Персона: Писарев, Александр Александрович
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Сравнение удержания дейтерия в вольфрамовых пленках различной толщины
Проведено сравнительное исследование содержание дейтерия в со-осажденных из плазмы магнетронного разряда на молибденовые подложки вольфрам-дейтериевых слоях толщиной 50, 250 и 750 нм. Измерения проводились методом in vacuo термодесорбционной спектроскопии без контакта с атмосферой. Проведено моделирование экспериментальных данных в коде ТМАР7, получены концентрации и энергии ловушек, при которых достигается наилучшее согласие с экспериментом. Содержание дейтерия в пленках, осажденных при температуре ~100°С, составило 3–5 ат. %. Показано, что толщина пленок не оказывает существенного влияния на характеристики центров захвата удержания дейтерия, хотя вид спектров для наиболее толстых пленок несколько отличается.
HYDROGEN CO-DEPOSITION WITH METALS IN PLASMA DISCHARGE
Deposition of a single element film is always accompanied by co-deposition of a certain amount of other elements. This can be done properly to improve properties of the coating or due to contamination by impurities. In the field of thermonuclear fusion research, where hydrogen isotopes are used as a fuel, co-deposition with sputtered material from the wall is one of major mechanisms of hydrogen isotopes accumulation in the installation. Since D-T fuel will be used in ITER and future fusion reactors, accumulation of radioactive tritium will limit the lifespan of the installations due to safety concerns. For example, tritium accumulation in ITER is limited by 1 kg. This is why carbon materials were not accepted for the use in ITER. Basing on experiments, it was predicted that the safety limit could be reached after 100 of shots with tritium. Recent experiments in JET [1] demonstrated in the case of “ITER-like” wall (first wall – Be, divertor area - tungsten) accumulation of deuterium fuel in the co-deposits was 20 times lower than in the full-carbon wall campaign. This is both due to smaller amount of co-deposits and smaller concentration of deuterium in them.
Temperature dependence of hydrogen co-deposition with metals
© 2019 Elsevier B.V. The deuterium retention in aluminum and molybdenum films co-deposited in magnetron discharge has been measured in a wide range of substrate temperatures (from RT to 800 K) by means of thermal desorption spectroscopy. A multi-step temperature dependence was observed, which was compared with the diffusion based model and previous experimental data for tungsten films. The key role of lattice defects in deuterium trapping was demonstrated. The highest D retention among the investigated materials was observed in W films, which have the lowest H solubility, but the highest binding energies of hydrogen with defects. The detrapping energies and concentrations of defects in the films were estimated on the base of the model, and the energies are consistent with the data for bulk materials.
Isotope exchange in Li-D co-deposited layers at temperatures below 200 °C
© 2020 Elsevier B.V.Hydrogen-deuterium isotope exchange in Li-D co-deposited films was studied as a method of removal of heavy hydrogen isotopes from Li films deposited in fusion devices. The total H and D retention was measured using in-vacuo thermal desorption spectroscopy. Co-deposited Li-D films were kept in H2 gas at 1 Pa and 105 Pa at room temperature, 100 °C, and 200 °C. Some reduction of the D content was observed even at RT, while nearly full isotope substitution was achieved at 200 °C after 1 day at 105 Pa in H2 gas.
ВЛИЯНИЕ УЛЬТРАФИОЛЕТОВОГО ИЗЛУЧЕНИЯ НА СОДЕРЖАНИЕ И ДЕСОРБЦИЮ ДЕЙТЕРИЯ ИЗ СООСАЖДЕННЫХ ЛИТИЕВЫХ СЛОЕВ
The influence of ultraviolet irradiation of co-deposited lithium layers on the content and desorption of deuterium from them is considered. It was found that exposure to ultraviolet radiation suppresses desorption at high temperatures, facilitates desorption at low temperatures. Effects are considered that can form the basis for the development of methods for determining the places of accumulation of lithium hydride in tokamaks with lithium walls, as well as facilitating the removal of heavy hydrogen isotopes from the walls of installations.
