Trap Mutation Research Articles

Molecular dynamics study of interstitial He clusters in nickel

This study presents a molecular dynamics analysis focusing on the behavior of interstitial helium (He) clusters in nickel (Ni), examining their formation, stability, and migration energetics. We found that the binding energies of interstitial He with a He cluster are positive and increase with the cluster size, indicating a preference for He atoms to cluster together. While the formation energy increases monotonically with cluster size, binding energy shows non-monotonic. trend Importantly, small He clusters were found to be thermally unstable at reactor operational temperatures (approximately 600 K), with the He2 cluster exhibiting instability even at room temperature. With a binding energy of 0.44 eV for a He4 cluster, we hypothesize that for He bubbles to form via homogeneous nucleation (i.e., through trap mutation) at reactor operating temperatures, the He concentration must be high enough to facilitate the formation of He clusters of at least size 4 or larger. At finite temperatures, He clusters of size 7 and larger trap mutate immediately. However, clusters of size 10 and larger will trap mutate even at 0 K. As expected, interstitial He and small He clusters are highly mobile, and found to be mobile even at temperatures as low as 200 K. Furthermore, the mean squared displacement method has been utilized to determine the activation energies and the corresponding prefactors for clusters ranging from He1 to He6

Synergistic influence of dislocations and helium cluster on hydrogen atom in tungsten

Dislocations are common line defects in the preparation and irradiation processes of tungsten (W), and they can serve as trapping sites for hydrogen (H) and helium (He) atoms. Existing studies have indicated that dislocations preferentially capture He atoms to form dislocation-He cluster structures. The focus of this study was on the effects of this new structure on the trapped and diffusion behavior of H atoms using molecular dynamics (MD) method. The study considered the number of He atoms from 1 to 150, and they were classified as small He cluster and large He cluster. By calculating the binding energy, the study revealed that the capture strength is positively correlated with the He clusters’ size. However, it is negatively correlated with the distance from the dislocation core and the captured H atoms’ number. This study output the stable configurations to analyze the growth patterns of He cluster in dislocations, and it was observed that the interstitial atoms caused by trap mutation preferentially aggregate into dislocation loops due to the dislocation core structure. Moreover, the study investigated the diffusion behavior of H atoms around the dislocation-He cluster structure at 600 K within 1 ns. The hydrogen atoms tend to migrate towards the He cluster and distribute around it uniformly. The distribution range narrows with the helium cluster's size, and the distribution pattern depends on the dislocations’ type. This demonstrated the formation of dislocation-He cluster structure, promoting the increased H retention.

Atomistic modeling of helium bubble network formation in copper

The helium (He) bubble network formation process in bulk copper (Cu) is investigated using atomistic simulations. Self-interstitial atoms (SIAs) emitted by trap mutation during the growth process of a single He bubble change surrounding lattices and stress fields and affect the transport of nearby He atoms. Interstitial He atoms or dimers can become trapped by deformed lattices and become nucleation sites for new He bubbles. The interaction of He with SIAs mutually hinders their movements. Finally, a distributed network of He bubbles, which are mostly formed along stacking fault edges, is generated. System temperature and He insertion rate have significant influences on this network.

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.

An empirical potential for simulating hydrogen isotope retention in highly irradiated tungsten

We describe the parameterization of a tungsten-hydrogen empirical potential designed for use with large-scale molecular dynamics simulations of highly irradiated tungsten containing hydrogen isotope atoms, and report test results. Particular attention has been paid to getting good elastic properties, including the relaxation volumes of small defect clusters, and to the interaction energy between hydrogen isotopes and typical irradiation-induced defects in tungsten. We conclude that the energy ordering of defects changes with the ratio of H atoms to point defects, indicating that this potential is suitable for exploring mechanisms of trap mutation, including vacancy loop to plate-like void transformations.

Microstructural evolution of Ti2ZrHfV0.5Mo0.2 refractory multi-principal alloy under low-energy, high-flux He ion irradiation

