Персона: Огородникова, Ольга Вячеславовна
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Deuterium and helium retention in W with and without He-induced W ‘fuzz’ exposed to pulsed high-temperature deuterium plasma
© 2018 Elsevier B.V. In the present work, helium (He) was incorporated into tungsten (W) samples by inductively coupled plasma (ICP) source above the threshold of He-induced W ‘fuzz’ formation on W surface. Then, W samples with and without nano-structured W ‘fuzz’ were exposed to pulsed heat loads using deuterium (D) plasma in quasi-stationary high-current plasma gun QSPA-T. The pulse duration was 1 ms and number of pulses was varied from one to thirty to simulate ITER transient events with surface heat load parameters relevant to edge-localized-mode (ELM) impacts. The irradiation was performed below and above the W melting threshold. The D and He retention in each irradiated sample was measured by a method of thermal desorption spectroscopy. We examined the impact of (i) ELMs-like events and (ii) formation of He-induced nano-structured ‘fuzz’ on the D retention in W. We found that the D retention was the highest for samples irradiated by plasma gun above the melting threshold after thirty pulses. Moreover, the D retention after 10 pulses of deuterium plasma gun exposure was higher than that after stationary low-energy plasma exposure at sample temperature of either 600 or 700 K indicating the dominate influence of ELM's-like events on the D retention compared to normal operation regime. The D retention in W samples with the presence of He-induced W ‘fuzz’ was slightly smaller than without that after one pulse of plasma gun exposure with heat load below the W melting temperature. The W ‘fuzz’ was not disappear in this loading conditions, only the length and thickness of nano-structured W fibres were reduced by factors of ∼4 and ∼2, respectively. The He concentration in W with W ‘fuzz’ was decreased by a factor of about 3 after one pulse of plasma gun exposure. The results obtained give possibility to assess the particle retention in divertor areas subjected to high thermal loads at different operation regimes.
Surface Structure Modification and Deuterium Retention in Tungsten under Pulsed Plasma Loads
© 2019, Pleiades Publishing, Ltd.Modification of the surface layer and deuterium accumulation in tungsten targets under plasma irradiation in a quasi-stationary plasma accelerator with an intrinsic magnetic field QSPA-T, which reproduces the conditions (plasma thermal load of 0.2-5 MJ/m2, pulse duration of 0.1-1.2 ms) typical of ELM events in ITER, are studied. Using a scanning electron microscope, structure modifications at the surface and in the bulk after deuterium plasma irradiation are analyzed. The observed changes in the near-surface layer are compared with the calculated data on the change in the internal structure of tungsten under intense thermal action obtained as a result of the numerical solution of the heat conduction problem. The total deuterium retention in the samples was measured using thermal desorption spectroscopy, and it was in the range of (3-4) × 1016 particles/cm2 for the samples melted during plasma exposure. These numbers exceed by an order of magnitude the values obtained for samples without traces of melting.
Helium retention in tungsten under plasma and ion beam irradiation and its impact on surface morphology
Helium (He) is a product of deuterium-tritium (DT)-fusion reaction and will be a natural impurity in DT plasma in future fusion devices. He retention in tungsten irradiated by plasma and mass-separated ions in a wide temperature range (300-1200 K) was investigated by means of thermal desorption spectroscopy (TDS). He retention did not exceed the level of 1.5 x 10(21) He m(-2) for all investigated samples. A significant effect of air exposure on TDS spectra was demonstrated. In contrast to in situ TDS measurements, He release after interaction with the air started from similar to 400 K, even in the case of high temperature irradiation. Changes in surface morphology were analyzed by secondary electron microscopy. Blisters were found at the surface after ion irradiation at low temperatures. Acceleration of surface modification and more complex surface morphology was observed in the case of irradiation at temperatures above 1000 K.
Effect of the presence of helium in tungsten on deuterium retention
© 2021Tungsten (W) samples were pre-irradiated with low-energy helium (He) ions with an energy of 80 eV, flux of 1021 He/m2s, at a sample temperature of 1200–1250 K in a inductively coupled plasma (ICP) source facility. Such irradiation conditions led to a formation of a nano-structured surface layer of tungsten, called ‘fuzz’. These samples were then irradiated with D3+ ions with an energy of 2 keV and small doses of 1019 D/m2 at room temperature and in-situ thermal desorption analysis (TDS) was performed. It was found that the main factor determining the deuterium (D) retention in W samples pre-irradiated with helium is the concentration of helium below the surface, namely, the D retention was increased with decreasing the He concentration until the nanostructured W ‘fuzz’ was removed. The effect of nano-structured tungsten ‘fuzz’ is only in a decrease of the reflection coefficient of deuterium ions compared to a smoother surface, resulting in the increase of the D influx into W and, consequently, increase of the D retention. However, this increase has a minor effect on the D retention compared to the He concentration in the subsurface layer of W.
Verification of the theory of primary radiation damage by comparison with experimental data
© 2019 Elsevier B.V.Understanding of radiation damage in a solid is important for development of advanced material technologies, namely, for space application, electron microscopy, semiconductor processing, fission and nuclear fusion. Space materials must withstand exposure to high-energy protons. Construction materials of future thermonuclear reactors must withstand exposure to high-energy neutrons. Many properties of materials including mechanical properties are governed by the presence and behavior of lattice defects. Hence, the study of the radiation-induced defects is an important task combining two fields: radiation physics and solid state physics. The paper presents new knowledge on primary defect formation in the main materials for advanced fission and nuclear fusion reactors, bcc tungsten (W) and bcc iron (Fe). The objective of this work is to compare the new experimental data of neutron- and proton-induced defects in W and Fe using well-established method of positron-annihilation lifetime-spectroscopy (PALS) in combination with the literature data with two models of radiation damage, the classical Norgett-Robinson-Torrens (NRT-dpa) model and recently developed athermal recombination corrected (arc-dpa) model. It is shown that experimental data for neutron- and proton-irradiated Fe are better described by arc-dpa model than NRT-dpa model. Whereas experimental data for neutron- and proton-irradiated W are between the NRT-dpa and arc-dpa predictions. The obtained results shed new light on the formation of the primary radiation defects in materials and indicate the need for further development of the theory of radiation damage in a solid.
