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C twin limiter in TEXTOR-94
Fusion Engineering and Design 49 – 50 (2000) 355 – 362 www.elsevier.com/locate/fusengdes
Material mixing on W/C twin limiter in TEXTOR-94 T. Tanabe a,*, T. Ohgo b, M. Wada c, M. Rubel d, V. Philipps e, J. von Seggern e, K. Ohya f, A. Huber e, A. Pospieszczyk e, B. Schweer e, TEXTOR team e a
CIRSE, Nagoya Uni6ersity, Nagoya 464 -8603, Japan Department of Physics, Fukuoka Uni6ersity of Education, Fukuoka 811 -4192, Japan c Department of Electronics, Doshisha Uni6ersity, Kyotanabe, Kyoto 610 -0321, Japan d Physics Department Frescati, Royal Institute of Technology, S-100 44 Stockholm, Sweden e Institute of Plasma Physics, Juelich Reserach Center, Juelich D-52425, Germany f Department of Electrical Engineering and Electronics, Tokushima Uni6ersity, Tokushima 770 -8506, Japan b
Abstract In order to investigate the effect of mutual contamination between tungsten (W) and carbon (C) and its influence on the plasma, a W–C twin test limiter, half made of W and the other half of C, was inserted into the edge plasma of TEXTOR-94 under ohmic and NBI heating conditions. The contamination process was observed by spectroscopy, and the intensity distribution of WI showed migration of W onto the C side by the successive cycles of sputtering and prompt redeposition. On the other hand, the deposition of C on the W surface was not obvious. Most of the hydrogen (deuterium) on the limiter was found to be retained in the deposited layers and that in the deposited C layer much higher than that in the deposited W layer. This indicates that tritium retention is smaller in metallic deposits above 500 K. The AES analysis conducted after the exposure of the test limiter showed that W deposited on C reacted with the substrate to form carbides at higher temperatures. The thickness of carbide layer, and/or the content of W in C were influenced by the temperature and flux distributions, and no carbide layer was formed at the limiter edge where the temperature was relatively low. © 2000 Elsevier Science B.V. All rights reserved. Keywords: TEXTOR; Tungsten; Carbon
1. Introduction In the present international thermonuclear reactor (ITER) divertor design W and C will be simultaneously used as plasma facing materials * Corresponding author. Tel.: + 81-52-7895177/5200; fax: + 81-52-7895177/3791. E-mail address: [email protected] (T. Tanabe).
(PFM) [1]. Although, the recent extensive studies on high Z materials in TEXTOR, ASDEX and Alcator C-mod have shown that the plasma collapse due to high Z accumulation by self sputtering is not serious [2,3], a substantial amount of high Z materials is found to be released by sputtering due to low Z impurities, i.e. carbon and oxygen, present in the boundary plasma. The released high Z impurities are transported in the
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edge plasma to be deposited on the surface of other parts of the PFM. At the same time, low Z impurities are deposited on high Z materials and modify their surface characteristics. Such material mixing has been observed in all tokamaks. However, due to the lack of proper diagnostics, the influence of the material mixing on the materials themselves and that on the plasmas has not been systematically investigated. In order to investigate the effect of mutual contamination between tungsten (W) and carbon (C) and its influence on the plasma, we have conducted a W – C twin test limiter experiment in TEXTOR. The W – C twin limiter, half made of W and the other half of C, was inserted into the edge plasma of TEXTOR-94 under ohmic and NBI heating conditions. The contamination process under the plasma exposure was monitored by optical spectroscopy, and the limiter surface was investigated by various surface analysis techniques after the exposure.
Fig. 1. Experimental setup.
