A.V. Markin

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.

Thermal desorption of deuterium implanted into beryllium

Abstract By means of TDS measurements it is shown that the desorption of deuterium from Be implanted with 5 keV D-ions to fluences, Φ, from 1 × 10 20 D/m 2 to 1 × 10 21 D/m 2 proceeds in one high-temperature stage B, while at Φ ≥ 1.2 × 10 21 D/m 2 one more stage A appears. The desorption maximum A is narrow and consists of two peaks A 1 and A 2 at about 460 K and 490 K, respectively. Peak A 1 is attributed to the desorption of deuterium from the walls of opened channels formed under D-ion implantation. Peak A 2 is a consequence of the opening of a part of bubbles/closed channels to the outer surface. The position of maximum B, T m (B), shifts noticeably and nonsteadily on the fluence in a range 850–1050 K. The origin of this maximum is the liberation of D-atoms bound at vacancy complexes discussed previously by Wampler. The dependence of T m (B) on the fluence is governed by the interaction of freely migrating D-atoms with arrangements of fully closed or partly opened gas cavities which are created under temperature ramping, but differently in specimens implanted with D-ions to different fluences.

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 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.

Role of grain boundaries and carbon deposition in deuterium retention behavior of deuterium plasma exposed tungsten

Data on deuterium retention in W single and polycrystals exposed to D plasma at 500 K are presented. The energy of D ions was well below the threshold of damage production. In a W polycrystal the deuterium spreads to a depth of about 4 μm. In contrast, no deuterium was found even at a depth of 2 μm in a W single crystal. This is an indication of significant grain boundary transport of deuterium at relatively low temperature. It has been found that ∼10 nm thin carbon layers grown on the plasma facing surface can be a barrier for D penetration into the bulk of tungsten.

Laboratory study of the transport and condensation of hydrocarbon radicals and its consequences for mitigating the tritium inventory in the ITER-FEAT divertor

Abstract The surface loss probability β of CH 3 radicals at a-C:H surfaces has been determined by analysis of the carbon deposition profiles along a tube flow reactor directly coupled with a methane RF discharge. β is equal to (1.0±0.2)×10 −3 for a methyl to atomic hydrogen flux ratio of 10:1. β remains the same in the temperature range 300–800 K and decreases slightly between 800 and 1200 K. The deposition rate drops drastically in the range 400–800 K. Above 800 K the carbon deposition is regained. From separate experiments on magnetron sputtering of graphite by D ions, it is inferred that at 300 and 900 kinetic CH x species react at the C:D surface with the same probability. In contrast, the deposition of thermal C x H y radicals decreases by a factor of 25 with a temperature rise from 320 to 400 K, remaining at this low level up to 1000 K. The implications of these results for co-deposition in the ITER-FEAT divertor are discussed.

Experimental Modeling of Transport and Deposition of Hydrocarbon Radicals on ITER-FEAT Cold Trap

Abstract The surface loss probability, β , of methyl (CH 3 ) radicals and atomic (H) hydrogen on a growing film surface was measured at lowered temperatures. The loss of reactive species was investigated by analysis of the deposition profiles along a quartz tube reactor directly coupled with a methane RF discharge (so-called ‘stream’ technique). The experiments were performed using special external containers with dry ice (∼200 K) and liquid nitrogen (77 K). It has been found that β (CH 3 ) on the growing film surfaces increases from 1.6×10 −4 at 300 K to 6.2×10 −4 at 200 K. Based on the data analysis, β for H can be deduced. The β (H) values were 3×10 −4 , 1×10 −3 and at least 10 −2 at 300 K, ∼200 K and 77 K, respectively. Film deposition from hydrocarbon radicals generated in a magnetron discharge is studied at temperatures from 300 K down to 120 K. Transition from formation of film-like to drop-like deposit is observed with cooling the substrate from 220 to 120 K. The application of the results to the understanding of transport and deposition of neutral reactive species on ITER-FEAT cold trap have been given.

Effect of oxidation on thermal desorption of deuterium sorbed in graphite

Abstract By means of thermal desorption spectroscopy (TDS), effect of air oxidation on deuterium release from carbon fiber composite SEP N112-A and fine grain graphite POCO AXF-5Q has been studied. Carbon samples were loaded with deuterium by exposure to 133 Pa D 2 at 1170 K for 2 h. Thermal desorption from SEP proceeds over a wide range, 1200 to 1900 K, without any sharp peaks. There are two distinct release stages visible in spectra of POCO. A minor portion of deuterium is evolved in a relatively narrow peak at 1350 K, while the rest is in the wide peak centered at 1800 K. Air oxidation of loaded samples at 1000 K to a weight-loss of ∼5% leads to a disappearance of the low temperature part of spectra while the amounts of deuterium released above 1600 K remain unchanged. It is suggested that air oxygen removes chemisorbed deuterium from the walls of pores accessible to O 2 molecules. The rest of the sorbed deuterium is apparently trapped at the sites accessible to D atoms only, not to D 2 and O 2 , and therefore is stable at the initial stage of oxidation.

Deuterium trapping by carbon materials for tokamak plasma-facing components

Abstract The experiments on large tokamaks on measuring the hydrogen isotope pressure in the near-wall or divertor plasma are briefly described. Based on the analysis of exchange fluxes between the plasma and the wall it is stated that the plasma action, with respect to hydrogen sorption by graphite components of the tokamak first wall, is identical to materials exposure in the atmosphere of molecular hydrogen at pressures of 1–10 3 Pa and temperatures of 1200–1700 K. Within these ranges of pressures and temperatures RG-Ti-91 and USB-15 graphites are shown to be impermeable to molecular hydrogen in contrast to easily permeable POCO-AXF-5Q. Sorption capacity of well-treated dense graphites is about 10 2 appm. Sorption capacity of materials is increased to (1–3)×10 4 appm with defects like unsaturated carbon bonds, introduced into the above materials by mechanical treatment, ion irradiation, graphite gasification (exposure at 1200 K, 133 Pa during 1 h). At the same time permeability of graphites is increased to molecular hydrogen. Graphite modification by boron retards the rise of sorption capacity. The hydrogen sorption process is described as the filling of two sets of traps, i.e. weak traps arranged over the boundaries of carbon materials and deep traps arranged in the volume of separate crystallises. Russian-made graphites RG-Ti-91 and USB-15 can be used in large tokamaks as divertor and first wall tiles, respectively.