Field emission property of nanostructured tungsten formed by helium plasma irradiation

Field emission property of nanostructured tungsten formed by the exposure to a helium plasma is measured to investigate the initiation process of arcing in nuclear fusion devices. Field emission current from the nanostructured tungsten and tungsten with helium bubbles was significantly higher than that from the polished tungsten specimen. However, field enhancement factor was on the order of 100, which was almost comparable to that of bulk tungsten; the increase in the field emission current is attributed to primarily an increase in the effective emission surface area. From the calculation of thermo-field emission current from a heated tungsten, it is thought that a significant current emission can be still initiated even when the field enhancement factor is not so high when tungsten is exposed to high density plasmas such as the one accompanied with ELMs.     

Temperature dependence of surface morphology and deuterium retention in polycrystalline ITER-grade tungsten exposed to low-energy,high-flux D plasma

Surface topography and deuterium retention in polycrystalline ITER-grade tungsten have been examined after exposure to a low-energy (38 eV/D), high-flux (10²² D/m² s) deuterium plasma with ion fluences of 10²⁶ and 10²⁷ D/m² at various temperatures. The methods used were scanning electron microscopy equipped with focused ion beam, thermal desorption spectroscopy, and the D(³He,p) ⁴He nuclear reaction at ³He energies varied from 0.69 to 4.0 MeV. During exposure to the D plasma at temperatures in the range from 320 to 815 K, small blisters of size in the range from 0.2 to 5 μm, depending on the exposure temperature and ion fluence, are formed on the W surface. At an ion fluence of 10²⁷ D/m², the deuterium retention increases with the exposure temperature, reaching its maximum value of about 10²² D/m² at 500 K, and then decreases below 10¹⁹ D/m² at 800 K.     

Energetics of He and H atoms with vacancies in tungsten: First-principles approach

The formation energies of various defect configurations of He and H atoms in W were estimated based on the density functional theory. A special consideration was given to the coexistence of the He and H atoms at the presence of the vacancy and vacancy cluster in W. A single He atom favors a substitutional site, while a H atom spontaneously incorporates at an interstitial site with the negative formation energy. When He and H are present close to each other, they form an interstitial pair, occupying relaxed tetrahedral sites. When He, H and a vacancy coexist within a unit cell of W, however, He occupies the vacancy site then the He_sub-H_tet pair is predicted to be the lowest energy configuration. At the presence of a nearby vacancy cluster, He atoms occupy the vacant space while H atoms move slightly toward W.     

低能氦离子辐照对钨和钼材料的表面损伤作用

钨和钼材料具有高熔点、高热导率、低溅射率等优点成为国际热核实验反应堆计划中面向等离子体材料的候选材料。因此研究钨和钼材料的辐照损伤行为对于认识面向等离子体材料的辐照损伤机制具有重要意义。本文采用120 e V的He+在873 K对钨和钼材料进行辐照实验,利用纳米压痕仪与导电模式原子力显微镜(Conductive Atomic Force Microscopy,CAFM)相结合,原位比较了钨和钼材料在辐照前后的表面形貌、表面微结构以及表层缺陷分布的变化特征。结果表明,低能He+辐照会导致钨和钼材料的近表面产生纳米量级氦泡缺陷,这些氦泡缺陷的存在使得样品表面产生绒毛或波浪状结构。纳米压痕深度分析和扫描电镜的分析结果表明,低能He+辐照会对Mo材料产生明显的刻蚀作用。本工作对于进一步认识低能氦离子辐照对面向等离子体材料的辐照损伤作用具有一定的科学参考意义。     

Masses of the H,D, He4 and C12 isotopes

Results are given of the measurements of the masses of the H, D, He⁴ and C¹² isotopes carried out by means of a mass-spectrograph with a resolving power of 70,000–100,000. The data obtained agree well with the corresponding mass values obtained from the energy balance of nuclear reactions.The author consider it to be their duty to express their gratitude to M. Ardenne for valuable advice and for his Interest In this work, and also to G. Jaeger, I. A. Chukhtn, attd V. Roggenbuk for the help given by thgm in adjusting the equipment.After the present article was completed a communication by Nier and collaborators [10] appeared giving new preliminary values of masses obtained on a new mass-spectrometer with a maximum resolving power of approximately 100,000: H = 1.0081439 ± 5; D = 2.0147380 ± 10; C¹² = 12.0038174 ± 18.     