The dependence of lithium emissivity from temperature in vacuum ЗАВИСИМОСТЬ СТЕПЕНИ ЧЕРНОТЫ ЛИТИЯ ОТ ТЕМПЕРАТУРЫ В ВАКУУМЕ
© 2019 National Research Center Kurchatov Institute. All rights reserved.Correct interpretation of IR video observation data of the surfaces of plasma-facing elements in fusion devices requires detailed knowledge about the emissivity factor of these surfaces in different conditions. In this work, results of emissivity measurements for free metallic lithium surface and a lithium surface supported by the capillary-porous system (CPS) are measured as a function of temperature in the range from 400 to 800 K. Emissivity of solid lithium changed from ~0.04 at 400 K to ~0.09 at 453 K. During melting a sudden drop of emissivity down to ~0.04 was observed. Emissivity increased linearly from 0.04 to ~0.15 with temperature increasing from 455 to 800 K. For fully wetted CPS, emissivity was close to that of free lithium surface for temperature up to ~570 K, while at higher temperature it was lower, probably due to changes in microrelief at high temperatures.
A setup for study of co-deposited films
© 2020 IOP Publishing Ltd and Sissa Medialab.A setup for investigation of thermal desorption spectra of gases accumulated in thin films deposited by plasma sputtering of solid targets is described. Deposition and thermal desorption spectroscopy (TDS) are performed in two different vacuum chambers separated by a gate valve with the sample transferred between the chambers in-vacuo. The temperature of the substrate for deposited films can be varied in the range of 300-800 K; and the deposition rate is controlled by a quartz microbalance. Thermal desorption of co-deposited gases is analyzed by a quadrupole mass spectrometer. Sputtering rate and evaporation of the film during TDS are measured by quartz microbalances. Three experiments are described 1) trapping of deuterium by the growing chemically active Li film with subsequent decomposition and evaporation of the film, 2) temperature dependent deuterium trapping in the growing W film resulting in trapping with several binding energies, and 3) chemical interaction of D-Li layer with water vapor leading to isotopic H-D exchange and chemical transformation of the deposited film.
Deuterium trapping in co-deposited layers of ITER-relevant materials
Hydrogen isotope accumulation in fusion devices is a serious problem. Because deuterium-tritium mixture will be a working gas in future fusion devices, including ITER tokamak, tritium accumulation is an issue from the perspective of radiation safety. In total, only 700 grams of tritium are allowed to be present in ITER vessel at any time, with additional 120 in the cryopumps, and 180 grams allocated to measurement error, to the total of 1000 grams.
Elastic backscattering as a method for the measurement of the integral lithium content in thin films on fusion-relevant substrates
© 2019 Elsevier B.V.Different ion beam analysis techniques for the study of thin lithium-containing layers on top of fusion relevant materials are discussed and compared to each other. Elastic backscattering analysis (EBS) with protons is determined to be one of the most promising techniques and allows measurements of Li layers with thicknesses from ∼100 nm up to ∼600 μm, as shown by SIMNRA simulations. The best sensitivity for thin films (∼100 nm) can be achieved using 4 MeV protons with 170° scattering detection angle for layers on Mo and W substrates, and 2 MeV for C substrates. Experimentally EBS measurements were successfully tested for Li films with thicknesses from ∼50 nm up to ∼400 nm after air exposure. The Li films become strongly inhomogeneous and require averaging over multiple measurements in nearby areas. This necessitates averaging over multiple nearby measurement points, and limits the overall precision of the measurement.
Model for hydrogen accumulation in co-deposited layers
© 2020 The Author(s)An improved diffusion based model for prediction of the hydrogen content in co-deposited layers depending on the deposition conditions (the properties of the material which is co-deposited with hydrogen, rate of deposition, hydrogen flux and particle energy, substrate temperature) is presented. The model is validated using experimental data for W, Mo and Al, and is compared to empirical scaling equations currently in use. It is shown that a good agreement is observed in regards to hydrogen content vs substrate temperature experimental data, and no disagreement with scaling equations can be seen in regards to the role of hydrogen flux and the rate of deposition. It is shown that the influence of hydrogen particle energy on the hydrogen content requires further investigation. In addition to hydrogen, models for other mobile impurities both in co-deposited layers and in layers under steady-state net erosion conditions are given.