The irradiation response of refractory multi-principal element alloys (RMPEAs) has gained increasing attention for the potential applications in future fusion devices. In controlled fusion reactors, plasma-facing materials have to suffer continuous low-energy high flux helium (He) ion irradiations. During high flux He irradiation, vacancy-interstitial pairs can be generated through trap mutation mechanism even without cascade collision. To simplify the model, Ti2ZrHfV0.5Mo0.2 RMPEA was irradiated with 50 eV He+ (less than the minimum He displacement energy of the alloy EdTi = 67 eV) under 1200 K to various fluences of 5 × 1024 - 1 × 1026 ions/m2, and the microstructural evolution was investigated. In the irradiated RMPEA, the intense sputtering of Ti, the growth and degradation of fuzz mainly composed of Zr, Hf elements on the surface, as well as the formation of V, Mo enriched band and high density large He cavities in the subsurface layer were observed. Although the sputtering threshold energy for He of V is similar to that of Ti (ETi = 24 eV, EV = 29 eV), V showed anomalous enrichment in the subsurface layer. Moreover, unlike the W fuzz growth process, the adatoms used to form fuzz in Ti2ZrHfV0.5Mo0.2 were derived from the Zr, Hf atoms diffusing through the vacancy-solute exchange mechanism, rather than from the atoms formed by the loop-punching mechanism. The anomalous enrichment of V atoms and the different fuzz growth mechanism were attributed to (i) different migration rates for different species of atoms in the alloy under vacancy diffusion mechanism and (ii) the formation of AB2 (A = Ti, Zr, Hf; B = V, Mo) phase blocking the continued formation of prismatic dislocation loops mainly composed of V and Mo.

Effect of Re and Ta on self-trapping of helium in tungsten: a first-principles calculation

Self-trapping may play an important role in helium radiation damage of W material in fusion reactors. But the effect of rhenium (Re) and tantalum (Ta) on the self-trapping process in the defect-free tungsten is not clear at present, which has been investigated by the first-principles calculation in this work. The results suggest that He can be trapped by other He atoms without prior defects in the presence of Re/Ta substitutional atom. The substitutional Re atom acts as a trap for the interstitial He clusters and enhances their nucleation, while the Ta atom repulses the large He clusters (n ⩾ 5) and weakens the cluster bonding, thus the growth of He clusters in the vicinity of Ta is not energetically favorable. Moreover, the critical content of He atoms that may trigger the trap mutation decreases when Re exists but increases when Ta exists. These results suggest Re can facilitate the growth of He clusters by self-trapping while Ta has the opposite effects.

On the interactions of interstitial helium atom with helium bubble in tungsten: A molecular dynamics study

Tungsten (W) has been chosen as the primary plasma-facing material in nuclear fusion reactors. However, when helium (He) ions are continuously irradiated, enormous amounts of He atoms react with W generating He clusters and bubbles. The interactions of interstitial He atoms with He bubbles of various helium-to-vacancy ratios (RHe/V) in W were investigated using molecular dynamics simulation. Single He atoms were discovered to cause trap mutations in the periphery of He bubbles, particularly those with relatively low RHe/V. The processes of these trap mutations were analyzed, and the substitutional He atoms formed in these trap mutations were generally divided into two types: the T type and the S type. Afterward, the detrapping behaviors of the S-type He atoms were investigated using a linearly increasing temperature method, and their average lifetime was also calculated. The results suggest that these substitutional He atoms’ average lifetime depends on their neighboring W atoms and the RHe/V of He bubbles. Additionally, the average duration for a He atom colliding with He bubbles in the fusion environment was estimated using He atom distributions obtained using rate theory. At 500 K, most of the average lifetime of the S-type He atoms is longer than the colliding time for the fluxes of around 1020–1024 m−2s−1. At 1000 K, although the average lifetime decreases compared with 500 K, some S-type He atoms, especially the one trapped around the He bubble with RHe/V = 1, can be trapped longer than the colliding time. Our results demonstrate that it is likely to form satellite nanobubble around the He bubble during its continuous irradiation in W. This study provides new insights into the growth of the He bubble in W in the fusion environment.

Effect of helium flux on near-surface helium accumulation in plasma-exposed tungsten

We report results of object kinetic Monte Carlo (OKMC) simulations to understand the effect of helium flux on the near-surface helium accumulation in plasma-facing tungsten, which is initially pristine, defect-free, and has a (100) surface orientation. These OKMC simulations are performed at 933 K for fluxes ranging from 1022 to 4 × 1025 He/m2 s with 100 eV helium atoms impinging on a (100) surface up to a maximum fluence of 4 × 1019 He/m2. In the near-surface region, helium clusters interact elastically with the free surface. The interaction is attractive and results in the drift of mobile helium clusters towards the surface as well as increased trap mutation rates. The associated kinetics and energetics of the above-mentioned processes obtained from molecular dynamics simulations are also considered. The OKMC simulations indicate that in pristine tungsten, as the flux decreases, the retention of implanted helium decreases, and its depth distribution shifts to deeper below the surface. Furthermore, the fraction of retained helium diffusing into the bulk increases as well, so much so that for the flux of 1022 He/m2 s, almost all of the retained helium diffused into the bulk with minimal/negligible near-surface helium accumulation. At a given flux, with increasing fluence, the fraction of retained helium initially decreases and then starts to increase after reaching a minimum. The occurrence of the retention minimum shifts to higher fluences as the flux decreases. Although the near-surface helium accumulation spreads deeper into the material with decreasing flux and increasing fluence, the spread appears to saturate at depths between 80 and 100 nm. We present a detailed analysis of the influence of helium flux on the size and depth distribution of total helium and helium bubbles.

