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Helium and hydrogen trapping in tungsten deposition layers formed by helium plasma sputtering
Fusion Engineering and Design 82 (2007) 1645–1650
Helium and hydrogen trapping in tungsten deposition layers formed by helium plasma sputtering K. Katayama ∗ , K. Imaoka, T. Okamura, M. Nishikawa Department of Advanced Energy Engineering Science, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan Received 31 July 2006; received in revised form 13 April 2007; accepted 13 April 2007 Available online 12 June 2007
Abstract Tungsten deposition layers were formed by helium plasma sputtering utilizing a capacitively coupled RF plasma. For comparison, hydrogen plasma was also used for the formation of the deposition layer. It was found that non-negligible amount of helium and hydrogen were trapped in the tungsten deposition layer formed helium plasma sputtering. It is considered that the hydrogen emitted from the plasma chamber wall by helium plasma discharge was trapped in the layer. Atomic ratio of helium to tungsten (He/W) in the layer was estimated to be 0.08. This value is not quite small compared with that of hydrogen in the tungsten deposition layer formed by hydrogen plasma sputtering. The release behavior of helium from the layer formed by helium plasma sputtering was similar to that of hydrogen from the layer formed by hydrogen plasma sputtering. © 2007 Elsevier B.V. All rights reserved. Keywords: Plasma facing material; Tungsten deposition layer; Helium retention
1. Introduction Understanding of tritium behavior in plasma facing materials is an important issue for design of a fusion reactor from viewpoints of fuel control and radiation safety. Tungsten and graphite material are used as plasma facing materials in the divertor region of ITER [1]. It is well known that a carbon deposition layer can trap a large amount of tritium [2,3]. Therefore, ∗ Corresponding author. Tel.: +81 92 642 3785; fax: +81 92 642 3784. E-mail address: [email protected] (K. Katayama).
0920-3796/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2007.04.026
many studies on tritium behavior in carbon deposition layers have been done so far [4–9]. On the other hand, the investigation of hydrogen isotope behavior in a tungsten deposition layer has not been performed sufficiently. The one reason is that a sputtering yield of tungsten by hydrogen isotope is estimated to be quite lower than that of carbon. That is to say, the deposition rate of tungsten on plasma-facing surfaces in a fusion reactor is considered to be small [10,11]. The other reason is a quit low solubility of hydrogen into tungsten bulk [12]. However, it was observed in recent study by the present authors that a larger amount of hydrogen than expected from solubility of
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tungsten bulk is released from the tungsten deposition layer formed by hydrogen plasma sputtering [13]. The mechanism of hydrogen trapping in the tungsten deposition layer has not been clarified yet. The averaged erosion/deposition rate of tungsten in ITER is considered to be low because the ion energy in the divertor region is small. However, it is important to evaluate tritium behavior in tungsten deposition layers considering a long-term plasma operation. Thus, it is necessary to obtain more data for the formation of deposition layer and the trapping phenomena of environmental gas. In a plasma vessel of a fusion reactor, not only hydrogen isotope but also the helium generated by the nuclear reaction exists. Some studies on helium implantation into plasma-facing materials have been carried out so far [14–17]. They have pointed out that the implanted helium affects the release behavior of hydrogen isotopes. Therefore, we need to evaluate an influence of helium on the trapping and release behavior of hydrogen isotopes. In this study, tungsten deposition layers were formed by sputtering method using helium RF plasma. An erosion rate and a deposition rate of tungsten were estimated by weight measurement. The release behaviors of helium and hydrogen were observed by temperatureprogrammed desorption (TPD) experiment.
Table 1 Deposition conditions He (◦ C)
Gas temperature Total gas pressure (Pa) Gas flow rate (cm3 /min) RF power (W) Target-substrate distance (cm) Discharge period (h)
H2
125
154 10 1.2 200 11 120
2. Experimental
as a target material. The energy of the incident particle to the target is estimated to be about 2 keV from DC self bias on the target [13]. Tungsten chips (20 mm × 5 mm, 0.2 mm in thickness, 99.5% in purity, Nilaco Co.) were put on the ground electrode as substrates. The deposition conditions are shown in Table 1. The erosion rate was derived from the weight change of the target plate before and after plasma discharge. The deposition rate was derived from the weight change of the substrates. Weight measurements were performed by an electric balance with a sensitively of 0.01 mg. The surface observation of tungsten deposition layer was performed by the scanning electron microscopy (SEM: SS-550, SHIMAZU Co.). Atomic concentration (at.%) on the deposition layer was analyzed by an energy dispersive X-ray (EDX: Genesis2000, EDAX Inc.) equipment. The SEM and EDX used in this study were installed at the Center of Advanced Instrumental Analysis, Kyushu University.
