Characterization Of The Dynamic Formation Of Nano Tendril Surface Morphology On Tungsten While Exposed To Helium Plasma
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Characterization Of The Dynamic Formation Of Nano Tendril Surface Morphology On Tungsten While Exposed To Helium Plasma
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Author : Kevin Benjamin Woller
language : en
Publisher:
Release Date : 2017
Characterization Of The Dynamic Formation Of Nano Tendril Surface Morphology On Tungsten While Exposed To Helium Plasma written by Kevin Benjamin Woller and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2017 with categories.
Tungsten undergoes surface morphology changes on the nanometer scale when subjected to low energy helium ion bombardment. This is due in part to the ion bombardment causing tungsten atoms to move on the surface, but also because of helium implantation and bubble development in the near surface at a depth 30 nm. At high enough surface temperatures, T/TM /~ 0.2, where TM is the melting temperature, nanoscale tendrils form on the surface and grow longer with additional bombardment by helium, but will decompose at the same temperature without helium bombardment. A tungsten surface that develops a densely packed layer of nano-tendrils over macroscopic areas greater than the grain size is referred to as tungsten fuzz, and is under intense study in fusion energy research, both for better understanding of how tungsten fuzz forms and of how tungsten fuzz affects the performance of plasma-facing components. The necessity of helium irradiation of the surface to induce nano-tendril growth motivates investigation into the dynamic process of helium implantation and accumulation in the surface. In this thesis, in situ elastic recoil detection is developed and used to measure the dynamic concentration of helium within a tungsten surface during the active growth of tungsten fuzz. During the development of in situ elastic recoil detection analysis, a variant of tungsten nano-tendril growth was discovered featuring drastically isolated bundles of nano-tendrils that grow at a higher rate than tungsten fuzz. The variation in nano-tendril morphology is correlated with incident helium ion energy modulation. The dependence on ion energy modulation and isolated nature of the nano-tendril bundles reveals clearly that nano-tendril growth is sensitive to surface kinetic effects. In this thesis, the structure and parameter space of the newly discovered nano-tendril bundle growth is analyzed with a suite of electron-based surface science techniques.
Surface Response Of Tungsten To Helium And Hydrogen Plasma Flux As A Function Of Temperature And Incident Kinetic Energy
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Author : Faiza Sefta
language : en
Publisher:
Release Date : 2013
Surface Response Of Tungsten To Helium And Hydrogen Plasma Flux As A Function Of Temperature And Incident Kinetic Energy written by Faiza Sefta and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2013 with categories.
Tungsten is a leading candidate material for the diverter in future nuclear fusion reactors. Previous experiments have demonstrated that surface defects and bubbles form in tungsten when ex- posed to helium and hydrogen plasmas, even at modest ion energies. In some regimes, between 1000K and 2000K, and for He energies below 100eV, "fuzz" like features form. The mechanisms leading to these surfaces comprised of nanometer sized tungsten tendrils which include visible helium bubbles are not currently known. The role of helium bubble formation in tendril morphology could very likely be the starting point of these mechanisms. Using Molecular dynamics (MD) simulations, the role of helium and hydrogen exposure in the initial formation mechanisms of tungsten "fuzz" are investigated. Molecular dynamics simulations are well suited to describe the time and length scales associated with initial formation of helium clusters that eventually grow to nano-meter sized helium bubbles. MD simulations also easily enable the modeling of a variety of surfaces such as single crystals, grain boundaries or "tendrils". While the sputtering yield of tungsten is generally low, previous observations of surface modification due to plasma exposure raise questions about the effects of surface morphology and sub-surface helium bubble populations on the sputtering behavior. Results of computational molecular dynamics are reported that investigate the influence of sub-surface helium bubble distributions on the sputtering yield of tungsten (100) and (110) surfaces induced by helium ion exposure in the range of 300 eV to 1 keV. The calculated sputtering yields are in reasonable agreement with a wide range of experimental data; but do not show any significant variation as a result of the pre-existing helium bubbles. Molecular dynamics simulations reveal a number of sub-surface mechanisms leading to nanometer- sized "fuzz" in tungsten exposed to low-energy helium plasmas. We find that during the bubble formation process, helium clusters create self-interstitial defect clusters in tungsten by a trap mutation process, followed by the migration of these defects to the surface that leads to the formation of layers of adatom islands on the tungsten surface. As the helium clusters grow into nanometer sized bubbles, their proximity to the surface and extremely high gas pressures can cause them to rupture the surface thus