Material migration in W and Mo during bubble growth and fuzz formation

Growth of helium (He) induced bubbles and fuzz in tungsten (W) and molybdenum (Mo) is investigated using samples of W films on Mo substrates and Mo films on W substrates exposed to He-containing plasma in the temperature range of 340 to 1075 K, fluence range of 1.0–14 × 1025 He·m−2, and incident ion energy of <50 eV. No fuzz (only up to 2 nm diameter bubbles) and no material transport occur in W films at ⩽750 K, while precursors-of or fully-developed fuzz and material mixing occur in W and Mo films at ⩾800 K. This suggests that fuzz forms in multi-material systems as long as one material meets the conditions for fuzz formation, namely T s/T m ∼ 0.27–0.5 where T s and T m are the sample exposure and material melting temperatures, respectively. Larger He bubbles, more material mixing, and further-developed fuzz occur at higher temperature due to increased mobility of He atoms and small He clusters. Accumulation of substrate material at the surface of fuzzy W and Mo thin-film (<80 nm) samples suggests fuzz growth by material transport from the bubble layer in the bulk up to the fiber tip, likely by a two-step process: (i) diffusion of punched dislocation loops in the bulk toward the fuzz base and (ii) diffusion of adatoms along the fuzz base and fiber surface (with effective transport of adatoms upwards due to trapping of adatoms at curved surfaces of fiber tips and/or due to the continuous generation of adatoms at the fuzz base). While the bubble size and fuzz thickness increase with reduced W concentration in Mo thin-film samples at 838 K likely due to an increase in trap mutation and dislocation loop punching in Mo compared to W, the fuzz thickness decreases with reduced W concentration at 1075 K despite an increase in the bubble size likely due to slower diffusion of interstitial loops in Mo.

Study on the interaction between He and defects induced by He-ion irradiation in W and W5Re alloy

To investigate the interaction between He atom and irradiation-induced defects in W and W5Re, multiple energetic He ions with damage dose ranging from 0.84 dpa to 1.63 dpa were selected in the present study to implant into W and W5Re samples. Doppler broadening spectroscopy (DBS) and coincidence Doppler broaden spectroscopy (CDB) based on slow positron beam and synchrotron X-ray diffraction technology were employed in the current work. Under the same dose (≤ 1 × 1021 ions m−2) irradiation at room temperature, the depth distribution of vacancy-type defects in W and W5Re is similar. With the increase of irradiation dose, the vacancy-type defects in W continue to grow through absorbing and merging. However, Re atoms are more likely to hinder this growth mechanism, leading to an accumulation of high density but small size vacancy-type defects in W5Re. The HenVm complexes in W continuously attract the mobile interstitial He atom and then induce the trap mutation in the implanted region. Micro-voids and large He bubbles (∼ μm) exceed the recognition of positron. Re atoms has strong the attraction to vacancies and positron affinity. The possible compound of Re and HenVm complexes makes it easier for positron to characterize the He-related and Re-related information around the defect sites. The lattice swelling is similar for the W and W5Re because the structure of HenVm cluster or He bubble can destroy the lattice periodicity and increase the lattice stress in the implanted region.

Comparative studies of helium behavior in copper and tungsten using molecular dynamics simulations

Helium (He) behavior in copper (Cu) and tungsten (W) bulk has been investigated and compared by molecular dynamics simulations. In contrast to the diffusion of He in W, interstitial He and He2 in Cu have higher diffusion coefficients and lower migration energies, which causes He atoms to rapidly coalesce into helium clusters. Once a Hen cluster (n ≥ 3) forms, trap mutation occurs in a short time. W atoms are less likely to be projected by Hen clusters. The binding energies are calculated to prove the results. For He2 in Cu, the dissociation of He2 and trap mutation both occur above 700 K. The total phenomenon exhibits Arrhenius behavior. The effective capture radii (ECRs) of an interstitial He trapping another one in Cu are obtained, which differ slightly from that in W. They decrease as the temperature increases, which deserves more attention, as the ECRs are required in MC and RT models.