2. Experimental setup The experimental setup is schematically shown in Fig. 1. The test limter was inserted from the top into TEXTOR-94 edge plasma through a limiterlock manifold. The limiter was 12 and 8 cm long in toroidal and poloidal directions, respectively. The spherical face of the limiter in contact with the plasma has the radius of 7 cm. One half of the limiter was made of W and the other half was made of graphite (EK-98). The entire limiter can be rotated so as to change the tungsten and graphite faces to either the ion-drift-side or the electron-drift-side. TEXTOR-94 plasma was operated with 360 kA plasma current, 2.25 T toroidal magnetic field, and 6 s discharge duration. Neutral beam (NBI) heating with the power of 1.4 MW was applied to the plasma for 2 s. Plasma main radius was determined by ALT-II toroidal belt-limiter made of graphite to be 46 cm. The contamination process was observed by spectroscopy, i.e. by monitoring line spectra of W and C neutral species from both halves of the test limiter. Radial distributions of spectra line intensities emitted from ions and neutrals around the test limiter were measured by an image intensified CCD-camera coupled to a monochromator. The spectra were recorded in the wavelength range from 409–435 nm where we observed CII (426.7 nm), WI (429.5 nm), Dg (434.0 nm) and OII (434.6 nm). The two-dimensional intensity distribution of WI and Da emissions were observed from the direction tangential to the limiter surface with another CCD-camera through interference filters at 400.8 nm for W, and 656.3 nm for Dg with 1.5 nm band width. The third CCD-camera viewed the limter surface from the bottom through an infrared transmission (850–1100 nm) filter to construct the temperature distribution on the limter surface. Thermocouples were inserted into drilled holes in both halves of the limiter at a location 1 cm from the center and 0.7 cm beneath the surface. After the plasma exposure the contaminated limiter surface was analyzed by various techniques including RBS, NRA, SIMS, AES. Details of the surface analysis is given elsewhere [4]
T. Tanabe et al. / Fusion Engineering and Design 49–50 (2000) 355–362
Fig. 2. WI line intensity distribution in front of the twin limter for shots with the different cumulative numbers of discharges.
Fig. 3. Photograph of the twin limiter after plasma exposure.
Fig. 4. Comparison of deposited heat between the C and W sides.
3. Results and discussion
3.1. Spectroscopic measurements Fig. 2a and b show the WI line intensity distributions in front of the twin limiter for shots with the different cumulative numbers of discharges. The figure clearly indicates that W neutrals pene-
357
trated into the plasma beyond the scrape-offlayer. Comparison between the two figures gives an insight of how the W neutrals diffused towards the carbon side in accordance with the increase in the number of discharges. Since a substantial amount of carbon flux from the ALT-II limiter is present in the TEXTOR plasma, carbon ions and neutrals strike the surface of the W side of the limiter and sputter W. The migration of WI lines into the carbon side of the limiter did not occur homogeneously, but the distribution of WI lines had gradually extended to the carbon side along the toroidal direction. This observation indicates that the successive cycle of prompt redeposition [5,6] and sputtering is the dominant contamination (or transport) process. On the other hand, contamination of the W side by C was not clear. The carbon flux to the test limiter surface from the plasma was homogeneous as the main source of carbon into the plasma is the ALT-II limiter surrounding the entire torus of TEXTOR. Meanwhile, C produced on the C-side can be redeposited on the adjacent W surface with relatively small energy as W are deposited on C. However, redeposited C is easily removed by physical sputtering, and as a consequence, the top region of W surface is left metallic as seen in Fig. 3. Namely, the top region of the test limiter is erosion dominated against the carbon flux unlike the case of W deposition on C which is caused by the smaller physical and chemical sputtering yields of W on C. Thus, the contamination on the smaller mass materials is more serious than that on the larger mass materials. Higher reflection of C from a high Z surface rather than a low Z one might also have some contribution to the smaller deposition. One can see the clear difference in the absorbed (deposited) heat flux between the C side and the W side as shown in Fig. 4. Although the absorbed heat flux in the C side stayed relatively constant with increasing line-averaged density, that in W showed steeper decrease. Such density dependence of the absorbed heat on the density is very likely governed by the reflection phenomena. Due to the much smaller mass of C than W, the effect due to the C deposition over W upon hydrogen reflection is rather small, whereas W deposition on C signifi-
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cantly enhances the hydrogen reflection. As a result, the difference of the heat depositions between the C and W sides are not quite different compared with the difference in the energy reflection coefficients between pure C (0.15) and W (0.6) [7]. In the standard TEXTOR operation, when the electron density is higher, the electron temperature is lower. The reflection coefficient of W increases with decreasing impact energy, which means the decreasing electron temperature. Accordingly, the deposited heat on the W side decreases with increasing density. On the other hand, the heat load to the C side was not severely influenced by the reflection owing to its small reflection, and showed little dependence upon density. Difference in the reflection is also seen in the Da intensity distribution in front of the limiter as shown in Fig. 5. Although the Da intensity was higher on the C side, the penetration length (efolding length) of Da was larger at the W side. Two reasons are possible, (1) the energy of
reflected D at the W side is much higher than that at the C side, whereas (2) reemission as low energy D2 and/or CD dominates the C side, which is easily excited to give Da emission near the surface. Again the deposited C on W has a reduced role in reflection as long as the deposited layer remains thinner than the range of the injected ions.