Annealing of radiation-induced defects in tungsten: Positron annihilation spectroscopy study
© 2019 Elsevier B.V. Positron annihilation lifetime spectroscopy (PALS) was applied to study the annealing of radiation-induced defects in polycrystalline tungsten (W) irradiated with 21.6 MeV protons at 100 °C up to a fluence of 5 × 10 15 p/cm 2 . Three components were observed in the measured spectra: short-lifetime of 100–120 ps (positron annihilation in the defect-free W lattice), medium-lifetime of ∼190–330 ps (annihilation at mono-vacancies and small vacancy cluster containing ∼ 2–4 vacancies) and long-lifetime of ∼500 ps (annihilation in large vacancy clusters containing more than 10 vacancies). The irradiation of W with protons at 100 °C, primary, led to the formation of mono-vacancies, self-interstitial defects were created as well but migrated towards sinks during the irradiation. Onset of vacancy diffusion in W starts already at 200 °C before defect recovery stage III. After annealing at ∼400 °C, a sharp drop in the intensity of the positron medium-life component together with a simultaneous increase in positron lifetime from ∼220 to ∼280 ps is observed, and a long-life component appears. This indicates migration and annealing of vacancies and their agglomeration in large vacancy clusters. After annealing at 500–700C, the intensity of long-life component increases indicating the growth of large vacancy clusters but at 900 °C they anneal completely as the mean lifetime recovers nearly to the value measured in the un-irradiated material.
Deuterium Retention in a Nanostructured Tungsten Surface Layer Formed during High-Temperature Irradiation with Helium Plasma
The preliminary irradiation of a tungsten sample with low-energy helium ions (80 eV, flux of 10(21) m(-2) s(-1)) at a temperature of 1200-1250 K in a facility with an inductive RF discharge leads to the formation of a nanostructured tungsten surface layer, which is referred to as fuzz. After that, the sample is subjected to a set of annealing procedures and irradiations with ions with an energy of 2 keV (0.67 keV per D) at low fluences of 10(19)-10(20) m(-2). Deuterium retention at each stage is analyzed by in-situ thermal desorption spectroscopy. An increase in the helium concentration in the sample leads to a significant change of deuterium retention. At high helium concentrations, deuterium retention becomes low. Annealing in the temperature range of 1000-1400 K leads to helium desorption, modification of the surface layer and defects, and, as a consequence, an increase in the amplitude of the main deuterium desorption peak and a shift of the peak to higher temperatures. Annealing at a temperature of 1600 K leads to removal of the nanostructured fuzz from the tungsten surface and a decrease in the deuterium retention on account of an increase in the reflection coefficient from a smoother surface.
Recombination coefficient of hydrogen on tungsten surface
© 2019 Elsevier B.V. The recombination coefficient is an important parameter for modelling hydrogen-metal interaction. It is responsible for hydrogen desorption from the surface of the metal and, therefore, significantly affects the hydrogen penetration into the metal, accumulation in and permeation through the metal. In the present work, the recombination coefficient of hydrogen (H) on tungsten (W) surface is examined. It is shown that the recombination coefficient of H on a clean W surface is extremely high which indicates the rapid desorption of the hydrogen molecule from the surface. Simulation using a high recombination coefficient well describes a wide range of experimental data of gas and ions interaction of hydrogen isotopes with tungsten. Proof of incorrectness of the Anderl's recombination coefficient is presented by comparing it with both theory and experiment.
Influence of plasma heat loads relevant to ITER transient events on deuterium retention in tungsten
Deuterium (D) retention in Tungsten (W) under plasma heat loads relevant to edge localized modes in ITER was experimentally investigated at the QSPA-T plasma gun facility. Samples were exposed to 1.0 ms D plasma pulses with different heat loads in the range of 0.4-3.7 MJm(-2) (heat flux factor P root t = 13.3-123 MJm(-2)s(-0.5)). A significant D retention was observed already after one pulse. Moreover, the D retention grew up continuously with increasing the power load, although the surface was melted at highest loads (above 1.4 MJ m(-2)). The D retention was higher than that in the case of stationary plasma irradiation at 600-700 K, indicating possible significant contribution of ELM's-like events to the total D retention. All stages of the experiments (irradiation, storage time and TDS) have been simulated using the TMAP 7 code.
Deuterium trapping in the subsurface layer of tungsten pre-irradiated with helium ions
© 2021The effect of He-induced defects in tungsten on the efficiency of trapping of deuterium ions in the subsurface layer was studied using thermal desorption spectroscopy (TDS). The W sample was pre-irradiated with 3 keV helium ions at room temperature and various fluences in the range of 1019 – 5 × 1021 He/m2. Then, it was exposed to a probe fluence of 1019 D/m2 of 2 keV D3+ (670 eV/D) ions, and in-situ TDS was performed. The de-trapping energy for D atoms increased with the increase of the He pre-irradiation fluence. On the other hand, a strong decrease in the D retention was observed if the He fluence increased above 1021 He/m2. At the highest He fluence of 5 × 1021 He/m2 deuterium trapping was possible only after partial release of He atoms. By comparison of experimental TDS spectra with modeling, the de-trapping energies of D atoms from various defects were estimated.