- Email: [email protected]
Study of co-deposited carbon layers and of mixed (W+C) layers on tungsten and graphite in a plasma accelerator
Vacuum 67 (2002) 253–260
Study of co-deposited carbon layers and of mixed (W+C) layers on tungsten and graphite in a plasma accelerator M.I. Gusevaa, V.M. Gureeva, L.S. Danelyana, V.S. Kulikauskasb, S.N. Korshunova, Yu.V. Martynenkoa,*, P.G. Moskovkina, I.D. Skorlupkina, V.V. Zatekinb a
Institute of Nuclear Fusion, Russian Research Center, Kurchatov Institute, Kurchatov Square 1, Moscow 123098, Russian Federation b Skobeltcyn Nuclear Physics Institute, M.V. Lomonosov Moscow State University, Vorobievyi Goryi 1, Moscow 119899, Russian Federation Received 24 December 2001; received in revised form 28 February 2002; accepted 2 April 2002
Abstract 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 1023 m2) to a globular one under high doses (1024 m2). Films with a globular structure appear on graphite already at a dose of 2 1023 m2. 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 1021 to 3.2 1021 m2 for a dose increase from 1 1023 to 1 1024 m2. 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. Keywords: Carbon; Tungsten; Divertor; Co-deposition layer; Hydrogen content
1. Introduction Tungsten and CFC composites are expected to be used in different parts of the ITER divertor. The presence of the two different materials, as the *Corresponding author. Tel.: +7-95-196-7041; fax: +7-95943-0073. E-mail address: martyn@nfi.kiae.ru (Y.V. Martynenko).
divertor components, will unavoidably result in the production of co-deposited mixed (W+C) layers, as already shown in many studies [1–4]. Formation of the mixed (W+C) layers on tungsten promotes an increase of the deuterium isotope content in the surface layer of tungsten [1–4]. The problems related to the production of the mixed W and C layers and with the accumulation
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 2 7 1 - 3
254
M.I. Guseva et al. / Vacuum 67 (2002) 253–260
of hydrogen isotopes in tungsten and in graphite under the conditions in the gaseous divertor, when the deuterium ion energy is below the threshold energy for physical sputtering for W and C, are of a particular interest. At the same time, the hydrocarbon molecules will enter into the plasma due to a chemical graphite erosion by H+-ions. This means that carbon co-deposition on W and CFC surfaces will be observed in spite of the below threshold energy of H+-ions. Recently, the problems related to the chemical graphite erosion at hydrogen isotope ion energies lower than the threshold energy of physical sputtering [5–10] were studied. In most of the publications, the production of molecular hydrocarbon during chemical graphite erosion was investigated as a dependence of ion beam intensity and irradiation temperature. In the present work, the samples of W and graphite were exposed in C2H2 plasma to model the co-deposition process in the divertor. The energy of C2H2 ions, bombarding the surface was 300 eV, i.e. the H+-ion energy was lower than the threshold energy for W and C sputtering.
2. Experimental technique The experiments were done at the VITA accelerator which includes, along with an ion source of high-energy ions, a stationary plasma accelerator with closed electron drift and with an extended acceleration zone, developed at the Kurchatov Institute [11]. Samples of tungsten (W-99.96%, Mo-0.04%) and MPG-8 graphite were used in the experiments. The samples were placed in the vacuum chamber of the VITA accelerator, as shown in Fig. 1. For studying the process of carbon co-deposition the C2H2 plasma flow was directed straight to the samples of W or graphite (Fig. 1a). The schematic diagram of the experiment on carbon co-deposition simultaneously with W resputtering onto the targets of tungsten or graphite is shown in Fig. 1b. As seen in Fig. 1b, the C2H2 ion flow simultaneously interacts with the tungsten sample (or MPG-8 graphite sample) and the tungsten target being sputtered is a cone with a vertex angle of 451 surrounding the
Fig. 1. Scheme of sample disposition in a vacuum chamber of VITA-accelerator: (a) co-deposition, (b) co-deposition simultaneously with resputtering.
sample. The C2H2 ion energy is equal to 300 eV. The ion flux is 1021/m2 s. The sample temperature was determined by the plasma flux power on the target and was measured with a thermocouple. In the process of irradiation it reached a constant 670 K due to the heat conduction from the sample holder. The irradiation doses were varied from 1 1023 to 1 1024 m2 (see Table 1). After each plasma simulation, studies on the target surface microstructure were made using a JEOL scanning electron microscope. The chemical composition of the surface layer was studied by Rutherford back scattering (RBS) using a
M.I. Guseva et al. / Vacuum 67 (2002) 253–260
255
Table 1 Irradiation conditions for W and graphite Sample No.