3.2. Surface analysis As clearly seen in Fig. 3, most of the W side is shiny except the very edge where C deposition occurs. Fig. 6 shows deposition profiles of C, W and D of the whole limiter along the center in the toroidal direction measured by an ion surface analysis technique [4]. The shiny part of the W side had more or less no deposition. There are two possible reasons for such small deposition of C on the W side. Those are (1) high reflection of low Z incident on high Z target and (2) efficient momentum transfer from underlying high Z atom
Fig. 5. Comparison of Da intensity decay in front of the limiter between the C and W side. The limiter is at 45.5 cm.
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Fig. 6. Deposition profiles of C, W and D on the twin limiter [4].
to surface low Z atom during the process of collision cascade in sputtering [8], i.e. sputter enhancement. Nevertheless, the C deposition at the very edge of the W side, which appears in Fig. 3 shows a very sharp boundary with the width of only 2 mm. Such clear boundary formation between the erosion dominated and the deposition dominated regions is often observed in walls of tokamaks, and is explained to be caused by the ion temperature difference [9]. Deuterium content in the shinny part was below the detection limit. The main reason could be the high reflection of W. But there are another possibilities like deuterium penetration deep into W metal or rapid reemission due to high temperature (during the shot, average temperature of the limiter was over 800 K). The deuterium content in the deposited part of the C side is roughly proportional to the C amount, probably because the deuterium was codeposited with carbon. On the other hand, the deuterium content in the C side coincides to the surface temperature, and is the highest at the edge mainly due to the codeposition and the smallest in the highest temperature region. The top tip of the limiter, where the temperature is not the highest, shows low concentration probably due to high erosion. JET reported that the Be surface retained similar level of hydrogen as graphite [10]. On the other hand, the present results indicate that if W
PFMs are used at higher temperatures, hydrogen retention is substantially smaller than that of C even with the layer of C deposition. W deposition profile on the C side confirms the spreading of W by the successive cycling of prompt redeposition and sputtering as indicated in Section 3.1. It is interesting to note that the Si profile is very similar to W (TEXTOR occasionally employs siliconization and boronization, boron (B) was not detected either on W or C by RBS, but AES showed B deposition as described below). However, this does not necessarily mean that the deposition process of Si is similar to that of W. Depth profiles of W deposited on the C side are measured by a sputter AES analysis. In the AES analysis, the deposition of B is observed and B KLL AES peak overlaps over some of the W AES peaks. Even so we could make a quantitative analysis with an error of less than around 30%. The obtained surface concentrations at different positions coincide well with the RBS results. Fig. 7 shows several cases of such depth profiles. At the positions of 1 and 7 mm, apart from the center (the top) of the limiter in the toroidal direction, surface W concentration was roughly 1.0 in W/C ratio, maintaining the same ratio till the depth of about 90 nm. In addition, the C KLL Auger spectra showed a carbide form, which indicates the existence of tungsten carbide (WC) on the surface. When the depth profiles at the posi-
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Fig. 7. Depth profiles of deposited W on the C side given by AES analysis. The sputtering rate of W/C layer was estimated to be around 4.5 nm/min. However, it might include large error (less than 30%) because of large uncertainty in sputtering yield of W/C mixed layer.
