Surface Morphology Of Tungsten Research Articles

Influence of helium plasma on the structural state of the surface carbide layer of tungsten

<abstract> <p>This paper presents the results of the experimental studies of the helium plasma interaction with a surface carbide layer of tungsten. The experiments were carried out on a plasma beam installation (PBI) at a constant energy of incoming ions of 2 keV and at a surface temperature of the tungsten carbide layer of ~905 and ~1750 ℃. The local parameters (T<sub>e</sub>, n<sub>0</sub>) of the helium plasma were evaluated using the probe method and spectrometric analysis of the plasma composition. The helium plasma irradiated two types of the carbide layer on the tungsten surface, WC and W<sub>2</sub>C. The mechanisms of changing the tungsten surface morphology in the result of the plasma irradiation have been described. The study of the surface structure of the tungsten samples with a carbide layer of two types (WC, W<sub>2</sub>C) after the exposure to the helium plasma has revealed two different types of the formation of helium bubbles and changes in the surface morphology. The physical mechanism of the formation of helium bubbles consists in the capture of helium atoms by the thermal vacancies generated at high temperature by the material surface. However, with a significant increase in temperature to 1750 ℃, the formation of the bubbles was no longer observed and the sample surface had a developed coral-like structure with crystallographically oriented grains.</p> </abstract>

Hole formation effect on surface morphological response of plasma-facing tungsten

We report simulation results on the effect of helium (He) bubble bursting-mediated surface hole formation on the surface morphological response of tungsten plasma-facing components (PFCs) in nuclear fusion devices. Our analysis is based on an atomistically informed, continuum-scale model, which is capable of accessing the spatiotemporal scales relevant to the fuzz nanostructure formation process on the surface of PFC tungsten. Our simulations account, in an empirical fashion, for two types of subsurface bubble dynamical phenomena in the nanobubble region of PFC tungsten during He plasma irradiation, involving bubble bursting and surface crater formation. We demonstrate that the hole formation effect on the PFC tungsten surface accelerates the growth rate of nanotendrils and the onset of fuzz formation. As a result, the predicted incubation time for surface nanotendril growth is shortened, in agreement with experimental data of incubation fluence at comparable plasma exposure conditions. We also explore systematically the dependence of the PFC surface morphological response on the areal density of holes introduced at regular time intervals onto the He-implanted tungsten surface, a parameter in our analysis that serves as a proxy for the rate of He bubble bursting. More importantly, our simulations capture fine surface features in the PFC tungsten surface morphology and predict that the average spacing between nanotendrils is on the order of 100 nm, consistent with the experimental findings.

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.

Migration energy barriers and diffusion anisotropy of point defects on tungsten surfaces

Tungsten is one of the most promising candidates for plasma-facing materials in future fusion devices, owing to its high performance under extreme irradiation conditions. However, irradiation-induced surface morphology varies significantly, depending on the irradiation type, fluence, and flux. Therefore, it is critical to examine the dynamics of point defects on tungsten (W) surfaces to understand how irradiation affects surface morphology. In this study, we employ the Self-Evolving Atomistic Kinetic Monte Carlo (SEAKMC) method to search for potential migration paths of point defects on W (100), (110) and (111) surfaces. The first-principles calculations are then used to accurately determine the migration energy barriers. The obtained paths and barriers are incorporated into a kinetic Monte Carlo (KMC) model to determine trajectories and diffusivities, which are described by diffusion tensors to demonstrate their anisotropic features. Multiple diffusion mechanisms have been identified on different surfaces, with various anisotropy factors. Particularly, point defects on the W (110) surface have the highest diffusivities, with anisotropy factors independent of temperatures. In comparison, the anisotropy of the W (100) surface decreases as temperature increases, while the W (111) surface is isotropic for point defects. This study provides insights into defect transport properties on different surfaces, which are essential for understanding the early stages of irradiation-induced microstructural evolutions and surface morphology of tungsten.

