W-enriched surface layer.

Defect formation in multiwalled carbon nanotubes under low-energy He and Ne ion irradiation.

The mechanical, structural, electronic and magnetic properties of carbon nanotubes can be modified by electron or ion irradiation. In this work we used 25 keV He+ and Ne+ ion irradiation to study the influence of fluence and sample thickness on the irradiation-induced damage of multiwalled carbon nanotubes (MWCNTs). The irradiated areas have been characterised by correlative Raman spectroscopy and TEM imaging. In order to preclude the Raman contribution coming from the amorphous carbon support of typical TEM grids, a new methodology involving Raman inactive Au TEM grids was developed. The experimental results have been compared to SDTRIMSP simulations. Due to the small thickness of the MWCNTs, sputtering has been observed for the top and bottom side of the samples. Depending on thickness and ion species, the sputter yield is significantly higher for the bottom than the top side. For He+ and Ne+ irradiation, damage formation evolves differently, with a change in the trend of the ratio of D to G peak in the Raman spectra being observed for He+ but not for Ne+. This can be attributed to differences in stopping power and sputter behaviour.

Erosion of EUROFER steel by mass-selected deuterium ion bombardment

The erosion behaviour of EUROFER steel due to mono-energetic deuterium (D) ion bombardment in the energy range of 100–1000 eV/D was investigated. At low fluences, the sputtering yield is comparable to that of pure iron (Fe). It then decreases with increasing fluence and tends toward a steady state at larger fluences. The largest experimentally investigated fluences are of the order of several 1024 D/m2. The yield reduction is more pronounced for lower D impinging energies. A simple model is presented within which the evolution of the yield can be described by an exponential decay, using empirical values for the fitting parameters. In this model, the yield reduction is caused by preferential sputtering of Fe and the consequent development of a tungsten- (W-) enriched surface layer. SDTrimSP simulations also confirm the appearance of a W-enriched surface layer. However, the experimentally observed fluence dependence of the sputtering yield could not satisfactorily be reproduced in these simulations. The resulting enrichment layer thicknesses below 1 Å stretch the physics model implemented in SDTrimSP beyond its validity range. Experimentally, surface enrichment of W was qualitatively confirmed in Rutherford backscattering and sputter X-ray photoelectron spectroscopy measurements. However, the measured depth profiles are very likely influenced by the observed surface roughening caused by the D irradiation. Electron microscopy revealed the inhomogeneous W distribution on the un-irradiated sample as well as a grain-dependence of erosion and grain-dependent topography development. The inclusion of surface topography in the description of the erosion could be the key to improve the agreement between model predictions and the experiment.

Transient effects during erosion of WN by deuterium ions studied with the quartz crystal microbalance technique

Transient effects during erosion of polycrystalline tungsten-nitride (WN) films by mono-energetic deuterium projectiles are studied using a quartz crystal microbalance technique. The evolution of the mass removal rate of a 360nm thin WN film under 500eV/D and 1000eV/D bombardment is investigated at a temperature of 465K in situ and in real-time as a function of the deuterium fluence. The measurements are performed at a typical flux of 1018m−2s−1. A strong dependency of the observed mass change rate on the deuterium fluence is found. The mass loss is initially higher than for pure tungsten (W) and drops with fluence, finally reaching the same steady state value as for pure W sputtering. Steady state surface conditions are obtained at a fluence of about 0.2×1023D/m2 for 500eV/D and 0.6×1023D/m2 for 1000eV/D. SDTrimSP simulations indicate a preferential removal of N and a corresponding W enrichment of the surface.

Experimental analysis and WallDYN simulations of the global nitrogen migration in ASDEX Upgrade L-mode discharges

This work presents ASDEX Upgrade experiments, where the nitrogen deposition and re-erosion on divertor manipulator samples and the effect of its transport through the plasma were studied. These results are compared to WallDYN-DIVIMP simulations based on SOLPS plasma backgrounds and employing an improved WallDYN model, which includes the effusion of nitrogen from saturated surfaces. On one hand, this allows the WallDYN code and the new saturation model with a comprehensive data set to be benchmarked, on the other hand the simulations help in the interpretation of the experimental results. Both, experimental results and simulations, show that the N content in the region of the outer strike line reaches its steady-state value within one discharge. The simulations also reproduce the experimentally observed nitrogen content in samples exposed to N2-seeded discharges. With respect to the boron deposition, the nitrogen deposition in a non-seeded discharge and the re-erosion of nitrogen discrepancies to the WallDYN-DIVIMP simulations are observed. Based on SDTrimSP simulations, these are attributed to the missing depth resolution of the WallDYN surface model. A detailed comparison of spectroscopic measurements to WallDYN simulations, based on a novel synthetic spectroscopy diagnostic for WallDYN, shows that the nitrogen fluxes in the plasma are well described by the simulations. From a comparison of several WallDYN-DIVIMP simulations employing customized onion-skin model plasma backgrounds the physical processes controlling the nitrogen concentration in the core plasma and the applicability of onion-skin model plasma backgrounds are discussed. From these simulations the private flux zone with the gas valve, the outer baffle and the high field side main wall are identified as the main sources for the nitrogen content of the core plasma.

Interaction of deuterium plasma with sputter-deposited tungsten nitride films

Magnetron-sputtered tungsten nitride (WNx) films were used as a model system to study the behaviour of re-deposited WNx layers which could form in fusion devices with tungsten (W) wall during nitrogen seeding. The interaction of such WNx layers with deuterium (D) plasmas was investigated in dedicated laboratory experiments. D retention and N removal due to D plasma exposure (D flux: 9.9 × 1019 D m−2 s−1, ion energy 215 eV) at different temperatures were measured with ion beam analysis (IBA). Low-energy argon sputtering followed by IBA was applied to resolve the D distribution in the top-most surface of WNx with significantly improved depth resolution compared with the standard D depth profiling method by nuclear reaction analysis. Experimentally determined thicknesses for the penetration of D in WNx were compared with the penetration depth for D calculated in SDTrimSP simulations. Results show that D is only retained within the ion penetration range for samples exposed at 300 K. In contrast to the 300 K case, D diffuses beyond the implantation depth in a sample exposed at 600 K. However, the D penetration depth is much lower than in pure W at comparable conditions. The total amount of retained D in WNx at 600 K is by 50% lower than for implantation at 300 K with the same D fluence. Nitrogen is removed only within the D ion range.

MD simulations of low energy deuterium irradiation on W, WC and [formula omitted] surfaces

According to the present design beryllium (Be), tungsten (W) and carbon (C) will be the plasma facing materials in the ITER fusion reactor. Due to sputtering and subsequent re-deposition, mixing of these materials will occur. In this context, molecular dynamics simulations of cumulative, low energy and high flux D bombardment of pure W and tungsten carbides (WC, W2C) were carried out. The retention and sputtering properties as well as the structural deformation were analysed and comparisons to SDTrimSP simulations were made. Almost no tungsten is sputtered in the energy range considered and the D backscattering is lower in pure tungsten than in any of the tungsten carbides. In WC and W2C, the deuterium is mainly trapped forming small molecules, whereas mostly atomic D is present in pure W. The C sputtering increases with C content in the material, and shows a peak at the bombardment energy ∼50eV, most likely due to the swift chemical sputtering mechanism. Pure W is seen to lose its crystallinity in the areas where D is present. After the D irradiation, the composition of both WC and W2C is mostly W in the topmost layers, due to preferential sputtering of C, an amorphous D–C mixture underneath and an undisturbed lattice in the rest of the cell.