S. N. Korshunov

Sputtering of beryllium, tungsten, tungsten oxide and mixed W-C layers by deuterium ions in the near-threshold energy range

Abstract An experimental method of determination of sputtering yield for current-conducting materials under ion bombardment of light gases in the near-threshold energy range has been developed. Such an information is very important in both the purely scientific and applied aspects. This method is based on the use of special regimes of field ion microscopic analysis. The procedure of measuring the sputtering yield includes cleaning of the surface in situ by desorption and evaporation of atoms by the field in order to make atomic-clean and atomic-smooth surface. This method permits to observe single vacancies in the irradiated surface, i.e., directly to count the single sputtered atoms. It has been used for beryllium, technically pure tungsten, tungsten oxide and mixed W–C layer on the tungsten irradiated by deuterium ions. The energy dependence of sputtering yield of those materials by deuterium ions at energies ranging from 10 to 500 eV is investigated. Experimental results for beryllium are in a satisfactory agreement with the calculations of Eckstein et al. Substantial connection between threshold energy of the sputtering and condition of oxidized surface of tungsten has been ascertained. The threshold energy for sputtering of oxidized tungsten surface is equal to 65 eV. The threshold energy for sputtering of mixed W–C layer has almost the same value as for the pure W.

Selfsputtering of beryllium and sputtering and erosion of CC composite in the experiments on plasma disruption simulation

Abstract The energy dependence of the Be-selfsputtering yield in the energy range of 1.5–10 keV was measured. The yield of Be-sputtering by Be + -ions attains its maximal value at the ion energy of 1.5 keV, being equal to 0.31 ± 0.02 atoms/ion; at the further increase in the energy the yield monotonously decreases. The temperature dependence of the sputtering yield of CC composite with H + ions are given. Results from disruption simulaiton experiments are described in which CC composite specimens were exposed to 0.5–1.0 MJ m −2 energy deposition during 50 μs from hydrogen plasma with a density of ∼ 10 15 cm −3 . The microstructure of redeposited layers, deposits morphology and the deposits size distribution were studied by the methods of transmission and scanning electron microscopy.

Peculiarity of deuterium ions interaction with tungsten surface in the condition Imitating combination of normal operation with plasma disruption in ITER

Tungsten is a candidate material for the ITER divertor. For the simulation of ITER normal operation conditions in combination with plasma disruptions samples of various types of tungsten were exposed to both steady-state and high power pulsed deuterium plasmas. Tungsten samples were first exposed in a steady-state plasma with an ion current density 10 21 m -2 s -1 up to a dose of 10 25 m - 2 at a temperature of 770 K. The energy of deuterium ions was 150 eV. The additional exposure of the samples to 10 pulses of deuterium plasma was performed in the electrodynamical plasma accelerator with an energy flux 0.45 MJ/m 2 per pulse. Samples of four types of tungsten (W-1%La 2 O 3 , W-131. monocrystalline W(1 1 1) and W-10%Re) were investigated. The least destruction of the surface was observed for W(1 1 1). The concentration of retained deuterium in tungsten decreased from 2.5 x 10 19 m - 2 to 1.07 × 10 19 m -2 (for W(1 1 1)) as a result of the additional pulsed plasma irradiation. Investigation of the tungsten erosion products after the high power pulsed plasma shots was also carried out.

Fusion safety studies in Russia from 1996 to 2000

Abstract The paper is a review of results of some fusion safety studies performed in Russia during 1996–2000 (after the 6th IAEA Technical Committee Meeting on ‘Developments in Fusion Safety’). In particular, changes of hydrogen isotope inventory in co-deposited layers on tungsten and graphite under simulation of normal tokamak operation and plasma disruptions are reported. Size distribution of graphite and W-Be-dust under the experiments at issue revealed two maximums. Erosion products were collected within tokamak T-10. Their microstructure and chemical composition were studied and are reported in the paper. Chemical interaction of beryllium powder with steam (in temperature range of 400–600°C) and air (at 500–1000°C) was investigated. Mathematical models describing it were developed. Chemical interaction of Be-powder with carbon was studied too. A dependence of beryllium carbide layer thickness growth versus time at 950°C was found on the basis of the experiments.

Investigation of the beryllium ion-surface interaction

Abstract The energy and temperature dependence of self-sputtering yields of beryllium were measured. The energy dependence of the beryllium self-sputtering yield agrees well with that calculated by Eckstein et al. Below 770 K the self-sputtering yields are temperature independent; at T irr. > 870 K the yield increases steeply. Beryllium samples were implanted at 370 K with monoenergetic 5 keV hydrogen ions and with a stationary hydrogen plasma power flux of about 5 MW/m 2 . In the fluence range of 5 × 10 22 -1.5 × 10 25 m −2 the depth profile is shifted towards the surface with increasing fluence and the concentration of trapped hydrogen atoms is reduced from 3.3 × 10 21 to 7.4 × 10 20 m −2 . About 95% of the trapped hydrogen is located within bubbles and only ∼ 5% is trapped as atoms. With increasing implantation fluence the bubbles coalesce, producing channels through which hydrogen escapes.

