Correlating Grain Size to Radiation Damage Tolerance of Tungsten Materials Exposed to Relevant Fusion Conditions

Tungsten remains a leading candidate for plasma facing component (PFC) in future fusion devices. This is in large part due to its strong thermal and mechanical properties. The ITER project has already chosen to use an all tungsten divertor. Despite having a high melting temperature and low erosion rate, tungsten faces a large variety of issues when subject to fusion like conditions. These include embrittlement, melting, and extreme morphology change (growth of fuzz nanostructure). The work presented here investigates mechanisms that drive surface morphology change in tungsten materials exposed to fusion relevant plasmas. Specifically, tungsten materials of different grain sizes are studied to elucidate the impact of grain boundaries on irradiation damage. Exposure of ultrafine (\u3c 500 nm) and nanocrystalline (\u3c 100 nm) grain materials are exposed to high flux helium plasmas at the Dutch Institute for Fundamental Energy Research (DIFFER) in the Netherlands. These samples are then compared to large grain (1-5 microns) tungsten materials exposed to similar conditions at DIFFER or tungsten samples from other published studies. After exposing the ultrafine grain materials to a variety of helium plasmas to different fluences between 1 x 10 23 - 1 x 1027 ions-m-2 , temperatures between 600-1500 °C, and ion energies between 25-70 eV, it is observed that ultrafine grained tungsten samples develop fuzz at an order of magnitude larger fluence when compared to large grained tungsten. These observations suggest that grain boundaries play a role in dictating damage accumulation and damage rate caused by ion bombardment of tungsten surfaces. These experiments are complemented by In-situ TEM analysis during 8 keV Helium irradiation of ultrafine tungsten samples to see damage propagation in different sized grains in real time. The in-situ TEM work was completed in a JEOL JEM-2000FX TEM at the Microscope and Ion Accelerator for Materials Investigation (MIAMI) facility at the University of Huddersfield. The TEM results show a strong dependence on grain size and defect production rate. Images also suggest that smaller grains tend to form helium bubbles at the grain boundaries. The distribution of bubble size and location is significantly different in nanocrystalline grains

Effect of Helium Ions Energy on Molybdenum Surfaces Under Extreme Conditions

Plasma facing components (PFCs) in fusion devices must be able to withstand high temperatures and erosion due to incident energetic ion radiations. Tungsten has become the material of choice for PFCs due to its high strength, thermal conductivity, and low erosion rate. However, its surface deteriorates significantly under helium ion irradiation in fusion-like conditions and forms nanoscopic fiber-like structures, or fuzz. Fuzz is brittle in nature and has relatively lower thermal conductivity than that of the bulk material. Small amounts of fuzz may lead to excessive contamination of the plasma, preventing the fusion reaction from taking place. Despite recent efforts, the physical mechanism of the surface deterioration is still not clear. This necessitates finding alternative materials for PFCs. In this report, the effect of helium ion energy on molybdenum surfaces is investigated. Helium ion irradiations on mirror finished polished molybdenum samples are performed as a function of helium ion energy from 100-1600eV with fixed values of ion-flux (7.2 x 1020 ions m-2 s-1), ion-fluence (2.6 x 1024 ions m-2), and temperature (923K). The surface modifications were studied using scanning electron and atomic force microscopy along with X-ray photoelectron spectroscopy and optical-reflectivity measurements. Reduction in the “protrusion” of fuzz from the surface and fuzz density at increased energy have been seen from microscopy and optical reflectivity studies. These findings further the understanding of fuzz formation on high-Z refractory metals for fusion applications. KEYWORD

Effect of Carbon Impurity on Molybdenum Nanostructure Evolution under Helium Ion Irradiation in Extreme Conditions

The performance of plasma facing components (PFC) is of great important for the realization of prototype nuclear fusion. Tungsten has been considered as the leading high-Z PFC material for these reactors and tokamaks due to its superior thermophysical properties, high melting point, low sputtering yield, and low tritium inventory. However, its surface deteriorates significantly under helium ion irradiation in extreme (fusion) conditions and forms nanoscopic fiber like structures (fuzz) Recent studies show that the formation of fuzz nanostructure on tungsten can be suppressed by the presence of plasma impurities such as carbon and beryllium. In the present study, the effects of carbon impurity on molybdenum nanostructure evolution under extreme condition helium ion irradiation have been investigated. For mixing the carbon impurity on molybdenum surface, a mixture of helium and methane (CH4) gas has been used. Separate experiments with 100% pure helium and with mixture gas have been performed. Ion energy (100eV), ion-flux (7.2 1020 ions m-2 s-1), ion-fluence (2.6 1024 ions m-2) and target temperatures (923K) were chosen from our previous studies and fixed constant during the whole study, for all the samples. The surface modification and compositional analysis, due to 100% pure helium ion and “helium+ carbon” ion irradiations, will be studied using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), respectively. In addition, optical-reflectivity measurements will also be performed for monitoring the surface deterioration due to energetic pure helium ion and mixture “helium+carbon” ion irradiations. Our results indicate that 0.5 % carbon impurity (a mixture of 97.5 % helium and 2.5% methane gas) may prevent almost all the molybdenum fuzz formation and deposit a thin carbon layer on molybdenum surface