Surface morphology in tungsten and RAFM steel exposed to helium plasma in PSI-2

Impact of the helium plasma exposure on the surface modification in tungsten and reduced activation ferritic/martensitic (RAFM) steel have been investigated on the linear plasma device PSI-2 assuming the condition of DEMO first wall. In tungsten, a nanoscale undulating surface structure, which has a periodic arrangement, is formed under low temperature conditions below fuzz nanostructure formation threshold ∼1000 K. Interval and direction of the undulation shows dependence on the crystal orientation. A large variation in surface level up to 200 nm has been observed among grains at a fluence of He m−2 showing dependence of the surface erosion rate on the crystal orientation. The {100} plane in which the undulating surface structure is not formed shows the highest erosion rate. This significant erosion is due to the multistage sputtering through impurity. In RAFM steel, sponge-like nanostructure is developed and it grows with increasing helium fluence beyond 1 μm. In the sponge-like nanostructure, a composition change from the base material is observed in which the tungsten ratio increases while the iron ratio decreases showing differences in sputtering ratio depending on the atomic mass.

Baseline high heat flux and plasma facing materials for fusion

In fusion reactors, surfaces of plasma facing components (PFCs) are exposed to high heat and particle flux. Tungsten and Copper alloys are primary candidates for plasma facing materials (PFMs) and coolant tube materials, respectively, mainly due to high thermal conductivity and, in the case of tungsten, its high melting point. In this paper, recent understandings and future issues on responses of tungsten and Cu alloys to fusion environments (high particle flux (including T and He), high heat flux, and high neutron doses) are reviewed. This review paper includes; Tritium retention in tungsten (K. Schmid and M. Balden), Impact of stationary and transient heat loads on tungsten (J.W. Coenen and Th. Loewenhoff), Helium effects on surface morphology of tungsten (Y. Ueda and A. Ito), Neutron radiation effects in tungsten (A. Hasegawa), and Copper and copper alloys development for high heat flux components (C. Hardie, M. Porton, and M. Gilbert).

Isolated nano-tendril bundles on tungsten surfaces exposed to radiofrequency helium plasma

The DIONISOS experiment is used to study the impact of RF helium (He) plasma on the surface morphology of tungsten (W) at a frequency of 13.56MHz. Helium ion energy distributions with a span of 70–75eV, while still below the sputtering threshold result in nano-tendril bundles (NTBs) and free-standing W whiskers on surfaces at 1020K. The NTBs are distributed intragranularly with coverage of less than 10% while reaching up to 30µm normal to the surface for He ion fluence of 7.6 ×1025m−2 and flux density of 1022m−2s−1. Analysis of the NTB interior and sub-surface structure is provided through focused ion beam cross section.

Study of plasma-surface interaction at the GOL-3 facility

The review presents experimental studies of plasma-surface interaction and materials behavior under plasma loads done in the multiple-mirror trap of the GOL-3 facility. In the experiments for the PSI, the energy density in the extracted plasma stream varies from 0.5 to 30MJ/m2. Parameters of near-surface plasma measured by a set of diagnostics are reviewed. Surface patterns of targets exposed to the plasma are analyzed. The erosion depth depends on the energy loads—it rises from 0 to 600μm at 0.5 and 30MJ/m2, correspondingly. Cracking and evolution of graphite and tungsten surface morphology are discussed. The enthalpy of brittle destruction of graphite (10kJ/g), which determines the threshold of bulk damage of targets irradiated with a charged-particle flux with large penetration depth, was determined. Comparison of different facilities for PSI studies are presented. Heat flux play a key role to the target surface erosion.

Surface morphology of tungsten exposed to helium plasma at temperatures below fuzz formation threshold 1073 K

Impact of crystal orientation on the surface morphology of the helium plasma exposed tungsten has been investigated on the linear device PSI-2. A nanoscale undulating surface structure having a periodic arrangement is formed for temperatures below 1073 K, in contrast to the fuzz nanostructure formation in a higher temperature range. The crests of undulation align with the direction. The interval of the undulation is narrowest at the crystal grain of {1 1 0} surface. The interval becomes wider as the crystal grain surface tilts away from the {1 1 0} surface, and the undulating surface structure is not formed near the {1 0 0} surface. The height of undulations is nm, independently of the interval of the undulations, and it corresponds to the depth of the layer heavily damaged due to helium plasma exposure.

