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Author :
Publisher :
ISBN 13 :
Total Pages : 19 pages
Book Rating : 4.:/5 (96 download)
Download or read book Characterization of Helium Bubble Growth in Tungsten Exposed to Low-flux Plasmas written by and published by . This book was released on 2014 with total page 19 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Author : Edward Xiang Gao
Publisher :
ISBN 13 :
Total Pages : 150 pages
Book Rating : 4.:/5 (15 download)
Download or read book Investigation of Helium Bubble Behavior in Plasma-Facing Micro-Engineered Tungsten: Theory & Experiment written by Edward Xiang Gao and published by . This book was released on 2018 with total page 150 pages. Available in PDF, EPUB and Kindle. Book excerpt: Understanding the interaction mechanisms between low-energy helium plasma and solid materials has significant implications in several advanced technologies; primary amongst them are fusion energy and space electric propulsion. Since the penetration of low-energy helium is very shallow, it may be advantageous to manipulate the surface of the solid by micro-engineering techniques so as to provide a degree of control over the resulting plasma-induced damage. Helium plasma exposure of tungsten in fusion devices causes extensive damage due to the creation of nanometer-scale subsurface bubbles. We investigate here the formation mechanisms of helium bubbles in tungsten that has been micro-engineered utilizing both theory and experiments. Experiments on four different micro-engineered surfaces were conducted at the PISCES facility at the University of California, San Diego (UCSD), where they were exposed to various fluences of low-energy helium plasma, with planar surface as control. These samples included two surfaces covered with uniform micro-pillars; one with cylindrical pillars 1 micron in diameter and 25 micron in height, and one with dendritic conical pillars 4 - 10 micron in diameter and 20 micron in height. Additionally, two samples with reticulated open-cell foam geometry, one at 45 pores per inch (PPI), and the other at 80 PPI were fabricated with Chemical Vapor Deposition (CVD). The samples were exposed to helium plasma at 30 - 100 eV ion energy, 823 - 1123 K temperature, and 5 x 10^25 - 2 x 10^26 m^-2 ion fluence. Results show that whereas nanometer-scale tendrils (fuzz) would grow on planar tungsten surfaces, the fuzz formation on micro-engineered surfaces was greatly reduced due to backscattering of ions and the increased surface area. A 20% decrease in the ion incident angle on pillar type surfaces leads to ~30% decrease in bubble size, down to 30 nm in diameter. W fuzz was found to be absent from pillar sides due to high ion reflection. In foam samples, 28% higher PPI is observed to have 24.7% - 36.7% taller fuzz, and 17.0% - 25.0% larger subsurface bubbles. These are found to be an order of magnitude smaller than those in planar surfaces of similar environment. The helium bubble density was found to increase with ion energy in pillars, roughly from 8.2% to 48.4%, and to increase with increasing PPI, from 36.4% - 116.2%, and with bubble concentrations up to 9.1 x 10^21 m^-3. Geometric shadowing effects in or near surface ligaments were observed in all foam samples, with near absence of helium bubbles or fuzz in deeper layers of the foam. Then 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. The novel features of the multiscale model are: (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, results are compared with the above mentioned experimental exposure on micro-pillars at 100 eV. The predicted average bubble radius and density are in qualitative agreement with experimental results.
