Low-energy Helium Plasma Research Articles

Effects of surface anisotropy on the surface morphological response of plasma-facing tungsten

Surface free energy has been hypothesized as the cause behind the experimental evidence (Parish et al., 2014) of a strong correlation between crystallographic orientation and surface morphology changes in helium ion-irradiated tungsten at the early stages of ‘fuzz’ growth in fusion plasma-facing tungsten. Here, this hypothesis is tested through self-consistent dynamical simulation based on an atomistically informed model of surface evolution in plasma-irradiated tungsten. Low-energy helium plasma irradiation of a tungsten surface, even when the ion energy is below the sputtering threshold, generates surface vacancy-adatom pairs in addition to adatoms from the flux of self-interstitial atoms to the surface resulting from the growth of high-pressure helium bubbles. Along with the stress-driven surface morphological instability as the primary driving force, preferential diffusion of these surface adatoms modulated by the associated Ehrlich–Schwoebel (ES) barriers, combined with the surface free energy anisotropy drive the anisotropic surface nanostructure growth. Simulation results for helium irradiation on W(100) show growth of pyramidal surface features, akin to experimental observations. This work confirms the hypothesis on the effect of tungsten surface crystallographic orientation on helium-ion-induced surface morphology changes, where surface mound formation is the outcome of a stress-driven surface morphological instability, while the anisotropic growth of the mounds is controlled by the ES-barrier-modulated anisotropic surface adatom diffusion and faceting of the mounds is controlled by the surface free energy anisotropy.

Formation of orientation-dependent surface morphologies on tungsten after low energy helium plasma exposure: multiscale characterization and new insights

In ITER, the helium (He) impurity produced by the deuterium-tritium reaction will bombard the tungsten (W) divertor armor at the strike points. Consequently, strong interaction occurs therein that both impact the performance of the plasmas and the lifetime of the divertor. Despite an ever-increasing understanding of this interaction, some experimental phenomena remain mysterious, especially the formation of orientation-dependent surface morphologies. Here, we combine multiscale experimental characterization and theoretical models to shed new light on this problem. After low-energy He plasma exposure in a linear plasma generator, the polycrystalline W surface developed various morphologies. Through electron backscatter diffraction analysis, we found that the {111} grains developed cube-corner structures, the {110} grains developed ripple structures, whereas the {100} grains remained smooth. Then, electron-transparent lamellae were extracted from such grains to observe the subsurface He bubbles by transmission electron microscopy. The volume density, size distribution, and depth range of the He bubbles weakly depend on the crystallographic orientation, suggesting that the migration of W atoms causes the morphology variety. Accordingly, we proposed a two-stage formation mechanism. First, W atoms generated by over-pressurized He bubbles glide on the slip plane and in the slip direction to reach the surface, forming characteristic patterns that are enclosed by the slip traces. Second, morphological instability drives the evolution of the surface patterns, in which the initial surface structure and surface self-diffusion kinetics mediate. The proposed mechanism has been incorporated into a topographical instability model to enable asemi-quantitative analysis. The obtained new insights are valuable to the impurity control of the core plasmas and the lifetime analysis of the divertor for ITER.

Surface modification of W–Ta–V–Cr multi-component alloy after low-energy He plasma irradiation

This study explores a hypothetical scenario where low-activation refractory multi-component alloys (RMCAs) are considered for use as divertor target materials in fusion reactors. To investigate the surface modifications under divertor service conditions, a multi-phase W–Ta–V–Cr RMCA is irradiated with low-energy helium (He) plasma at varying temperatures to approximately 5.0 × 1025 He·m−2. The W-rich and Cr-rich phases in the multi-phase RMCA mimic the mono-phase W–Ta–V–Cr RMCA and segregation at grain boundaries, respectively. Following irradiation, fuzz layer formation is observed for all phases at temperatures lower than pure W requires. Additionally, nano He bubbles are identified in the fuzz tendrils at 920 °C. The modified layers exhibit reduced V and Cr content with increasing temperature, transitioning into W-Ta binary alloys at 920 °C. Notably, the fuzz layer on the W-rich phase is thinner than that on pure W at 920 °C. While a thinner fuzz layer suggests the alloy’s potential as a divertor target material, challenges include a lower fuzz formation temperature and potential high V and Cr sputtering yield, raising concerns for plasma contamination during fusion reactor operation. This dual perspective highlights both the promise and challenges of utilizing RMCAs as divertor target materials in severe fusion reactor environments.

