Study of the impact of steady-state plasma on radiation-damaged tungsten

Experimental studies of tungsten (as a candidate plasma-facing material for a fusion reactor) whose properties will degrade as a result of its contact with near-wall plasma and irradiation with neutrons are performed. The effect of a high level of radiation damages (1–100 displacements per atom) on deuterium accumulation and erosion caused by tungsten irradiation with deuterium plasma was studied. Radiation damages are obtained as a result of the irradiation of tungsten samples with high-energy ions in an accelerator (He⁺², C⁺³, 4–10 MeV). Then the samples are exposed to steady-state deuterium plasma at the LENTA facility (National Research Centre Kurchatov Institute). The effects of the erosion of tungsten and accumulation of hydrogen isotopes in it are studied. Modification of the surface microstructure and radiation swelling is observed. The helium and deuterium concentrations were measured using the methods of nuclear elastic backscattering and elastic recoil detection analysis. An increased accumulation of deuterium in the damaged layer to a depth of about 5 μm is revealed.     

Simulation of deuterium reflection from plasma-sprayed tungsten

A method of computer simulation of the formation of plasma-sprayed tungsten surface is proposed. The dependencies of the reflection coefficients of hydrogen isotopes on the energy of incident particles in the range from 5 to 10⁴ eV were obtained for this surface. It is shown that, at energies about 10 eV, the reflection coefficient for the surface of plasma-sprayed tungsten is smaller than that for a smooth surface by a factor of 1.5; this decrease corresponds to the increase in the deuterium incorporation coefficient by a factor of 2.     

Ion-implanted deuterium accumulation in a deposited tungsten coating

The effects of trapping of ion-implanted deuterium and its thermal desorption on the structure and stressed state of a tungsten coating deposited on a composite substrate is studied. The amount of accumulated deuterium, macrostresses of the coating, and the shape of thermal desorption spectra are shown to depend on the D⁺ ion fluence and the irradiation temperature. Possible mechanisms of these processes are proposed.     

Reduced blister quantity in damaged tungsten exposed to deuterium plasma

正Tungsten (W) is a promising candidate for plasma-facing materials (PFMs) in fusion reactors owing to its excellent thermal conductivity, sputtering resistance, and low hydrogen isotope solubility [1]. W PFMs encounter the challenges of triple irradiations generated by high-heat loads; high-flux particles, including helium/deuterium (D)/tritium (T); and high-energy neutrons. High-flux plasma exposure degrades     

Sputtering of tungsten, tungsten oxide, and tungsten-carbon mixed layers by deuterium ions in the threshold energy region

An original experimental method is developed for determining the sputtering coefficients of electrically conducting materials during bombardment by light gas ions at threshold energies. This information is very valuable in both purely scientific and practical terms. The basis of the method is a special field-ion-microscopic analysis regime. The procedure for measuring the sputtering coefficients includes cleaning the surface by field desorption and evaporation, with the subsequent work on an atomically clean and atomically smooth surface. The method permits identification of single vacancies on the irradiated surface, i.e., it is possible to count individual sputtered atoms. The method is tested on commercially pure tungsten, tungsten oxide, and a W-C mixed layer on tungsten under deuterium ion bombardment. The energy dependences of the sputtering coefficients of these materials for sputtering by deuterium ions at energies of 10–500 eV are obtained and analyzed. An important relationship between the energy threshold for sputtering and the conditions for oxidation of tungsten is found. The energy threshold for sputtering of an oxidized tungsten surface is 65 eV. The energy threshold for sputtering of the W-C mixed layer is almost equal to the corresponding value for pure tungsten. Zh. Tekh. Fiz. 69, 137–142 (September 1999)     

Investigation of a fine structure of deuterium thermal desorption spectra from tungsten

The results of deuterium thermal desorption from tungsten after irradiation by 10 keV ions are discussed. A special heating procedure allows features of the fine structure of the thermal-desorption spectra maxima to be revealed.     

Analysis of a tungsten surface irradiated by fast ions and deuterium plasma

The effects occurring on the surface of tungsten under irradiation with fast ions with an energy in the megaelectrolvolt range and with high fluxes of hydrogen (deuterium) plasma are considered. These effects are radiation damage of the surface layer of the material, its erosion and deuterium retention in it. Irradiation with helium ⁴He²⁺ (3.2–4.0 MeV) and carbon ¹²C³⁺ (10 MeV) ions is performed using a cyclotron at the National Research Center Kurchatov Institute. The thickness of the damaged layer is 3.5–6 μm. The irradiated samples are exposed to steady-state deuterium plasma using a LENTA linear plasma facility to reach a plasma ion fluence of 10²¹–10²² cm^?2. Tungsten erosion and modification of the structure of the damaged layer are analyzed at a plasma-ion energy of 250 eV. Deuterium retention in the damaged layer is studied by elastic recoil detection analysis. The deuterium concentration and its penetration depth into the material are measured. The data obtained for different kinds of fast ions used in the work are compared.     