Material of the sample
Material of the target sputtered simultaneously
Irradiation dose (m2)
Duration of irradiation (s)
1 2 3 4 5 6
W W W W MPG-8 graphite MPG-8 graphite
— — — W — W
1 1023 4 1023 1 1024 2 1023 2 1023 2 1023
100 400 1000 200 200 200
Van-de-Graaf accelerator, where the 1.6 MeV He+-ions scattered through an angle of 1701 were registered using a surface barrier detector. The samples were weighed with a microbalance before and after irradiation. The deuterium distribution profiles in the irradiated targets were determined by elastic recoil detection analysis. For this a He+-ion beam with an energy of 2.2 MeV was incident on the sample at an angle of 151 to its surface. The recoil atoms were registered at an angle of 301 to the initial He+-ion direction. In order to produce the absolute values of deuterium atom concentration energy spectra measurements of standard calibration samples were performed.
3. Experimental results 3.1. Study of the co-deposited carbon layers on tungsten and on graphite 3.1.1. Microstructures of the co-deposited carbon layers Some typical surface microphotographs of codeposited layers on graphite (at a dose of 2 1023 m2) and on W at various irradiation doses by C2H2 ions with an energy of 300 eV are given in Fig. 2. At a irradiation dose of 1023 m2 (Sample No. 1) a relatively homogeneous smooth golden-coloured film is observed on the surface. With an increase in the irradiation dose, the growth of some nuclei of a new globular structure (Fig. 2b, Sample No. 2) appear on the surface and develop over the whole W surface under an irradiation dose of 1024 m2 (Fig. 2c, Sample No.
3). The globule size varies from 1.5 to 20 mm. The globular structure on the graphite surface is already produced at a dose of 2 1023 m2 (Fig. 2d). The globule size on the graphite is of the same order. One should note that a similar globular structure on the graphite layer surface was observed in tokamaks TEXTOR [12], DIII-D [13] and in T-10 [14]. Fig. 3 illustrates a typical globular structure of the film surface produced in the T-10 tokamak. This confirms the results of our experiments: carbon film surfaces acquire a globular structure under higher irradiation doses. 3.1.2. Accumulation of hydrogen in the co-deposited carbon layers under C2H2 ion irradiation The hydrogen distribution profiles in the codeposited carbon layers on tungsten for various irradiation doses by C2H2 ions and on MPG-8 graphite for a dose of 2 1023 m2 (curve 3) are given in Fig. 4. The hydrogen concentration at the carbon film surface on tungsten decreases from 23.5 at% for a smaller dose to 10 at% for the maximum dose. As seen from the graphs, for all the irradiation doses the hydrogen is distributed practically uniformly in the carbon layer, 300 nm thick, on tungsten. The concentration of hydrogen (curve 3) in the carbon layer co-deposited from plasma on the MPG-8 graphite surface coincides, practically, with the hydrogen concentration in the carbon layer on the tungsten with the globular structure when developed over the whole irradiated surface (curve 4, Fig. 4). In this case, the concentration on the carbon-containing layer surface is about 1.5 times smaller than that at a depth of 300 nm, where it is close to the hydrogen
256
M.I. Guseva et al. / Vacuum 67 (2002) 253–260
Fig. 2. Typical microphotographs of the co-deposited layer surfaces on tungsten: (a) D ¼ 1023 ; (b) D¼ 4 1023 ; (c) D ¼ 1024 m2 and on MPG-8 graphite: (d) D¼ 2 1023 m2 :
H concentration, 10 28 atoms/m
3
2. 5
1
2. 0
2
1. 5 3
1. 0 4
0. 5
0. 0 0
100
200
300
Depth, nm
Fig. 3. Typical globular structure of the film produced in T-10 tokamak.
Fig. 4. Hydrogen distribution in depth in the co-deposited carbonic layer on tungsten under various irradiation doses by C2H2 ions: (1) D¼ 1 1023 ; (2) D¼ 4 1023 ; (3) D¼ 1 1023 m2 and on MPG-8 graphite: (4) D¼ 2 1023 m2 :
concentration in the globules produced on tungsten at an intermediate irradiation dose (curve 2, Fig. 4). A reduction in the hydrogen concentration towards the surface confirms the fact that the hydro-
gen desorption on the surface rises with the globule production and with an increase of globule density. From the spectra of back-scattered protons from the carbon films on W for various irradiation
M.I. Guseva et al. / Vacuum 67 (2002) 253–260
257
Table 2 Experimental results for the co-deposited carbonic layers on tungsten and on MPG-8 graphite Sample No.