enabling helium release. Helium bubble bursting induces additional surface damage and tungsten mass loss which varies depending on the nature of the surface. We then show tendril-like geometries have surfaces that are more resilient to helium clustering and bubble formation and rupture. Finally, the study includes hydrogen to reveal the effect of a mixed 90%H-10%He plasma mix on the tungsten surface. We find that hydrogen greatly affects the tungsten surface, with a near surface hydrogen saturation layer, and that helium clusters still form and are attractive trapping sites for hydrogen. Molecular dynamics simulations have also investigated the effect of sub-surface helium bubble evolution on tungsten surface morphology. The helium bubble/tungsten surface interaction has been systematically studied to determine how parameters such as bubble shape and size, temperature, tungsten surface orientation and ligament thickness above the bubble impact bubble stability and surface evolution. The tungsten surface is roughened by a combination of adatom islands, craters and pinholes. The study provides insight into the mechanisms and conditions leading to various tungsten topology changes, most notably the formation of nanoscale fuzz. An atomistic study of the mechanisms behind initial phases of tungsten nano-fuzz growth has determined that tungsten surfaces are affected by sub-displacement energy helium and hydrogen fluxes through a series of mechanisms. Sub-surface helium atom clustering, bubble nucleation, growth and rupture lead to tungsten surface deformation. Helium clustering processes vary near grain boundaries or in tendril-like surface geometries. In the presence of hydrogen, these mechanisms are coupled with hydrogen surface saturation. Finally, further investigation to connect these atomistic mechanisms to nano-size tungsten fuzz growth is needed to get a comprehensive under- standing of the effects of low energy helium and hydrogen on tungsten.
Helium Nano Bubble Formation In Tungsten
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Author : Matt Thompson
language : en
Publisher: Springer
Release Date : 2018-08-01
Helium Nano Bubble Formation In Tungsten written by Matt Thompson and has been published by Springer this book supported file pdf, txt, epub, kindle and other format this book has been release on 2018-08-01 with Science categories.
This PhD thesis characterises the damage that occurs in tungsten when it is exposed to a fusion-like environment. The book presents pioneering work on the use of grazing-incidence small-angle X-ray scattering (GISAXS) to measure nano-bubble formation in tungsten exposed to helium plasma. The phenomenon of nanoscale bubble formation within metals during helium plasma exposure can lead to undesirable changes in the material properties, such as complex nanoscale surface modification or a reduction in thermal conductivity. As a result of this work, it is now possible to quantify how nanobubble behaviour changes within different materials, and under different plasma conditions. In 2015 the author published the first GISAXS study of helium-induced nanobubble formation in tungsten, demonstrating the viability of using GISAXS for this work. This paper has generated significant interest from the international fusion community and was selected as one of the highlights for the journal Nuclear Fusion.
Morphologies Of Tungsten Nanotendrils Grown Under Helium Exposure
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Author :
language : en
Publisher:
Release Date : 2017
Morphologies Of Tungsten Nanotendrils Grown Under Helium Exposure written by and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2017 with categories.
Nanotendril "fuzz" will grow under He bombardment under tokamak-relevant conditions on tungsten plasma-facing materials in a magnetic fusion energy device. We have grown tungsten nanotendrils at low (50 eV) and high (12 keV) He bombardment energy, in the range 900-1000 °C, and characterized them using electron microscopy. Low energy tendrils are finer (~22 nm diameter) than high-energy tendrils (~176 nm diameter), and low-energy tendrils have a smoother surface than high-energy tendrils. Cavities were omnipresent and typically ~5-10 nm in size. Oxygen was present at tendril surfaces, but tendrils were all BCC tungsten metal. Electron diffraction measured tendril growth axes and grain boundary angle/axis pairs; no preferential growth axes or angle/axis pairs were observed, and low-energy fuzz grain boundaries tended to be high angle; high energy tendril grain boundaries were not observed. We speculate that the strong tendency to high-angle grain boundaries in the low-energy tendrils implies that as the tendrils twist or bend, strain must accumulate until nucleation of a grain boundary is favorable compared to further lattice rotation. Finally, the high-energy tendrils consisted of very large (>100 nm) grains compared to the tendril size, so the nature of the high energy irradiation must enable faster growth with less lattice rotation.
Surface Damage And Microstructure Deformation Of Resolidified Tungsten In High Heat Flux Conditions
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Author : Minsuk Seo
language : en
Publisher:
Release Date : 2021
Surface Damage And Microstructure Deformation Of Resolidified Tungsten In High Heat Flux Conditions written by Minsuk Seo and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2021 with categories.
