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Retention and desorption of hydrogen and helium in stainless steel wall by glow discharge
Fusion Engineering and Design 72 (2005) 339–344
Retention and desorption of hydrogen and helium in stainless steel wall by glow discharge T. Hinoa,∗ , Y. Yamauchia , S. Satoha , Y. Hirohataa , A. Komorib , N. Ashikawab , A. Sagarab , N. Nodab , N. Ohyabub , O. Motojimab a
Laboratory of Plasma Physics and Engineering, Hokkaido University, Sapporo 060-8628, Japan b National Institute for Fusion Science, Toki-shi, Gifu-prefecture 509-5292, Japan Received 6 January 2004; received in revised form 9 January 2004; accepted 9 January 2004 Available online 18 November 2004
Abstract In order to evaluate the retention of hydrogen and helium and the ion impact desorption in stainless steel, the hydrogen and helium glow discharge were alternately conducted in the glow discharge apparatus with a stainless steel liner. The retained amount of hydrogen or helium in the stainless steel wall was measured based upon a residual gas analysis. The wall temperature was taken in the range from room temperature to 473 K. The retained amount of helium was observed to be large, comparable with that of hydrogen. The helium ion impact desorption for the retained hydrogen was observed to be effective, and increased with the wall temperature. The fraction of desorbed hydrogen became approximately a half of the retained amount of hydrogen. The hydrogen ion impact desorption for the retained helium, however, was observed to be negligible small. The impurity desorption owing to the hydrogen or helium glow discharge increased with the wall temperature. For reduction of the surface impurities, the hydrogen glow discharge was more effective than the helium glow discharge. The present results are useful to understand the retention and desorption of helium and hydrogen in stainless steel wall. © 2004 Elsevier B.V. All rights reserved. Keywords: Hydrogen retention; Helium retention; Stainless steel; Glow discharge; Ion impact desorption
1. Introduction Glow discharge cleaning has been widely employed in fusion devices such as the large helical device (LHD) ∗ Corresponding author. Tel.: +81 11 706 7195; fax: +81 11 709 6413. E-mail address: [email protected] (T. Hino).
0920-3796/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2004.01.002
[1–5] and other tokamak devices [6–9], in order to reduce the retention of discharge gas such as hydrogen and helium, and impurities at the wall surface. Numerous laboratory experiments using glow discharge were conducted to investigate hydrogen and/or helium retention in stainless steel [10–13], boron film [10], graphite [13] and molybdenum [14]. The material damage owing to the hydrogen and/or helium ion was also exam-
340
T. Hino et al. / Fusion Engineering and Design 72 (2005) 339–344
ined [15,16]. In a case that hydrogen or helium emits to the plasma, the plasma confinement characteristics and plasma heating condition such as ion cyclotron heating become different from the case of pure hydrogen or helium discharge. Thus, it is required to investigate the behavior of gas retention and desorption. First, the retained amounts of hydrogen and helium have to be examined. Before the hydrogen or helium main discharge, the helium or hydrogen glow discharge may be conducted to reduce the hydrogen or helium retention. Then, the ion impact desorption for retained hydrogen or helium also has to be evaluated. In the present study, hydrogen and helium glow discharges were alternately conducted in a glow discharge apparatus with a stainless steel liner made by SS 316L at Hokkaido University. Retained amounts and desorbed amounts of hydrogen and helium were measured by using a residual gas analysis, RGA. The gas retention in molybdenum was investigated by alternate irradiations of hydrogen and helium [14]. However, for stainless steel, the alternate irradiation experiment has not been conducted so far.
2. Experiments In order to measure the retained amounts of hydrogen and helium, the glow discharge apparatus with a stainless steel liner made by SS 316L was fabricated. This stainless steel has been widely employed for the vacuum vessel of fusion devices such as LHD [17,18].