Sample material
D (C2H2/m2)
N ( 1021 H/m2)
N (C/m2)
1 2
W W
1 1023 4 1023
7.2 5.8
6 1022 3 1023
3 4
W MPG-8 graphite
1024 2 1023
3.2 3.8
9 1023
doses 1 1023, 4 1023 and 1 1024 m2, the thickness of the carbon films produced was determined (see Table 2). From Table 2 it follows that the carbon layer thickness and the total carbon content rise with increase in the irradiation dose whereas the total hydrogen content falls. The thickness of the co-deposited carbon films in the dose range (1–10) 1023 m2 rises from 1 to 10 mm. One should emphasize this unexpected result that a reduction of total hydrogen content occurs in the carbon layers on W with dose increase and with an increase of the layer thickness. 3.2. Study of co-deposited mixed (W+C) layers on tungsten and on graphite 3.2.1. Component distribution in the mixed (W+C) layers In the experiments made according to the schematic diagram in Fig. 1b along with the sample irradiation by C2H2 ions, tungsten sputtering by C+-ions took place. In this way the mixed (W+C) layers were produced on surfaces of tungsten and graphite [15]. The distributions of W and C atoms in depth within the mixed layers deposited on W are given in Fig. 5. The distribution is typical for the implantation mixed layer. The mixed layer is about 500 nm thick. The concentration of carbon atoms decreases from 96 at% on the surface to 4 at% at 500 nm and the concentrations of C and W become equal to each other at a depth of 230 nm (50 at% of C-atoms and 50 at% of Watoms). The (W+C) layer on graphite is also 500 nm thick (Fig. 5b). In this case, the concentra-
d (mm)
rs =ro
Structure
1.0 3.3
0.64 0.96
10.0
0.96
Homogeneous Globular and homogeneous Globular Globular
tion of tungsten atoms in the (W+C) layer is reduced from 18 at% in the layer 100 nm thick to o1 at% at a depth of 500 nm. Probably, the distribution typical for the mixed implantation layer, where up to 50 at% for each element is attained at some depth, on tungsten is formed by the mixing of sputtered W atoms in the growing (W+C) layer. 3.2.2. Microstructure of the mixed (W+C) layers The mixed (W+C) layer topography on tungsten (a) and that on graphite (b) is shown in Fig. 6. The mixed layer surface microstructure on tungsten, where 96 at% of C atoms is contained on the surface, is similar to the globular structure produced under co-deposition of carbon layers (Figs. 2b–d). The (W+C) layer microstructure on graphite with 18 at% of W at the surface layer is characterized by the presence of the spiral structures usually produced under ion bombardment of a surface including impurities [16]. One can see that the film has poor adhesion to the graphite substrate. There are some parts, where exfoliation of the film has occurred. 3.2.3. Accumulation of hydrogen in the mixed (W+C) layers The hydrogen distribution profiles in the mixed (W+C) layers on tungsten (curve 1, Sample No. 4) and on graphite (curve 2, Sample No. 6) are given in Fig. 7. The hydrogen distribution profiles in the (W+C) layers on tungsten (curve 1) has a smooth maximum at a depth of 100–200 nm, confirming the fact that the content of hydrogen in it is reduced with increase in tungsten concentration
M.I. Guseva et al. / Vacuum 67 (2002) 253–260
258 100
Concentration, at.%
80
W 60
40
C 20
0 0
250
(a)
500 Depth, nm
750
100
Concentration, at.%
80
C 60
40
W
20
Fig. 6. Surface topography of the mixed (W+C) layers on (a) tungsten, (b) graphite.
0 0
(b)
250
500
750
Depth, nm
Fig. 5. Element distribution in depth within the co-deposited (C+W) layer on tungsten (a) and that within the (C+H+W) layer on graphite (b) under a dose of 2 1023 m2.
(see, Fig. 5) in a range of 200–300 nm. As known, the ratio of H/W in tungsten is smaller than H/C in carbon [17]. The decrease of the hydrogen concentration towards the surface confirms the hydrogen desorption from that layer. The hydrogen distribution within sample No. 6 (curve 2) has the same kind as the distribution in the (W+C) layer on tungsten (curve 1). At the
same time, curve 2 is significantly lower. The maximum of this curve corresponds to a hydrogen concentration of 7 1021 at/cm3, whereas the concentration maximum in the (W+C) layer on tungsten is 1.6 1022 at/cm3. The total hydrogen content in the (W+C) layer on graphite is 4.7 1021 m2 whereas that on tungsten is 2.1 1021 m2. It is evident that such an effect is provided by the different structure (Fig. 6) and by the different composition (Fig. 5) of the films produced on the surfaces of tungsten and graphite. As seen in Fig. 6, the presence of tungsten (up to 18 at% in the surface layer) results in the embrittlement, in bad adhesion and thermal contact of the
M.I. Guseva et al. / Vacuum 67 (2002) 253–260
H concentration,10
28
atoms/m
3
2.5
2.0
1 1.5
1.0
2 0.5
0.0 0
100
200
300
Depth,nm
Fig. 7. Hydrogen distribution in depth in the co-deposited (C+W) layers on tungsten (1) and on graphite (2) under a dose of 2 1023 m2.
film with the substrate, in film temperature growth and, as a consequence, in an increase of the hydrogen desorption rate from the (W+C) film on graphite.
4. Discussion In all the experiments the sample irradiation was by the C2H2 ions with an energy of 300 eV. The kinetic energy of C+- and H+-ions is divided in accordance with the ratio of their masses: C(12/13) and H(1/13). As a result of this, the H+-ion energy in our experiments is equal to 11.54 eV; that of C+-ions is 138.46 eV. It means that the H+-ion energy is lower than the threshold energy for physical sputtering of graphite, tungsten and tungsten oxide. Hence, only chemical sputtering of graphite by hydrogen ions takes place in these experiments. The more energetic C+-ions sputter both graphite and tungsten. The mixed (W+C) layers are produced as a result of chemical sputtering of graphite and physical sputtering of graphite and tungsten. In principle, the process of chemical sputtering for the CFC composite will take place in a gaseous divertor, and the presence of tungsten atoms can be provided by evaporation of tungsten during plasma disruptions.