Nuclear fusion is a potential energy source for the next generation. To make a tokamak reactor, magnetic confinement of hot plasma and durable plasma facing components are necessary for the operation. In the reactor, the divertor is the component that experiences the highest heat flux (10-20MW/m2). Tungsten (Tm=3422°C) is a candidate divertor material for many tokamak reactors and has the potential to survive in high heat flux conditions. However, plasma-material interactions and edge localized modes (1-10GW/m2) can cause significant hardening and melting damage. The motivation of this dissertation is to investigate surface damage of tungsten in extreme heat load conditions, and, further, understand the materials degradation process in fusion relevant environments. Morphological and nanomechanical alternation of tungsten in extreme environment, like those of edge localized modes in nuclear fusion environments, up to 46.3GW/m2 heat fluxes were experimentally simulated using electrothermal plasma. Surface and subsurface damage to the tungsten is seen mainly in the form of pore formation, cracks, and resolidified melt instabilities. Micro voids, dendrite-type microfeatures, core-shell structures, particle enrichment, and submicron channels, all manifest in the damaged subsurface. The formation of voids in the subsurface was determined to originate from the ductile fracture of hot tungsten by plastic flow but not developed to cracking. The voids were preferentially settled in grain boundaries, interfaces. The directionality of elongated voids and grains is biased to the heat flow vector or plasma pathway, which is the likely consequence of the thermally driven grain growth and sliding in high-temperature conditions. The presence of a border between the transient layer and heat-affected zone is observed and attributed to plasma shock and thermal spallation of fractural tungsten at high temperature. Plasma peening-like hardening effects in tungsten were observed in the range of 22.7GW-46.3GW/m2 but least in the case of the lowest heat flux, 12.5GW/m2. Next, tungsten in a high heat flux (46.3GW/m2) and helium plasma environment was investigated. We studied possible residual helium and microstructure deformation in resolidified tungsten. Following this, a 1-30keV Ga focused ion beam was used for TEM sample milling. In as-received tungsten, manufactured dark spots of possible lattice strain and defects were in the grains. After heat exposure under plasma pressure, intergranular features, dark spots in TEM, dislocations in IFFT images, and elongated FIB induced pore-artifact structures (second phase) formed. Helium appears to be absent from the resolidified tungsten matrix due to erosive melting damage and high temperature conditions. A simple four-step microstructure deformation process from the elastic bulk side to the near-pore environment is proposed. A range of measured high dislocation density from IFFT images, grain size reduction, and microstructure deformation affected hardening. The near-pore environment likely experienced grain refinement, and further disordered nanophases are possible under severe deformation conditions. Tungsten and tungsten carbide were exposed to high heat flux (59.6GW/m2) using a femtosecond laser with different incident angles (0°, 30°, 45°, 60°) up to approximately 5,000 shots in ambient air. The aim of these experiments was to simulate high heat flux, off normal events in fusion reactors, particularly Edge Localized Modes (ELMs) (1-10GW/m2) and Loss of Vacuum Accident (LOVA) relevant conditions. At low pulse numbers (1-8 laser pulses on the same spot), the tungsten surface was more durable than tungsten carbide, but at very high pulse numbers (~5,000) the opposite was true. The surface damage mostly took the form of craters that were near-circular at low impact angles and became more elongated at higher laser pulse impact angles. A cluster of tungsten oxide debris also formed on the tungsten surface. It also appeared that areas of tungsten oxide, cobalt oxide, or cobalt tungstate formed on the tungsten carbide surfaces. Laser Induced Periodic Surface Structures (LIPSS) and grooves were formed, during laser exposure, and their geometries varied with laser intensity (roughly with the gaussian spot shape) and with laser impact angle. The period of laser induced surface changes increased for both tungsten and tungsten carbide surfaces as the incident angle increased. More mass was lost in from tungsten than tungsten carbide, which agrees on the morphological responses. The mass loss by laser ablation overwhelmed the possible mass gains from surface oxidation.
Morphology Changes Due To Energetic Helium Ion Irradiation Of Tungsten Surfaces At High Temperatures
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Author : Karla Brittany Hall
language : en
Publisher:
Release Date : 2019
Morphology Changes Due To Energetic Helium Ion Irradiation Of Tungsten Surfaces At High Temperatures written by Karla Brittany Hall and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2019 with categories.
