S.L. Kanashenko

Hydrogen adsorption on and solubility in graphites

The experimental data on sorption and solubility of hydrogen isotopes in graphite in a wide ranges of temperature and pressure are reviewed. The Langmuir type adsorption is proposed for the hydrogen -- graphites interaction with taking into account dangling sp{sup 2}{minus}bonds relaxation. Three kinds of traps are proposed: Carbon interstitial loops with the adsorption enthalpy of {minus}4.4 eV/H{sub 2} (Traps l); carbon network edge atoms with the adsorption enthalpy of {minus}2.3 eV/H{sub 2} (Traps 2): Basal planes adsorption sites with enthalpy of +2.43 eV/H{sub 2} (Traps 3). The sorption capacity of every kind of graphite could be described with its own unique set of traps. The number of potential sites for the ``true solubility`` (Traps 3) we assume as 1E+6 appm, or HC=l, but endothermic character of this solubility leads to negligible amount of inventory in comparison with Traps 1 and Traps 2. The irradiation with neutrons or carbon atoms increases the number of Traps 1 and Traps 2. At damage level of {approximately}1 dpa under room temperature irradiation the number of these traps was increased up to 1500 and 5000 appm respectively. Traps 1 and Traps 2 are stable under high temperature annealing.

Gas-induced swelling of beryllium implanted with deuterium ions

Abstract 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 × 10 20 to 8 × 10 21 D/m 2 at temperatures, T irr = 300, 500 and 700 K has been carried out. Depth distributions of deuterium in a form of separate D atoms and D 2 molecules 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 T irr = 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 T irr ≥ 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.

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.

Trapping of deuterium in boron and titanium modified graphites before and after carbon ion irradiation

Nuclear reaction analysis was used to investigate the retention of deuterium (D) at radiation damage in new graphites USB-15, PGI, RG-Ti-91, RG-Ti-91 with 0.5 wt%B made in Russia and in POCO AXF-5Q graphite. Samples were irradiated with C+ ions to produce damage up to doses of ~ 10 displacements per atom (dpa), and then soaked in deuterium gas at 1473 K. The concentration of D in the damaged region increased with damage and saturated for damage levels above about 1 dpa. At saturation the smallest D concentration was found in USB-15 (~ 60 appm), and the largest D concentration was found in RG-Ti-91 (~ 1500 appm). Based on microstructural investigations done by TEM, an attempt was made to relate differences in concentrations of retained D to variations in radiation-induced trap concentration and to kinetic limitations for D atom access to traps available. The effect of boron on D trapping in graphite was also examined.

Hydrogen retention in plasma-facing materials and its consequences on tokamak operation

Abstract 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.

Deuterium trapping in graphites irradiated with C+ ions at 350 and 673 K

By means of NRA method the retention of D atoms was investigated in USB, RGT, PGI and POCO graphites after irradiation at 350 and 673 K with 200 keV C + ions to different doses up to about 10 dpa followed by deuterium loading from D 2 gas at 1473 K under pressure of 0.66 Pa for 1 h. The aim was to understand the nature of deep traps for D atoms with E D b ≃ 4.5 eV, differences in deuterium accumulation in different graphites and the role of the irradiation temperature. After irradiation at 673 K the concentration, C D , of D atoms trapped in the ion stopping range in all the graphites except for USB is 1.5-2.0 times as low as in those after irradiation at 350 K and amounts to 600-800 appm. USB demonstrates a minimum gas retention (≃ 100 appm), but it is more than that after implantation at 350 K. It was shown that deep traps are available in trace amounts in all graphites in their original state (about 10 appm) and can be created in the course of plastic deformation and/or fracture (up to about 100 appm). Deep traps are considered to be dangling bonds of C atoms along peripheral edges of interstitial clusters formed mainly due to radiation damage of graphites. Differences in the dose dependences of D retention, C D (Kt), for different graphites irradiated at 350 and 673 K were discussed in frames of available knowledge on the damage buildup and relevant effects which greatly depend on the irradiation temperature

Model for deuteron retention and reemission in graphites in a wide range of temperatures and energies

Abstract The experimental data available on hydrogen isotope trapping and release in graphite irradiated with deuterium ions under various conditions (energies 0.05–20 keV, temperatures 300–1300 K) are discussed. A new model of radiation-induced sequential reactions for hydrogen trapping and molecules reemission is proposed. The principal difference of our model from earlier models derives from the proposal that hydrogen retention occurs in two systems of traps (CD and CD2 complexes) that are formed locally and consequently in the irradiated area, and the molecular reemission takes place as a result of interaction between incoming ions and the already filled CD and CD2 traps. In our model the temperature dependence of retention and reemission is based on the assumption of the increase, at different rates, of reaction volumes with the irradiation temperature rising. Numerical simulations of hydrogen isotopes retention and reemission through the model equations agree well enough with the accumulated existing experimental data. The model could be used for the analysis of hydrogen isotopes behavior in graphite plasma-facing components.

Interaction of deuterium atoms with radiation defects in graphite

Abstract A thermal desorption mass spectrometry (TDS) study of deuterium trapping in single crystal graphite as a result of ion irradiation at low doses is presented. TDS spectra reveal several desorption peaks attributed to the dissociation of multiply occupied deuterium-vacancy complexes. A mechanism for the dissociation of these complexes is proposed. The resulting dissociation energy values are in fairly good agreement with those determined experimentally.

Vacancy deficit in metals implanted with inert gases

Abstract On the basis of the thermodynamic approach an improved expression for the calculation of real vacancy concentration in a metal (Me) matrix containing nonequilibrium inert gas (IG) bubbles is presented. In contrast to previous theoretical treatments elastic properties of both the Me matrix (characterized by its shear modulus μ) and the IG (characterized by its bulk modulus BT) were taken into consideration. Errors due to ignoring the above mentioned elastic properties appear to be significant mainly when the stiffness of the Me matrix is small (μ is low), and also when the IG pressure in overpressurized bubbles is high. Taking into account the cavity volume relaxation a useful equation is derived for the evaluation of internal pressure changes in nonequilibrium IG bubbles under temperature variations.

Evolution of hydrogen-containing traps in graphite under helium ion bombardment

Abstract The steady-state CD and CD2 complexes profiles in single crystalline graphite (0002) irradiated with 3 keV D+ to a fluence of 1018 D/cm2 (T = 300 K) have been obtained. The CD and CD2 contents turned out to be close to 2 × 1017 cm−2. After replacing the deuterium beam by a 10 keV 3He beam the D2 molecules emission flux was observed. For fluences of (1–10) × 1017 He/cm2 the emission was conditioned by radiation-induced decay of CD2 complexes. The CD complex profiles and their content varied insignificantly For interpretation of the data a model is developed for radiation-induced sequential reactions as proposed earlier. Additional reactions for radiation-induced decay of CD2 complexes which leads to the release of deuterium atoms, being interacted afterwards with the traps, is considered. The attempt has been made to generalize known data of helium induced deuterium emission from graphite.