259
From a comparison of the hydrogen accumulation in the co-deposited carbon layers on tungsten and on graphite with the corresponding film surface microstructure (see, Table 2), one can conclude that the maximum hydrogen concentration in the co-deposited layers is observed in a uniform smooth film on the surface. With the development of a globular structure, the hydrogen concentration in the co-deposited layers on the surfaces of tungsten and graphite is reduced and also the hydrogen concentration decreases towards the surface. This tendency is clearly observed on the MPG-8 graphite, where a difference in the hydrogen concentration in depth (under low irradiation doses) exceeds 1.5-times the hydrogen concentration on the surface. The very fact that the structure of co-deposited films in our experiments under irradiation of W and graphite by low-energy C2H2 ions is similar to that produced in tokamaks confirms the efficiency of modelling the resputtering and co-deposited process in a high-intensity acetylene plasma. One should note that the thickness of the layers deposited on the collector was in the range 40– 300 nm which is comparable with the thickness of the layers produced on the chamber wall after a long exposure (B2000 s) of the graphite tiles in the tokamak plasma [13]. In our experiments we produce layers, 1–10 mm thick in a shorter time (100–1000 s). 5. Conclusions 1. Carbon co-deposited layers with globular structure similar to that obtained in tokamaks on tungsten and on graphite were produced by the co-deposition process from a high-intensity C2H2 plasma with a molecular ion energy of 300 eV. 2. The structure of co-deposited films on tungsten varies from a uniformly smooth one at low irradiation doses (p1 1023 m2) to a globular structure at high dose (1024 m2). The film with a globular structure is already formed on graphite at a dose of 2 1023 m2. 3. The hydrogen content in the co-deposited films is determined by their structure. The total hydrogen content in the co-deposited films is reduced with an increase in the globular
260
M.I. Guseva et al. / Vacuum 67 (2002) 253–260
structure fraction on the tungsten surface from 7.2 1021 to 3.2 1021 m2 for a dose increase from 1 1023 to 1 1024 m2. 4. The globular film production occurs on the smooth film by the appearance of separate small globules and by a gradual increase in their density and size on the surface with an increase in the irradiation dose (or equivalent tokamak run duration). 5. Under simultaneous irradiation of graphite and tungsten by C2H2 ions mixed (W+C) layers, 500 nm thick, were produced on tungsten and graphite. The mixed layers on tungsten and graphite essentially differ from each other in composition, microstructure and hydrogen content. 6. The total hydrogen content in the (W+C) film on graphite is 2.3 times smaller, than the hydrogen content in the mixed layer on tungsten, as a result of desorption due to poor adhesion of the film to the substrate. References [1] Haasz AA, Davis JW, Poon M. First IAEA RCM on ‘‘PMI Data of Mixed Plasma-Facing Materials’’, Vienna, 19–20 October, 1998. p. 1–6. [2] Anderl RA, Pawelko RJ, Schuetz ST. 14 PSI Rosenheim, Germany, 22–26 May, 2000.
[3] Longhust G, Anderl RA, Cadwallader LC, Carmack WJ, Gorman SV, McCarty KA, et al. Fusion Safety Program, Annual Report Fiscal Year, 1998. [4] Mech BV, Haasz AA, Davis JW. J Appl Phys 1998;84: 1655–69. [5] Mech BV, Haasz AA, Davis JW. J Nucl Mater 1998;255: 153–64. [6] Roth J. J Nucl Mater 1999;266–269:51–7. [7] Haasz AA, Mech BV, Davis JW. J Nucl Mater 1996;231:170–2. [8] Roth J, Balden M. EU/JCT Meeting on Plasma–Wall Interaction, Kloster Seeon, December 1997. [9] Grote H, Bohmeyer W, Kornejew R, Reiner HD, Fussmann G, Schlugl R, Weinberg G, Wu CH. J Nucl Mater 1999;266–269:245–51. [10] Grote H, Bohmeyer W, Reiner HD, Fuchs T, Kornejew R, Steinbrink J. J Nucl Mater 1999;241–243:1152–8. [11] Veselovzorov AN, Barkalov EE, Pogorelov AA, Pokrovskii IB, Terentjev AR. Russian Patent N 1367829 15.09.1987. [12] Scholtz T, et al. J Nucl Mater 1997;241–243:848–51. [13] Buzjinskij OI, Opimach IV, Barsuk VA, et al. J Nucl Mater 1996;233–237:787–90. [14] Alimov VH, Goncharov VL, Guseva MI, Kolbasov BN, Romanov PV, Chimtchenko LN. Issledovanie strukturi pereosagdennich v vakuumnoi kamere tokamaka T-10 uglerodnich plenok i soderganie v nich deiterij. VANT Termojadernii sintez 2000;2:45–54. [15] Guseva MI, Korshunov SN, Lazarev NE, Suvorov AL. J Nucl Mater 1999;266:222–7. [16] Rossnagel SM, Robinson RS. J Vac Sci Technol 1982;20:506–11. [17] Haasz AA, Davis JW. J Nucl Mater 1997;241–243: 1076–81.
