V.M. Sharapov

Interaction of hydrogen with radiation defects in metals

The interaction of hydrogen (deuterium) with radiation defects in Mo and Ni was observed by the hydrogen permeation method from a glow discharge plasma for self-interstitial atoms (SIAs) and by depth profiling of implanted gas for vacancies (V). On the basis of the analogies with the free metal surface the model of interaction of hydrogen with vacancy defects is suggested. Using the known values of the adsorption and solution heats, the binding energies of hydrogen with vacancy and vacancy clusters are estimated for some metals (V, Nb, Ta, α-Fe, W, Pd, Cu, Au, Al). Good correlation is observed for calculated and experimental values.

Characterization of a-B/C:H films deposited from different boron containing precursors

This paper describes the characteristics of a-B/C:H films deposited by PCVD processes with a new harmless precursor-carborane (C2B10H12) in the model device as well as in the T-11M tokamak. Deposition conditions, density, thickness, structure, chemical bonds, content and depth distribution of elements, and some other properties of the films, were investigated in detail. The structure and properties of the films were compared with those for films deposited by other boronization techniques with different precursors (diborane and trimethylborane) and also with a-C:H films. Application of carborane for boronization leads us to the increase of B/C ratio in the films up to a value of 3–4 which is optimal for the erosion process. The structure of the nearest order became icosahedral instead of a mixture of diamond-like and icosahedral as in the case of deposition with diborane. All amorphous a-B/C:H films are homogeneous, and uniform in depth. The hydrogen content is the same as for other B containing films.

Boronization of Russian tokamaks from carborane precursors

Abstract A new and cheap boronization technique using the nontoxic and nonexplosive solid substance carborane has been developed and succesfully applied to the Russian tokamaks T-11M, T-3M, T-10 and TUMAN-3. The glow discharge in a mixture of He and carborane vapor produced the amorphous B C coating with the B C ratio varied from 2.0–3.7. The deposition rate was about 150 nm/h. The primary effect of boronization was a significant reduction of the impurity influx and the plasma impurity contamination, a sharp decrease of the plasma radiated power, and a decrease of the effective charge. Boronization strongly suppressed the impurity influx caused by additional plasma heating. ECR- and ICR-heating as well as ECR current drive were more effective in boronized vessels. Boronization resulted in a signficant extension of the Ne- and q-region of stable tokamak operation. The density limit rose strongly. In Ohmic H-mode energy confinement time increased significantly (by a factor of 2) after boronization. It rose linearly with plasma current Ip and was 10 times higher than Neo-Alcator time at maximum current.

Deuterium accumulation in beryllium oxide layer exposed to deuterium atoms

Abstract The interaction of deuterium atoms with beryllium TIP-30 was studied at temperatures of 340, 500 and 740 K. After D atom exposure, the depth distributions of deuterium atoms and molecules in Be were measured using combined Secondary Ion Mass Spectrometry (SIMS) and Residual Gas Analysis (RGA) methods. It was shown that deuterium is mainly accumulated in the oxide layer although long tails are also observed. Deuterium is retained in two states – atomic and molecular forms. The amount of trapped deuterium in samples decreases during the sample storage in vacuum or air at room temperature. The results were explained by the chemical bonding of D atoms in BeO oxide with beryllium hydroxide formation and the trapping of deuterium molecules in bubbles which are formed at growth defects in the oxide layer.

Deuterium accumulation in beryllium in contact with atomic deuterium at 740 K

The interaction of deuterium atoms with type TIP-30 beryllium has been studied. A plasma source with a heated cathode was used to produce deuterium atoms. The sample was under positive potential of +80 V to repulse the deuterium ions. The atomic flux was estimated at 1016 cm−2 s−1. After exposures to different doses at 740 K, the deuterium distribution and inventory in the samples were measured by means of elastic recoil detection (ERD) and thermal desorption spectroscopy (TDS) methods. The experiments showed that when in contact with atomic deuterium, beryllium absorbs deuterium up to concentrations of (6–8) × 1021 D/cm3. Deuterium penetrates deeply into the beryllium bulk. The penetration depth depends on the exposure time and reaches 400 nm after 240 min exposure. The main features of deuterium accumulation in beryllium during the atom exposure are very similar to those during the deuterium ion implantation, and this points out the similarity of the mechanisms of deuterium spread into beryllium, perhaps through the formation of the bubbles.