Migration behaviors of helium atoms near tungsten surfaces: A molecular dynamics study

Under the fusion environment, the behavior of helium atoms near tungsten (W) surfaces plays a crucial role in surface morphological evolution and near-surface structural evolution of tungsten. In this paper, the migration behaviors of single helium atoms near W (2 1 1) and W (3 1 1) surfaces were investigated using molecular dynamics simulations. The results show that these single atoms can be well-described by the theory of continuous diffusion of particles in a semi-infinite medium, and different types of trap mutations occur for both surfaces. Although the temperature has an impotent impact on the occurrence probabilities of different types of trap mutations and the probabilities of helium atoms escaping from trap sites, there is no a strict relationship between the depth and temperature for trap mutations occurring. For the W (2 1 1) surface, an approximately one-dimensional diffusion may take place along the 〈1 1 1〉 directions when the helium atoms are trapped near 2 layers below the W surface. For the W (3 1 1) surface, the behaviors of trap mutations seem more significant than (2 1 1) and (1 1 1) surfaces at T = 1000 K. To investigate the diffusion and trap mutation processes, the nudged elastic band (NEB) method was also applied. These results can be helpful for understanding of helium release, helium retention, subsurface helium clustering, as well as surface morphological evolution.

Temperature dependent He-enhanced damage and strain in He-implanted AlN

He implantation was performed at RT and 550 °C into AlN epitaxial films. The resulting microstructural damage and elastic strain were investigated using TEM and XRD. Basal stacking faults, clusters of point defects and bubbles aligned along (0001) planes are observed at both temperatures. Apart from slight variations of size and densities of defects, no significant difference is observed on the microstructure after implantation at RT and 550 °C. In particular, He bubbles are observed with similar size, showing that the long-range migration of vacancies is not triggered at 550 °C. An important effect of He on the enhancement of damage and strain build-up during implantation is evidenced. He bubbles are observed to strongly favor the nucleation of extended defects, both basal stacking faults and point defect clusters, which results from the increase of free interstitial concentration due to stabilization of vacancies by He that could be enhanced by the bubble growth by loop punching or trap mutation to relieve the gas pressure. At an earlier stage of implantation-induced damage, the presence of He is also seen to induce a strong increase of strain with increasing implantation temperature, whilst efficient point defect recombination takes place in the regions free of He, or with low concentration of He, resulting in a decrease of strain. The strong increase of strain with increasing temperature in the deep implanted region, concomitant with a strong reduction of scattered intensity, is ascribed to the growth of He–V complexes, probably induced by a thermally enhanced mobility of He, and only occurs for sufficient He concentration.

Effects of temperature and surface orientation on migration behaviors of helium atoms near titanium surfaces

The effects of temperature and surface orientation on migration behaviors of helium (He) atoms near titanium (Ti) surfaces have been investigated by molecular dynamics (MD) simulation. For the Ti (0001) surface, He atoms can quickly escape out from the substrate and no trap mutation was observed near the surface at the temperature range of 500–1250 K. While for the Ti (011¯0) and (21¯1¯0) surfaces, trap mutations can occur within the neighboring region of the surfaces, which hinder the escape of He atoms. It is also found that the occurring probability of trap mutation near the surface depends on the temperature. The increasing temperature usually induces more trapped He atoms in the surface region. These results are useful for understanding the structural evolution and He thermal release on Ti surfaces.

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.

Diffusion behavior of hydrogen isotopes in tungsten revisited by molecular dynamics simulations**Project supported by the National Magnetic Confinement Fusion Program of China (Grant No. 2013GB109002).

Molecular dynamics simulations were performed to study the diffusion behavior of hydrogen isotopes in single-crystal tungsten in the temperature range of 300–2000 K. The simulations show that the diffusion coefficient of H isotopes exhibits non-Arrhenius behavior, though this deviation from Arrhenius behavior is slight. Many-body and anharmonic effects of the potential surface may induce slight isotope-dependence by the activation energy; however, the dependence of the pre-factor of the diffusion coefficient on the isotope mass is diminished. The simulation results for H-atom migration near W surfaces suggest that no trap mutations occur for H atoms diffusing near either W{100} or W{111} surfaces, in contrast to the findings for He diffusion near W surfaces. Based on the H behavior obtained by our MD simulations, the time evolution of the concentration distribution of interstitial H atoms in a semi-infinite W single crystal irradiated by energetic H projectiles was calculated. The effect of H concentration on H diffusion is discussed, and the applicability of the diffusion coefficients obtained for dilute H in W is assessed.