Study of co-deposited carbon layers and of mixed (W+C) layers on tungsten and graphite in a plasma accelerator M.I. Gusevaa, V.M. Gureeva, L.S. Danelyana, V.S. Kulikauskasb, S.N. Korshunova, Yu.V. Martynenkoa,*, P.G. Moskovkina, I.D. Skorlupkina, V.V. Zatekinb a
Institute of Nuclear Fusion, Russian Research Center, Kurchatov Institute, Kurchatov Square 1, Moscow 123098, Russian Federation b Skobeltcyn Nuclear Physics Institute, M.V. Lomonosov Moscow State University, Vorobievyi Goryi 1, Moscow 119899, Russian Federation Received 24 December 2001; received in revised form 28 February 2002; accepted 2 April 2002
Abstract 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 1023 m2) to a globular one under high doses (1024 m2). Films with a globular structure appear on graphite already at a dose of 2 1023 m2. 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 1021 to 3.2 1021 m2 for a dose increase from 1 1023 to 1 1024 m2. 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. Keywords: Carbon; Tungsten; Divertor; Co-deposition layer; Hydrogen content
1. Introduction Tungsten and CFC composites are expected to be used in different parts of the ITER divertor. The presence of the two different materials, as the *Corresponding author. Tel.: +7-95-196-7041; fax: +7-95943-0073. E-mail address: martyn@nfi.kiae.ru (Y.V. Martynenko).
divertor components, will unavoidably result in the production of co-deposited mixed (W+C) layers, as already shown in many studies [1–4]. Formation of the mixed (W+C) layers on tungsten promotes an increase of the deuterium isotope content in the surface layer of tungsten [1–4]. The problems related to the production of the mixed W and C layers and with the accumulation
0042-207X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 2 ) 0 0 2 7 1 - 3
254
M.I. Guseva et al. / Vacuum 67 (2002) 253–260
of hydrogen isotopes in tungsten and in graphite under the conditions in the gaseous divertor, when the deuterium ion energy is below the threshold energy for physical sputtering for W and C, are of a particular interest. At the same time, the hydrocarbon molecules will enter into the plasma due to a chemical graphite erosion by H+-ions. This means that carbon co-deposition on W and CFC surfaces will be observed in spite of the below threshold energy of H+-ions. Recently, the problems related to the chemical graphite erosion at hydrogen isotope ion energies lower than the threshold energy of physical sputtering [5–10] were studied. In most of the publications, the production of molecular hydrocarbon during chemical graphite erosion was investigated as a dependence of ion beam intensity and irradiation temperature. In the present work, the samples of W and graphite were exposed in C2H2 plasma to model the co-deposition process in the divertor. The energy of C2H2 ions, bombarding the surface was 300 eV, i.e. the H+-ion energy was lower than the threshold energy for W and C sputtering.
2. Experimental technique The experiments were done at the VITA accelerator which includes, along with an ion source of high-energy ions, a stationary plasma accelerator with closed electron drift and with an extended acceleration zone, developed at the Kurchatov Institute [11]. Samples of tungsten (W-99.96%, Mo-0.04%) and MPG-8 graphite were used in the experiments. The samples were placed in the vacuum chamber of the VITA accelerator, as shown in Fig. 1. For studying the process of carbon co-deposition the C2H2 plasma flow was directed straight to the samples of W or graphite (Fig. 1a). The schematic diagram of the experiment on carbon co-deposition simultaneously with W resputtering onto the targets of tungsten or graphite is shown in Fig. 1b. As seen in Fig. 1b, the C2H2 ion flow simultaneously interacts with the tungsten sample (or MPG-8 graphite sample) and the tungsten target being sputtered is a cone with a vertex angle of 451 surrounding the
Fig. 1. Scheme of sample disposition in a vacuum chamber of VITA-accelerator: (a) co-deposition, (b) co-deposition simultaneously with resputtering.
sample. The C2H2 ion energy is equal to 300 eV. The ion flux is 1021/m2 s. The sample temperature was determined by the plasma flux power on the target and was measured with a thermocouple. In the process of irradiation it reached a constant 670 K due to the heat conduction from the sample holder. The irradiation doses were varied from 1 1023 to 1 1024 m2 (see Table 1). After each plasma simulation, studies on the target surface microstructure were made using a JEOL scanning electron microscope. The chemical composition of the surface layer was studied by Rutherford back scattering (RBS) using a
M.I. Guseva et al. / Vacuum 67 (2002) 253–260
255
Table 1 Irradiation conditions for W and graphite Sample No.
Material of the sample
Material of the target sputtered simultaneously
Irradiation dose (m2)
Duration of irradiation (s)
1 2 3 4 5 6
W W W W MPG-8 graphite MPG-8 graphite
— — — W — W
1 1023 4 1023 1 1024 2 1023 2 1023 2 1023
100 400 1000 200 200 200
Van-de-Graaf accelerator, where the 1.6 MeV He+-ions scattered through an angle of 1701 were registered using a surface barrier detector. The samples were weighed with a microbalance before and after irradiation. The deuterium distribution profiles in the irradiated targets were determined by elastic recoil detection analysis. For this a He+-ion beam with an energy of 2.2 MeV was incident on the sample at an angle of 151 to its surface. The recoil atoms were registered at an angle of 301 to the initial He+-ion direction. In order to produce the absolute values of deuterium atom concentration energy spectra measurements of standard calibration samples were performed.