Evolution of irradiation damage in tungsten matrix and Y2O3 phase after low energy helium plasma exposure

Tungsten is the preferred material for the first wall. In this paper, W-0.5 %wt Y2O3 was selected for the exposure test of helium plasma, so that fuzz tendrils of considerable thickness were formed on its surface. By using ultrasonic dispersion, the tendril dispersion was obtained and observed through TEM. The fuzz tendrils on the W matrix surface were seen to have a complete tungsten crystal structure, along with bubbles of different sizes. The outer layer of the tendril is composed of an amorphous material, which is visible on the surface of the fuzz tendril. This is likely due to the subsequent of helium plasma exposure damage. Fuzz tendrils only appeared on the W matrix, whereas Y2O3 particles created nano-protrusions and samll cracks on the surface. The nano-protrusion morphology and EDS analysis, including the lack of Y in fuzz tendrils, indicate that Y2O3 particles were unable to form a fuzzy structure. It is evident that the defects distribution of W matrix and Y2O3 particles is altered due to helium plasma exposure. Near the surface of Y2O3 particles, fewer but larger bubbles were observed, whereas further away, smaller but denser bubbles were seen. In W matrix, bubbles were formed at a deeper level and were larger than the deepest bubbles in Y2O3 particles, likely caused by the lower migration energy. The different responses of W matrix and Y2O3 particles to helium plasma exposure have combined to further reveal the potential of increasing the plasma exposure resistance of plasma-oriented tungsten based materials.

Enhanced radiation resistance of W-based HEA under helium-ion irradiation conditions

Recently, high entropy alloy (HEA) has demonstrated superior radiation resistance to energetic particles. In this paper, we examine the radiation response of W-based HEA (WMoTaVNb) under the low-energy helium plasma exposure. As compared to pure W, WMoTaVNb demonstrated significant suppression in surface nanofuzz growth after the 65 eV helium plasma exposure to the same fluence (2 × 1024 He/m2). In addition, the 1 MeV helium ion implantation results present the sluggish helium bubble growth in W-based HEA, which is suspected to be the dominant cause to delay the nanofuzz growth. These results are further interpreted by comparing the transport of interstitial defects toward the surface regions in both pure W and W-based HEA via the molecular dynamic (MD) atomistic simulations. Our results show that due to chemical complexity, defect diffusion is highly localized in HEA, suggesting that the sluggish motion of interstitial defects may play another critical role in delaying surface nanofuzz growth if trap mutation and surface adatom formation mechanism dominates the fuzz growth.

Irradiation-enhanced superficial modification and evolution of mechanical behavior in TaTiNbZr refractory high entropy alloy films exposed to low energy helium plasma

In this work, TaTiNbZr refractory high entropy alloy (RHEA) films constructed with nano-crystalline and amorphous composite structures were irradiated by helium (He) plasma with a fixed flux of 5 × 1022 m−2·s−1 under bias-voltage driven plasma energy (0–70 eV) to systematically explore the evolution of morphological modification and mechanical properties. The results indicated that the RHEA exhibited composite structures after He plasma irradiation, and the irradiation-enhanced surface blisters were strongly dependent on He plasma energy. After exposure to He plasma, the size and density of blisters on the surface of RHEA films gradually increased with the enhancement of plasma energy, these blisters originated from the formation of bulges underneath the surface which were induced by the large-sized bubbles. Some blister bursts were initiated at a critical energy of 50 eV (irradiation temperature was 600 K). Non-obvious superficial sputtering can be observed for the RHEA films irradiated at energy less than 50 eV. At the He plasma energy of 70 eV, cracks and abundant blister exfoliation were produced on the RHEA surface due to the synergistic effect of enhanced energy and irradiation temperature (650 K). Surface sputtering and the thermal diffusion-driven mass flow were induced by He plasma due to the sputtering threshold energies in Ti and Zr elements than that of Ta and Nb elements, which were responsible for the growth of fiber-like structures on the surface of RHEA film as the energy was fixed at 70 eV. Moreover, the maximum hardening ratio of 21.4% was revealed from the TaTiNbZr RHEA nano-film irradiated at plasma energy of 50 eV, the role of plasma energy on the evolution of mechanical properties, and the corresponding mechanisms in response to irradiation damage were also discussed in detail.