Author : Faiza Sefta
Publisher :
ISBN 13 :
Total Pages : 128 pages
Book Rating : 4.:/5 (94 download)
Download or read book Surface Response of Tungsten to Helium and Hydrogen Plasma Flux as a Function of Temperature and Incident Kinetic Energy written by Faiza Sefta and published by . This book was released on 2013 with total page 128 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Author : Kevin Benjamin Woller
Publisher :
ISBN 13 :
Total Pages : 140 pages
Book Rating : 4.:/5 (11 download)
Download or read book Characterization of the Dynamic Formation of Nano-tendril Surface Morphology on Tungsten While Exposed to Helium Plasma written by Kevin Benjamin Woller and published by . This book was released on 2017 with total page 140 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Author : Matt Thompson
Publisher : Springer
ISBN 13 : 3319960113
Total Pages : 112 pages
Book Rating : 4.3/5 (199 download)
Download or read book Helium Nano-bubble Formation in Tungsten written by Matt Thompson and published by Springer. This book was released on 2018-08-01 with total page 112 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Author : Matt Thompson
Publisher :
ISBN 13 : 9783319960128
Total Pages : pages
Book Rating : 4.9/5 (61 download)
Download or read book Helium Nano-bubble Formation in Tungsten written by Matt Thompson and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book A Multi-technique Analysis of Deuterium Trapping and Near-surface Precipitate Growth in Plasma-exposed Tungsten written by and published by . This book was released on 2015 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: We examine how deuterium becomes trapped in plasma-exposed tungsten and forms near-surface platelet-shaped precipitates. How these bubbles nucleate and grow, as well as the amount of deuterium trapped within, is crucial for interpreting the experimental database. Here, we use a combined experimental/theoretical approach to provide further insight into the underlying physics. With the Tritium Plasma Experiment, we exposed a series of ITER-gradetungsten samples to high flux D plasmas (up to 1.5 × 1022 m-2 s-1) at temperatures ranging between 103 and 554 °C. Retention of deuterium trapped in the bulk, assessed through thermal desorption spectrometry, reached a maximum at 230 °C and diminished rapidly thereafter for T> 300 °C. Post-mortem examination of the surfaces revealed non-uniform growth of bubbles ranging in diameter between 1 and 10 [mu]m over the surface with a clear correlation with grain boundaries. Electron back-scattering diffraction maps over a large area of the surface confirmed this dependence; grains containing bubbles were aligned with a preferred slip vector along the 111 directions. Focused ion beam profiles suggest that these bubbles nucleated as platelets at depths of 200 nm-1 [mu]m beneath the surface and grew as a result of expansion of sub-surface cracks. Furthermore, to estimate the amount of deuterium trapped in these defects relative to other sites within the material, we applied a continuum-scale treatment of hydrogen isotope precipitation. Additionally, we propose a straightforward model of near-surface platelet expansion that reproduces bubble sizes consistent with our measurements. For the tungsten microstructure considered here, we find that bubbles would only weakly affect migration of D into the material, perhaps explaining why deep trapping was observed in prior studies with plasma-exposed neutron-irradiated specimens. We foresee no insurmountable issues that would prevent the theoretical framework developed here from being extended to a broader range of systems where precipitation of insoluble gases in ion beam or plasma-exposed metals is of interest.
Author : H. Ullmaier
Publisher : Springer
ISBN 13 : 9783540514350
Total Pages : 0 pages
Book Rating : 4.5/5 (143 download)
Download or read book Atomic Defects in Metals / Atomare Fehlstellen in Metallen written by H. Ullmaier and published by Springer. This book was released on 1991-09-30 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Atomic or point defects are disturbances of the periodicity of the crystal lattice extending over only a few atomic distances. Many physical and mechanical properties of solids are sensitive to their presence. Furthermore other defects which are crucial to material behaviour are influenced by their interaction with atomic defects. A detailed knowledge of production mechanisms and properties of point defects is therefore essential for assessing and for understanding the atomistic as well as the macroscopic behaviour of materials. As a result of the use of new research methods in recent years, such information is now available for almost all pure metals, for many dilute alloys and for some concentrated alloys. However, a critical and comprehensive collection of these data has so far been missing. The aim of the present volume is to close this gap by sifting, evaluating and compiling data on vacancies and self-interstitial atoms in solid metals. A chapter on the element helium is included because of the exeptional position of He as an "alloying" element and its role in radiation damage phenomena.