The Role of Vanadium in Reduction of Helium Diffusion in Tungsten Based Alloys

Helium diffusion is a very critical factor for designing a plasma facing components of fusion reactor. Because it not only reduce the thermal conductivity but also degrade the surface morphology of tungsten based materials. In this numerical study the specimens of tungsten, vanadium and tungsten vanadium alloys have been simulated under thermo-diffusion conditions. The finite element commercial software Abaqus was used for simulation to obtain helium diffusion profile at temperature of 800 K for concentration of1024number of atoms in tungsten and vanadium metals. Due to its relatively low helium diffusion coefficient, vanadium has shown better resistance against helium penetration as compared to tungsten. Subsequently the analytical effect of helium diffusion in tungsten, vanadium and different grades of tungsten vanadium alloys has been investigated. To study the distribution of vanadium in tungsten, tungsten vanadium alloys were fabricated by spark plasma sintering technique and its morphology has been analyzed by scanning electron microscopy.

Helium bubble bursting in tungsten

Molecular dynamics simulations have been used to systematically study the pressure evolution and bursting behavior of sub-surface helium bubbles and the resulting tungsten surface morphology. This study specifically investigates how bubble shape and size, temperature, tungsten surface orientation, and ligament thickness above the bubble influence bubble stability and surface evolution. The tungsten surface is roughened by a combination of adatom “islands,” craters, and pinholes. The present study provides insight into the mechanisms and conditions leading to various tungsten topology changes, which we believe are the initial stages of surface evolution leading to the formation of nanoscale fuzz.

Observations of orientation dependence of surface morphology in tungsten implanted by low energy and high flux D plasma

Surface modification by formation of blistering and nanostructures with pronounced orientation dependence has been observed on surfaces of rolled tungsten and recrystallized tungsten after exposure to a low energy (38eV) deuterium (D) plasma with a high flux of 1024m−2s−1. The correlation between blisters and nanostructures with grain orientation was examined on recrystallized tungsten to exclude the influence of defects introduced during plastic deformation on the pattern of surface modification. The amount of blistering changed from the most in grains oriented close to 〈111〉 to the least in grains oriented close to 〈001〉. Three kinds of typical nanostructures were observed, with a clear dependence on grain orientation. Triangular structures were observed on grains oriented near the 〈111〉 corner of the inverse pole figure, with lamellar structures formed for grains oriented near the 〈011〉 corner, and spongy structures for grains near the 〈001〉 corner. Possible reasons for the orientation dependence of both the blisters and nanostructures are discussed.

Surface modification and droplet formation of tungsten under hot plasma irradiation at the GOL-3

The paper presents experimental investigations of tungsten surface modification after plasma loads at the multimirror trap GOL-3. Energy loads on tungsten surface varied from 2 up to 4 MJ/m 2 per shot with sets from 1 to 9 repetitive irradiations that corresponds to loads higher than tungsten melting threshold and is close to ITER giant ELM loads. Targets surface modification and transverse microsections after irradiation was studied by optical microscopy , SEM and hardness tester. Formation on tungsten surface of three different crack networks with typical cell sizes of 1000, 10 and 0.3 μm and bubbles are identified. The network of large cracks extend perpendicularly to the irradiated sample surface to a depth of 50–350 μm. Erosion depth depends on energy loads – rises from 20 to 200 μm at 2 and 4 MJ/m 2 correspondingly. Cracking, development of tungsten surface morphology and droplets splashing are discussed.

Helium effects on tungsten surface morphology and deuterium retention

Recent experimental results on tungsten surface morphology, especially nano-structure (fuzz), induced by helium plasma exposure at temperatures between 1000K and 2000K are reviewed. This structure was firstly reported in 2006. In this review, most of experimental results reported so far including characteristics and formation conditions of the nano-structure in both linear plasma devices and magnetic confinement devices, erosion and arcing by steady-state plasma exposure and ELM-like pulsed heat or pulsed plasma exposure by a laser and a plasma gun are summarized. In addition, He effects on D retention under simultaneous D/He irradiation on tungsten are presented.

Plasma-Surface Interaction during ITER Type 1 ELMs: Comparison of Simulation with QSPA KH-50 and the GOL-3 Facilities