2.1. Formation of tungsten deposition layer
2.2. Temperature-programmed desorption
Tungsten deposition layers were formed by utilizing a capacitively coupled helium RF plasma. For comparison, hydrogen plasma was also used for the formation of the deposition layer. The RF plasma apparatus and the experimental procedure have been mentioned in previous paper [13]. The plasma chamber has two parallel-plate electrodes. One electrode is connected with the plasma chamber at ground potential. This electrode is called a ground electrode. When an RF power is applied to the other electrode, which is called an RF electrode, plasma is generated between the electrodes. The electrode plate to which an RF power is impressed is eroded and the sputtered particles are deposited on the inner surfaces of the plasma chamber. A tungsten plate (50 mm × 50 mm, 1 mm in thickness, 99.5% in purity, Nilaco Co.) was mounted on the RF electrode
In order to observe the helium released from the tungsten deposition layer, temperature-programmed desorption (TPD) experiment was conducted using a quadruple mass spectrometer (QMS: REGA, ULVAC Inc.). The substrate on which tungsten deposition layer was formed was installed in a test tube of stainless steel. The pressure in the test tube was previously evacuated to approximately 10−4 Pa by a turbo molecular pump. The temperature was elevated from room temperature to 450 ◦ C with a ramping rate of 10 ◦ C/min. Additionally, the TPD experiment using a cryogenic gas chromatograph was carried out in order to quantify the amount of released helium. The substrate was heated in a test tube of stainless steel from room temperature to 500 ◦ C in 100 ◦ C steps at 1-h intervals. Then, the temperature was maintained at 500 ◦ C. For compar-
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ison, the tungsten deposition layer formed by hydrogen plasma sputtering was heated to 800 ◦ C in 100 ◦ C steps at 1-h intervals. The amount of released hydrogen was quantified by an ordinary gas chromatograph. 2.3. Observation of species in the plasma chamber There is a possibility that the impurity gases are released from the plasma chamber wall during plasma discharge. Thus, the observation of species in the plasma chamber was performed using QMS.
3. Results and discussions Fig. 1 shows SEM image of the surface of the tungsten deposition layer formed by helium plasma sputtering. This surface structure was similar to that by hydrogen plasma sputtering at the RF power of 200 W. No blister and no bubble were observed on the surface of the layer. Some particulate protrusions seem to exist on the surface. It was found from EDX analysis that the deposition layer contains trace amount of iron (1.3 at.%). This is because a part of stainless steel mesh surrounding the tungsten target was sputtered. Oxygen was not detected in this analysis though there is a possibility that oxygen is also contained as a form of tungsten oxide. Fig. 2 shows SEM image of the surface of the tungsten deposition layer that was heated up to 450 ◦ C. A considerable flaking was observed. It is considered that the flaking was caused by an expan-
Fig. 2. SEM image of the surface of the tungsten deposition layer that was heated up to 450 ◦ C.
sion while heating and a subsequent contraction. This suggests that the tungsten dust and flake are generated from the deposition layer in a plasma vessel of a fusion reactor. The effective sputtered area of the tungsten target was determined to be 12 cm2 from the hollow formed on the tungsten target during the plasma discharge. The erosion rates of tungsten target by helium plasma and by hydrogen plasma were obtained to be 8.8 × 10−4 g/cm2 h and 4.9 × 10−4 g/cm2 h, respectively. It was found that the erosion rate of tungsten by helium plasma is 1.8 times larger than that by hydrogen plasma. The erosion rate is considered to depend on the energy of an incident particle. When the ion incident to the target material is assumed to be the perfectly elastic collision, the following relationship is given: U=
Fig. 1. SEM image of the surface of the tungsten deposition layer formed by helium plasma sputtering.