Author : Luxherta Buzi
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (931 download)
Download or read book Influence of the Particle Flux on Surface Modifications of Tungsten written by Luxherta Buzi and published by . This book was released on 2015 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Tungsten is the selected material to be used in the ITER divertor due to its favorable thermal and physical properties. Particle flux densities and energies, and surface temperature will vary by several orders of magnitude along the divertor surface, with values in the range 1020-1024 m2s-1, 0.1-100 eV and 370-1370 K, respectively. Exposed to such conditions, tungsten may undergo erosion, cracking and other surface modifications affecting its thermal and mechanical properties. Another concern is the retention of implanted radioactive fuel atoms (tritium) in the material surface and their diffusion through the bulk. A considerable amount of studies have addressed retention and plasma induced surface modifications, focusing mainly on the effect of ion energy, ion fluence and surface temperature while very little knowledge exists on the influence of the plasma flux. These results are largely scattered and occasionally bear a lack of consistency. The aim of this thesis is to provide a coherent picture of the behavior of tungsten exposed to plasma conditions relevant for future fusion reactors. A systematic investigation assessing the impact of the plasma flux density and exposure temperature on surface modifications and hydrogen accumulation in tungsten was performed by means of experiments carried out in the linear plasma devices PSI2 at Forschungszentrum Juelich, Pilot-PSI and Magnum-PSI at DIFFER, and PISCES-A at UCSD. The correlation between the particle flux density, exposure temperature, surface modifications and hydrogen retention in tungsten was investigated for different material microstructures. Three types of polycrystalline tungsten (thermally treated at 1273 and 2273 K) and single crystal tungsten samples (110 crystal orientation) were exposed to deuterium plasmas at surface temperatures of 530-1170 K to two different ranges of deuterium ion fluxes (low and high flux: ~1022 and ~1024 m2s-1). All the exposures were performed at the same incident ion energy of 40 eV and particle fluence of ~1026 m2. The exposed samples were analyzed postmortem utilizing various surface imaging and analyses techniques (microscopy, thermal desorption spectroscopy and ion beam analysis). Increasing the particle flux by two orders of magnitude caused blister formation at temperatures above 700 K for which blistering is usually absent under low flux exposure conditions. Small blisters of several tens of nanometers and up to 1 micrometer of lateral size were detected on the annealed polycrystalline and in single crystal tungsten samples, respectively. On the contrary, blisters were absent on the recrystallized samples except for the low flux and low temperature case where large blisters of about 10 micrometer and cavities along the grain boundaries appeared. The total deuterium retention was measured by means of thermal desorption spectroscopy (TDS). In the cases with low exposure temperatures, the retained fraction of deuterium was one to two orders of magnitude higher after exposure to the low flux compared to the high flux. On the contrary, an opposite tendency of the total deuterium retention at high exposure temperatures was observed. Hence, the maximum of the total deuterium retention was observed to occur at a higher temperature in the case of high incident particle flux (~850 K) compared to low flux exposures (~650 K). Overall, experimental results on deuterium retention were similar for all the investigated tungsten microstructures. Deuterium retention decreased at high temperatures and the maximal retention was lower for high flux exposures. However, due to the shift of the maximal retention to higher temperatures, the amount of deuterium retained at temperatures above 800 K was higher at high flux rather than at low flux, being still about one order of magnitude lower than the maximal retention at low flux.