Plasma formation and first OH experiments in the Globus-M tokamak

The article reports the results of experimental campaigns on plasma ohmic heating performed during 1999-2000 on the spherical tokamak Globus-M. Later experimental results with the tokamak fed by thyristor rectifiers are presented in detail. The toroidal magnetic field and plasma pulse duration in these experiments were significantly increased. The method of stray magnetic field compensation is described. The technology of vacuum vessel conditioning, including boronization of the vessel performed at the end of the experiments, is briefly discussed. Specific features of neutral gas column breakdown in spherical tokamaks by applied inductive voltage are outlined. Also discussed is the influence of ECR preionization on the breakdown conditions. Experimental data on plasma column formation and current ramp-up in different regimes of operation with the magnetic flux of the central solenoid limited to ~100 mWb are presented. A significant reduction of the background density after boronization (below 2 × 1018m-3) allows the density to be completely controlled with external gas puffing and makes the influence of the wall negligible. The magnetic flux consumption efficiency is discussed. The results of magnetic equilibrium simulations are presented and compared with experiment. Ramp-up of the plasma current of 0.25 MA for a time interval of about 0.03 s with about 0.02 s flat-top at a toroidal field strength of 0.35 T allows the conclusion that the target design parameters of Globus-M could be achieved in a double swing regime.

Erosion of a-B/C : H films under deuterium plasma irradiation

The erosion of amorphous boronized carbon a-B/C: H films under low energy deuterium plasma irradiation was investigated. The films were deposited on a silicon substrate by the PCVD-method using the carborane vapour C 2 B 10 H 12 as a precursor. The deuterium plasma parameters in the erosion experiments were the following: ion flux density 3 × 10 16 cm −2 s − 1, ion energy 50 eV, thermal atom flux density 1 × 10 16 cm −2 s −1 . The erosion yields were calculated on the basis of mass thickness losses which were measured by electron probe micro-analysis. The erosion yields of a-B/C: H films do not depend on the temperature in the range 100-550°C and are more than one order of magnitude lower compared to those of boronized graphite USB-15. It was shown that the film erodes through the developing porosity which initially exists in the film and that the erosion has a chemical nature

Hydrogen permeation through metal membrane with protective coating in contact with atomic or ionized hydrogen

Abstract The use of protective coatings is one of the most common methods to reduce the hydrogen permeation through structural materials. Considering the hydrogen permeation through such multilayer membrane it is usually suggested that the ratio of hydrogen concentrations at the interface is equal to that of the hydrogen equilibrium solubilities in these materials. However, during diffusion permeation the hydrogen concentrations at the interface are not equilibrium values but they are determined by diffusion input D1/d1 and output D2/d2 rates and by passage rate through interface which is proportional to the ratio of equilibrium solubilities S1 and S2 (di, Di – thickness of and hydrogen diffusivity in i-layer, respectively). Therefore the ratio of hydrogen concentrations at the interface can be adopted by the ratio (S1D1/d1)/(S2D2/d2). By this approximation the analysis of hydrogen permeation through the duplex membrane in a diffusion-limited regime showed that the coating of the same materials can both decrease and increase the hydrogen permeability of the system depending on the ratio of these parameters. In this case it is possible to explain the contradictory results of experiments on hydrogen permeation through duplex membranes.

Microstructure and some properties of boron modified graphite USB-15

Boronized graphites, in particular USB-15, have some unique properties which make them applicable in thermonuclear confinement experiments. The optimization of USB-15 manufacture technology demands more detailed knowledge of its properties and, first of all, its microstructure. In this connection microstructure of USB-15 was studied in detail by means of X-ray diffraction, microanalysis, analytical TEM, SEM and other methods, both in original state and after annealing at 2300, 2700 and 3100 K. The erosion resistance to low energy bombardment from deuterium plasma was investigated between 350 and 900 K. In the course of 3 keV D+-ion implantation up to a dose ~ 5 × 1020 D+ m−2 deuterium is mainly trapped in vacancy complexes, which were studied by means of TDS and deuterium profiling using SIMS and RGA. The ratio of CD4 to D2 molecules emitted from USB-15 during post-implantation thermal desorption is considerably less than that from other types of graphites, indicating that the recombination desorption of deuterium is greatly enhanced by B in solid solution. Summary is given on property evolution of USB-15 (before and after thermal treatment) as a result of irradiation up to a fast neutron fluence (E > 0.18 MeV) of 2.5 × 1025 n m−2 in a temperature range 360–1100 K. Thermal annealing of USB-15 in the range 2200–2400 K is recommended for improvement of its thermal strength and radiation stability.

Plasma deposited boronized carbon films

A new method of plasma deposition of boron-carbon films is described. Since the precursors used the vapours of safe, solid carboranes, there is no neccessity for special safety precautions. The analysis of films showed that they are amorphous hydrogenated films α-B/C:H with ratio B/C = 2 and more.