4M1 M2 E (M1 + M2 )2
(1)
where U is the energy transferred from the incident particle to the target atom, E the energy of an incident particle and M1 and M2 are the weight of an incident particle and a target atom, respectively. When a same RF power was impressed to the same target material, EHe in the helium plasma and EH2 in the hydrogen plasma are almost equal. Therefore, the value of U depends on the weight of an incident particle. When He+ ion and H2 + ion are assumed as an incident particle, the ratio of UHe to UH2 is derived to be 1.96. This value is near to the difference of the erosion rate by helium plasma and hydrogen plasma. It is estimated
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Fig. 3. He and H2 release curves from the tungsten deposition layer formed by helium plasma sputtering.
that a number of hydrogen ion impinges tungsten target as a form of H2 + under the experimental condition. The deposition rates of tungsten on the ground electrode by helium plasma and by hydrogen plasma were obtained to be 1.3 × 10−5 g/cm2 h and 3.2 × 10−6 g/cm2 h, respectively. This large difference cannot be explained only by the difference of the erosion rate. The tungsten atoms spattered to hydrogen plasma might be easily scattered to the surrounding compared with that to helium plasma. In near future, we will investigate the distribution of tungsten deposition in the plasma chamber. Fig. 3 shows the result of TPD experiment using QMS. The obvious release of helium (m/e = 4) can be observed. It was revealed that some amounts of helium were incorporated into the deposition layer though helium is inert gas. In this experiment, not only the helium release but also the hydrogen release (m/e = 2) was observed as shown in this figure. It had been confirmed previously that the amount of hydrogen released from a test tube is negligible. Therefore, the hydrogen seems to be released from the tungsten deposition layer certainly. The QMS system was connected with the plasma chamber to investigate a presence of hydrogen during the helium plasma discharge. The measurement of species in the plasma chamber was performed at the time that the plasma discharge began and at just before the plasma discharge was terminated. The results of the measurement were shown in Fig. 4(a) and (b). It
Fig. 4. Observed species in the plasma chamber at the time that the plasma discharge began (a) and at just before the plasma discharge was terminated (b).
was revealed that when the plasma discharge begins hydrogen rapidly appears and then slowly falls off. This figure also shows gradual decreases of nitrogen and water vapor in addition to hydrogen. The emission of those species, however, has been already completed about 120 h later. We also confirmed that when the plasma discharge was turned off, hydrogen disappears immediately. The emission source of hydrogen that cannot be removed only by evacuation seems to exist in the plasma chamber. On account of the installation of target material and substrates, the inner surface of the plasma chamber is exposed to the air for a while. In this period, some amounts of the water vapor contained in the air are surely adsorbed on the inner surface of the plasma chamber. This adsorbed water vapor is guessed to be the main source of hydrogen emission. It has been reported by the present authors that the tungsten depo-
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Fig. 5. Release behaviors of helium and hydrogen from the tungsten deposition layer formed by helium plasma sputtering.
Fig. 6. Release behavior of hydrogen from the tungsten deposition layer formed by hydrogen plasma sputtering.
sition layer formed by hydrogen plasma can trap a large amount of hydrogen [13]. That is to say, if hydrogen exists in the plasma the sputtered tungsten deposits on the substrate with some amounts of hydrogen. It is concluded that the hydrogen emitted from the inner surface of the plasma chamber at the discharge beginning was trapped into tungsten deposition layers with helium, and it was released by the TPD experiment. Ion currents of helium and hydrogen in QMS have not been calibrated yet in this study so that the amounts of released hydrogen and helium were measured by a cryogenic gas chromatograph. Fig. 5 shows the observed release behaviors of helium and hydrogen. The helium release began at 200 ◦ C and the peak appeared at the 400 ◦ C. On the other hand, the hydrogen release began at 400 ◦ C. For comparison, the hydrogen release from the tungsten deposition layer formed by hydrogen plasma is shown in Fig. 6. The hydrogen release began at 200 ◦ C and the peak appeared at the 400 ◦ C. This release behavior is similar to the helium release behavior from the layer formed by helium plasma. Therefore, the trapping mechanism of helium and hydrogen into the tungsten deposition layer might be same. The hydrogen release behavior shown in Fig. 5 and that shown in Fig. 6 are apparently different. It is speculated that hydrogen concentration distribution is formed in the deposition layer because hydrogen concentration in helium plasma decreases with the plasma discharge time. That is to say, hydrogen concentration in the deep part of the layer is high and that in the shallow part of the layer is low. On the other hand, the tungsten deposition layer formed by hydrogen plasma
contains hydrogen uniformly. Therefore, it is considered that the difference of the hydrogen release curve is due to the difference of hydrogen concentration distribution in the layer. This means that the hydrogen release process is the diffusion controlling. In near future, it is necessary to conduct the TPD experiment varying the layer thickness in order to confirm these estimations. The numbers of helium atom and hydrogen atom in the layer were derived from the release amount in the TPD experiments. The number of tungsten atom in the layer was derived from the weight of the layer assuming the layer consists of tungsten, helium and/or hydrogen. That is to say, the impurities in the layer were ignored in the estimation of atomic ratio in the layer, He/W or H/W. The He/W and H/W in the tungsten deposition layer formed by helium plasma were estimated to be 0.080 and 0.075, respectively. On the other hand, the H/W in the tungsten deposition layer formed by hydrogen plasma was estimated to be 0.15. This value is near to the (He + H)/W: 0.155 in the layer formed by helium plasma. When the hydrogen emission during helium plasma discharge can be repressed, the He/W might attain to around 0.15. In a fusion reactor having tungsten as a plasma facing material, tungsten deposition layers will be formed under various conditions. Hydrogen isotope and helium retention in the layer is considered to depend on ion flux, tungsten atom flux, incident energy and temperature. Therefore, there is possibility that some amounts of the layers formed in the reactor contain a large amount of hydrogen isotopes and helium.