H+ and He+ stopping in a degenerate electron gas

Calculations on the stopping power of an electron gas for slow H⁺ and H⁺ ions are presented using a frequency dependent local-field correction function within the linear-response theory. Protons are also treated in the elastic binary collisions theory using a screened model potential. For He⁺ ions, the wave-function of the bound electron is determined self-consistently. Comparisons with Lindhard's linear theory and with the results of nonlinear local density-functional calculations are made.     

Erosion of tungsten surfaces in He and Ar/He plasma

Irradiation tests of tungsten surface were performed with He and He/Ar plasma generated by microwave electron cyclotron resonance. Thickness loss was used as the erosion rate of tungsten surface under the plasma irradiation. The results revealed that the thickness loss increased linearly with negative bias. SEM images proved that the addition of Ar apparently increased the plasma erosion. The thickness loss increased sharply with the Ar fraction of Ar/He mixture when it was \20 %,where the increasing slope of thickness loss lowered down gradually.     

Observation of micro-cavities in nickel during He and D irradiation and post-irradiation annealing

Changes in sizes and morphology of small cavities in nickel irradiated by 25 keV helium ions and 20 keV deuterons were investigated during irradiation and on annealing after irradiation by means of transmission electron microscopy. In the early stage of He⁺ irradiations at 600 and 700° C, roundish cubes appeared, gradually changed to octahedra. and, then, by the truncation of apexes, finally reached cubo-octahedra. Nucleation and growth behavior of cubic cavities in D⁺ irradiated nickel was different from the case of He⁺ irradiation. On annealing of the He⁺ irradiated specimen, only octahedral cavities showed marked growth, finally changing to roundish cubes at 750° C. Cavities of roundish cubes and cubo-octahedra did not grow nor change their shapes remarkably by the annealing. The cubic cavities formed by D⁺ irradiation at 360° C showed gradual shrinkage on annealing at 600° C and disappeared at 625° C. The changes of cavities during irradiation and on annealing were interpreted by the effect of the internal gas pressure.     

Measurement of He and H depth profiles in tungsten using ERDA with medium heavy ion beams

Elastic recoil detection analysis method (ERDA) with medium heavy analyzing ion beam and its application for the simultaneous measurement of light elements in a very heavy substrate is presented. The availability of cross section data and the method of cross section calculation for recoiled particles are discussed. Different ion species for analyzing beam are discussed with respect to the cross-section data availability, sensitivity of the method, and the depth resolution. Calculations of depth resolution for each element and maximum depth of analysis for tungsten substrate are presented. The influence of the detector geometry and multiple scattering effects on the depth resolution is discussed. An example spectrum measured on tungsten implanted with He seeded D plasma is shown.     

Kinetic electron emission yield induced by H and He ions versus stopping power for Al, Cu, Ag and Au

For H⁺ and He²⁺ ions impinging on Al, Cu, Ag and Au targets we measured simultaneously the yield, γ, of emitted electrons and the electronic energy loss, S_e, in the energy range 0.5 to 4.8 MeV. The targets were prepared under high-vacuum conditions before they were transferred to an ultra-high-vacuum chamber without breaking the vacuum. The targets were sputter cleaned and their composition was examined by Auger electron spectrometry. The values of γ were obtained by current integration and S_e was determined from the energy width of Rutherford backscattering spectra. For H⁺ ions impinging on Cu, Ag or Au and He²⁺ on Al and Cu, the expected proportionality between γ and S_e was found within the experimental errors of 2%. For H⁺ ions on Al and He²⁺ ions on Ag and Au targets, significant deviations were observed.     

Transmission electron microscopy of αZrO2 films formed in 573 K oxygen

Oxide films between 10 and 362 nm thickness were removed from zirconium substrates and examined by selected area electron diffraction and transmission electron microscopy. Evidence was found for a dynamic process of oxide crystallite nucleation, recrystallization and growth. Investigations were limited to oxide film thicknesses of less than 365 nm due to the presence of a dense network of amorphous oxide protusions at the oxide/metal interface.     