Author : Guinevere Chamberlain Shaw
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (137 download)
Download or read book Quantification of Hydrogen-helium Retention in Tungsten Using Laser Induced Breakdown Spectroscopy Coupled with Laser Ablation Mass Spectrometry written by Guinevere Chamberlain Shaw and published by . This book was released on 2018 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Tungsten (W) has been selected for the ITER divertor because of its high melting temperature, low sputtering yield, and high thermal conductivity. During deuterium/tritium (D/T) plasma exposure in ITER, a large flux (1024m−2s−1) of relatively low-energy (100 eV) of D/T plus helium (He) will strike the divertor. The resulting plasma surface interactions (PSI) will lead to surface damage and deformation such as bubble formation, surface blistering and/or erosion, and tritium retention. Experiments have shown that the formation of helium bubbles can have a direct effect on hydrogen retention, although the extent is not fully known. This dissertation has developed and demonstrated two new, complementary laser-based characterization techniques (LIBS and LAMS) for assessing gas concentrations in nuclear materials as a function of spatial position (depth below the surface), with an emphasis on assessing the He-H interaction synergies in tungsten that are expected to impact tritium retention in the ITER divertor and future fusion reactors. In the newly established ultrahigh vacuum setup, the LIBS capability is coupled with the ability to simultaneously pump the ablated gases into a quadrupole mass spectrometer in an existing thermal desorption system (TDS) to simultaneously (although with a small time delay) measure the ion current of the detected gas species in a QMS, this capability we define as LAMS. The gas fluxes measured in LAMS are converted to an absolute quantity of measured gas per laser ablation pulse through calibration with known leak volumes in the TDS. Results of gas concentration as a function of depth in tungsten are shown following exposure to various fluences and plasma configurations, as well as compared to other surface gas evaluation techniques. Altogether this dissertation provides significant new results that: demonstrate the ability of LIBS and LAMS to perform depth dependent gaseous species concentration measurements in nuclear materials; offer new data that comprehensively reflects the complex He-H synergistic interactions and the role of He on tritium retention expected in the ITER tungsten divertor; and provide depth dependent concentration measurements (in addition to integrated retention values) for validation of multiscale models.
Author : Yafang Han
Publisher : Springer
ISBN 13 : 9811301581
Total Pages : 939 pages
Book Rating : 4.8/5 (113 download)
Download or read book Advances in Energy and Environmental Materials written by Yafang Han and published by Springer. This book was released on 2018-04-17 with total page 939 pages. Available in PDF, EPUB and Kindle. Book excerpt: This proceedings volume gathers selected papers presented at the Chinese Materials Conference 2017 (CMC2017), held in Yinchuan City, Ningxia, China, on July 06-12, 2017. This book covers a wide range of energy conversion and storage materials, thermoelectric materials and devices, nuclear materials, solar energy materials and solar cells, minerals and oil and gas materials, photocatalytic materials for energy production, eco-materials, and environmental engineering materials. The Chinese Materials Conference (CMC) is the most important serial conference of the Chinese Materials Research Society (C-MRS) and has been held each year since the early 1990s. The 2017 installment included 37 Symposia covering four fields: Advances in energy and environmental materials; High performance structural materials; Fundamental research on materials; and Advanced functional materials. More than 5500 participants attended the congress, and the organizers received more than 700 technical papers. Based on the recommendations of symposium organizers and after peer reviewing, 490 papers have been included in the present proceedings, which showcase the latest original research results in the field of materials, achieved by more than 300 research groups at various universities and research institutes.
Author : Yongqiang Wang
Publisher : MDPI
ISBN 13 : 303936362X
Total Pages : 196 pages
Book Rating : 4.0/5 (393 download)
Download or read book Radiation Damage in Materials written by Yongqiang Wang and published by MDPI. This book was released on 2020-12-28 with total page 196 pages. Available in PDF, EPUB and Kindle. Book excerpt: The complexity of radiation damage effects in materials that are used in various irradiation environments stems from the fundamental particle–solid interactions and the subsequent damage recovery dynamics after the collision cascades, which involves multiple length and time scales. Adding to this complexity are the transmuted impurities that are unavoidable from accompanying nuclear processes. Helium is one such impurity that plays an important and unique role in controlling the microstructure and properties of materials used in fast fission reactors, plasma-facing and structural materials in fusion devices, spallation neutron target designs, actinides, tritium-containing materials, and nuclear waste. Their ultra-low solubility in virtually all solids forces He atoms to self-precipitate into small bubbles that become nucleation sites for further void growth under radiation-induced vacancy supersaturations, resulting in material swelling and high-temperature He embrittlement, as well as surface blistering under low-energy and high-flux He bombardment. This Special Issue, “Radiation Damage in Materials—Helium Effects”, contains review articles and full-length papers on new irradiation material research activities and novel material ideas using experimental and/or modeling approaches. These studies elucidate the interactions of helium with various extreme environments and tailored nanostructures, as well as their impact on microstructural evolution and material properties.