Fig. 1 shows schematic view of the glow discharge apparatus. The diameter and height of the vacuum chamber are 406 mm and 600 mm, respectively. The diameter and height of the liner are 246 mm and 533 mm, respectively. The area of the liner is 0.4 m2 . The liner can be heated and the temperature kept up to 523 K. The hydrogen or helium gas pressure is adjusted by using both the mass flow controller and orifice with a diameter of 2 mm. The total and partial pressures were measured by Schlutz gauge (SG) and quadruple mass spectrometer (QMS). The movable surface station is attached to the chamber in order to prepare the sample exposed to the plasma. Before the installation of the sample to the surface station, mechanical polishing and cleaning with ethanol was conducted. The product of the effective pumping speed and the sensitivity of QMS for hydrogen and helium were measured by using standard leaks of hydrogen and helium, respectively. In the experiment, the hydrogen and helium glow discharges were alternately repeated and then the retained and desorbed amounts were measured by the change of partial pressures. In the measurement of helium or hydrogen ion impact desorption for the retained hydrogen or helium, the helium or hydrogen glow discharge was conducted after the hydrogen or helium glow discharge, respectively. Two types of discharge experiments were conducted: first, either hydrogen or helium discharge was carried out using a new liner. After the first discharge, the hydrogen or helium discharge was alternately repeated. Namely, two procedures were
Fig. 1. Schematic view of glow discharge apparatus with stainless steel liner and movable surface station.
T. Hino et al. / Fusion Engineering and Design 72 (2005) 339–344
341
Table 1 Parameters for hydrogen and helium glow discharges
Hydrogen discharge Helium discharge
Gas pressure (Pa)
Anode voltage (V)
Average current density (A/cm2 )
8 8
280 230
4 × 10−6 3 × 10−5
taken by changing discharge gas, one is in the order of hydrogen, helium and hydrogen, and the other in the order of helium, hydrogen and helium. A new liner mechanically polished was installed before the experimental procedure. The discharge time was taken 2 h, since the retention and the desorption saturated within this time in both the hydrogen and helium glow discharges. The discharge parameters were summarized in Table 1. The retained amount of helium and desorbed amount of hydrogen were measured from the change in partial pressure as shown in Fig. 2. In this example, the helium discharge was conducted after the hydrogen discharge. The partial pressure of helium dropped in the initial discharge phase, while the partial pressure of hydrogen increased. The decrease of helium partial pressure and the increase of hydrogen partial pressure correspond to the rates of helium retention and the hydrogen desorption, respectively. The retained amount of hydrogen and the desorbed amount of helium were similarly obtained when the hydrogen discharge was conducted after the helium discharge. It is noted that in the present helium or hydrogen discharge, the helium or hydrogen partial pressure remained the same before
Fig. 2. Changes of helium and hydrogen partial pressures during helium glow discharge. The reduction of helium partial pressure and the increase of hydrogen partial pressure correspond to the rates of helium retention and hydrogen desorption.
and after the discharge. The pressure rise or drop was observed just after the discharge. This amount was not included in the retained amount. In order to measure temperature dependences of hydrogen and helium retention, the helium and hydrogen discharges were repeated for the wall temperature in the range from RT to 473 K. A new liner mechanically polished was installed in the vacuum chamber before the experimental procedure with different wall temperature. In addition to the helium and hydrogen, the desorbed amount of impurity was also measured.
3. Results Fig. 3 shows the retained amounts of helium and hydrogen when the initial discharge was the helium and the wall temperature was RT. The retained amount of helium was approximately half of the retained amount of hydrogen. In the second discharge of hydrogen, the desorbed amount of helium was very small and then the retained amount of helium remained roughly the same. The helium glow discharge followed by hydrogen glow discharge was several times repeated and measured the desorbed amount of helium. It was confirmed that the fraction of desorbed helium was of order of 0.1%. In
Fig. 3. Retained amounts of hydrogen and helium vs. discharge number. Here, the first, second and third discharges were helium, hydrogen and helium discharges.