Diffusion of hydrogen and deuterium on the (111) plane of tungsten

The diffusion of ¹H and ²H on the (111) plane of a W field emitter has been studied by the fluctuation method at various coverages. Both activated and unactivated diffusion is observed; the latter shows very little isotope effect, suggesting that coupling to the substrate is so strong that mass renormalization makes the effective masses of ¹H and ²H nearly identical. Values of D in the tunneling, i.e. temperature independent, regime are 10^?13?5 × 10^?14 cm²/s depending on coverage. For activated diffusion at high coverages, corresponding to population of the β₁ state E = 2.4?3.2 kcal/mol and D₀ = 2 × 10^?8 ?5 × 10^?7 cm²/s, depending on coverage. For lower coverages, corresponding to β₂ population, E = 7–9 kcal/mol, D₀ = 9 × 10^?6 ?2 × 10^?3 cm²/s, again depending on coverage. Similar values are obtained for ²H, with E and D₀ values slightly reduced. An exponentially decaying correlation signal for clean W was also seen and interpreted in terms of flip-flop of W atoms.     

Angular distributions of muon pairs produced by negative pions on deuterium and tungsten

We present the angular distributions of high-mass muon pairs produced in a high-statistics experiment by 140 and 194 GeV/c π^? beams impinging on a tungsten target, and by 286 GeV/c π^? beam on deuterium and tungsten targets. We find no evidence for a center-of-mass energy dependence or a nuclear dependence of the angular distribution parameters. The two parameters λ and μ are found to be essentially independent of any kinematical variable. In contrast, the parameterv increase with the dimuon transverse momentumP _T , at variance with recent perturbative QCD predictions. Our statistics at largex ₁ are insufficient to substantiate the highertwist prediction.     

Angular distribution of muon pairs produced by negative pions on deuterium and tungsten in terms of coherent states

Recently published data on thep _T dependence of the angular distributions of muon pairs produced in collisions of negative pions with deuterium and tungsten nuclei reveal an unexpected tendency. We show that the quantum statistical approach involving expansions in terms of coherent states leads in this case to an exact, non-perturbative result which in the lowest order approximation describes the observed trends with sufficient accuracy. In this procedure, the non-vanishing correlations between the coordinates and momenta of dimuons play a decisive role.     

Autocompensation doping in light-soaked and in radiation-damaged a-Si:H

n-type a-Si:H films have been irradiated with light, electrons, protons and heavy ion beams. It is shown that the non-thermal creation of dangling-bond defects activates significant densities of previously inactive phosphorus dopants. The relevance of these results is discussed with respect to equilibration phenomena in doped material and with respect to degradation phenomena in a-Si:H solar cells.     

Emission and capture processes in radiation-damaged silicon semiconductor diodes

Silicon diodes irradiated with 1 MeV neutrons show relaxation behaviour, which results from deep levels that cause the material to become semi-insulating. The signature of these levels has been determined using the powerful technique of deep level transient spectroscopy. In particular, two deep levels are found not to anneal out with increases in fluence. It is possible to relate the activity of one of the levels to the relaxation behaviour of the diodes. The suggestion is that the relaxation behaviour of radiation-damaged silicon diodes is a consequence of the activity of those radiation-induced deep levels that are situated to act as generation–recombination centres.     

Modification of optical properties of recrystallized tungsten due to the change in surface morphology induced by deuterium ion bombardment

Using ellipsometry and reflectometry, an anomalous change in the optical properties of recrystallized tungsten (W) as a result of bombardment with deuterium (D) ions was discovered at a sample temperature of ∼535 K. There is a qualitative difference between the value of reflectivity obtained from reflectometry measurements and that calculated from the ellipsometric data. A physical model of the discovered effect is proposed. It is shown that two processes take place at the surface of W exposed to plasma at 535 K: the appearance of blisters and a modification of the electronic structure of the surface layer.     