The paper presents experimental investigations of plasma-surface interaction and materials behavior under plasma loads relevant to type I ITER ELMs. The experiments were performed with quasi-stationary plasma accelerator QSPA Kh-50 and multi-mirror trap GOL-3 devices located in Kharkov (Ukraine) and Novosibirsk (Russia) respectively. QSPA generated repetitive plasma streams of duration 0.25 ms and the energy density up to 2.5 MJ/m2. In GOL-3 multi-mirror trap plasma was heated up to temperature of 2-4 keV by a high power relativistic electron beam. Energy density in the exhaust plasma stream vary from 0.5 to 30 MJ/m2. Surface patterns of the targets exposed by QSPA and GOL-3 plasma are analyzed. Cracking, development of tungsten surface morphology and droplets splashing are discussed. It is shown that under an applied energy density loads (>1 MJ/m2) the evolution of surface morphology due to plasma irradiation are similar for two devices in spite of the qualitative differences of particles energy of the impact plasma streams. Formation of three different crack networks with typical cell sizes of 1000, 10 and 0.3 μm are identified after irradiation of tungsten surface. Experiments show that major cracks (cell size of 1000 μm) are attributed to a ductile-to-brittle transition. The key role of heat loads magnitude on development of surface due to powerful plasma impacts is demonstrated.

Status of Plasma Facing Material Studies and Issues toward DEMO

This review paper presents status of research activity of plasma facing materials, mostly tungsten and critical issues towards DEMO reactors. A helium effect on tungsten surface morphology and its impact on fusion reactors and a pulsed heat effect to tungsten are briefly summarized. For DEMO, effects of steady-state operation and heavy neutron irradiation are important subjects to investigate. Present understandings on these are briefly summarized. Finally, issues of helical system towards DEMO will be discussed.

A description of bubble growth and gas release of helium implanted tungsten

Bubble growth and gas release during annealing of helium implanted tungsten is described using a Kinetic Monte Carlo approach. Implantation of tungsten with low energy He ions results in the unusual formation of large numbers of oversized surface pores, which eventually lead to drastic changes of the tungsten surface morphology. Traditional rate-theory models are not suitable to simulate surface pore formation. Results of the KMC simulations illustrate the rapid formation of very large helium bubbles and surface pores. The simulation results agree well with observed results of low energy He implanted tungsten experiments, and thus provide an explanation for the observed surface morphology changes caused by low energy He implantation of tungsten.

High heat load properties of ultra fine grained tungsten

Ultra fine grained tungsten samples with average grain size of 0.2, 1 and 3 μm were fabricated by resistance sintering under ultra-high pressure. The annealing experiments for the investigation of the material resistance against grain growth showed that recrystallization and grain growth occur obviously at heating temperature of 1500 °C. The finer the initial grain sizes of tungsten, the smaller its grain growth. The effects of transient high heat loads on tungsten surface morphology have been performed in an electron beam test facility with 4 ms pulses at different power density of 0.11, 0.16, 0.22, 0.27 and 0.44 GW/m 2, respectively. Particle erosions occurred for tungsten with 3 μm size at 0.16 GW/m 2, but for tungsten with 0.2 and 1 μm size at 0.27 GW/m 2. The weight losses of tungsten with 0.2, 1 and 3 μm size are 2, 0.1 and 0.6 mg, respectively, at 0.44 GW/m 2.

Formation of subwavelength periodic structures on tungsten induced by ultrashort laser pulses

The evolution of surface morphology of tungsten irradiated by single-beam femtosecond laser pulses is investigated. Ripplelike periodic structures have been observed. The period of these ripples does not show a simple relation to the wavelength and angle of incidence. The orientation of ripples is aligned perpendicularly to the direction of polarization for linearly polarized light. Surprisingly, we find that the alignment of the ripple structure turned left or right by 45 degrees with respect to the incident plane when using right and left circularly polarized light, respectively. The period of the ripple can be controlled by the pulse energy, the number of pulses, and the incident angle. We find a clear threshold for the formation as a function of pulse energy and number of pulses. The mechanism for the ripple formation is discussed, as well as potential applications in large-area structuring of metals.

Effects of repetitive ELM-like heat pulses on tungsten surface morphology

The effects of ELM-like heat pulses on tungsten surface morphology were studied by using a fundamental wave (wavelength of 1.06 μm) of a YAG laser, which was operated in a multi-pulsed mode with an effective pulse length of about 100 μm, typical pulse length of ELMs. Tungsten base temperature was 640 °C, heated by a sheath heater and laser pulse heating. The energy absorption rate of the laser light was about 30% for mirror finished surface. Under non-melting conditions (laser pulse energy fluence: ≤0.1 kJ/cm 2), roughening and cracking formation within the laser spot appeared. As the energy fluence decreased, the larger shot number was necessary for the surface morphology change to appear. Even at the highest temperature of around 1240 °C, surface roughening and fine cracking appeared at the large shot number (200,000 shots).