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Fig. 4. Surface morphologies of stainless steel, SS 316L, before exposure (a), after exposure to hydrogen discharge (b) and after exposure to helium discharge (c). The formation of blisters was clearly seen in the surface after the helium discharge.
the third discharge of helium, the hydrogen desorption was clearly observed. The fraction of desorbed hydrogen was approximately 20% of retained amount in the second discharge. The surface morphologies after the helium and hydrogen discharges were observed by atomic force microscope (AFM). The samples used for AFM were prepared by using the movable surface station. Fig. 4 shows the surface morphologies before the exposure (a), after the exposure to hydrogen discharge (b) and after the exposure to helium discharge (c). In the sample exposed to helium discharge, the formation of blisters with a size of 100 nm was observed. The helium retained by forming the blisters. When the first discharge was hydrogen in the other experimental procedure, the retained amount of hydrogen was 3.2 × 1016 cm−2 , which was larger than the value appeared in Fig. 3. This result suggests that the retention of hydrogen may be reduced after the formation of blisters. The retained amounts of helium and hydrogen were measured for the different wall temperature. The experimental sequence was similar for the case of RT. Fig. 5 shows the retained amounts of helium and hydrogen in the first discharge to the wall temperature. The retained amount of hydrogen decreased with increase of the wall temperature while the retained amount of helium remained roughly the same. In order to examine this difference, the thermal desorption spectroscopy (TDS) [19] was conducted for the samples exposed to the hydrogen and helium discharges. In the sample exposed to the hydrogen discharge, the desorption peak appeared only at 500 K. In the sample exposed to the helium discharge, two desorption peaks appeared at around 570 K and 1070 K. The desorbed amount at lower temperature was approximately double of that at higher temperature. The peak temperature of helium desorption was at least 100 K higher than a highest wall
Fig. 5. Retained amounts of hydrogen and helium vs. wall temperature. The retained amount of hydrogen decreased as increase of wall temperature although that of helium remained roughly the same.
temperature in the present experiment, 473 K. This is the reason that the helium retention remained roughly the same. The result shown in Fig. 5 shows that the helium retention becomes comparable with the hydrogen retention at 473 K. Fig. 6 shows the fraction of desorbed hydrogen or helium due to the helium or hydrogen discharge versus
Fig. 6. Fractions of desorbed hydrogen due to helium discharge and desorbed helium due to hydrogen discharge. The desorbed amount of hydrogen increased as wall temperature although that of helium remained roughly the same.
T. Hino et al. / Fusion Engineering and Design 72 (2005) 339–344
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Fig. 7. Desorbed amounts of impurity gases during hydrogen discharge (a) and helium discharge (b). Major species of impurity was CO in both the discharges.
the wall temperature. The fraction of desorbed hydrogen increased with the wall temperature. When the wall temperature was 473 K, the fraction of desorbed hydrogen became large, approximately 40%. The fraction of desorbed helium slightly increased with the wall temperature. However, the fraction of desorbed helium was still very small, as high as about 0.2%. In the helium discharge, the momentum and energy transfer from helium ion to atoms of the stainless steel is larger than that of hydrogen ion by a factor of 3.6. The sputtering yield of helium ion for the stainless steel is also larger by a factor of 20. These may be the reasons that the helium ion impact for hydrogen was dominant. During the hydrogen or helium discharge, the desorption of impurities such as CO, H2 O, CH4 and CO2 were observed. Fig. 7 shows the desorbed amount of impurity gasses during the hydrogen discharge (a) and helium discharge (b). In both cases, the total desorption amount of impurities increased with the temperature. In the helium discharge, the major impurity species was CO. In the hydrogen discharge, the desorptions of H2 O, CH4 and CO2 were also observed in addition of CO. The total amount desorbed by the hydrogen discharge was larger than that by the helium discharge, by a factor of 4.4 at RT and 1.3 at 473 K.