3. Experimental results 3.1. Study of the co-deposited carbon layers on tungsten and on graphite 3.1.1. Microstructures of the co-deposited carbon layers Some typical surface microphotographs of codeposited layers on graphite (at a dose of 2 1023 m2) and on W at various irradiation doses by C2H2 ions with an energy of 300 eV are given in Fig. 2. At a irradiation dose of 1023 m2 (Sample No. 1) a relatively homogeneous smooth golden-coloured film is observed on the surface. With an increase in the irradiation dose, the growth of some nuclei of a new globular structure (Fig. 2b, Sample No. 2) appear on the surface and develop over the whole W surface under an irradiation dose of 1024 m2 (Fig. 2c, Sample No.
3). The globule size varies from 1.5 to 20 mm. The globular structure on the graphite surface is already produced at a dose of 2 1023 m2 (Fig. 2d). The globule size on the graphite is of the same order. One should note that a similar globular structure on the graphite layer surface was observed in tokamaks TEXTOR [12], DIII-D [13] and in T-10 [14]. Fig. 3 illustrates a typical globular structure of the film surface produced in the T-10 tokamak. This confirms the results of our experiments: carbon film surfaces acquire a globular structure under higher irradiation doses. 3.1.2. Accumulation of hydrogen in the co-deposited carbon layers under C2H2 ion irradiation The hydrogen distribution profiles in the codeposited carbon layers on tungsten for various irradiation doses by C2H2 ions and on MPG-8 graphite for a dose of 2 1023 m2 (curve 3) are given in Fig. 4. The hydrogen concentration at the carbon film surface on tungsten decreases from 23.5 at% for a smaller dose to 10 at% for the maximum dose. As seen from the graphs, for all the irradiation doses the hydrogen is distributed practically uniformly in the carbon layer, 300 nm thick, on tungsten. The concentration of hydrogen (curve 3) in the carbon layer co-deposited from plasma on the MPG-8 graphite surface coincides, practically, with the hydrogen concentration in the carbon layer on the tungsten with the globular structure when developed over the whole irradiated surface (curve 4, Fig. 4). In this case, the concentration on the carbon-containing layer surface is about 1.5 times smaller than that at a depth of 300 nm, where it is close to the hydrogen
256
M.I. Guseva et al. / Vacuum 67 (2002) 253–260
Fig. 2. Typical microphotographs of the co-deposited layer surfaces on tungsten: (a) D ¼ 1023 ; (b) D¼ 4 1023 ; (c) D ¼ 1024 m2 and on MPG-8 graphite: (d) D¼ 2 1023 m2 :
H concentration, 10 28 atoms/m
3
2. 5
1
2. 0
2
1. 5 3
1. 0 4
0. 5
0. 0 0
100
200
300
Depth, nm
Fig. 3. Typical globular structure of the film produced in T-10 tokamak.
Fig. 4. Hydrogen distribution in depth in the co-deposited carbonic layer on tungsten under various irradiation doses by C2H2 ions: (1) D¼ 1 1023 ; (2) D¼ 4 1023 ; (3) D¼ 1 1023 m2 and on MPG-8 graphite: (4) D¼ 2 1023 m2 :
concentration in the globules produced on tungsten at an intermediate irradiation dose (curve 2, Fig. 4). A reduction in the hydrogen concentration towards the surface confirms the fact that the hydro-
gen desorption on the surface rises with the globule production and with an increase of globule density. From the spectra of back-scattered protons from the carbon films on W for various irradiation
M.I. Guseva et al. / Vacuum 67 (2002) 253–260
257
Table 2 Experimental results for the co-deposited carbonic layers on tungsten and on MPG-8 graphite Sample No.