Investigating permeation and transport of H isotopes in tungsten by first-principles

We have investigated permeation and transport of hydrogen (H) isotopes in tungsten (W) single crystal employing first-principles calculations in junction with Fick’ law. Permeability was approximately evaluated according to the solubility and diffusion coefficient of H. The solubility for H in bulk W from present calculation is consistent with the experimental results measured by Frauenfelder. The permeation fluxes of H isotopes are examined at the different thickness of W wall. The permeation fluxes of deuterium with the W thickness of 21 μm at the temperature of 770 K and with the W thickness of 50 μm at the temperature of 893 K were 0.68 × 10¹³ atom/m²s and 0.34 × 10¹⁴ atom/m²s, respectively. The dissociation coefficients of H isotopes are also evaluated. We believe that the present first-principles combined with Fick’ law method can be also generalized to investigate permeation and transport of H isotopes in most metals since such H isotopes behaviors in most metals are similar to those of H isotopes in W.     

Electron microscopy and Rutherford backscattering spectrometry characterisation of 6H SiC samples implanted with He

6H SiC single crystals were implanted at room temperature with 1 MeV He⁺ up to a fluence of 2 × 10¹⁷ at./cm². RBS-channeling analysis with a 2 MeV He⁺ beam indicated the formation of extended defects or the generation of point defects at a constant concentration over a depth of about 1 μm. Electron microscopy characterisation revealed the presence of two amorphous buried layers at depths of about 1.75 and 4.8 μm. They are due to the implantation and to the analysing RBS beam, respectively. No extended planar or linear faults were found in the region between the surface and the first amorphous layer. However, at the surface, a 50 nm thick amorphous layer was observed in which crystalline inclusions were embedded. Electron diffraction and HREM data of the inclusions were typical for diamond. These inclusions were even found in the crystalline SiC material below this layer, however at a reduced density.     

Investigating microstructure of Longmaxi shale in Shizhu area,Sichuan Basin,by optical microscopy,scanning electron microscopy and micro-computed tomography

Microstructure of shale formation is the key to understanding its petrophysical and chemical properties.Optical microscopy, scanning electron microscopy and micro-computed tomography(μ-CT) have been combined for characterization of microstructure of Longmaxi(LMX)shale from Shizhu area, Sichan Basin. The results indicate that laminated LMX shale consists of mineral matrix-rich layers and organic matter(OM)-rich layers at micrometer scale in two and three dimensions. Mineral matrix layers,mainly consisting of interparticle pores and intraplatelet pores, are approximately parallel to the bedding plane.Pyrite-rich layer, mainly containing intercrystalline pores,shows a strong preferred orientation parallel to the bedding plane. OM-rich layer, mainly containing OM pores, seems to be discontinuous. In addition, intercrystalline pores are enriched in some layers, while OM pores are distributed irregularly in matrix layers. This vertical heterogeneity of pore microscopic structures in LMX shale is of great importance to understand its petrophysical and chemical properties.     

Evaluation of D（d,n）^3 He reaction neutron source models for BNCT irradiation system design

A mathematical method was developed to calculatc the yield.energy spectrum and angular distribution of neutrons from D(d,n)3 He(D-D)reaction in a thick deuterium-titanium target for incident deuterons in energies lower than 1.0MeV.The data of energy spectrum and angular distribution wefe applied to set up the neutron source model for the beam-shaping-assembly(BSA)design of Boron-Neutron-Capture-Therapy(BNCT)using MCNP-4C code.Three cases of D-D neutron source corresponding to incident deuteron energy of 1000.400 and 150 kaV were investigated.The neutron beam characteristics were compared with the model of a 2.45 MeV mono-energetic and isotropic neutron source using an example BSA designed for BNCT irradiation.The results show significant differences in the neutron beam characteristics,particularly the fast neutron component and fast neutron dose in air,between the non-isotropic neutron source model and the 2.5 MeV mono-energetic and isotropic neutron source model.     