Modeling tungsten response under helium plasma irradiation: a review

Tungsten, a leading candidate for plasma-facing materials (PFM) in future fusion devices, will be exposed to high-flux low-energy helium plasma under the anticipated fusion operation conditions. In the past two decades, experiments have revealed that exposure to helium plasma strongly modifies the surface morphology and hence the sputtering, thermal and other properties of tungsten, posing a serious danger to the performance and lifetime of tungsten and the steady-state operation of plasma. In this article, we provide a review of modeling and simulation efforts on the long-term evolution of helium bubbles, surface morphology, and property changes of tungsten exposed to low-energy helium plasma. The current gap and outstanding challenges to establish a predictive modeling capability for dynamic evolution of PFM are discussed.

Molecular dynamics simulations of cascade overlap with Void/Helium bubble

When tungsten (W) is used as a plasma-facing material in fusion reactors, it will suffer irradiation from the high-energy neutron and high-flux low-energy helium (He) plasma, affording voids and He bubbles with varying helium-to-vacancy ratios (He/V). With the accumulation of irradiation dose, an energetic neutron is very likely to trigger collision cascades close to the pre-existing voids/He bubbles. Therefore, in this study, we ran systematic molecular dynamics simulations of the cascades that overlapped with voids/He bubbles in W. We investigated the relationship between the evolution of the stress field and defect formation. Cascades overlapping with the bubbles with He/V values between 1 and 2 produced the least defects; alternatively, other bubbles could facilitate defect formation. Furthermore, two mechanisms for generating dislocation loops, the cascade-destruction and the over-pressured bubble-assist types, were identified and can be attributed to subcascade formation. When there are voids or under-pressured bubbles, the cascade-destruction mechanism can yield vacancy-type dislocation loops; however, the existence of He atoms can inhibit this behavior. In contrast, when the bubbles are over-pressured, the bubble-assist mechanism can contribute to the formation of self-interstitial atom-type dislocation loops. This study helps to improve our understanding of radiation damage in a complicated environment in experiments with multiple irradiations or fusion reactors.

Defect evolution in tungsten exposed to helium plasma and deuterium plasmas studied by slow positron beam

• W samples have been exposed to He plasma and D plasmas. • DBS-PA technique has been used to characterize the defects in W. • Low energy plasma exposure could create defects in W. • D plasmas exposure could reduce the detectable defects in He plasma exposed W. Comparison of defect formation in tungsten samples exposed only to a helium plasma and those exposed to both helium plasma and subsequent deuterium plasmas is performed via Doppler broadening spectroscopy in the positron annihilation technique (DBS-PA). The S parameters of tungsten samples after helium plasma exposure increased with the increase of the incident fluence, which indicates that low energy helium plasma could induce the formation of defects such as nano-sized helium bubbles and dislocations in tungsten. The S parameters of samples exposed to helium incident fluence of 7.9 × 10 21 He/m 2 enhanced after deuterium plasmas exposure, indicating that low energy deuterium plasmas could form defects in tungsten too. Unexpectedly, deuterium plasmas exposure did not significantly influence the S parameters of the tungsten sample with helium incident fluence of 3.9 × 10 22 He/m 2 , and reduced the S parameters of the tungsten sample with helium incident fluence of 7.9 × 10 22 He/m 2 . These variations of S parameters could be associated with the occupation of nano-sized helium bubbles by deuterium atoms. This indicates possible competition between the increased density of vacancy-type defects by deuterium plasmas exposure and the filling of nano-sized helium bubbles by deuterium atoms, reducing detectability of these defects.