Primary lymphomatous presentation of hairy cell leukemia as osteolytic vertebral lesions: a case report

Hairy cell leukemia (HCL) is a rare and indolent mature B cell neoplasm. Most patients with HCL have excellent response to purine analogs and BRAF inhibitors. Therefore, it is clinically important to identify and differentiate HCL from other B cell neoplasms. HCL is predominantly found in the peripheral blood, bone marrow, or spleen. HCL presenting initially or solely in unusual sites is extremely rare (approximately ten or fewer). We hereby report a unique case of HCL presenting as vertebral osteolytic and epidural mass lesions without bone marrow, spleen, or peripheral blood involvement. The patient was a 53-year-old male with acute and chronic radicular pain down the posterior aspect of bilateral thighs. Imaging study with MRI demonstrated a L4 posterior vertebral osteolytic mass lesion, extending into the spinal canal, and a second right sacral S1 lesion extending into the sacral iliac joint. These two noncontiguous lesions were thought to be consistent with metastatic cancer or sarcoma, and the specimen was submitted to non-hematopoietic pathologist for diagnosis without flow cytometry. Initial immunohistochemical stains were ordered for solid tumor markers, which were all negative. As part of the workup, immunostain for BRAF V600E (VE1) was ordered for melanoma, and CD138 and cyclin D1 for myeloma, which returned positive and suggested the possibility of hairy cell leukemia. Additional workup for hairy cell leukemia confirmed the diagnosis. HCL typically presents with peripheral blood, bone marrow, or spleen involvement, while mass lesions in other sites are rare. High index of suspicion is essential for the correct diagnosis of HCL in mass lesions at unusual sites. In the absence of initial hematopathological workup and flow cytometry, HCL presenting as solid mass in unusual locations is misdiagnosed most likely as marginal zone B cell lymphoma (mucosa-associated lymphoid tissue (MALT) lymphoma). The expression of cyclin D1, TRAP, annexin A1, and BRAF V600E mutation would confirm the diagnosis of HCL. The main differentials include MALT lymphoma, mantle cell lymphoma (MCL), and hairy cell leukemia variant (HCLv). This case report will be interesting for the clinicians and pathologists alike and broaden the spectrum of clinical presentation of lymphomatous HCL.

Dynamics of Small Mobile Helium Clusters Near a Symmetric Tilt Grain Boundary of Plasma-Exposed Tungsten

We report the results of a systematic atomic-scale analysis of small helium cluster dynamics near a Σ3<111>{121} symmetric tilt grain boundary (GB) in tungsten based on molecular-dynamics simulations according to a reliable interatomic interaction potential. We find that small, mobile helium clusters (Hen, 1 ≤ n ≤ 7) in the near-GB region are attracted to the GB due to an elastic cluster-GB interaction force. Moreover, as the clusters drift toward the GB, cluster trap mutation (TM) reactions in the near-GB region are activated at rates much higher than those in the bulk of the material's grains. This near-GB cluster dynamics has significant effects on the near-GB defect structures and the amount of helium retained in the material upon plasma exposure. Each TM reaction generates a tungsten vacancy, which traps helium by forming an immobile helium-vacancy complex, and an interstitial tungsten atom in the form of an extended tungsten interstitial complex on the GB. This interstitial configuration is characterized by mobility that depends on the location where the TM reaction occurs: It is immobile when the vacancy produced by the TM reaction is located a few lattice planes away from the GB plane and highly mobile along a specific direction when the produced vacancy is located on the GB. The latter mechanism initiates a potentially fast migration path for W atoms along the GB toward a free surface, which may influence significantly the surface morphology of plasma-exposed tungsten.

Modeling Helium Segregation to the Surfaces of Plasma-Exposed Tungsten as a Function of Temperature and Surface Orientation

We provide a description of the dependence on surface crystallographic orientation and temperature of the segregation of helium implanted with energies consistent with low-energy plasma exposure to tungsten surfaces. Here, we describe multiscale modeling results based on a hierarchical approach to scale bridging that incorporates atomistic studies based on a reliable interatomic potential to parameterize a spatially dependent drift-diffusion-reaction cluster-dynamics code. An extensive set of molecular dynamics (MD) simulations has been performed at 933 K and/or 1200 K to determine the probabilities of desorption and modified trap mutation that occurs as small, mobile Hen (1 ≤ n ≤ 7) clusters diffuse from the near-surface region toward surfaces of varying crystallographic orientation due to an elastic interaction force that provides the thermodynamic driving force for surface segregation. These near-surface cluster dynamics have significant effects on the surface morphology, the near-surface defect structures, and the amount of helium retained in the material upon plasma exposure, for which we have developed an extensive MD database of cumulative evolution during high-flux helium implantation at 933 K, which we compare to our properly parameterized cluster-dynamics model. This validated model is then used to evaluate the effects of temperature on helium retention and subsurface helium clustering.