Mechanical modeling of creep, swelling and damage under irradiation for polycrystalline metals

A constitutive equation of creep, swelling and damage under irradiation for polycrystalline metals applicable to structural analyses in multiaxial state of stress is developed. After reviewing microscopic mechanisms of irradiation creep and swelling, the relevant theories proposed so far from the view point of metallurgical physics and their applicability are discussed first. Then a constitutive model is developed by assuming that creep under irradiation can be decomposed into irradiation-affected thermal creep and irradiation-induced creep. By taking account of the Stress-Induced Preferential Absorption (SIPA) mechanism, the irradiation-induced creep is represented by an isotropic tensor function of order one and zero with respect to stress, which is, at the same time, the function of neutron flux and neutron fluence. The volumetric part of the irradiation-induced creep is identified with swelling. The irradiation-affected thermal creep is described by modifying Kachanov-Rabotnov theory for stress-controlled creep and creep damage by incorporating the effect of irradiation. Finally irradiation creep and swelling of 20% cold-worked type 316 stainless steel at elevated temperature are predicted by the proposed constitutive equations, and the numerical results are compared with the corresponding experimental results.     

Thermal annealing behavior of hydrogen and surface topography of H2+ ion implanted tungsten

Tungsten (W) has been proposed as a plasma-facing material in fusion reactors due to its outstanding properties. Degradation of the material properties is expected to occur as a result of hydrogen (H) isotope permeation and trapping in W. In this study, two polycrystalline W plates were implanted with 80 keV H₂⁺ ions to a fluence of 2 × 10²¹ H⁺/m² at room temperature (RT). Time-of-flight secondary ion mass spectrometry (ToF-SIMS), focused ion beam (FIB), and scanning electron microscopy (SEM) were used for sample characterization. The SIMS data shows that H atoms are distributed well beyond the ion projected range. Isochronal annealing appears to suggest two H release stages that might be associated with the reported activation energies. H release at RT was observed between days 10 and 70 following ion implantation, and the level was maintained over the next 60 days. In addition, FIB/SEM results exhibit H₂ blister formation near the surface of the as-implanted W. The blister distribution remains unchanged after thermal annealing up to 600 ?C.     

Effect of ion and electron beam irradiation on surface morphology and optical properties of PVA

Polyvinyl alcohol(PVA) is a well-known friendly polymer for paper-making, textiles, and a variety of coatings, biomedical applications such as artificial pancreas, synthetic vitreous body, wound dressing, artificial skin,and cardiovascular device. In this paper, ion/electron beam is employed to get insight into the irradiation effect on surface morphology and optical properties of PVA polymer. UV-Vis spectra are recorded to investigate the effect of induced defects on the optical band gap and the formed carbon clusters size. Scanning electron microscopy(SEM) is used to relate and investigate surface morphology and optical properties of the target polymer with different doses(15, 30 and 60 min). Also, PVA polymer is subjected to theoretical studies by using semi-empirical PM7 quantum chemical method.     

Probing cytotoxicity of nanoparticles and organic compounds using scanning proton microscopy, scanning electron microscopy and fluorescence microscopy

Scanning proton microscopy, scanning electron microscopy (SEM) and fluorescence microscopy have been used to probe the cytotoxicity effect of benzo[a]pyrene (BaP), ethidium bromide (EB) and nanoparticles (ZnO, Al₂O₃ and TiO₂) on a T lymphoblastic leukemia Jurkat cell line. The increased calcium ion (from CaCl₂) in the culture medium stimulated the accumulation of BaP and EB inside the cell, leading to cell death. ZnO, Al₂O₃ and TiO₂ nanoparticles, however, showed a protective effect against these two organic compounds. Such inorganic nanoparticles complexed with BaP or EB which became less toxic to the cell. Fe₂O₃ nanoparticles as an insoluble particle model scavenged by macrophage were investigated in rats. They were scavenged out of the lung tissue about 48 h after infection. This result suggest that some insoluble inorganic nanoparticles of PM (particulate matters) showed protective effects on organic toxins induced acute toxic effects as they can be scavenged by macrophage cells. Whereas, some inorganic ions such as calcium ion in PM may help environmental organic toxins to penetrate cell membrane and induce higher toxic effect.