High-quality graphene as a coating for polycrystalline tungsten in low-energy helium and deuterium plasma exposures

• Graphene acts as a sputter shield against low energy helium and deuterium plasmas. • High ion fluences can limit the functional range for graphene coatings. • Tungsten fuzz growth can be slowed down by coating polished surfaces with graphene. In the presence of irradiation by energetic ions, plasma facing components (PFCs) tend to develop surface morphologies that lead to mass loss of the wall material, potentially diminishing their lifetime and plasma performance. We explore the performance of graphene as a coating for plasma facing components to protect against sputtering due to low-energy ions. We show that graphene can slow changes in surface morphology of tungsten subjected to energetic-helium and deuterium ion bombardment over a wide range of energies, as tested in the PISCES-A facility at UC-San Diego. We exposed tungsten samples half-coated with graphene in the PISCES-A facility, at energies of 40 and 140 eV and fluences ranging from 1 × 10 24 - 3.6 × 10 25 ions/m 2 . We found that tungsten “fuzz” growth at high fluences (10 24 –10 25 He-D/m 2 ) can be reduced by approximately 30%. At the lower fluence and energy, no fuzz formed. Using scanning electron microscopy and focused-ion beam machining, we investigate these changes in surface morphology. Deuterium results do not show the graphene defect production to be as energy or fluence dependent as helium bombardment. We determine damage to and the lifetime of the graphene membrane with Raman spectroscopy for vacuum components subjected to such extreme environmental conditions.

Modeling of the impurity-induced silicon nanocone growth by low energy helium plasma irradiation

The formation mechanism of nanocone structure on silicon (Si) surface irradiated by helium plasma has been investigated by experiments and simulations. Impurity (molybdenum) aggregated as shields on Si was found to be a key factor to form a high density of nanocone in our previous study. Here to concrete this theory, a simulation work has been developed with SURO code based on the impurity concentration measurement of the nanocones by using electron dispersive x-ray spectroscopy. The formation process of the nanocone from a flat surface was presented. The modeling structure under an inclining ion incident direction was in good agreement with the experimental result. Moreover, the redeposition effect was proposed as another important process of nanocone formation based on results from the comparison of the cone diameter and sputtering yield between cases with and without the redeposition effect.

The influence of impurities on the formation of nanocone structures on silicon surface irradiated by low energy helium plasma

Forming nanocone structures on a silicon (Si) surface by low (<100 eV) energy helium plasma has been proposed in recent years as a simple method for fabricating black Si, which is an attractive material for photon absorption through the solar spectrum. In this study, different appearances of the Si surface were observed and analyzed with a scanning electron microscope. By introducing impurities of molybdenum and tungsten during plasma irradiation, it was revealed that the formation and the distribution of nanocones have a clear dependence on the amount of impurities on the surface.

Evolution of vacancy defects in heavy ion irradiated tungsten exposed to helium plasma

Evolution of vacancy-type defects has been investigated in undamaged and copper ion pre-damaged tungsten exposed to low-energy and high-flux helium plasma (60 eV, 1 × 1022 He/m2s). The results measured by Doppler broadening positron annihilation spectroscopy (DB-PAS) indicate that helium-vacancy complexes generate due to intense self-trapping in the undamaged tungsten after helium plasma exposure. In contrast, the occupation of pre-existing vacancies and vacancy clusters, caused by the presence helium atoms, plays a dominant role in the pre-damaged tungsten, but the density of vacancy-type defects in the pre-damaged case is still higher than that in the undamaged case. This means that the dominating process of helium-vacancy complexes formation in tungsten changes from self-trapping to vacancy-trapping in the case of high vacancy density. Meanwhile, the elastic recoil detection analysis (ERDA) and transmission electron microscopy (TEM) results reveal that these pre-existing defects can increase helium retention and helium nano-bubble size. However, it is surprising that in the case of pre-damaged sample, the density/volume of vacancy-type defects also decreased even outside of the helium distribution depth. We attribute this phenomenon to the interstitial dislocation loops punched by helium clusters preferentially diffusing into the pre-damage regions of tungsten far beyond the helium distribution depth, resulting in the significant recombination with vacancies or vacancy clusters. This intrinsic mechanism is further verified by TEM observations and molecular dynamics (MD) simulations.