Sample material
D (C2H2/m2)
N ( 1021 H/m2)
N (C/m2)
1 2
W W
1 1023 4 1023
7.2 5.8
6 1022 3 1023
3 4
W MPG-8 graphite
1024 2 1023
3.2 3.8
9 1023
doses 1 1023, 4 1023 and 1 1024 m2, the thickness of the carbon films produced was determined (see Table 2). From Table 2 it follows that the carbon layer thickness and the total carbon content rise with increase in the irradiation dose whereas the total hydrogen content falls. The thickness of the co-deposited carbon films in the dose range (1–10) 1023 m2 rises from 1 to 10 mm. One should emphasize this unexpected result that a reduction of total hydrogen content occurs in the carbon layers on W with dose increase and with an increase of the layer thickness. 3.2. Study of co-deposited mixed (W+C) layers on tungsten and on graphite 3.2.1. Component distribution in the mixed (W+C) layers In the experiments made according to the schematic diagram in Fig. 1b along with the sample irradiation by C2H2 ions, tungsten sputtering by C+-ions took place. In this way the mixed (W+C) layers were produced on surfaces of tungsten and graphite [15]. The distributions of W and C atoms in depth within the mixed layers deposited on W are given in Fig. 5. The distribution is typical for the implantation mixed layer. The mixed layer is about 500 nm thick. The concentration of carbon atoms decreases from 96 at% on the surface to 4 at% at 500 nm and the concentrations of C and W become equal to each other at a depth of 230 nm (50 at% of C-atoms and 50 at% of Watoms). The (W+C) layer on graphite is also 500 nm thick (Fig. 5b). In this case, the concentra-
d (mm)
rs =ro
Structure
1.0 3.3
0.64 0.96
10.0
0.96
Homogeneous Globular and homogeneous Globular Globular
tion of tungsten atoms in the (W+C) layer is reduced from 18 at% in the layer 100 nm thick to o1 at% at a depth of 500 nm. Probably, the distribution typical for the mixed implantation layer, where up to 50 at% for each element is attained at some depth, on tungsten is formed by the mixing of sputtered W atoms in the growing (W+C) layer. 3.2.2. Microstructure of the mixed (W+C) layers The mixed (W+C) layer topography on tungsten (a) and that on graphite (b) is shown in Fig. 6. The mixed layer surface microstructure on tungsten, where 96 at% of C atoms is contained on the surface, is similar to the globular structure produced under co-deposition of carbon layers (Figs. 2b–d). The (W+C) layer microstructure on graphite with 18 at% of W at the surface layer is characterized by the presence of the spiral structures usually produced under ion bombardment of a surface including impurities [16]. One can see that the film has poor adhesion to the graphite substrate. There are some parts, where exfoliation of the film has occurred. 3.2.3. Accumulation of hydrogen in the mixed (W+C) layers The hydrogen distribution profiles in the mixed (W+C) layers on tungsten (curve 1, Sample No. 4) and on graphite (curve 2, Sample No. 6) are given in Fig. 7. The hydrogen distribution profiles in the (W+C) layers on tungsten (curve 1) has a smooth maximum at a depth of 100–200 nm, confirming the fact that the content of hydrogen in it is reduced with increase in tungsten concentration
M.I. Guseva et al. / Vacuum 67 (2002) 253–260
258 100
Concentration, at.%
80
W 60
40
C 20
0 0
250
(a)
500 Depth, nm
750
100
Concentration, at.%
80
C 60
40
W
20
Fig. 6. Surface topography of the mixed (W+C) layers on (a) tungsten, (b) graphite.
0 0
(b)
250
500
750
Depth, nm
Fig. 5. Element distribution in depth within the co-deposited (C+W) layer on tungsten (a) and that within the (C+H+W) layer on graphite (b) under a dose of 2 1023 m2.
(see, Fig. 5) in a range of 200–300 nm. As known, the ratio of H/W in tungsten is smaller than H/C in carbon [17]. The decrease of the hydrogen concentration towards the surface confirms the hydrogen desorption from that layer. The hydrogen distribution within sample No. 6 (curve 2) has the same kind as the distribution in the (W+C) layer on tungsten (curve 1). At the
same time, curve 2 is significantly lower. The maximum of this curve corresponds to a hydrogen concentration of 7 1021 at/cm3, whereas the concentration maximum in the (W+C) layer on tungsten is 1.6 1022 at/cm3. The total hydrogen content in the (W+C) layer on graphite is 4.7 1021 m2 whereas that on tungsten is 2.1 1021 m2. It is evident that such an effect is provided by the different structure (Fig. 6) and by the different composition (Fig. 5) of the films produced on the surfaces of tungsten and graphite. As seen in Fig. 6, the presence of tungsten (up to 18 at% in the surface layer) results in the embrittlement, in bad adhesion and thermal contact of the
M.I. Guseva et al. / Vacuum 67 (2002) 253–260
H concentration,10
28
atoms/m
3
2.5
2.0
1 1.5
1.0
2 0.5
0.0 0
100
200
300
Depth,nm
Fig. 7. Hydrogen distribution in depth in the co-deposited (C+W) layers on tungsten (1) and on graphite (2) under a dose of 2 1023 m2.
film with the substrate, in film temperature growth and, as a consequence, in an increase of the hydrogen desorption rate from the (W+C) film on graphite.
4. Discussion In all the experiments the sample irradiation was by the C2H2 ions with an energy of 300 eV. The kinetic energy of C+- and H+-ions is divided in accordance with the ratio of their masses: C(12/13) and H(1/13). As a result of this, the H+-ion energy in our experiments is equal to 11.54 eV; that of C+-ions is 138.46 eV. It means that the H+-ion energy is lower than the threshold energy for physical sputtering of graphite, tungsten and tungsten oxide. Hence, only chemical sputtering of graphite by hydrogen ions takes place in these experiments. The more energetic C+-ions sputter both graphite and tungsten. The mixed (W+C) layers are produced as a result of chemical sputtering of graphite and physical sputtering of graphite and tungsten. In principle, the process of chemical sputtering for the CFC composite will take place in a gaseous divertor, and the presence of tungsten atoms can be provided by evaporation of tungsten during plasma disruptions.