4. Conclusion In the present experiments, the retained amount of hydrogen or helium was measured for the stainless steel wall in the temperature range from RT to 473 K. It was shown that the helium retention was comparable with the hydrogen retention. This result has not been clearly
known so far. In fusion devices, the helium and hydrogen glow discharge cleanings are conducted before the main discharges of fuel hydrogen. The retained helium emits to the fusion plasma during the main discharge since the wall receives the charge exchanged hydrogen atoms with high energy and high heat flux from the plasma. The dilution of helium into the plasma takes place, and then the condition of plasma heating such as ion cyclotron heating changes and the alpha heating power decreases in the fusion reactor. The helium or hydrogen ion impact desorption for the retained hydrogen or helium was measured for the wall temperature in the range from RT to 473 K. The amount of retained hydrogen was significantly reduced by the helium glow discharge. The reduction of retained hydrogen increased with the wall temperature. The present result showed that the fraction of desorbed hydrogen became approximately 40% when the wall temperature was 473 K. In opposition, it was observed that the reduction of helium retention due to hydrogen ion impact desorption was quite small. Hence, the other scheme is required for reduction of helium retention. In the stainless steel, the desorption peak of retained helium in the lower temperature regime was approximately 570 K. If the baking with the temperature higher than 570 K is conducted for the vacuum vessel, the helium retention is significantly reduced. The impurity desorption was observed to increase with the wall temperature. For the reduction of surface impurities, the use of hydrogen discharge was superior for that of helium discharge. In the present study, the retention and desorption of helium and hydrogen in stainless steel wall were
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systematically investigated for the wall temperature in the range from RT to 473 K. The present results are useful to understand the retention and desorption of helium and hydrogen by glow discharge in stainless steel wall.
Acknowledgement This work was supported by LHD Collaboration Study Program of National Institute of Fusion Science.
References [1] S. Masuzaki, K. Akaishi, H. Funaba, M. Goto, K. Ida, et al., J. Nucl. Mater. 290–293 (2001) 12. [2] H. Suzuki, N. Ohyabu, A. Komori, T. Morisaki, J. Miyazawa, et al., J. Nucl. Mater. 313–316 (2003) 297. [3] T. Hino, A. Sagara, Y. Nobuta, N. Inoue, Y. Hirohata, et al., Nucl. Fusion 44 (2004) 496. [4] T. Hino, Y. Nobuta, A. Sagara, T. Ohuchi, Y. Hirohata, et al., J. Nucl. Mater. 313–316 (2003) 167. [5] Y. Yamauchi, S. Satoh, Y. Hirohata, T. Hino, A. Komori, et al., J. Vac. Soc. Jpn. 47 (2004) 480.
[6] S. Ishida, K. Yatsu, T. Hino, Y. Takase, H. Ninomiya, et al., Nucl. Fusion 43 (2003) 606. [7] G.L. Jackson, T.S. Taylor, P.L. Taylor, Nucl. Fusion 30 (1990) 2305. [8] V. Philipps, H.G. Esser, J. von Seggern, H. Reimer, M. Freisinger, et al., J. Nucl. Mater. 266–269 (1999) 386. [9] N.M. Zhang, E.Y. Wang, M.X. Wang, W.Y. Hong, C.H. Cui, et al., J. Nucl. Mater. 266–269 (1999) 747. [10] K. Tsuzuki, M. Nastir, N. Inoue, A. Sagara, N. Noda, et al., J. Nucl. Mater. 241–243 (1997) 241. [11] A.B. Antoniazzi, W.T. Shmayda, R.A. Surette, Fusion Technol. 21 (1992) 867. [12] M. Ogura, N. Yamaji, T. Higuchi, M. Imai, A. Itoh, et al., J. Vac. Sci. Technol. A6 (1988) 2421. [13] K. Akaishi, S. Watanabe, Y. Kubota, Y. Funato, J. Vac. Sci. Technol. A9 (1991) 733. [14] R. Shulz, R. Behrisch, B.M.U. Scherzer, J. Nucl. Mater. 93–94 (1980) 608. [15] P. Wang, Y. Li, J. Liu, G. Zhang, R. Ma, et al., J. Nucl. Mater. 169 (1989) 167. [16] A.A. Pisarev, B.A. Gurovich, Nucl. Instrum. Methods Phys. Res., Sect. B 115 (1996) 540. [17] O. Motojima, A. Komori, N. Ohyabu, A. Sagara, T. Hino, et al., Nucl. Fusion 43 (2003) 1674. [18] A. Sagara, N. Noda, N. Ashikawa, A. Komori, et al., J. Nucl. Mater. 313–316 (2003) 1. [19] T. Hino, T. Ohuchi, Y. Yamauchi, Y. Hirohata, et al., J. Nucl. Mater. 290–293 (2001) 1176.