Helium-plasma–induced straight nanofiber growth on HCP metals

Low-energy helium (He) plasma irradiations were conducted on ruthenium (Ru) and rhenium (Re), which have hexagonal close packed (HCP) crystal structures. Growth of linear shaped fiberform nanostructures were identified on the surfaces of the both metals after the He plasma irradiation. We also conducted He plasma irradiation while Re particles were deposited on tungsten substrate; 3-mm-thick large scale fiberform nanostructures were grown on the surface. The crystal orientation was analyzed using diffraction patterns of Re and Ru nanofibers together with detailed transmission electron microscope observations. It was found that the growth of linear nanofibers has a preferential crystal orientation in the growth direction and it is always in the c-direction of the HCP crystals. Potential growth processes and mechanisms are proposed based on the experimental observations.

Nanoscale modification of silicon and germanium surfaces exposed to low-energy helium plasma

Complex surface nanostructures were observed in germanium and silicon samples exposed to low energy (24 or 36 eV ion kinetic energy) helium plasma. Pyramidal growth is observed in germanium across the temperature range studied (185 °C to 336 °C), while significant modification in silicon was only observed at 630 °C. Nano-wire growth was observed in both germanium and silicon, and appears to be linked to the strength of the electric field, which in turn determines the implantation energy of the helium ions. Nanostructure formation is proposed to be driven by surface adatom migration which is strongly influenced by an Ehrlich-Schwoebel-type surface instability. The role of helium in this model is to drive germanium interstitial formation by ejecting germanium atoms from lattice sites, leading to germanium interstitial diffusion towards the sample surface and subsequent adatom and surface nanostructure formation.

Selective Engineering of Chalcogen Defects in MoS2 by Low-Energy Helium Plasma

Structural defects in two-dimensional transition-metal dichalcogenides can significantly modify the material properties. Previous studies have shown that chalcogen defects can be created by physical sputtering, but the energetic ions can potentially displace transition-metal atoms at the same time, leading to ambiguous results and in some cases, degradation of material quality. In this work, selective sputtering of S atoms in monolayer MoS2 without damaging the Mo sublattice is demonstrated with low-energy helium plasma treatment. Based on X-ray photoelectron spectroscopy analysis, wide-range tuning of S defect concentration is achieved by controlling the ion energy and sputtering time. Furthermore, characterization with scanning transmission electron microscopy confirms that by keeping the ion energy low, the Mo sublattice remains intact. The properties of MoS2 at different defect concentrations are also characterized. In situ device measurement shows that the flake can be tuned from a semiconducting to metallic-like behavior by introducing S defects due to the creation of mid-gap states. When the defective MoS2 is exposed to air, the S defects are soon passivated, with oxygen atoms filling the defect sites.

Black silicon with nanostructured surface formed by low energy helium plasma irradiation

Black silicon formed by helium plasma irradiation has attractive surface morphologies in terms of solar cell fabrication, because it has good photon absorption property across the solar spectrum. It also does not need any masking process, which is sometimes employed in the fabrication of pyramid-like microstructures. Black silicon formed by helium plasma irradiation has potential not only for solar cell applications, but also in the fields of photonics, optics, and ultrasonic wave generation. Here, the nano-cone structure formation process, detailed observations of surface morphologies with TEM as well as SEM, the optimal He irradiation conditions, surface reflectivity over UV–visible-NIR wavelengths, and scattered light enhancement with Raman spectroscopy in addition to X-ray diffraction measurements were studied. Investigation of electronic properties is reported, including good resistivity of the Si black surface without an ITO electrode.

Low-energy helium plasma effects on textured micro-porous tungsten

The effects of low-energy helium plasma on surface evolution and restructuring of micro-porous tungsten and tungsten-rhenium micro-engineered materials was experimentally investigated. Tungsten foam samples of 45 and 80 Pores Per Inch (PPI) of three types of textured surface architecture (nodular W, dense Re-pillars, and W-coated Re-pillars) were exposed to low-energy helium plasma at 80 and 200 eV to fluence levels of 1, 2, and 7 × 1025 m−2 at 1123 K. The sidewalls of Re pillars exhibited surface holes due to trapped helium while their tips were found to form complex nano-scale web structures, 100's of microns across, rather than nano-tendrils (fuzz) that are often observed in flat W samples. A nano-necklace bead/string structure was observed for the first time on Re pillars, likely driven by a high density of nano-meter size helium bubbles crowded near pillar tips. W-coated Re pillars exhibited typical fuzz nanostructures seen on flat and untextured W-foam, albeit much shorter. While the height of W-fuzz was found to depend on the square root of ion fluence, the nano-web height was found to be linearly dependent on ion fluence. The fuzz was found to grow much slower on nodular foams as compared to planar tungsten, by as much as 25% slower in the W-coated Re pillar samples. Weight loss due to ion sputtering was found to be significantly reduced in nodular foam as compared to planar tungsten, but was found to be more pronounced with Re pillars. Surface coverage by nano-webs may provide insulation against plasma disruptions and act as a passive Thermal Protection System (TPS) that may be ablated during severe plasma disruptions, thus protecting the underlying surface. W-pillars or W-coated Re-pillars are found to allow trapped helium to diffuse out of the surface instead of building up sub-surface stress.

