Персона: Гаспарян, Юрий Микаэлович
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Руководитель научной группы -Международный центр ядерных компетенций (МЦЯДКОМ)
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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.
Modeling of COMPASS tokamak divertor liquid metal experiments
© 2020 The Author(s)Two small liquid metal targets based on the capillary porous structure were exposed to the divertor plasma of the tokamak COMPASS. The first target was wetted by pure lithium and the second one by a lithium-tin alloy, both releasing mainly lithium atoms (sputtering and evaporation) when exposed to plasma. Due to poorly conductive target material and steep surface inclination (implying the surface-perpendicular plasma heat flux 12–17 MW/m2) for 0.1–0.2 s, the LiSn target has reached 900 °C under ELMy H-mode. A model of heat conduction is developed and serves to evaluate the lithium sputtering and evaporation and, thus, the surface cooling by the released lithium and consequent radiative shielding. In these conditions, cooling of the surface by the latent heat of vapor did not exceed 1 MW/m2. About 1019 lithium atoms were evaporated (comparable to the COMPASS 1 m3 plasma deuterium content), local Li pressure exceeded the deuterium plasma pressure. Since the radiating Li vapor cloud spreads over a sphere much larger than the hot spot, its cooling effect is negligible (0.2 MW/m2). We also predict zero lithium prompt redeposition, consistent with our observation.
DETERMINATION OF HYDROGEN BINDING ENERGIES WITH DEFECTS IN METALS FROM THERMAL DESORPTION MEASUREMENTS WITH DIFFERENT HEATING RATES
Investigation of hydrogen isotope behaviour in plasma-facing and structural components is of great importance for successful development of fusion reactors with magnetic confinement. The hydrogen inventory (especially in the metals with a positive heat of solution) is determined by the presence of defects, which act as trapping sites for hydrogen. Therefore precise determination of defects’ parameters, namely binding energies for hydrogen atoms and their concentration is essential. Thermal desorption spectroscopy (TDS) is a commonly used method for such purposes. The trap parameters are usually determined by fitting of simulated TDS spectra to experimental ones using numerical diffusion-trapping codes, such as TMAP7 [1]. In such approach, the determination of the characteristics of the trapping sites is ambiguous – they act as free parameters in the model and, as a result, one spectrum can be fitted by using several combinations of parameters.
ИЗОТОПНЫЙ ЭФФЕКТ В НАКОПЛЕНИИ ВОДОРОДА В ОСАЖДЁННЫХ ИЗ ПЛАЗМЫ ВОЛЬФРАМОВЫХ СЛОЯХ
Важность изучения накопления водорода в соосаждённых из плазмы металлических слоях во многом обусловлена опасностью избыточного накопления радиоактивного изотопа трития в термоядерных установках, где со-осаждение материалов стенки с частицами работчего газа – один из основных каналов накопления водорода в установке. Так как эксперименты с использованием трития крайне затруднены технически, опасны и дороги, подавляющее большинство работ в данной области проводится с использованием дейтерия как симулятора трития. При этом предполагается, что накопление трития будет происходить идентично накоплению дейтерия. В связи со сложностью проведения экспериментов с тритием, возможно провести опыты по сравнению свойств протия и дейтерия и сделать некоторые оценки в части трития на основе полученных данных.
Post-mortem analyses of gap facing surfaces of tungsten tiles of T-10 ring limiter
© 2020 Elsevier B.V.Surfaces facing the gap between W tiles of the ring limiter of tokamak T-10 were analyzed after T-10 decommissioning using LIBS, SEM/EDA, XRD, TDS, and NRA techniques. Gaps with the width of 5 mm and 0.1 mm were nearly completely covered to their full depths of 22 and 15 mm, respectively, by a deposited film. The film was formed mainly by deposition of lithium that came from Li limiter and transformed in air to Li2CO3 and Li2O. Carbon was deposited from volatile hydrocarbons sputtered from the tokamak walls. Besides, carbon appeared due to chemical reaction with lithium in air. Chemical interactions of W with C, O, and Li led to formation W2C, WC, WO2, and Li2WO4. Carbides formed in W over the entire surface to the full depth of the gaps. Trapping of deuterium and helium in tiles was demonstrated. Possible influence of auto-oscillating discharges on ionization and ion trapping of C,D, and He in gaps is discussed.
Assessment of laser induced breakdown spectroscopy accuracy for determination of hydrogen accumulation in tungsten
© 2021 The Author(s)Laser-induced breakdown spectroscopy (LIBS) is an in situ method of determining hydrogen (H) content in plasma-facing materials in tokamak fusion reactors. Observing radiation from the plasma plume produced by a powerful laser pulse during the target exposition characterizes the sample composition. This is typically accomplished using the Saha-Boltzmann (SB) plot technique under local thermodynamic equilibrium (LTE) conditions. Despite many experimental studies dedicated to applying LIBS to determine H isotope retention in fusion reactor materials, the current understanding of this method's intrinsic accuracy remains inadequate. In this report, we use numerical calculations to estimate the relative error of determining H content in a sample using LIBS. As an example. we consider LIBS to study a W sample loaded with H in a vacuum. Under typical LIBS pulse parameters (109 W/cm2 and 12 ns duration), the error can be quite large, approximately 70%. We demonstrate that the error tends to decrease as the laser pulse intensity increases. Various factors contributing to the relative error are examined and their dependence on the LIBS plasma parameters is discussed. The SB plot remains a straight line even when LTE conditions are violated, making it difficult to anticipate the experimental results’ error.
INTERACTION OF LITHUM-DEUTERIUM FILMS WITH ATMOSPHERIC GASES
A choice of plasma-facing materials is one of the key issues in thermonuclear fusion reactor design. Lithium as an element with the low atomic number is a promising material for plasma-facing components (PFC) in fusion installations and a number of experiments at tokamaks already demonstrated many positive effects on plasma operation [1,2]. Lithium can be used for conditioning, PFC on the base of of capillary porous system with liquid lithium are also considered. In any way, one can expect lithium co-deposition with hydrogen isotopes at the surface of PFC and at remote areas. Lithium as a good getter can accumulate high amount of hydrogen isotopes (deuterium, tritium) that can be a problem from the safety reason. Deuterium retention and thermal desorption from lithium films formed in plasma discharge were investigated in this work.
Analytical approach for description of deuterium content in deuterium-beryllium co-deposited layers
© 2021 The Author(s)Available literature data for beryllium content in D-Be co-deposited layers was modeled using newly developed model. It was shown that the model adequately describes the experimental results. An empirical scaling for the available trap concentrations as a function of deuteron energy was introduced in order to improve the agreement between the experimental data and model predictions. The limited recombination rate describes the co-deposition better than the unlimited one. Assuming two types of traps in the film with detrapping energies of 0.8–0.9 and 1.2–1.3 eV and trap concentrations of 10 at. % and 1 at. %, respectively, and the deuterium implantation energy of 15.6 eV, the average relative deviation between experimental and predicted D/Be values of 1.9 was observed, which is better than the empirical D/Be scalings.
COMPARISON OF EROSION AND DEPOSITION IN JET DURING CARBON AND ITER-LIKE CAMPAIGNS
Erosion of plasma facing elements in fusion devices is an important question affecting their lifetimes. Redeposition of eroded materials may lead to accumulation of hydrogen isotopes, which makes it an important problem from the perspective of radiological safety. Redeposition in layers with carbon and beryllium could be a channel for hydrogen isotope accumulation.