259
From a comparison of the hydrogen accumulation in the co-deposited carbon layers on tungsten and on graphite with the corresponding film surface microstructure (see, Table 2), one can conclude that the maximum hydrogen concentration in the co-deposited layers is observed in a uniform smooth film on the surface. With the development of a globular structure, the hydrogen concentration in the co-deposited layers on the surfaces of tungsten and graphite is reduced and also the hydrogen concentration decreases towards the surface. This tendency is clearly observed on the MPG-8 graphite, where a difference in the hydrogen concentration in depth (under low irradiation doses) exceeds 1.5-times the hydrogen concentration on the surface. The very fact that the structure of co-deposited films in our experiments under irradiation of W and graphite by low-energy C2H2 ions is similar to that produced in tokamaks confirms the efficiency of modelling the resputtering and co-deposited process in a high-intensity acetylene plasma. One should note that the thickness of the layers deposited on the collector was in the range 40– 300 nm which is comparable with the thickness of the layers produced on the chamber wall after a long exposure (B2000 s) of the graphite tiles in the tokamak plasma [13]. In our experiments we produce layers, 1–10 mm thick in a shorter time (100–1000 s). 5. Conclusions 1. Carbon co-deposited layers with globular structure similar to that obtained in tokamaks on tungsten and on graphite were produced by the co-deposition process from a high-intensity C2H2 plasma with a molecular ion energy of 300 eV. 2. The structure of co-deposited films on tungsten varies from a uniformly smooth one at low irradiation doses (p1 1023 m2) to a globular structure at high dose (1024 m2). The film with a globular structure is already formed on graphite at a dose of 2 1023 m2. 3. The hydrogen content in the co-deposited films is determined by their structure. The total hydrogen content in the co-deposited films is reduced with an increase in the globular
260
M.I. Guseva et al. / Vacuum 67 (2002) 253–260
structure fraction on the tungsten surface from 7.2 1021 to 3.2 1021 m2 for a dose increase from 1 1023 to 1 1024 m2. 4. The globular film production occurs on the smooth film by the appearance of separate small globules and by a gradual increase in their density and size on the surface with an increase in the irradiation dose (or equivalent tokamak run duration). 5. Under simultaneous irradiation of graphite and tungsten by C2H2 ions mixed (W+C) layers, 500 nm thick, were produced on tungsten and graphite. The mixed layers on tungsten and graphite essentially differ from each other in composition, microstructure and hydrogen content. 6. The total hydrogen content in the (W+C) film on graphite is 2.3 times smaller, than the hydrogen content in the mixed layer on tungsten, as a result of desorption due to poor adhesion of the film to the substrate. References [1] Haasz AA, Davis JW, Poon M. First IAEA RCM on ‘‘PMI Data of Mixed Plasma-Facing Materials’’, Vienna, 19–20 October, 1998. p. 1–6. [2] Anderl RA, Pawelko RJ, Schuetz ST. 14 PSI Rosenheim, Germany, 22–26 May, 2000.
[3] Longhust G, Anderl RA, Cadwallader LC, Carmack WJ, Gorman SV, McCarty KA, et al. Fusion Safety Program, Annual Report Fiscal Year, 1998. [4] Mech BV, Haasz AA, Davis JW. J Appl Phys 1998;84: 1655–69. [5] Mech BV, Haasz AA, Davis JW. J Nucl Mater 1998;255: 153–64. [6] Roth J. J Nucl Mater 1999;266–269:51–7. [7] Haasz AA, Mech BV, Davis JW. J Nucl Mater 1996;231:170–2. [8] Roth J, Balden M. EU/JCT Meeting on Plasma–Wall Interaction, Kloster Seeon, December 1997. [9] Grote H, Bohmeyer W, Kornejew R, Reiner HD, Fussmann G, Schlugl R, Weinberg G, Wu CH. J Nucl Mater 1999;266–269:245–51. [10] Grote H, Bohmeyer W, Reiner HD, Fuchs T, Kornejew R, Steinbrink J. J Nucl Mater 1999;241–243:1152–8. [11] Veselovzorov AN, Barkalov EE, Pogorelov AA, Pokrovskii IB, Terentjev AR. Russian Patent N 1367829 15.09.1987. [12] Scholtz T, et al. J Nucl Mater 1997;241–243:848–51. [13] Buzjinskij OI, Opimach IV, Barsuk VA, et al. J Nucl Mater 1996;233–237:787–90. [14] Alimov VH, Goncharov VL, Guseva MI, Kolbasov BN, Romanov PV, Chimtchenko LN. Issledovanie strukturi pereosagdennich v vakuumnoi kamere tokamaka T-10 uglerodnich plenok i soderganie v nich deiterij. VANT Termojadernii sintez 2000;2:45–54. [15] Guseva MI, Korshunov SN, Lazarev NE, Suvorov AL. J Nucl Mater 1999;266:222–7. [16] Rossnagel SM, Robinson RS. J Vac Sci Technol 1982;20:506–11. [17] Haasz AA, Davis JW. J Nucl Mater 1997;241–243: 1076–81.