Dehydration and recrystallization of radiation-damaged titanite under thermal annealing

Abstract

Dehydration of oriented sections of a radiation-damaged titanite crystal, CaTiSiO₅, at temperatures up to 1500K was analysed using infrared spectroscopy. The IR spectra of the untreated sample show only a very weak orientational dependence. The absence of sharp absorption peaks at wave number near 3486 cm^?1 in metamict titanite shows that the local environmental configurations of OH species in the metamict titanite differ strongly from that of crystalline titanite. The OH spectra of radiation-damaged titanite can be decomposed into two components: the first component shows anisotropic and sharp spectral features while the second component consists of a broad spectral feature like those observed in disordered silica glasses. It is proposed that the first component is related to the crystalline part of the titanite sample while the second is from the defected and disordered part which suffered strong radiation damage. With increasing annealing temperature, a decrease in the broad absorption between 2500 and 3200 cm^?1 is accompanied by a recovery of sharp IR bands near 3486 cm^?1 which display the same orientational dependence as undamaged single crystals. Annealing the sample at 1000K leads to the line profiles and orientational dependence of the main OH stretching bands near 3486cm^?1 that are virtually identical with those of crystalline, undamaged titanites. At temperatures above 1500 K, the crystal starts to melt and the orientational dependence of the IR absorption is destroyed. The recrystallization processes are quantified and discussed in terms of a percolation behaviour of amorphous and crystalline titanite. It is proposed that hydrogen transport is strongly enhanced during recrystallization.     

The current and capacitance response of radiation-damaged silicon PIN diodes

The current–voltage (I–V) and capacitance–voltage (C–V) characteristics of silicon p–i–n diodes have been investigated both prior to and after radiation-induced damage by 1 MeV neutrons. The results have been analysed and several rates of damage evaluated. The indication is mainly that radiation damage occurs only up to certain fluencies. Beyond these, the material becomes resistant to further damage. Thus, initial heavy radiation damage can be used to achieve radiation-hardness of detector diodes. This result is contrary to previous suggestions that continued irradiation renders the detectors inoperable but is in good agreement with our results on radiation-hardness induced by gold-doping.     

Fresnel diffraction in deuterium

In Glauber's theory of high energy scattering the wavefunction inside the nucleus is given by geometrical optics. The theory is extended to lower energies by incorporating the effects due to Fresnel diffraction, and applied to elastic and inelastic scattering off deuterium.     

Cosmic deuterium

The knowledge of the primordial deuterium to hydrogen ratio provides one of the most reliable tests of the early Universe nucleosynthesis models and a direct estimate of the cosmic baryon density. Evaluations have been traditionally made using D/H estimations in the interstellar medium, extrapolated backwards in time with the use of galactic evolution models. Direct primordial D/H measurements have been carried out only recently in the direction of quasars. These measurements of deuterium abundances along with observations made in the solar system and in the interstellar medium are presented.New results that indicate spatial variations of the deuterium abundance in the interstellar medium at the level of 50% over scales possibly as small as 10 pc, may question our global vision of deuterium evolution until the causes for the origin of these variations are understood. With a conservative point of view, observations thus suggest that the primordial D/H value should be within the range 1. × 10⁻⁵−3. × 10⁻⁴, leading to a relatively low baryon content Universe.Since the actual evolution of deuterium from primordial nucleosynthesis to now is not known in details, more observations, hopefully to be made with the Hubble Space Telescope, FUSE the Far Ultraviolet Spectroscopic Explorer (launched in 1999). or from the ground with the largest telescopes (Keck, VLT,), should reveal the evolution of that key element, and better constrain its primordial abundance.     

Pionic deuterium

The strong-interaction shift ε _1s ^πD and broadening Γ _1s ^πD in pionic deuterium have been determined in a high statistics study of the πD(3p-1s) X-ray transition using a high-resolution crystal spectrometer. The pionic deuterium shift will provide constraints for the pion-nucleon isospin scattering lengths extracted from measurements of shift and broadening in pionic hydrogen. The hadronic broadening is related to pion absorption and production at threshold. The results are ε _1s ^πD = (?2356 ± 31) meV (repulsive) and Γ _1s ^πD meV yielding for the complex πD scattering length a _πD = [?(24.99±0.33)+i(6.22 _?0.26 ^+0.12 )] × 10^?3 m _π ^?1 . From the imaginary part, the threshold parameter for pion production is obtained to be α = (251 _?11 ⁺⁵ ) μb. This allows, in addition, and by using results from pion absorption in ³He at threshold, the determination of the effective couplings g ₀ and g ₁ for s-wave pion absorption on isoscalar and isovector NN pairs.     

Pionic deuterium

Data taking of the PIONIC HYDROGEN project has been completed with a high statistics study of the strong-interaction effects in πD by measuring the Kβ X-radiation for three different target densities with a high resolution Bragg crystal spectrometer. The πD hadronic shift will provide a constraint for the πN isospin scattering lengths extracted from the πH measurement. The hadronic width is directly related to pion production at threshold.