The Materials Irradiation Experiment (MITE-E) at the University of Wisconsin-Madison in the Inertial Electrostatic Confinement Laboratory was used to simulate helium ion (He+) irradiation of tungsten at fusion reactor relevant ion fluences and temperatures. Single and polycrystalline tungsten (SCW and PCW) samples were irradiated with 30-55 keV He+ at normal incidence with fluences of 3x1017 to 1019 He+/cm2 at temperatures from 500-900 oC. Post-irradiation analysis of the samples irradiated in the MITE-E device revealed several unique surface morphologies when the ion energy, temperature, and fluence were varied. Surface erosion due to energetic He+ irradiation was determined by mass loss measurements of samples pre- and post-irradiation. Past studies in the MITE-E revealed a surface orientation near {001} on PCW had considerably less erosion from He+ irradiation than surrounding grains Morphology development and mass loss measurements on [110] and [100] SCW revealed that the [100] crystal orientation lead to decreased surface erosion below 10^18 He+/cm2. Simulating what a fusion reactor component would likely experience, a sequentially increasing and decreasing multi-energy He+ irradiation on PCW was explored. Samples irradiated with increasing and decreasing multi-energy (35, 45, and 55 keV) He+ showed more surface damage than samples irradiated with mono-energetic (30 keV) He+. However, the multi-energy ion irradiated samples had less mass loss than the mono-energetic ion irradiated samples under similar parameters. Varying the temperature of samples under mono-energetic and multi-energy He+ irradiation at the same fluence caused an increase in mass loss as the temperature was decreased. The amount of mass loss and morphologies that developed on all samples point to He+ as a contributing mechanism in the undesired creation of W dust in a fusion reactor.
Numerical Simulations Of Tungsten Under Helium Irradiation
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Author : Thibault Faney
language : en
Publisher:
Release Date : 2013
Numerical Simulations Of Tungsten Under Helium Irradiation written by Thibault Faney and has been published by this book supported file pdf, txt, epub, kindle and other format this book has been release on 2013 with categories.
Magnetic confinement fusion is a promising technology for electricity production due to available fuel and low waste products. However, the construction of a nuclear fusion reactor remains a scientific challenge. One of the main issues is the resistance of the plasma facing materials exposed to very harsh operating conditions. Tungsten is the leading candidate for the divertor, a crucial plasma facing component. This dissertation focuses on modeling the behavior of tungsten under irradiation conditions relevant to the divertor operations using a multi-scale modeling approach. In particular, high fluxes of helium ions at low energy impact the divertor and are responsible for changes in the tungsten microstructure such as the formation of helium blisters and ''fuzz"--Like structures which can ultimately lead to erosion, degradation of materials performance and materials failure. A spatially dependent cluster dynamics model is introduced in order to model the evolution of the tungsten microstructure under irradiation. This continuum model is based on kinetic rate theory and handles each material defect type independently. Under the assumptions of a low dilute limit and no spatial correlation between defects, this leads to a large system of non-linear reaction-diffusion equations. Hence, the results addressed in this thesis consist in the determination of the kinetic parameters for the cluster dynamics model, the construction of a solver which efficiently deals with the large non-linear system of partial differential equations, the determination of the applicability of the model to fusion relevant conditions, and the model results for a variety of irradiation conditions. The input kinetic parameters to the cluster dynamics model are the defects' diffusion coefficients, binding energies and capture radii. These can be determined using a molecular dynamics and density functional theory simulations as well as empirical data. The challenge lies in obtaining a consistent set of kinetic parameters. Therefore, a method to determine the value of the diffusion coefficients for small helium, interstitial and vacancy defects at various temperatures using only molecular dynamics simulations is presented. Binding energies are also determined using molecular dynamics, and when combined with the diffusion coefficients they form a consistent set of kinetic parameters. An efficient implementation of a parallel solver is presented to deal with the large number of stiff non linear reaction diffusion equations. The implementation of a SDIRK scheme using a modified version of the SPIKE algorithm gives excellent parallelization results and suggests that this implementation would also be efficient for an extension of the model to two or three dimensions. Convergence results for a variety of SDIRK schemes show a convergence order reduction of the numerical scheme due to the stiffness of the reaction and diffusion terms. A comparison between simulation results using the cluster dynamics model and experimental results is essential to assess the validity of the model. Comparison with thermal helium desorption spectrometry experiments at low flux and fluence shows an excellent agreement between simulation and experiments and indicate that the model captures the key physical properties affecting the evolution of the tungsten microstructure. Further comparison with molecular dynamics simulations at extremely high fluxes provides an insight in the expected limitations of the model due to surface effects and dilute limit approximations breakdown when applied to fusion relevant conditions. Results of the model under fusion relevant conditions show the formation of large helium bubbles under the surface at a temperature dependent depth. The results are very sensitive to both irradiation flux and temperature. At large temperatures, a small concentration of large bubbles forms first deep under the tungsten surface, and forms a ``plug" which moves towards the surface until eventually the dilute limit approximation breaks down, indicating that the sub-surfaces bubbles become interlinked. At small temperatures, a larger concentration of smaller bubbles forms close to the surface until eventually surface effects such as bubble bursting are expected to occur. These results are found to be in good agreement with a similar analytical reaction diffusion model for fusion relevant conditions. More work is needed to simulate past the dilute limit breakdown and examine the possibility of taking into account surface effects.