tions of 1 and 7 mm (Fig. 7a and b) are compared, the WC layer at the former position with a thickness of about 90 nm was followed by a thick
layer having nearly constant W concentration of about 20%. As the result, the total amount of the deposited W was the highest at this position. The
T. Tanabe et al. / Fusion Engineering and Design 49–50 (2000) 355–362
thickness of the surface WC layer at 7 mm position is nearly the same, but the W concentration continuously decreased to zero without showing the second layer that appeared at 1 mm. At 13.5 mm no WC layer was observed, but 20% W region continued from the surface to a depth of more than 300 nm. Such W profiles correspond well to the surface temperature distributions, where the surface temperature at the position of 7 – 13 mm was the highest. Accordingly, deposited W tends to diffuse deeper. Only when sufficient W flux is supplied compared with the diffusion flux, WC is formed. Otherwise either carbides containing less W such as WC2 (g phase only stable at above 2800 K [11]) is formed, or the W concentration decays continuously into deeper site from the surface. In the central gap of the twin limiter between C and W, a significant amount of redeposited material containing both C and W was observed of which detail will be given elsewhere [4]. The reason is not clear. However, such deposition in plasma shadow is very important from the viewpoint of safety issues, because they can contain a substantial amount of hydrogen (tritium).
3.
4.
5.
4. Conclusions The W/C twin limiter experiments are found to give very useful information on how the lowZ and high-Z materials behave for the case of simultaneous utilization as PFM under the cross-contamination. Results are summarized as follows. 1. Spectroscopic measurements in front of the twin limiter clearly show that the WI line emission, which was initially seen only in front of the W side, very rapidly developed to the C side shot by shot. This is attributed to the successive cycling of prompt redeposition and sputtering. 2. On the other hand C contamination on the W side was not appreciable owing to large sputtering. Correspondingly most part of the W side remained shiny indicating very small deposition, except the very edge of the lim-
6.
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iter where C deposition was seen with a very sharp boundary indicting a transition from erosion dominated region to deposition dominated one in a short distance. Due to the much smaller mass of C than W, the effect of deposited C on W upon hydrogen reflection is small, but W on C enhances the reflection. As the result, the difference of the heat deposition between the C and W sides was not as large as that expected from the difference in the energy reflection coefficients of pure C and W. On the other hand, sputtering of the deposited C on W is very likely enhanced due to the efficient momentum transfer from a W substrate. Thus, the contamination on the smaller mass materials is more serious than that on the larger mass materials. Most of the hydrogen (deuterium) on the limiter was found to be retained on the deposited layers and that in deposited C layer was much higher than that in the deposited W layer. This indicates smaller tritium retention in metallic deposits above 500 K. The AES analysis showed the deposited W reacted with substrate C to form carbides at higher temperatures. And the thickness of W and/or penetration of W are determined by the temperature and the incident flux. The C deposited at the limiter edge where the temperature was not high enough did not make carbide. In the gap at the center (between the two sides), both C and W depositions have been observed, for which mechanism is not clear yet. Such deposition in the plasma shadow is a very important safety issue, because the deposits can retain high concentrations of tritium.
Acknowledgements This work was done under IEA contract for TEXTOR cooperation and was partly supported by a Grant-in-aid for scientific research of Education, Science and Culture, Japan.
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References [1] R. Aymar, Present status and future prospect of the ITER project, J. Nucl. Mater. 258-263 (1998) 56 – 64. [2] T. Tanabe, M. Akiba, Y. Ueda, et al., On the utilization of high Z materials as plasma facing component, Fusion Eng. Des. 39-40 (1998) 275–285. [3] N. Noda, V. Philipps, R. Neu, A review of recent experiments on W and high Z materials as plasma-facing components in magnetic fusion devices, J. Nucl. Mater. 241-243 (1997) 227–243. [4] M. Rubel, V. Philipps, A. Huber, T. Tanabe, et al., Formation of carbon containing layers on tungsten limiters, Phys. Scripta T81 (1999) 61–63. [5] D. Naujoks, J. Roth, K. Krieger, et al., Erosion and redeposition in ASDEX upgrade divertor, J. Nucl. Mater. 210 (1994) 43–50.