Erosion products of ITER divertor materials under plasma disruption simulation

Abstract Candidate ITER divertor armor materials: carbon–fiber-composite and four tungsten grades/alloys as well as mixed re-deposited W+Be and W+C layers were exposed in electrodynamic plasma accelerator MKT which provided a pulsed deuterium plasma flux simulating plasma disruptions with maximum ion energy of 1–2 keV, an energy density of 300 kJ/m2 per shot and a pulse duration of ∼60 μs. The number of pulses was from 2 to 10. The resultant erosion products were collected on a basalt filter and Si-collectors and studied in terms of morphology and size distribution using both scanning and transmission electron microscopy. Metal erosion products usually occurred in the form of spherical droplets, sometimes flakes. Their size distribution depended on the positioning of the collector. Simultaneously irradiated W, CFC and mixed W+Be targets appeared to have undergone a greater erosion than the same targets irradiated individually. Particles sized from 0.01 to 30 μm were found on collectors and on a molten W-surface. A model of droplet emission and behavior in shielding plasma is provided.

Subthreshold sputtering at high temperatures

The sputtering of tungsten from a target at a temperature of 1470 K during irradiation by 5-eV deuterium ions in a steady-state dense plasma is discovered. The literature values of the threshold for the sputtering of tungsten by deuterium ions are 160–200 eV. The tungsten sputtering coefficient measured by the loss of weight is found to be 1.5×10−4 atom/ion at a deuterium ion energy of 5 eV. Previously, such a sputtering coefficient was usually observed at energies of 250 eV. The sputtering is accompanied by a change in the target surface relief, i.e., by the etching of the grain boundaries and the formation of a wavy structure on the tungsten surface. The subthreshold sputtering at a high temperature is explained by the possible sputtering of adsorbed tungsten atoms that are released from the traps around the interstitial atoms and come to the target surface from the space between the grains. The wavy structure on the surface results from the merging of adsorbed atoms into ordered clusters.

Ion implantation into metals simultaneously irradiated by electrons

It has been revealed that concomitant irradiation by 1-keV electrons changes the depth distributions of the N and C atoms implanted with the energies of 10 and 30 keV in nickel and stainless steel samples. Instead of a single peak, the distribution has two peaks; one peak is closer to the surface and the other is located deeper than that in the absence of electron irradiation. The electron radiation effect is attributed to the formation of a band above the Fermi level in the atomic system whereto the metal electrons are transferred due to the electron bombardment excitation. The electrons in this band have a quite long lifetime and increase the mobility of impurity atoms.

Tungsten erosion under simulation of iter divertor operation

Under simulation of ITER gaseous divertor operation in a dense stationary plasma of the LENTA-facility, the tungsten sputtering by 5-eV deuterons at 1470 K has been detected. The sputtering yield was equal to 1.5 × 10−4 atom/ion. Below 1250 K the sputtering was not observed. The mechanism of subthreshold W-sputtering, based on a potential adatomic sputtering by slow deuterons, is proposed. Under plasma disruption simulation the W-droplet diameter distribution in respect to a collector position has been studied. Large droplets, up to 6 µm in diameter, fly away nearly perpendicular to the surface. Small droplets fly off in parallel to the surface or return to it. The minimal erosion is found for tungsten W (1 1 1) and the maximal one for W-10% Re.

Study of co-deposited carbon layers and of mixed (W + C) layers on tungsten and graphite in a plasma accelerator

In the present work samples of W and graphite were exposed in the C2H2 plasma for modelling the co-deposition process in a tokomak divertor. The energy of C2H2 ions, bombarding the surface was 300 eV. This means that the H + ion energy was lower than the threshold energy for W and C sputtering. Carbon co-deposited layers with globular structure similar to that obtained in tokamaks on W and C were produced. The structure of co-deposited films on W varied from uniformly smooth under low irradiation doses (p1 � 10 23 m � 2 ) to a globular one under high doses (10 24 m � 2 ). Films with a globular structure appear on graphite already at a dose of 2 � 10 23 m � 2 . The globular film production occurs by the appearance of separate small globules and by a gradual increase in their density and size. The film density is equal to 0.52 and 0.79 of that for crystalline graphite for the homogeneous structure and for the globular one, respectively. The integral H content in the co-deposited films is reduced with an increase in the globular structure fraction on the W surface from 7.2 � 10 21 to 3.2 � 10 21 m � 2 for a dose increase from 1 � 10 23 to 1 � 10 24 m � 2 . Under simultaneous irradiation of W and C by C2H2 ions mixed (W+C) layers, 500 nm thick, were produced on W and C surfaces. The integral hydrogen content in the (W+C) film on graphite is 2.3 times higher than the H content in the mixed layer on W. r 2002 Elsevier Science Ltd. All rights reserved.