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4. Conclusions The erosion rate of tungsten by helium plasma sputtering was 1.8 times larger than that by hydrogen plasma sputtering. This is due to the difference of the weight of He+ and H2 + . It was found that non-negligible amount of helium is trapped in the tungsten deposition layer formed by helium plasma sputtering. Atomic ratio of helium to tungsten (He/W) in the layer was estimated to be 0.08 under the plasma discharge condition in this study. This value is not quite small compared with that of hydrogen in the tungsten deposition layer formed by hydrogen plasma sputtering. The release behavior of helium from the layer formed by helium plasma sputtering was similar to that of hydrogen from the layer formed by hydrogen plasma puttering. Some amounts of tungsten deposition layers formed in a fusion reactor may contain a large amount of hydrogen isotopes and helium.
References [1] G. Federici, P. Andrew, P. Barabaschi, J. Brooks, R. Doerner, A. Geier, A. Herrmann, G. Janeschitz, K. Krieger, A. Kukushkin, A. Loarte, R. Nue, G. Saibene, M. Shimada, G. Strohmayer, M. Sugihara, Key ITER plasma edge and plasma-material inter action issues, J. Nucl. Mater. 313–316 (2003) 11– 22. [2] J.P. Coad, N. Bekris, J.D. Elder, S.K. Erents, D.E. Hole, K.D. Lawson, G.F. Matthews, R.-D. Penzhorn, P.C. Stangeby, Erosion/deposition issues at JET, J. Nucl. Mater. 290–293 (2001) 224. [3] C.H. Skinner, C.A. Gentile, G. Ascione, A. Carpe, R.A. Causey, T. Hayashi, J. Hogan, S. Langish, M. Nishi, W.M. Shu, W.R. Wampler, K.M. Young, Studies of tritiated codeposited layers in TFTR, J. Nucl. Mater. 290–293 (2001) 486–490. [4] A.A. Haasz, J.W. Davis, The removal of codeposited layers from TFTR tiles by O2 gas exposure, J. Nucl. Mater. 256 (1998) 65–68. [5] W. Jacob, B. Landkammer, C.H. Wu, Removal of codeposited layers by ECR discharge cleaning, J. Nucl. Mater. 266–269 (1999) 552–556.