[6] Y. Ueda, T. Tanabe, V. Philipps, et al., Effects of impurities released from high Z test limiter on plasma performance in TEXTOR, J. Nucl. Mater. 220-222 (1995) 224. [7] R. Ito, T. Tabata, N. Itoh, et al., Data on the backscattering coefficients of light ions from solids, IPPJ-AM-41 (1985). [8] K. Ohya, T. Tanabe, M. Wada, et al., Simulation and experimental studies of impurity release from tungsten exposed edge plasma in TEXTOR-94, Nucl. Instr. Methods Phys. Res. B153 (1999) 354 – 360. [9] V. Philipps, A. Pospieszczyk, H.G. Esser, et al., Impurity release and deposition process close to limiter surfaces in TEXTOR-94, J. Nucl. Mater. 241-243 (1997) 105 – 117. [10] R. Behrisch, A.P. Martinelli, S.G. Grigull, et al., Surface layer composition of the JET vessel walls, J. Nucl. Mater. 220-222 (1995) 590 – 594. [11] T.B. Massalski, et al., Binary alloy phase diagrams, Am. Soc. Metals, Ohio, 1986.
Material mixing on W/C twin limiter in TEXTOR-94 T. Tanabe a,*, T. Ohgo b, M. Wada c, M. Rubel d, V. Philipps e, J. von Seggern e, K. Ohya f, A. Huber e, A. Pospieszczyk e, B. Schweer e, TEXTOR team e a
CIRSE, Nagoya Uni6ersity, Nagoya 464 -8603, Japan Department of Physics, Fukuoka Uni6ersity of Education, Fukuoka 811 -4192, Japan c Department of Electronics, Doshisha Uni6ersity, Kyotanabe, Kyoto 610 -0321, Japan d Physics Department Frescati, Royal Institute of Technology, S-100 44 Stockholm, Sweden e Institute of Plasma Physics, Juelich Reserach Center, Juelich D-52425, Germany f Department of Electrical Engineering and Electronics, Tokushima Uni6ersity, Tokushima 770 -8506, Japan b
Abstract In order to investigate the effect of mutual contamination between tungsten (W) and carbon (C) and its influence on the plasma, a W–C twin test limiter, half made of W and the other half of C, was inserted into the edge plasma of TEXTOR-94 under ohmic and NBI heating conditions. The contamination process was observed by spectroscopy, and the intensity distribution of WI showed migration of W onto the C side by the successive cycles of sputtering and prompt redeposition. On the other hand, the deposition of C on the W surface was not obvious. Most of the hydrogen (deuterium) on the limiter was found to be retained in the deposited layers and that in the deposited C layer much higher than that in the deposited W layer. This indicates that tritium retention is smaller in metallic deposits above 500 K. The AES analysis conducted after the exposure of the test limiter showed that W deposited on C reacted with the substrate to form carbides at higher temperatures. The thickness of carbide layer, and/or the content of W in C were influenced by the temperature and flux distributions, and no carbide layer was formed at the limiter edge where the temperature was relatively low. © 2000 Elsevier Science B.V. All rights reserved. Keywords: TEXTOR; Tungsten; Carbon
1. Introduction In the present international thermonuclear reactor (ITER) divertor design W and C will be simultaneously used as plasma facing materials * Corresponding author. Tel.: + 81-52-7895177/5200; fax: + 81-52-7895177/3791. E-mail address: [email protected] (T. Tanabe).
(PFM) [1]. Although, the recent extensive studies on high Z materials in TEXTOR, ASDEX and Alcator C-mod have shown that the plasma collapse due to high Z accumulation by self sputtering is not serious [2,3], a substantial amount of high Z materials is found to be released by sputtering due to low Z impurities, i.e. carbon and oxygen, present in the boundary plasma. The released high Z impurities are transported in the
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edge plasma to be deposited on the surface of other parts of the PFM. At the same time, low Z impurities are deposited on high Z materials and modify their surface characteristics. Such material mixing has been observed in all tokamaks. However, due to the lack of proper diagnostics, the influence of the material mixing on the materials themselves and that on the plasmas has not been systematically investigated. In order to investigate the effect of mutual contamination between tungsten (W) and carbon (C) and its influence on the plasma, we have conducted a W – C twin test limiter experiment in TEXTOR. The W – C twin limiter, half made of W and the other half of C, was inserted into the edge plasma of TEXTOR-94 under ohmic and NBI heating conditions. The contamination process under the plasma exposure was monitored by optical spectroscopy, and the limiter surface was investigated by various surface analysis techniques after the exposure.