Retention and desorption of hydrogen and helium in stainless steel wall by glow discharge T. Hinoa,∗ , Y. Yamauchia , S. Satoha , Y. Hirohataa , A. Komorib , N. Ashikawab , A. Sagarab , N. Nodab , N. Ohyabub , O. Motojimab a
Laboratory of Plasma Physics and Engineering, Hokkaido University, Sapporo 060-8628, Japan b National Institute for Fusion Science, Toki-shi, Gifu-prefecture 509-5292, Japan Received 6 January 2004; received in revised form 9 January 2004; accepted 9 January 2004 Available online 18 November 2004
Abstract In order to evaluate the retention of hydrogen and helium and the ion impact desorption in stainless steel, the hydrogen and helium glow discharge were alternately conducted in the glow discharge apparatus with a stainless steel liner. The retained amount of hydrogen or helium in the stainless steel wall was measured based upon a residual gas analysis. The wall temperature was taken in the range from room temperature to 473 K. The retained amount of helium was observed to be large, comparable with that of hydrogen. The helium ion impact desorption for the retained hydrogen was observed to be effective, and increased with the wall temperature. The fraction of desorbed hydrogen became approximately a half of the retained amount of hydrogen. The hydrogen ion impact desorption for the retained helium, however, was observed to be negligible small. The impurity desorption owing to the hydrogen or helium glow discharge increased with the wall temperature. For reduction of the surface impurities, the hydrogen glow discharge was more effective than the helium glow discharge. The present results are useful to understand the retention and desorption of helium and hydrogen in stainless steel wall. © 2004 Elsevier B.V. All rights reserved. Keywords: Hydrogen retention; Helium retention; Stainless steel; Glow discharge; Ion impact desorption
1. Introduction Glow discharge cleaning has been widely employed in fusion devices such as the large helical device (LHD) ∗ Corresponding author. Tel.: +81 11 706 7195; fax: +81 11 709 6413. E-mail address: [email protected] (T. Hino).
0920-3796/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2004.01.002
[1–5] and other tokamak devices [6–9], in order to reduce the retention of discharge gas such as hydrogen and helium, and impurities at the wall surface. Numerous laboratory experiments using glow discharge were conducted to investigate hydrogen and/or helium retention in stainless steel [10–13], boron film [10], graphite [13] and molybdenum [14]. The material damage owing to the hydrogen and/or helium ion was also exam-
340
T. Hino et al. / Fusion Engineering and Design 72 (2005) 339–344
ined [15,16]. In a case that hydrogen or helium emits to the plasma, the plasma confinement characteristics and plasma heating condition such as ion cyclotron heating become different from the case of pure hydrogen or helium discharge. Thus, it is required to investigate the behavior of gas retention and desorption. First, the retained amounts of hydrogen and helium have to be examined. Before the hydrogen or helium main discharge, the helium or hydrogen glow discharge may be conducted to reduce the hydrogen or helium retention. Then, the ion impact desorption for retained hydrogen or helium also has to be evaluated. In the present study, hydrogen and helium glow discharges were alternately conducted in a glow discharge apparatus with a stainless steel liner made by SS 316L at Hokkaido University. Retained amounts and desorbed amounts of hydrogen and helium were measured by using a residual gas analysis, RGA. The gas retention in molybdenum was investigated by alternate irradiations of hydrogen and helium [14]. However, for stainless steel, the alternate irradiation experiment has not been conducted so far.
2. Experiments In order to measure the retained amounts of hydrogen and helium, the glow discharge apparatus with a stainless steel liner made by SS 316L was fabricated. This stainless steel has been widely employed for the vacuum vessel of fusion devices such as LHD [17,18].