Continuum-scale modeling of helium bubble bursting under plasma-exposed tungsten surfaces

We present a comparison between a continuum-scale drift-diffusion-reaction cluster dynamics prediction of helium retention in low-energy helium plasma exposed tungsten and experimental measurements, in a temperature regime that did not produce tungsten fuzz. Our cluster dynamics model, Xolotl, has been successfully benchmarked to high helium implantation flux MD simulations at relatively low implanted fluence. In this article, we also describe the extension of the Xolotl DDR model to incorporate the effect of bubble bursting, which is observed in very high rate MD simulations, as well as MD simulations at longer times than simulated in our prior benchmarking comparison. The bursting model parameters have been tuned by comparing to MD simulations at a flux of 5.0 × 1027 m−2 s−1, and also compared to lower implanted fluence simulations performed at ~4.0 × 1025 m−2 s−1. This article then reports on the consistency of the Xolotl predictions with respect to the size of the simulated cluster phase space (i.e. the maximum cluster size), initial vacancy concentration, and bubble growth trajectory (maximum number of helium atoms per vacancy). Finally, our simulation results are compared to helium plasma experiments that did not produce fuzz. While the Xolotl predictions including bubble bursting are in quantitative agreement with high-flux MD simulations, the initial comparison to plasma exposure experiments at a flux on the order of 1021 m−2 s−1 disagree by more than an order of magnitude, and in fact cannot reproduce the trends in helium retention with varying exposure temperature. Modifying the initial vacancy concentrations and helium cluster diffusion behavior in Xolotl leads to a reasonable agreement with the experimental observations, although the underlying physical explanation for these modifications remains unclear. The predicted helium content at experimentally relevant fluxes has been shown to be relatively insensitive to the parameters used in the bubble bursting model implemented in Xolotl, although these parameters have a larger influence at higher flux. More systematic comparisons between the modeling predictions with both experiments and MD simulation results is expected to improve the bubble bursting model in Xolotl in the future.

A coupled rate theory-Monte Carlo model of helium bubble evolution in plasma-facing micro-engineered tungsten

A multiscale model of helium bubble evolution in plasma-facing materials is developed. The model links different stages of helium bubble evolution: deposition, nucleation, growth, motion, and coalescence. Helium deposition is simulated with the SRIM Monte Carlo program to give spatial information on helium and displacement damage distributions near the surface. This deposition profile is then introduced into a space-dependent rate theory of bubble nucleation and growth to describe the early stages of the distribution and size of helium bubbles. The coarsening stage of bubble evolution as a result of whole bubble motion, interaction, and coalescence is modeled by a new Object Kinetic Monte Carlo (OKMC) model, for which initial conditions are taken from the mean-field rate theory calculations. The model is compared to experimental data on low-energy helium plasma interaction with micro-engineered tungsten (W), and on high-energy helium ion deposition in flat W samples. Novel features of the multiscale model include: (1) space-dependent rate theory; (2) OKMC model of bubble motion in stress and temperature fields; and (3) application of the model to micro-engineered materials, and comparison with experiments on the same time-scale. At low helium ion energy, it is found that the mechanism of trap mutation is essential in achieving good agreement with experimental measurements. On the other hand, good agreement with experiments at high incident ion energy and temperature showed the importance of bubble coalescence and coarsening as main mechanisms. The results of the model are compared with experiments on flat W surfaces irradiated at high ion energy (30 keV), and with micro-engineered W, where the surface is coated with high-density micro-pillars at low ion energy (around 100 eV). The predicted average bubble radius and density are in qualitative agreement with experimental results.