Fig. 1. Experimental setup.
2. Experimental setup The experimental setup is schematically shown in Fig. 1. The test limter was inserted from the top into TEXTOR-94 edge plasma through a limiterlock manifold. The limiter was 12 and 8 cm long in toroidal and poloidal directions, respectively. The spherical face of the limiter in contact with the plasma has the radius of 7 cm. One half of the limiter was made of W and the other half was made of graphite (EK-98). The entire limiter can be rotated so as to change the tungsten and graphite faces to either the ion-drift-side or the electron-drift-side. TEXTOR-94 plasma was operated with 360 kA plasma current, 2.25 T toroidal magnetic field, and 6 s discharge duration. Neutral beam (NBI) heating with the power of 1.4 MW was applied to the plasma for 2 s. Plasma main radius was determined by ALT-II toroidal belt-limiter made of graphite to be 46 cm. The contamination process was observed by spectroscopy, i.e. by monitoring line spectra of W and C neutral species from both halves of the test limiter. Radial distributions of spectra line intensities emitted from ions and neutrals around the test limiter were measured by an image intensified CCD-camera coupled to a monochromator. The spectra were recorded in the wavelength range from 409–435 nm where we observed CII (426.7 nm), WI (429.5 nm), Dg (434.0 nm) and OII (434.6 nm). The two-dimensional intensity distribution of WI and Da emissions were observed from the direction tangential to the limiter surface with another CCD-camera through interference filters at 400.8 nm for W, and 656.3 nm for Dg with 1.5 nm band width. The third CCD-camera viewed the limter surface from the bottom through an infrared transmission (850–1100 nm) filter to construct the temperature distribution on the limter surface. Thermocouples were inserted into drilled holes in both halves of the limiter at a location 1 cm from the center and 0.7 cm beneath the surface. After the plasma exposure the contaminated limiter surface was analyzed by various techniques including RBS, NRA, SIMS, AES. Details of the surface analysis is given elsewhere [4]
T. Tanabe et al. / Fusion Engineering and Design 49–50 (2000) 355–362
Fig. 2. WI line intensity distribution in front of the twin limter for shots with the different cumulative numbers of discharges.
Fig. 3. Photograph of the twin limiter after plasma exposure.
Fig. 4. Comparison of deposited heat between the C and W sides.
3. Results and discussion
3.1. Spectroscopic measurements Fig. 2a and b show the WI line intensity distributions in front of the twin limiter for shots with the different cumulative numbers of discharges. The figure clearly indicates that W neutrals pene-
357
trated into the plasma beyond the scrape-offlayer. Comparison between the two figures gives an insight of how the W neutrals diffused towards the carbon side in accordance with the increase in the number of discharges. Since a substantial amount of carbon flux from the ALT-II limiter is present in the TEXTOR plasma, carbon ions and neutrals strike the surface of the W side of the limiter and sputter W. The migration of WI lines into the carbon side of the limiter did not occur homogeneously, but the distribution of WI lines had gradually extended to the carbon side along the toroidal direction. This observation indicates that the successive cycle of prompt redeposition [5,6] and sputtering is the dominant contamination (or transport) process. On the other hand, contamination of the W side by C was not clear. The carbon flux to the test limiter surface from the plasma was homogeneous as the main source of carbon into the plasma is the ALT-II limiter surrounding the entire torus of TEXTOR. Meanwhile, C produced on the C-side can be redeposited on the adjacent W surface with relatively small energy as W are deposited on C. However, redeposited C is easily removed by physical sputtering, and as a consequence, the top region of W surface is left metallic as seen in Fig. 3. Namely, the top region of the test limiter is erosion dominated against the carbon flux unlike the case of W deposition on C which is caused by the smaller physical and chemical sputtering yields of W on C. Thus, the contamination on the smaller mass materials is more serious than that on the larger mass materials. Higher reflection of C from a high Z surface rather than a low Z one might also have some contribution to the smaller deposition. One can see the clear difference in the absorbed (deposited) heat flux between the C side and the W side as shown in Fig. 4. Although the absorbed heat flux in the C side stayed relatively constant with increasing line-averaged density, that in W showed steeper decrease. Such density dependence of the absorbed heat on the density is very likely governed by the reflection phenomena. Due to the much smaller mass of C than W, the effect due to the C deposition over W upon hydrogen reflection is rather small, whereas W deposition on C signifi-