Fig. 1 shows schematic view of the glow discharge apparatus. The diameter and height of the vacuum chamber are 406 mm and 600 mm, respectively. The diameter and height of the liner are 246 mm and 533 mm, respectively. The area of the liner is 0.4 m2 . The liner can be heated and the temperature kept up to 523 K. The hydrogen or helium gas pressure is adjusted by using both the mass flow controller and orifice with a diameter of 2 mm. The total and partial pressures were measured by Schlutz gauge (SG) and quadruple mass spectrometer (QMS). The movable surface station is attached to the chamber in order to prepare the sample exposed to the plasma. Before the installation of the sample to the surface station, mechanical polishing and cleaning with ethanol was conducted. The product of the effective pumping speed and the sensitivity of QMS for hydrogen and helium were measured by using standard leaks of hydrogen and helium, respectively. In the experiment, the hydrogen and helium glow discharges were alternately repeated and then the retained and desorbed amounts were measured by the change of partial pressures. In the measurement of helium or hydrogen ion impact desorption for the retained hydrogen or helium, the helium or hydrogen glow discharge was conducted after the hydrogen or helium glow discharge, respectively. Two types of discharge experiments were conducted: first, either hydrogen or helium discharge was carried out using a new liner. After the first discharge, the hydrogen or helium discharge was alternately repeated. Namely, two procedures were
Fig. 1. Schematic view of glow discharge apparatus with stainless steel liner and movable surface station.
T. Hino et al. / Fusion Engineering and Design 72 (2005) 339–344
341
Table 1 Parameters for hydrogen and helium glow discharges
Hydrogen discharge Helium discharge
Gas pressure (Pa)
Anode voltage (V)
Average current density (A/cm2 )
8 8
280 230
4 × 10−6 3 × 10−5
taken by changing discharge gas, one is in the order of hydrogen, helium and hydrogen, and the other in the order of helium, hydrogen and helium. A new liner mechanically polished was installed before the experimental procedure. The discharge time was taken 2 h, since the retention and the desorption saturated within this time in both the hydrogen and helium glow discharges. The discharge parameters were summarized in Table 1. The retained amount of helium and desorbed amount of hydrogen were measured from the change in partial pressure as shown in Fig. 2. In this example, the helium discharge was conducted after the hydrogen discharge. The partial pressure of helium dropped in the initial discharge phase, while the partial pressure of hydrogen increased. The decrease of helium partial pressure and the increase of hydrogen partial pressure correspond to the rates of helium retention and the hydrogen desorption, respectively. The retained amount of hydrogen and the desorbed amount of helium were similarly obtained when the hydrogen discharge was conducted after the helium discharge. It is noted that in the present helium or hydrogen discharge, the helium or hydrogen partial pressure remained the same before
Fig. 2. Changes of helium and hydrogen partial pressures during helium glow discharge. The reduction of helium partial pressure and the increase of hydrogen partial pressure correspond to the rates of helium retention and hydrogen desorption.
and after the discharge. The pressure rise or drop was observed just after the discharge. This amount was not included in the retained amount. In order to measure temperature dependences of hydrogen and helium retention, the helium and hydrogen discharges were repeated for the wall temperature in the range from RT to 473 K. A new liner mechanically polished was installed in the vacuum chamber before the experimental procedure with different wall temperature. In addition to the helium and hydrogen, the desorbed amount of impurity was also measured.
3. Results Fig. 3 shows the retained amounts of helium and hydrogen when the initial discharge was the helium and the wall temperature was RT. The retained amount of helium was approximately half of the retained amount of hydrogen. In the second discharge of hydrogen, the desorbed amount of helium was very small and then the retained amount of helium remained roughly the same. The helium glow discharge followed by hydrogen glow discharge was several times repeated and measured the desorbed amount of helium. It was confirmed that the fraction of desorbed helium was of order of 0.1%. In
Fig. 3. Retained amounts of hydrogen and helium vs. discharge number. Here, the first, second and third discharges were helium, hydrogen and helium discharges.