[6] R.-D. Penzhorn, N. Bekris, W. Hellriegel, H.-E. Noppel, W. N¨agele, H. Ziegler, R. Rolli, H. Werle, A. Haigh, A. Peacock, Tritium profiles in tiles from the first wall of fusion machines and techniques for their detritiation, J. Nucl. Mater. 279 (2000) 139–152. [7] C.H. Skinner, C.A. Gentile, A. Carpe, G. Guttadora, S. Langish, K.M. Young, W.M. Shu, H. Nakamura, Tritium removal from codeposits on carbon tiles by a scanning laser, J. Nucl. Mater. 301 (2002) 98–107. [8] Y. Gotoh, J. Yagyu, K. Masaki, K. Kizu, A. Kaminaga, K. Kodama, T. Arai, T. Tanabe, N. Miya, Analyses of erosion and re-deposition layers on graphite tiles used in the W-shaped divertor region of JT-60U, J. Nucl. Mater. 313–316 (2003) 370–376. [9] K. Katayama, H. Nagase, Y. Manabe, Y. Kodama, T. Takeishi, M. Nishikawa, Formation of graphite re-deposition layer by hydrogen RF plasma, Thin Solid Films 457 (2004) 151–157. [10] G. Federici, R. Anderl, J.N. Brooks, R. Causey, J.P. Coad, D. Cowgill, R. Doerner, A.A. Haasz, G. Longhurst, S. Luckhardt, D. Mueller, A. Peacock, M. Pick, C.H. Skinner, W. Wampler, K. Wilson, C. Wong, C. Wu, D. Youchison, Tritium inventory in the ITER PFCs: predictions, uncertainties, R&D status and priority needs, Fusion Eng. Des. 39/40 (1998) 445. [11] G. Janeschitz, ITER JCT and HTs, plasma–wall interaction issues in ITER, J. Nucl. Mater. 290–293 (2001) 1–11. [12] R. Frauenfelder, Solution and diffusion of hydrogen in tungsten, J. Vac. Sci. Technol. 6 (1969) 388–397. [13] T. Kawasaki, Y. Manabe, K. Katayama, T. Takeishi, M. Nishikawa, Hydrogen retention in a tungsten re-deposition layer formed by hydrogen RF plasma, Fusion Sci. Technol. 48 (2005) 581–584. [14] T. Takeda, A. Yoshikawa, M. Oyaidzu, T. Nakahata, Y. Nishikawa, H. Kimura, Y. Onishi, H. Miyauchi, Y. Oya, A. Sagara, N. Noda, K. Okuno, Helium irradiation effects for deuterium retention in boron coating films, Fusion Eng. Des. 81 (2006) 301–306. [15] T. Sugiyama, Y. Morimoto, K. Iguchi, K. Okuno, M. Miyamoto, H. Iwakiri, N. Yoshida, Effect of helium irradiation on chemical behavior of energetic deuterium in SiC, J. Nucl. Mater. 307–311 (2002) 1080–1083. [16] S. Nagata, B. Tsuchiya, N. Ohtsu, T. Sugawara, T. Shikama, K. Tokunaga, M. Takenaka, E. Kuramoto, Hydrogen and deuterium uptake in helium implanted layer of Mo and W, J. Nucl. Mater. 313–316 (2003) 279–283. [17] H. Iwakiri, K. Morishita, N. Yoshida, Effects of helium bombardment on the deuterium behavior in tungsten, J. Nucl. Mater. 307–311 (2002) 135–138.
Helium and hydrogen trapping in tungsten deposition layers formed by helium plasma sputtering K. Katayama ∗ , K. Imaoka, T. Okamura, M. Nishikawa Department of Advanced Energy Engineering Science, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan Received 31 July 2006; received in revised form 13 April 2007; accepted 13 April 2007 Available online 12 June 2007
Abstract Tungsten deposition layers were formed by helium plasma sputtering utilizing a capacitively coupled RF plasma. For comparison, hydrogen plasma was also used for the formation of the deposition layer. It was found that non-negligible amount of helium and hydrogen were trapped in the tungsten deposition layer formed helium plasma sputtering. It is considered that the hydrogen emitted from the plasma chamber wall by helium plasma discharge was trapped in the layer. Atomic ratio of helium to tungsten (He/W) in the layer was estimated to be 0.08. This value is not quite small compared with that of hydrogen in the tungsten deposition layer formed by hydrogen plasma sputtering. The release behavior of helium from the layer formed by helium plasma sputtering was similar to that of hydrogen from the layer formed by hydrogen plasma sputtering. © 2007 Elsevier B.V. All rights reserved. Keywords: Plasma facing material; Tungsten deposition layer; Helium retention
1. Introduction Understanding of tritium behavior in plasma facing materials is an important issue for design of a fusion reactor from viewpoints of fuel control and radiation safety. Tungsten and graphite material are used as plasma facing materials in the divertor region of ITER [1]. It is well known that a carbon deposition layer can trap a large amount of tritium [2,3]. Therefore, ∗ Corresponding author. Tel.: +81 92 642 3785; fax: +81 92 642 3784. E-mail address: [email protected] (K. Katayama).