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cantly enhances the hydrogen reflection. As a result, the difference of the heat depositions between the C and W sides are not quite different compared with the difference in the energy reflection coefficients between pure C (0.15) and W (0.6) [7]. In the standard TEXTOR operation, when the electron density is higher, the electron temperature is lower. The reflection coefficient of W increases with decreasing impact energy, which means the decreasing electron temperature. Accordingly, the deposited heat on the W side decreases with increasing density. On the other hand, the heat load to the C side was not severely influenced by the reflection owing to its small reflection, and showed little dependence upon density. Difference in the reflection is also seen in the Da intensity distribution in front of the limiter as shown in Fig. 5. Although the Da intensity was higher on the C side, the penetration length (efolding length) of Da was larger at the W side. Two reasons are possible, (1) the energy of
reflected D at the W side is much higher than that at the C side, whereas (2) reemission as low energy D2 and/or CD dominates the C side, which is easily excited to give Da emission near the surface. Again the deposited C on W has a reduced role in reflection as long as the deposited layer remains thinner than the range of the injected ions.
3.2. Surface analysis As clearly seen in Fig. 3, most of the W side is shiny except the very edge where C deposition occurs. Fig. 6 shows deposition profiles of C, W and D of the whole limiter along the center in the toroidal direction measured by an ion surface analysis technique [4]. The shiny part of the W side had more or less no deposition. There are two possible reasons for such small deposition of C on the W side. Those are (1) high reflection of low Z incident on high Z target and (2) efficient momentum transfer from underlying high Z atom
Fig. 5. Comparison of Da intensity decay in front of the limiter between the C and W side. The limiter is at 45.5 cm.
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Fig. 6. Deposition profiles of C, W and D on the twin limiter [4].
to surface low Z atom during the process of collision cascade in sputtering [8], i.e. sputter enhancement. Nevertheless, the C deposition at the very edge of the W side, which appears in Fig. 3 shows a very sharp boundary with the width of only 2 mm. Such clear boundary formation between the erosion dominated and the deposition dominated regions is often observed in walls of tokamaks, and is explained to be caused by the ion temperature difference [9]. Deuterium content in the shinny part was below the detection limit. The main reason could be the high reflection of W. But there are another possibilities like deuterium penetration deep into W metal or rapid reemission due to high temperature (during the shot, average temperature of the limiter was over 800 K). The deuterium content in the deposited part of the C side is roughly proportional to the C amount, probably because the deuterium was codeposited with carbon. On the other hand, the deuterium content in the C side coincides to the surface temperature, and is the highest at the edge mainly due to the codeposition and the smallest in the highest temperature region. The top tip of the limiter, where the temperature is not the highest, shows low concentration probably due to high erosion. JET reported that the Be surface retained similar level of hydrogen as graphite [10]. On the other hand, the present results indicate that if W
PFMs are used at higher temperatures, hydrogen retention is substantially smaller than that of C even with the layer of C deposition. W deposition profile on the C side confirms the spreading of W by the successive cycling of prompt redeposition and sputtering as indicated in Section 3.1. It is interesting to note that the Si profile is very similar to W (TEXTOR occasionally employs siliconization and boronization, boron (B) was not detected either on W or C by RBS, but AES showed B deposition as described below). However, this does not necessarily mean that the deposition process of Si is similar to that of W. Depth profiles of W deposited on the C side are measured by a sputter AES analysis. In the AES analysis, the deposition of B is observed and B KLL AES peak overlaps over some of the W AES peaks. Even so we could make a quantitative analysis with an error of less than around 30%. The obtained surface concentrations at different positions coincide well with the RBS results. Fig. 7 shows several cases of such depth profiles. At the positions of 1 and 7 mm, apart from the center (the top) of the limiter in the toroidal direction, surface W concentration was roughly 1.0 in W/C ratio, maintaining the same ratio till the depth of about 90 nm. In addition, the C KLL Auger spectra showed a carbide form, which indicates the existence of tungsten carbide (WC) on the surface. When the depth profiles at the posi-
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Fig. 7. Depth profiles of deposited W on the C side given by AES analysis. The sputtering rate of W/C layer was estimated to be around 4.5 nm/min. However, it might include large error (less than 30%) because of large uncertainty in sputtering yield of W/C mixed layer.