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Fig. 4. Surface morphologies of stainless steel, SS 316L, before exposure (a), after exposure to hydrogen discharge (b) and after exposure to helium discharge (c). The formation of blisters was clearly seen in the surface after the helium discharge.
the third discharge of helium, the hydrogen desorption was clearly observed. The fraction of desorbed hydrogen was approximately 20% of retained amount in the second discharge. The surface morphologies after the helium and hydrogen discharges were observed by atomic force microscope (AFM). The samples used for AFM were prepared by using the movable surface station. Fig. 4 shows the surface morphologies before the exposure (a), after the exposure to hydrogen discharge (b) and after the exposure to helium discharge (c). In the sample exposed to helium discharge, the formation of blisters with a size of 100 nm was observed. The helium retained by forming the blisters. When the first discharge was hydrogen in the other experimental procedure, the retained amount of hydrogen was 3.2 × 1016 cm−2 , which was larger than the value appeared in Fig. 3. This result suggests that the retention of hydrogen may be reduced after the formation of blisters. The retained amounts of helium and hydrogen were measured for the different wall temperature. The experimental sequence was similar for the case of RT. Fig. 5 shows the retained amounts of helium and hydrogen in the first discharge to the wall temperature. The retained amount of hydrogen decreased with increase of the wall temperature while the retained amount of helium remained roughly the same. In order to examine this difference, the thermal desorption spectroscopy (TDS) [19] was conducted for the samples exposed to the hydrogen and helium discharges. In the sample exposed to the hydrogen discharge, the desorption peak appeared only at 500 K. In the sample exposed to the helium discharge, two desorption peaks appeared at around 570 K and 1070 K. The desorbed amount at lower temperature was approximately double of that at higher temperature. The peak temperature of helium desorption was at least 100 K higher than a highest wall
Fig. 5. Retained amounts of hydrogen and helium vs. wall temperature. The retained amount of hydrogen decreased as increase of wall temperature although that of helium remained roughly the same.
temperature in the present experiment, 473 K. This is the reason that the helium retention remained roughly the same. The result shown in Fig. 5 shows that the helium retention becomes comparable with the hydrogen retention at 473 K. Fig. 6 shows the fraction of desorbed hydrogen or helium due to the helium or hydrogen discharge versus
Fig. 6. Fractions of desorbed hydrogen due to helium discharge and desorbed helium due to hydrogen discharge. The desorbed amount of hydrogen increased as wall temperature although that of helium remained roughly the same.
T. Hino et al. / Fusion Engineering and Design 72 (2005) 339–344
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Fig. 7. Desorbed amounts of impurity gases during hydrogen discharge (a) and helium discharge (b). Major species of impurity was CO in both the discharges.
the wall temperature. The fraction of desorbed hydrogen increased with the wall temperature. When the wall temperature was 473 K, the fraction of desorbed hydrogen became large, approximately 40%. The fraction of desorbed helium slightly increased with the wall temperature. However, the fraction of desorbed helium was still very small, as high as about 0.2%. In the helium discharge, the momentum and energy transfer from helium ion to atoms of the stainless steel is larger than that of hydrogen ion by a factor of 3.6. The sputtering yield of helium ion for the stainless steel is also larger by a factor of 20. These may be the reasons that the helium ion impact for hydrogen was dominant. During the hydrogen or helium discharge, the desorption of impurities such as CO, H2 O, CH4 and CO2 were observed. Fig. 7 shows the desorbed amount of impurity gasses during the hydrogen discharge (a) and helium discharge (b). In both cases, the total desorption amount of impurities increased with the temperature. In the helium discharge, the major impurity species was CO. In the hydrogen discharge, the desorptions of H2 O, CH4 and CO2 were also observed in addition of CO. The total amount desorbed by the hydrogen discharge was larger than that by the helium discharge, by a factor of 4.4 at RT and 1.3 at 473 K.