Effect of Dual Ion Beam Irradiation (Helium and Deuterium) on Tungsten–Tantalum Alloys Under Fusion Relevant Conditions

The selection of tungsten (W) as a divertor material in ITER is based on its high melting point, low erosion, and strong mechanical properties. However, continued investigation has shown W to undergo severe morphology changes in fusion-like conditions. Recent literature suggests alloying W with other ductile refractory metals, viz. tantalum (Ta) may resolve some of these issues. These results provide further motivation for investigating W–Ta alloys as a plasma-facing component (PFC) for ITER and future DEMO reactors. Specifically, how these alloy materials respond to simultaneous He+ and D+ ion irradiation, and what is the effect on the surface morphology when exposed to fusion relevant conditions. In the present study, the surface morphology changes are investigated in several W–Ta targets (pure W, W-1%Ta, W-3%Ta, and W-5% Ta) due to simultaneous He+ and D+ ion irradiations. This comprehensive work allows for deeper understanding of the synergistic effects induced by dual ion irradiation on W and W–Ta alloy surface morphology. Pure W and W–Ta alloys were irradiated simultaneously by 100 eV He+ and/or D+ ions at various mixture ratios (100% He+, 60% D+ + 40% He+, 90% D+ + 10% He+ ions and 100% D+ ions), having a total constant He fluence of 6 × 1024 ion m−2, and at a target temperature of 1223 K. This work shows that slight changes in materials composition and He/D content have significant impact on surface morphology evolution and performance. While both the pure W and W–Ta alloys exhibit very damaged surfaces under the He+ only irradiations, there is a clear suppression of the surface morphology evolution as the ratio of D+/He+ ions is increased

Real time x-ray studies during nanostructure formation on silicon via low energy ion beam irradiation using ultrathin iron films

Real time grazing incidence small angle x-ray scattering and x-ray fluorescence (XRF) are used to elucidate nanodot formation on silicon surfaces during low energy ion beam irradiation of ultrathin iron-coated silicon substrates. Four surface modification stages were identified: (1) surface roughening due to film erosion, (2) surface smoothing and silicon-iron mixing, (3) structure formation, and (4) structure smoothing. The results conclude that 2.5 x 10(15) iron atoms in a 50 nm depth triggers surface nanopatterning with a correlated nanodots distance of 25 nm. Moreover, there is a wide window in time where the surface can have correlated nanostructures even after the removal of all the iron atoms from the sample as confirmed by XRF and ex-situ x-ray photoelectron spectroscopy (XPS). In addition, in-situ XPS results indicated silicide formation, which plays a role in the structure formation mechanism. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4773202

Performance of Tungsten-Tantalum Alloys as Plasma Facing Components in Relevant Fusion Conditions

Currently nuclear fusion remains a major target for future energy production. However, numerous key issues still remain unresolved for both inertial confinement and magnetic confinement design concepts. Material selection for Plasma Facing Components (PFCs) is a major concern that needs further investigation and innovative solutions. Tungsten (W) has become a leading candidate for use in fusion devices because of several key thermal and physical properties like high melting point, high thermal conductivity, and low erosion rate. However, continued research on tungsten has revealed several major concerns when tungsten is exposed to relevant fusion conditions, including embrittlement, melting, and extreme morphology evolution leading to a nanostructure called ‘fuzz’. These issues have prompted the need for innovative solutions to design more robust PFC materials. The work in this thesis will investigate alloying tungsten with tantalum in order to elucidate the possible enhancements in tungsten PFC performance as a function of tantalum concentration. The scope of the experimental work discussed in this thesis covers three major areas. First, tungsten (W) and tungsten-tantalum alloys (W-Ta) were exposed low energy helium ion (He+) at various temperatures. The results of this experiment showed a significant difference in accumulated surface damage as a function of both temperature and Ta concentration. Scanning electron microscopy (SEM) and X-Ray Diffraction (XRD) data indicated that there may be a correlation between the observed morphology differences and the induced crystal structure change caused by the presence of Ta. These results were supported via X-Ray Photoelectron Spectroscopy (XPS) and Optical Reflectivity (OR). The second major area discussed is the exposure of pure W and W-Ta alloys to mixed and sequential He+/D+ ion beam irradiations. In these experiments the effect of dual ion irradiation is investigated by subjecting W and W-Ta samples to four different D+:He + ratios (100% He+, 60% D+ + 40% He+, 90% D+ + 10% He+ and 100% D+). SEM results revealed that increasing the D+ concentration leads to suppression of He + induced surface damage. Additional, sequential ion exposures were conducted to decouple the interaction between the He+ and D + during the dual ion irradiations. For the sequential experiments, W and W-Ta sample were first exposed to low energy He+ ions. This was then followed by exposures to low energy D+ ions at 1223 K. SEM results revealed similar response in the surface due to the dual ion beam irradiations. There was significant degradation and reintegration of the fuzz surface in response to low energy D+ irradiation at 1223 K. This experiment was repeated for W and W-Ta samples but the D+ exposure was conducted at 523 K. In this case, post irradiation SEM revealed that the D+ had no effect on the He+ induced morphology. This result indicated that the morphology suppression mechanism is based on a temperature dependent W-D interaction mechanism, like D desorption. This result is significant to the fusion community in that it suggests there may be an operation parameter space for future fusion devices which actively suppresses He+ induced surface damage on the PFCs. The final area discussed is the exposure of pure W and W-Ta samples to both single and dual ion beam irradiation along with simultaneous pulsed heat loading to simulate ELM-like transient events expected in future fusion devices. SEM data from this chapter revealed three main conclusions. First, there was a very apparent difference in the severity of the laser induced damage when comparing the W to the W-5Ta samples. This trend was consistent regardless of loading conditions. Further investigation suggested that the weaker W-5Ta samples fail more readily under the intense thermal stresses that are induced by the transient heat loading. Second, for the heat fluxes investigated in these experiments, there was essentially no significant difference in the resulting surface damage between the laser only exposures and the sequential ion irradiation followed by laser exposures. Finally, the presence of ion irradiation (single and dual) with simultaneous pulsed heat loading did show significant differences in the damage morphology. Specially, deeper trenches and pore formation were present and suggest a possible increase in the surface erosion. This increase seems smaller in the dual ion irradiation case. Additional erosion studies using a Mo witness plate revealed an increase in erosion of W when exposed to transient heat loading with simultaneous ion bombardment. This is important, and it details a critical decrease in PFC performance when synergistic loading effects are taking into account. In addition to the results presented, the work provided in this thesis creates a framework for the comprehensive analysis of alternative PFC materials to complex fusion conditions