Author : Mary Alice Cusentino
Publisher :
ISBN 13 :
Total Pages : 155 pages
Book Rating : 4.:/5 (112 download)
Download or read book Discovering Key Unknowns for Tungsten-hydrogen-helium Plasma Material Interactions Using Molecular Dynamics written by Mary Alice Cusentino and published by . This book was released on 2018 with total page 155 pages. Available in PDF, EPUB and Kindle. Book excerpt: Molecular dynamics simulations have been used to study plasma material interactions to better understand the performance of a tungsten divertor. A tendril-like geometry was modeled to study the diffusion of helium in nanotendrils and its relation to fuzz growth. The tendrils remain stable throughout the simulation and a modified helium release mechanism is found that allows the helium retention to reach a steady state within the tendril. The helium retention within the tendril inversely depends on the surface to volume ratio. There is limited diffusion deep into the tendril and extrapolating the flux calculated to experimentally relevant time scales indicates that helium diffusion is not sufficient to drive fuzz growth. Helium implantation near a grain boundary, but not directly on the grain boudary itself, was performed. Helium behavior within the implantation zone is consistent with previous simulations of helium in defect-free tungsten. Some helium diffuses to the grain boundary where it forms small helium clusters but virtually no helium atoms diffuse over the grain boundary. The sink strength of the grain boundary and helium bubbles are calculated and the values are comparable, indicating that the grain boundary sink strength only matters at the beginning of the simulation before the helium bubbles form. Simulations of hydrogen and helium were performed to assess the interaction between the two gas atom species in tungsten. Simulations of small subsurface mixed hydrogen-helium bubbles indicate that hydrogen diffuses to the helium bubble periphery region and becomes trapped there. A binding energy of 2 eV is calculated. Modeling of hydrogen implantation in helium pre-implanted tungsten were performed and the presence of helium modifies the depth distribution and blocks the deeper diffusion of hydrogen when compared with hydrogen implantation in pure tungsten. This could potentially have a significant impact on tritium retention and material performance.
Author : I. H. Hutchinson
Publisher : Cambridge University Press
ISBN 13 : 9780521675741
Total Pages : 460 pages
Book Rating : 4.6/5 (757 download)
Download or read book Principles of Plasma Diagnostics written by I. H. Hutchinson and published by Cambridge University Press. This book was released on 2005-07-14 with total page 460 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book provides a systematic introduction to the physics of plasma diagnostics measurements. It develops from first principles the concepts needed to plan, execute and interpret plasma measurements, making it a suitable book for graduate students and professionals with little plasma physics background. The book will also be a valuable reference for seasoned plasma physicists, both experimental and theoretical, as well as those with an interest in space and astrophysical applications. This second edition is thoroughly revised and updated, with new sections and chapters covering recent developments in the field.