0920-3796/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2007.04.026
many studies on tritium behavior in carbon deposition layers have been done so far [4–9]. On the other hand, the investigation of hydrogen isotope behavior in a tungsten deposition layer has not been performed sufficiently. The one reason is that a sputtering yield of tungsten by hydrogen isotope is estimated to be quite lower than that of carbon. That is to say, the deposition rate of tungsten on plasma-facing surfaces in a fusion reactor is considered to be small [10,11]. The other reason is a quit low solubility of hydrogen into tungsten bulk [12]. However, it was observed in recent study by the present authors that a larger amount of hydrogen than expected from solubility of
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tungsten bulk is released from the tungsten deposition layer formed by hydrogen plasma sputtering [13]. The mechanism of hydrogen trapping in the tungsten deposition layer has not been clarified yet. The averaged erosion/deposition rate of tungsten in ITER is considered to be low because the ion energy in the divertor region is small. However, it is important to evaluate tritium behavior in tungsten deposition layers considering a long-term plasma operation. Thus, it is necessary to obtain more data for the formation of deposition layer and the trapping phenomena of environmental gas. In a plasma vessel of a fusion reactor, not only hydrogen isotope but also the helium generated by the nuclear reaction exists. Some studies on helium implantation into plasma-facing materials have been carried out so far [14–17]. They have pointed out that the implanted helium affects the release behavior of hydrogen isotopes. Therefore, we need to evaluate an influence of helium on the trapping and release behavior of hydrogen isotopes. In this study, tungsten deposition layers were formed by sputtering method using helium RF plasma. An erosion rate and a deposition rate of tungsten were estimated by weight measurement. The release behaviors of helium and hydrogen were observed by temperatureprogrammed desorption (TPD) experiment.
Table 1 Deposition conditions He (◦ C)
Gas temperature Total gas pressure (Pa) Gas flow rate (cm3 /min) RF power (W) Target-substrate distance (cm) Discharge period (h)
H2
125
154 10 1.2 200 11 120
2. Experimental
as a target material. The energy of the incident particle to the target is estimated to be about 2 keV from DC self bias on the target [13]. Tungsten chips (20 mm × 5 mm, 0.2 mm in thickness, 99.5% in purity, Nilaco Co.) were put on the ground electrode as substrates. The deposition conditions are shown in Table 1. The erosion rate was derived from the weight change of the target plate before and after plasma discharge. The deposition rate was derived from the weight change of the substrates. Weight measurements were performed by an electric balance with a sensitively of 0.01 mg. The surface observation of tungsten deposition layer was performed by the scanning electron microscopy (SEM: SS-550, SHIMAZU Co.). Atomic concentration (at.%) on the deposition layer was analyzed by an energy dispersive X-ray (EDX: Genesis2000, EDAX Inc.) equipment. The SEM and EDX used in this study were installed at the Center of Advanced Instrumental Analysis, Kyushu University.
2.1. Formation of tungsten deposition layer
2.2. Temperature-programmed desorption
Tungsten deposition layers were formed by utilizing a capacitively coupled helium RF plasma. For comparison, hydrogen plasma was also used for the formation of the deposition layer. The RF plasma apparatus and the experimental procedure have been mentioned in previous paper [13]. The plasma chamber has two parallel-plate electrodes. One electrode is connected with the plasma chamber at ground potential. This electrode is called a ground electrode. When an RF power is applied to the other electrode, which is called an RF electrode, plasma is generated between the electrodes. The electrode plate to which an RF power is impressed is eroded and the sputtered particles are deposited on the inner surfaces of the plasma chamber. A tungsten plate (50 mm × 50 mm, 1 mm in thickness, 99.5% in purity, Nilaco Co.) was mounted on the RF electrode
In order to observe the helium released from the tungsten deposition layer, temperature-programmed desorption (TPD) experiment was conducted using a quadruple mass spectrometer (QMS: REGA, ULVAC Inc.). The substrate on which tungsten deposition layer was formed was installed in a test tube of stainless steel. The pressure in the test tube was previously evacuated to approximately 10−4 Pa by a turbo molecular pump. The temperature was elevated from room temperature to 450 ◦ C with a ramping rate of 10 ◦ C/min. Additionally, the TPD experiment using a cryogenic gas chromatograph was carried out in order to quantify the amount of released helium. The substrate was heated in a test tube of stainless steel from room temperature to 500 ◦ C in 100 ◦ C steps at 1-h intervals. Then, the temperature was maintained at 500 ◦ C. For compar-
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ison, the tungsten deposition layer formed by hydrogen plasma sputtering was heated to 800 ◦ C in 100 ◦ C steps at 1-h intervals. The amount of released hydrogen was quantified by an ordinary gas chromatograph. 2.3. Observation of species in the plasma chamber There is a possibility that the impurity gases are released from the plasma chamber wall during plasma discharge. Thus, the observation of species in the plasma chamber was performed using QMS.