tions of 1 and 7 mm (Fig. 7a and b) are compared, the WC layer at the former position with a thickness of about 90 nm was followed by a thick
layer having nearly constant W concentration of about 20%. As the result, the total amount of the deposited W was the highest at this position. The
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thickness of the surface WC layer at 7 mm position is nearly the same, but the W concentration continuously decreased to zero without showing the second layer that appeared at 1 mm. At 13.5 mm no WC layer was observed, but 20% W region continued from the surface to a depth of more than 300 nm. Such W profiles correspond well to the surface temperature distributions, where the surface temperature at the position of 7 – 13 mm was the highest. Accordingly, deposited W tends to diffuse deeper. Only when sufficient W flux is supplied compared with the diffusion flux, WC is formed. Otherwise either carbides containing less W such as WC2 (g phase only stable at above 2800 K [11]) is formed, or the W concentration decays continuously into deeper site from the surface. In the central gap of the twin limiter between C and W, a significant amount of redeposited material containing both C and W was observed of which detail will be given elsewhere [4]. The reason is not clear. However, such deposition in plasma shadow is very important from the viewpoint of safety issues, because they can contain a substantial amount of hydrogen (tritium).
3.
4.
5.
4. Conclusions The W/C twin limiter experiments are found to give very useful information on how the lowZ and high-Z materials behave for the case of simultaneous utilization as PFM under the cross-contamination. Results are summarized as follows. 1. Spectroscopic measurements in front of the twin limiter clearly show that the WI line emission, which was initially seen only in front of the W side, very rapidly developed to the C side shot by shot. This is attributed to the successive cycling of prompt redeposition and sputtering. 2. On the other hand C contamination on the W side was not appreciable owing to large sputtering. Correspondingly most part of the W side remained shiny indicating very small deposition, except the very edge of the lim-
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iter where C deposition was seen with a very sharp boundary indicting a transition from erosion dominated region to deposition dominated one in a short distance. Due to the much smaller mass of C than W, the effect of deposited C on W upon hydrogen reflection is small, but W on C enhances the reflection. As the result, the difference of the heat deposition between the C and W sides was not as large as that expected from the difference in the energy reflection coefficients of pure C and W. On the other hand, sputtering of the deposited C on W is very likely enhanced due to the efficient momentum transfer from a W substrate. Thus, the contamination on the smaller mass materials is more serious than that on the larger mass materials. Most of the hydrogen (deuterium) on the limiter was found to be retained on the deposited layers and that in deposited C layer was much higher than that in the deposited W layer. This indicates smaller tritium retention in metallic deposits above 500 K. The AES analysis showed the deposited W reacted with substrate C to form carbides at higher temperatures. And the thickness of W and/or penetration of W are determined by the temperature and the incident flux. The C deposited at the limiter edge where the temperature was not high enough did not make carbide. In the gap at the center (between the two sides), both C and W depositions have been observed, for which mechanism is not clear yet. Such deposition in the plasma shadow is a very important safety issue, because the deposits can retain high concentrations of tritium.
Acknowledgements This work was done under IEA contract for TEXTOR cooperation and was partly supported by a Grant-in-aid for scientific research of Education, Science and Culture, Japan.
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