Author : Ashok Gurung
Publisher :
ISBN 13 :
Total Pages : 83 pages
Book Rating : 4.:/5 (11 download)
Download or read book Molecular Dynamics Study of Hydrogen Trapping and Helium Clustering in Tungsten written by Ashok Gurung and published by . This book was released on 2018 with total page 83 pages. Available in PDF, EPUB and Kindle. Book excerpt: The field of plasma confinement and path toward achieving thermonuclear fusion started with experimental devices like tokamak and has evolved into other more complex variants of magnetic plasma confinement such as stellarator and spherical-tokamaks. As the plasma confinement machines advance towards higher temperature and plasma density (thermonuclear fusion conditions) the role and nature of plasma-wall interaction such as possible edge plasma regimes, particle recycling at the walls and its consequence for erosion, migration and re-deposition of wall material and impurity generation, transport and radiation as well as issues of particle exhaust continues to be a dominant limiting factor due to the close proximity of the wall. The selection of optimal wall-material for the plasma-wall components is a complex process, and till date, it continues to be an important and challenging area in the field of study of plasma-wall interaction. Various metals, ceramics or graphites with desirable response to severe thermal loads and varying mechanical properties towards elastic deformation, plastic deformation, fatigue, and toughness have been proposed. Sputtering and wall-erosion which results in plasma contamination is an important determining factor for wall-material selection. Tungsten is considered as a possible candidate for plasma facing material because of its high thermal conductivity, low hydrogen retention, high atomic mass (high-Z), and, high melting point. In both, limiter and divertor configurations there is substantial recycling of particles on the wall due to continuous bombardment of wall material by both charged and neutral particles. Experimental studies have shown that the light particle species such as hydrogen and helium are able to penetrate into the tungsten wall and substantial trapping of helium in tungsten has been observed. Among other issues, blistering, fuzz formation, tritium retention, surface roughening, and intergranular embrittlement are major issues to be addressed. Considerable effort is invested towards developing a better understanding of the interactions of hydrogen, helium in tungsten matrix. In the present study we use classical molecular dynamics (MD) approach to study (a) hydrogen retention, (b) helium bubble formation in tungsten, and, (c) study the effect of the presence of helium bubbles in tungsten matrix on hydrogen retention. The hydrogen bombardment simulations span an energy range from 30 eV to 100 eV at three different substrate temperatures - 500K, 1200K and 2000K. The variation of hydrogen trapping on tungsten matrix surface orientation is examined by performing MD simulation for 100 and 111 surface orientations. The growth of helium clusters in tungsten matrix as a function of temperature and a varying number of helium atoms at the start of the simulation is performed to study solute saturation effects. The trapping of hydrogen in the presence of helium is studied through molecular dynamics study of the bombardment of hydrogen atoms on tungsten substrate with helium cluster distributed throughout the tungsten matrix.The results of this study show that the hydrogen trapping fraction grows almost linearly over the intermediate bombarding energy range with the exception of low incident energy for which higher hydrogen trapping is observed. The effect of substrate temperature for low energy hydrogen bombardment is found to be different from the high-energy incidence indicating a complex dynamics of atomic diffusion within the tungsten matrix. The surface orientation of the substrate also affects the trapping percentage of hydrogen. The formation and growth of helium cluster are found to be dependent on the temperature and the number of helium atoms per unit cell. In the presence of helium cluster, the trapping percentage of hydrogen is significantly affected, especially at low energy
Author : GARY S. WAS
Publisher : Springer
ISBN 13 : 1493934384
Total Pages : 1014 pages
Book Rating : 4.4/5 (939 download)
Download or read book Fundamentals of Radiation Materials Science written by GARY S. WAS and published by Springer. This book was released on 2016-07-08 with total page 1014 pages. Available in PDF, EPUB and Kindle. Book excerpt: The revised second edition of this established text offers readers a significantly expanded introduction to the effects of radiation on metals and alloys. It describes the various processes that occur when energetic particles strike a solid, inducing changes to the physical and mechanical properties of the material. Specifically it covers particle interaction with the metals and alloys used in nuclear reactor cores and hence subject to intense radiation fields. It describes the basics of particle-atom interaction for a range of particle types, the amount and spatial extent of the resulting radiation damage, the physical effects of irradiation and the changes in mechanical behavior