3. Results and discussions Fig. 1 shows SEM image of the surface of the tungsten deposition layer formed by helium plasma sputtering. This surface structure was similar to that by hydrogen plasma sputtering at the RF power of 200 W. No blister and no bubble were observed on the surface of the layer. Some particulate protrusions seem to exist on the surface. It was found from EDX analysis that the deposition layer contains trace amount of iron (1.3 at.%). This is because a part of stainless steel mesh surrounding the tungsten target was sputtered. Oxygen was not detected in this analysis though there is a possibility that oxygen is also contained as a form of tungsten oxide. Fig. 2 shows SEM image of the surface of the tungsten deposition layer that was heated up to 450 ◦ C. A considerable flaking was observed. It is considered that the flaking was caused by an expan-
Fig. 2. SEM image of the surface of the tungsten deposition layer that was heated up to 450 ◦ C.
sion while heating and a subsequent contraction. This suggests that the tungsten dust and flake are generated from the deposition layer in a plasma vessel of a fusion reactor. The effective sputtered area of the tungsten target was determined to be 12 cm2 from the hollow formed on the tungsten target during the plasma discharge. The erosion rates of tungsten target by helium plasma and by hydrogen plasma were obtained to be 8.8 × 10−4 g/cm2 h and 4.9 × 10−4 g/cm2 h, respectively. It was found that the erosion rate of tungsten by helium plasma is 1.8 times larger than that by hydrogen plasma. The erosion rate is considered to depend on the energy of an incident particle. When the ion incident to the target material is assumed to be the perfectly elastic collision, the following relationship is given: U=
Fig. 1. SEM image of the surface of the tungsten deposition layer formed by helium plasma sputtering.
4M1 M2 E (M1 + M2 )2
(1)
where U is the energy transferred from the incident particle to the target atom, E the energy of an incident particle and M1 and M2 are the weight of an incident particle and a target atom, respectively. When a same RF power was impressed to the same target material, EHe in the helium plasma and EH2 in the hydrogen plasma are almost equal. Therefore, the value of U depends on the weight of an incident particle. When He+ ion and H2 + ion are assumed as an incident particle, the ratio of UHe to UH2 is derived to be 1.96. This value is near to the difference of the erosion rate by helium plasma and hydrogen plasma. It is estimated
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Fig. 3. He and H2 release curves from the tungsten deposition layer formed by helium plasma sputtering.
that a number of hydrogen ion impinges tungsten target as a form of H2 + under the experimental condition. The deposition rates of tungsten on the ground electrode by helium plasma and by hydrogen plasma were obtained to be 1.3 × 10−5 g/cm2 h and 3.2 × 10−6 g/cm2 h, respectively. This large difference cannot be explained only by the difference of the erosion rate. The tungsten atoms spattered to hydrogen plasma might be easily scattered to the surrounding compared with that to helium plasma. In near future, we will investigate the distribution of tungsten deposition in the plasma chamber. Fig. 3 shows the result of TPD experiment using QMS. The obvious release of helium (m/e = 4) can be observed. It was revealed that some amounts of helium were incorporated into the deposition layer though helium is inert gas. In this experiment, not only the helium release but also the hydrogen release (m/e = 2) was observed as shown in this figure. It had been confirmed previously that the amount of hydrogen released from a test tube is negligible. Therefore, the hydrogen seems to be released from the tungsten deposition layer certainly. The QMS system was connected with the plasma chamber to investigate a presence of hydrogen during the helium plasma discharge. The measurement of species in the plasma chamber was performed at the time that the plasma discharge began and at just before the plasma discharge was terminated. The results of the measurement were shown in Fig. 4(a) and (b). It
Fig. 4. Observed species in the plasma chamber at the time that the plasma discharge began (a) and at just before the plasma discharge was terminated (b).
was revealed that when the plasma discharge begins hydrogen rapidly appears and then slowly falls off. This figure also shows gradual decreases of nitrogen and water vapor in addition to hydrogen. The emission of those species, however, has been already completed about 120 h later. We also confirmed that when the plasma discharge was turned off, hydrogen disappears immediately. The emission source of hydrogen that cannot be removed only by evacuation seems to exist in the plasma chamber. On account of the installation of target material and substrates, the inner surface of the plasma chamber is exposed to the air for a while. In this period, some amounts of the water vapor contained in the air are surely adsorbed on the inner surface of the plasma chamber. This adsorbed water vapor is guessed to be the main source of hydrogen emission. It has been reported by the present authors that the tungsten depo-
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Fig. 5. Release behaviors of helium and hydrogen from the tungsten deposition layer formed by helium plasma sputtering.