Near sputter-threshold GaSb nanopatterning

Nanopatterning at sputter-threshold energies with Ar irradiation of GaSb (100) surfaces is presented. Comparison with high-energy irradiations up to 1000 eV is conducted measuring in-situ the composition evolution over irradiation time at early stages (e. g., \u3c 10(17) cm(-2)) and up to nanostructure saturation (e. g., similar to 10(18) cm(-2)). Low-energy irradiation is conducted for energies between 15-100 eV and a low-aspect ratio nanostructured dot formation is found. Furthermore, the role of oxide on GaSb is found to delay nanostructure formation and this is predominant at energies below 100 eV. In-situ quartz crystal microbalance measurements collect sputtered particles yielding the sputter rate at threshold energies indicating a correlation between erosion and surface composition consistent with recent theoretical models. Ion-induced segregation is also found and indicated by both compositional measurements of both the surface and the sputtered plume. (C) 2013 AIP Publishing LLC

Deuterium Retention in Silicon Carbide Materials

Silicon Carbide (SiC) is a low Z material discussed as an alternative to graphite for fusion devices. The retention of hydrogenous species is an important plasma-surface interaction property. Deuterium was implanted into SiC, SiCf/SiC, Cf/C-SiC and SiC coated graphite under various particle energy and substrate temperature conditions. A TDS process was used to characterise the deuterium retention of the implanted specimens. While all SiC materials show elevated retention levels compared to graphite, the differences are limited to about a factor of two over the range of parameters investigated

Deuterium retention in silicon carbide, SiC ceramic matrix composites, and SiC coated graphite

Silicon Carbide (SiC) is a low Z material discussed as an alternative to graphite for fusion devices. The retention of hydrogenous species is an important plasma-surface interaction property. Deuterium was implanted into SiC, SiCf/SiC, Cf/C-SiC and SiC coated graphite under various particle energy and substrate temperature conditions. A TDS process was used to characterise the deuterium retention of the implanted specimens. While all SiC materials show elevated retention levels compared to graphite, the differences are limited to about a factor of two over the range of parameters investigated

Nanopatterning of metal-coated silicon surfaces via ion beam irradiation: Real time x-ray studies reveal the effect of silicide bonding

We investigated the effect of silicide formation on ion-induced nanopatterning of silicon with various ultrathin metal coatings. Silicon substrates coated with 10 nm Ni, Fe, and Cu were irradiated with 200 eV argon ions at normal incidence. Real time grazing incidence small angle x-ray scattering (GISAXS) and x-ray fluorescence (XRF) were performed during the irradiation process and real time measurements revealed threshold conditions for nanopatterning of silicon at normal incidence irradiation. Three main stages of the nanopatterning process were identified. The real time GISAXS intensity of the correlated peaks in conjunction with XRF revealed that the nanostructures remain for a time period after the removal of the all the metal atoms from the sample depending on the binding energy of the metal silicides formed. Ex-situ XPS confirmed the removal of all metal impurities. In-situ XPS during the irradiation of Ni, Fe, and Cu coated silicon substrates at normal incidence demonstrated phase separation and the formation of different silicide phases that occur upon metal-silicon mixing. Silicide formation leads to nanostructure formation due the preferential erosion of the non-silicide regions and the weakening of the ion induced mass redistribution. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4797480