A.E. Gorodetsky

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.

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.

Progress in the development of deposition prevention and cleaning techniques of in-vessel optics in ITER

The lifetime of front optical components unprotected from reactor grade plasmas may be very short due to intensive contamination with carbon and beryllium-based materials eroded by the plasma from beryllium walls and carbon tiles. Deposits result in a significant reduction and spectral alterations of optical transmission. In addition, even rather thin and transparent deposits can dramatically change the shape of reflectance spectra, especially for mirrors with rather low reflectivity, such as W or Mo. The distortion of data obtained with various optical diagnostics may affect the safe operation of ITER. Therefore, the development of optics-cleaning and deposition-mitigating techniques is a key factor in the construction and operation of optical diagnostics in ITER. The problem is of particular concern for optical elements positioned in the divertor region. The latest achievements in protection of in-vessel optics are presented using the example of deposition prevention/cleaning techniques for in-machine components of the Thomson scattering system in the divertor. Careful consideration of well-known and novel protection approaches shows that neither of them alone provides guaranteed survivability of the first in-vessel optics in the divertor. Only a set of complementary prevention/cleaning techniques, which include special materials for mirrors and inhibition additives for plasma, is able to manage the challenging task. The essential issue, which needs to be addressed in the immediate future, is an extensive development of techniques tested under experimental conditions (exposure time and contamination fluxes) similar to those expected in ITER.

First mirrors in ITER: material choice and deposition prevention/cleaning techniques

We present here our recent results on the development and testing of the first mirrors for the divertor Thomson scattering diagnostics in ITER. The Thomson scattering system is based on several large-scale (tens of centimetres) mirrors that will be located in an area with extremely high (3?10%) concentration of contaminants (mainly hydrocarbons) and our main concern is to prevent deposition-induced loss of mirror reflectivity in the spectral range 1000?1064?nm. The suggested design of the mirrors?a high-reflective metal layer on a Si substrate with an oxide coating?combines highly stable optical characteristics under deposition-dominated conditions with excellent mechanical properties. For the mirror layer materials we consider Ag and Al allowing the possibility of sharing the Thomson scattering mirror collecting system with a laser-induced fluorescence system operating in the visible range. Neutron tests of the mirrors of this design are presented along with numerical simulation of radiation damage and transmutation of mirror materials. To provide active protection of the large-scale mirrors we use a number of deposition-mitigating techniques simultaneously. Two main techniques among them, plasma treatment and blowing-out, are considered in detail. The plasma conditions appropriate for mirror cleaning are determined from experiments using plasma-induced erosion/deposition in a CH4/H2 gas mixture. We also report data on the numerical simulation of plasma parameters of a capacitively-coupled discharge calculated using a commercial CFD-ACE code. A comparison of these data with the results for mirror testing under deuterium ion bombardment illustrates the possibility of using the capacitively-coupled discharge for in situ non-destructive deposition mitigation/cleaning.

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.

Types of traps for deuterium implanted into graphite

Abstract Single-crystalline graphite samples have been implanted with deuterium ions at 3 keV to fluences of 1 × 10 15 −9×10 18 D/cm 2 at 300 K. The measurements of deuterium containing species by SIMS and of the partial pressure of HD and D 2 molecules during Ar-ion sputtering of the implanted and post-irradiation heated samples revealed that at high fluences there are two types of carbon-deuterium trap configurations, whereas at low fluences there is only one type responsible for the trapping of the majority of the D atoms. The depth profiles of these configurations were determined. It was speculated that the formation of these configurations is caused by the bonds between s-electrons of deuterium atoms and sp 2 or sp 3 hybridized carbon atoms.

AIN heteroepitaxial and oriented films grown on (111), (110) and (100) natural diamond faces

Abstract CVD growth of A1N thin films on (100), (110) and (111) diamond Cα substrates was observed, and RHEED and EPMP were used to provide the evidence of growth of epitaxial (01 · 1) [11 · 3] A1N//(111) [110] Cα or oriented wurtzitic A1N on (111) diamond. Twinning in A1N films on (111) Cα, was found. In the case of the (111) substrate, it was shown that in starting from a 0.4 μm film thickness, a conversion of epitaxial growth to a growth texture with a [00 · 1] axis normal to substrate takes place. The same textures were observed on both (100) and (110) diamond faces for 0.2–0.3 μm and greater film thickness. For an A1N thickness of about 1 μm, the crystallite size near outer A1N film surface was 10–20 nm.

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.