Fig. 6. Release behavior of hydrogen from the tungsten deposition layer formed by hydrogen plasma sputtering.
sition layer formed by hydrogen plasma can trap a large amount of hydrogen [13]. That is to say, if hydrogen exists in the plasma the sputtered tungsten deposits on the substrate with some amounts of hydrogen. It is concluded that the hydrogen emitted from the inner surface of the plasma chamber at the discharge beginning was trapped into tungsten deposition layers with helium, and it was released by the TPD experiment. Ion currents of helium and hydrogen in QMS have not been calibrated yet in this study so that the amounts of released hydrogen and helium were measured by a cryogenic gas chromatograph. Fig. 5 shows the observed release behaviors of helium and hydrogen. The helium release began at 200 ◦ C and the peak appeared at the 400 ◦ C. On the other hand, the hydrogen release began at 400 ◦ C. For comparison, the hydrogen release from the tungsten deposition layer formed by hydrogen plasma is shown in Fig. 6. The hydrogen release began at 200 ◦ C and the peak appeared at the 400 ◦ C. This release behavior is similar to the helium release behavior from the layer formed by helium plasma. Therefore, the trapping mechanism of helium and hydrogen into the tungsten deposition layer might be same. The hydrogen release behavior shown in Fig. 5 and that shown in Fig. 6 are apparently different. It is speculated that hydrogen concentration distribution is formed in the deposition layer because hydrogen concentration in helium plasma decreases with the plasma discharge time. That is to say, hydrogen concentration in the deep part of the layer is high and that in the shallow part of the layer is low. On the other hand, the tungsten deposition layer formed by hydrogen plasma
contains hydrogen uniformly. Therefore, it is considered that the difference of the hydrogen release curve is due to the difference of hydrogen concentration distribution in the layer. This means that the hydrogen release process is the diffusion controlling. In near future, it is necessary to conduct the TPD experiment varying the layer thickness in order to confirm these estimations. The numbers of helium atom and hydrogen atom in the layer were derived from the release amount in the TPD experiments. The number of tungsten atom in the layer was derived from the weight of the layer assuming the layer consists of tungsten, helium and/or hydrogen. That is to say, the impurities in the layer were ignored in the estimation of atomic ratio in the layer, He/W or H/W. The He/W and H/W in the tungsten deposition layer formed by helium plasma were estimated to be 0.080 and 0.075, respectively. On the other hand, the H/W in the tungsten deposition layer formed by hydrogen plasma was estimated to be 0.15. This value is near to the (He + H)/W: 0.155 in the layer formed by helium plasma. When the hydrogen emission during helium plasma discharge can be repressed, the He/W might attain to around 0.15. In a fusion reactor having tungsten as a plasma facing material, tungsten deposition layers will be formed under various conditions. Hydrogen isotope and helium retention in the layer is considered to depend on ion flux, tungsten atom flux, incident energy and temperature. Therefore, there is possibility that some amounts of the layers formed in the reactor contain a large amount of hydrogen isotopes and helium.
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4. Conclusions The erosion rate of tungsten by helium plasma sputtering was 1.8 times larger than that by hydrogen plasma sputtering. This is due to the difference of the weight of He+ and H2 + . It was found that non-negligible amount of helium is trapped in the tungsten deposition layer formed by helium plasma sputtering. Atomic ratio of helium to tungsten (He/W) in the layer was estimated to be 0.08 under the plasma discharge condition in this study. This value is not quite small compared with that of hydrogen in the tungsten deposition layer formed by hydrogen plasma sputtering. The release behavior of helium from the layer formed by helium plasma sputtering was similar to that of hydrogen from the layer formed by hydrogen plasma puttering. Some amounts of tungsten deposition layers formed in a fusion reactor may contain a large amount of hydrogen isotopes and helium.
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