of irradiated metals and alloys. Updated throughout, some major enhancements for the new edition include improved treatment of low- and intermediate-energy elastic collisions and stopping power, expanded sections on molecular dynamics and kinetic Monte Carlo methodologies describing collision cascade evolution, new treatment of the multi-frequency model of diffusion, numerous examples of RIS in austenitic and ferritic-martensitic alloys, expanded treatment of in-cascade defect clustering, cluster evolution, and cluster mobility, new discussion of void behavior near grain boundaries, a new section on ion beam assisted deposition, and reorganization of hardening, creep and fracture of irradiated materials (Chaps 12-14) to provide a smoother and more integrated transition between the topics. The book also contains two new chapters. Chapter 15 focuses on the fundamentals of corrosion and stress corrosion cracking, covering forms of corrosion, corrosion thermodynamics, corrosion kinetics, polarization theory, passivity, crevice corrosion, and stress corrosion cracking. Chapter 16 extends this treatment and considers the effects of irradiation on corrosion and environmentally assisted corrosion, including the effects of irradiation on water chemistry and the mechanisms of irradiation-induced stress corrosion cracking. The book maintains the previous style, concepts are developed systematically and quantitatively, supported by worked examples, references for further reading and end-of-chapter problem sets. Aimed primarily at students of materials sciences and nuclear engineering, the book will also provide a valuable resource for academic and industrial research professionals. Reviews of the first edition: "...nomenclature, problems and separate bibliography at the end of each chapter allow to the reader to reach a straightforward understanding of the subject, part by part. ... this book is very pleasant to read, well documented and can be seen as a very good introduction to the effects of irradiation on matter, or as a good references compilation for experimented readers." - Pauly Nicolas, Physicalia Magazine, Vol. 30 (1), 2008 “The text provides enough fundamental material to explain the science and theory behind radiation effects in solids, but is also written at a high enough level to be useful for professional scientists. Its organization suits a graduate level materials or nuclear science course... the text was written by a noted expert and active researcher in the field of radiation effects in metals, the selection and organization of the material is excellent... may well become a necessary reference for graduate students and researchers in radiation materials science.” - L.M. Dougherty, 07/11/2008, JOM, the Member Journal of The Minerals, Metals and Materials Society.
Author : D. Caillard
Publisher : Elsevier
ISBN 13 : 0080542786
Total Pages : 453 pages
Book Rating : 4.0/5 (85 download)
Download or read book Thermally Activated Mechanisms in Crystal Plasticity written by D. Caillard and published by Elsevier. This book was released on 2003-09-08 with total page 453 pages. Available in PDF, EPUB and Kindle. Book excerpt: KEY FEATURES: A unified, fundamental and quantitative resource. The result of 5 years of investigation from researchers around the world New data from a range of new techniques, including synchrotron radiation X-ray topography provide safer and surer methods of identifying deformation mechanisms Informing the future direction of research in intermediate and high temperature processes by providing original treatment of dislocation climb DESCRIPTION: Thermally Activated Mechanisms in Crystal Plasticity is a unified, quantitative and fundamental resource for material scientists investigating the strength of metallic materials of various structures at extreme temperatures. Crystal plasticity is usually controlled by a limited number of elementary dislocation mechanisms, even in complex structures. Those which determine dislocation mobility and how it changes under the influence of stress and temperature are of key importance for understanding and predicting the strength of materials. The authors describe in a consistent way a variety of thermally activated microscopic mechanisms of dislocation mobility in a range of crystals. The principles of the mechanisms and equations of dislocation motion are revisited and new ones are proposed. These describe mostly friction forces on dislocations such as the lattice resistance to glide or those due to sessile cores, as well as dislocation cross-slip and climb. They are critically assessed by comparison with the best available experimental results of microstructural characterization, in situ straining experiments under an electron or a synchrotron beam, as well as accurate transient mechanical tests such as stress relaxation experiments. Some recent attempts at atomistic modeling of dislocation cores under stress and temperature are also considered since they offer a complementary description of core transformations and associated energy barriers. In addition to offering guidance and assistance for further experimentation, the book indicates new ways to extend the body of data in particular areas such as lattice resistance to glide.
