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AbstractThe interaction of deuterium atoms with beryllium TIP-30 at 740 K was studied. A plasma source with heated cathode was used to produce deuterium atoms. A positive potentials was applied to the beryllium sample during D atoms exposure to screen in from positive D ions. After D atom exposure for 1 and 4 hours the depth distributions of deuterium atoms and molecules in Be were measured using combined SIMS and RGA methods. It has been shown that deuterium is retained in Be both in atomie and in molecular forms. Oxide film grows on the beryllium surface during the D atom exposure and BeO thickness correlates with deuterium distribution. The retained deuterium is mainly located in the oxide layer. It is suggested that the gas fllled bubbles and cavities which are formed in grown oxide layer are the traps for molecular deuterium. A high concentration of D atoms in the near-surface layers is attributed to the chemical bonding of D atoms to BeO oxide with beryllium hydroxide formation.

In-depth concentration profiles of deuterium atoms and molecules in beryllium implanted with 9 keV D ions to fluences, Φ, in the range from 6 × 10 to 9 × 10 D/m at temperatures, T irr , of 300 and 700 K have been determined using SIMS and RGA (residual gas analysis) measurements in the course of surface sputtering. The microstructure of implanted specimens was studied by TEM. Implanted deuterium is retained in Be matrix in the form of both D atoms and D 2 molecules. The total amount of gas captured within the sub-surface layer of ∼ 700 nm in thickness as a result of implantation at 300 and 700 K reaches 4 × 10 and 1 × 10 D/m , correspondingly. The ratio of deuterium quantities retained in the form of atoms and molecules, Q D :Q D 2 , varies from 1:3 for T irr = 300 K to 1:4 for T irr = 700 K . At T irr = 300 K the concentration of D 2 molecules at the depth of the ion mean range reaches its maximum of 4 × 10 molecules/m at Φ ≈ 2 × 10 D/m . The molecules are present in tiny bubbles which show a tendency toward interconnection at higher fluences. At T irr = 700 K , along with relatively small facetted bubbles (near the very surface), large oblate gas-filled cavities and channels forming extended labyrinths appear and they accumulate most of the injected gas. The maximum D 2 concentration in the latter case is of 1 × 10 molecules/m . The high concentration of D atoms in the ion stopping zone after implantation at T irr = 300 and 700 K (about 2 × 10 and 1 × 10 atoms/m , respectively) is attributed to deuterium (i) trapped in radiation vacancies, (ii) adsorbed on the walls of bubbles and channels and (iii) bonded to/by BeO formed on the surface and present in the form of metallurgical inclusions in the bulk.

The current status of research in the area of hydrogen retention and release for the prime candidate plasma-facing materials is briefly reviewed. Physical understanding of the basic problems of hydrogen behavior in the surface layers and material bulk of graphite, beryllium and tungsten is emphasized. The data base in the field obtained in laboratory set-ups permits more thorough consideration of the recent experimental results in large tokamaks. It is known that hydrogen isotopes retention and release has a great influence on tokamak operating conditions and its plasma parameters. The processes occurring in tokamaks with hydrogen participation, such as recycling, fuelling, codeposition, isotopic exchange and conditioning, now can be better understood, explained and controlled. Some consequences of hydrogen isotopes behavior in plasma and plasma-facing materials are described for long pulse tokamak operation.

Trapping and release of deuterium implanted in tungsten is investigated by modeling the results of reemission, thermal and isothermal desorption experiments. Rate coefficients and activation energies for diffusion, trapping and detrapping are derived. Hydrogen atoms are able to diffuse deep into tungsten, establishing a solute amount of the same order of magnitude as the trapped one. This 'diffusion zone' exceeds the implantation zone by more than two orders of magnitude, even at room temperature. The solute amount of hydrogen in tungsten depends only slightly on the incident ion energy, but scales with implantation fluence. This high amount of solute hydrogen is the main difference of tungsten compared to graphite where nearly all hydrogen is trapped in the implantation zone, the solute amount being orders of magnitude lower. The resulting unlimited accumulation of hydrogen in tungsten deep in the material down to the backward surface disadvantages tungsten as fusion reactor material with regard to hydrogen recycling properties.

Tungsten carbide coating, W 60 C 40 and W 85 C 15 prepared by chemical vapor deposition and consisting of different carbide phases, are implanted with 0.8, 1.5 and 3 keV deuterium ions at room temperature and elevated temperatures. The trapping and release of deuterium are investigated by means of re-emission and thermal desorption spectroscopy. The results have been compared with previous data from graphite and from tungsten prepared by various methods. It is found that a few percent of carbon impurities strongly increases the deuterium inventory in tungsten. The thermal desorption of deuterium from tungsten carbide occurs at temperatures much lower than for graphite and the spectral form shows much more complicated features. The peak temperatures, in this case, mainly depends on the carbon concentration in different phases of the samples. Chemical erosion is only detected for sample W 60 C 40 . In this case all carbon atoms are eroded as volatile hydrocarbons while the physical sputtering of tungsten atoms limits the total erosion. Both fluence and temperature dependence of the chemical erosion are very similar to the results from the interaction of TiC with keV hydrogen ions.

An extensive TEM study of the microstructure of TIP-30 Be implanted with 3 and 10 keV D ions to fluences, Φ in the range from 3 × 1020 to 8 × 1021D/m2at temperatures, Tirr= 300, 500 and 700 K has been carried out. Depth distributions of separate D atoms and D2molecules have been investigated by means of SIMS and RGA methods, correspondingly. D ion irradiation, accompanied by blistering, gives rise to destructions dependent mainly on Tirr. Irradiation at 300 K leads to the formation of tiny D2bubbles of 1 run in size (reminiscent of He bubbles in Be). At Tirr≥ 500 K, along with small facetted bubbles, the development of larger oblate cavities occurs accumulating most of injected deuterium and providing for a much higher gas swelling compared to that at 300 K. D (He) ion implantation leads to the enhanced growth of microcrystalline layers of cph-BeO oxide with a microstructure differing from that on the electropolished Be surface. Based on the analysis of experimental data deuterium reemission, thermal desorption and trapping in defects are discussed.

An extensive TEM study of the microstructure of Be TIP-30 irradiated with 3 and 10 keV D ions up to fluences, Φ, in the range from 3 × 1020to 8 × 1021D/m2at temperatures, Tirr= 300, 500 and 700 K has been carried out. Depth distributions of deuterium in a form of separate D atoms and D2molecules have been investigated by means of SIMS (secondary ion mass spectrometry) and RGA (residual gas analysis) methods, correspondingly. D ion implantation is accompanied by blistering and gives rise to processes of gas-induced cavitation which are very sensitive to the irradiation temperature. At Tirr= 300 K tiny gas bubbles (about 1 nm in size) pressurized with molecular deuterium are developed with parameters resembling those of helium bubbles in Be. Irradiation at Tirr≥ 500 K leads to the appearance of coarse deuterium-filled cavities which can form in sub-surface layers different kinds of oblate labyrinth structures. Questions of reemission, thermal desorption and trapping of deuterium in Be have been discussed.