Food Mixing
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Author : P. J. Cullen
language : en
Publisher: John Wiley & Sons
Release Date : 2009-07-21
Food Mixing written by P. J. Cullen and has been published by John Wiley & Sons this book supported file pdf, txt, epub, kindle and other format this book has been release on 2009-07-21 with Technology & Engineering categories.
The mixing of liquids, solids and gases is one of the most commonunit operations in the food industry. Mixing increases thehomogeneity of a system by reducing non-uniformity or gradients incomposition, properties or temperature. Secondary objectives ofmixing include control of rates of heat and mass transfer,reactions and structural changes. In food processing applications,additional mixing challenges include sanitary design, complexrheology, desire for continuous processing and the effects ofmixing on final product texture and sensory profiles. Mixing ensures delivery of a product with constant properties. Forexample, consumers expect all containers of soups, breakfastcereals, fruit mixes, etc to contain the same amount of eachingredient. If mixing fails to achieve the requiredproduct yield, quality, organoleptic or functional attributes,production costs may increase significantly. This volume brings together essential information on theprinciples and applications of mixing within food processing. Whilethere are a number of creditable references covering generalmixing, such publications tend to be aimed at the chemical industryand so topics specific to food applications are often neglected.Chapters address the underlying principles of mixing, equipmentdesign, novel monitoring techniques and the numerical techniquesavailable to advance the scientific understanding of food mixing.Food mixing applications are described in detail. The book will be useful for engineers and scientists who need tospecify and select mixing equipment for specific processingapplications and will assist with the identification and solving ofthe wide range of mixing problems that occur in the food,pharmaceutical and bioprocessing industries. It will also be ofinterest to those who teach, study and research food science andfood engineering.
Spectral Methods For Uncertainty Quantification
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Author : Olivier Le Maitre
language : en
Publisher: Springer Science & Business Media
Release Date : 2010-03-11
Spectral Methods For Uncertainty Quantification written by Olivier Le Maitre and has been published by Springer Science & Business Media this book supported file pdf, txt, epub, kindle and other format this book has been release on 2010-03-11 with Science categories.
This book deals with the application of spectral methods to problems of uncertainty propagation and quanti?cation in model-based computations. It speci?cally focuses on computational and algorithmic features of these methods which are most useful in dealing with models based on partial differential equations, with special att- tion to models arising in simulations of ?uid ?ows. Implementations are illustrated through applications to elementary problems, as well as more elaborate examples selected from the authors’ interests in incompressible vortex-dominated ?ows and compressible ?ows at low Mach numbers. Spectral stochastic methods are probabilistic in nature, and are consequently rooted in the rich mathematical foundation associated with probability and measure spaces. Despite the authors’ fascination with this foundation, the discussion only - ludes to those theoretical aspects needed to set the stage for subsequent applications. The book is authored by practitioners, and is primarily intended for researchers or graduate students in computational mathematics, physics, or ?uid dynamics. The book assumes familiarity with elementary methods for the numerical solution of time-dependent, partial differential equations; prior experience with spectral me- ods is naturally helpful though not essential. Full appreciation of elaborate examples in computational ?uid dynamics (CFD) would require familiarity with key, and in some cases delicate, features of the associated numerical methods. Besides these shortcomings, our aim is to treat algorithmic and computational aspects of spectral stochastic methods with details suf?cient to address and reconstruct all but those highly elaborate examples.
Fusion Technology 1994
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Author : K. Herschbach
language : en
Publisher: Newnes
Release Date : 2012-12-02
Fusion Technology 1994 written by K. Herschbach and has been published by Newnes this book supported file pdf, txt, epub, kindle and other format this book has been release on 2012-12-02 with Technology & Engineering categories.
The objective of the Symposium on Fusion Technology (SOFT) conference is to set the stage for the exchange of information on the design, construction, and operation of fusion experiments and the technology which is being developed for the next-step devices and for fusion reactors. These proceedings therefore present an up-to-date and throrough review of the state-of-the art in this dynamic field.