4. Conclusion In the present experiments, the retained amount of hydrogen or helium was measured for the stainless steel wall in the temperature range from RT to 473 K. It was shown that the helium retention was comparable with the hydrogen retention. This result has not been clearly
known so far. In fusion devices, the helium and hydrogen glow discharge cleanings are conducted before the main discharges of fuel hydrogen. The retained helium emits to the fusion plasma during the main discharge since the wall receives the charge exchanged hydrogen atoms with high energy and high heat flux from the plasma. The dilution of helium into the plasma takes place, and then the condition of plasma heating such as ion cyclotron heating changes and the alpha heating power decreases in the fusion reactor. The helium or hydrogen ion impact desorption for the retained hydrogen or helium was measured for the wall temperature in the range from RT to 473 K. The amount of retained hydrogen was significantly reduced by the helium glow discharge. The reduction of retained hydrogen increased with the wall temperature. The present result showed that the fraction of desorbed hydrogen became approximately 40% when the wall temperature was 473 K. In opposition, it was observed that the reduction of helium retention due to hydrogen ion impact desorption was quite small. Hence, the other scheme is required for reduction of helium retention. In the stainless steel, the desorption peak of retained helium in the lower temperature regime was approximately 570 K. If the baking with the temperature higher than 570 K is conducted for the vacuum vessel, the helium retention is significantly reduced. The impurity desorption was observed to increase with the wall temperature. For the reduction of surface impurities, the use of hydrogen discharge was superior for that of helium discharge. In the present study, the retention and desorption of helium and hydrogen in stainless steel wall were
344
T. Hino et al. / Fusion Engineering and Design 72 (2005) 339–344
systematically investigated for the wall temperature in the range from RT to 473 K. The present results are useful to understand the retention and desorption of helium and hydrogen by glow discharge in stainless steel wall.
Acknowledgement This work was supported by LHD Collaboration Study Program of National Institute of Fusion Science.
References [1] S. Masuzaki, K. Akaishi, H. Funaba, M. Goto, K. Ida, et al., J. Nucl. Mater. 290–293 (2001) 12. [2] H. Suzuki, N. Ohyabu, A. Komori, T. Morisaki, J. Miyazawa, et al., J. Nucl. Mater. 313–316 (2003) 297. [3] T. Hino, A. Sagara, Y. Nobuta, N. Inoue, Y. Hirohata, et al., Nucl. Fusion 44 (2004) 496. [4] T. Hino, Y. Nobuta, A. Sagara, T. Ohuchi, Y. Hirohata, et al., J. Nucl. Mater. 313–316 (2003) 167. [5] Y. Yamauchi, S. Satoh, Y. Hirohata, T. Hino, A. Komori, et al., J. Vac. Soc. Jpn. 47 (2004) 480.
[6] S. Ishida, K. Yatsu, T. Hino, Y. Takase, H. Ninomiya, et al., Nucl. Fusion 43 (2003) 606. [7] G.L. Jackson, T.S. Taylor, P.L. Taylor, Nucl. Fusion 30 (1990) 2305. [8] V. Philipps, H.G. Esser, J. von Seggern, H. Reimer, M. Freisinger, et al., J. Nucl. Mater. 266–269 (1999) 386. [9] N.M. Zhang, E.Y. Wang, M.X. Wang, W.Y. Hong, C.H. Cui, et al., J. Nucl. Mater. 266–269 (1999) 747. [10] K. Tsuzuki, M. Nastir, N. Inoue, A. Sagara, N. Noda, et al., J. Nucl. Mater. 241–243 (1997) 241. [11] A.B. Antoniazzi, W.T. Shmayda, R.A. Surette, Fusion Technol. 21 (1992) 867. [12] M. Ogura, N. Yamaji, T. Higuchi, M. Imai, A. Itoh, et al., J. Vac. Sci. Technol. A6 (1988) 2421. [13] K. Akaishi, S. Watanabe, Y. Kubota, Y. Funato, J. Vac. Sci. Technol. A9 (1991) 733. [14] R. Shulz, R. Behrisch, B.M.U. Scherzer, J. Nucl. Mater. 93–94 (1980) 608. [15] P. Wang, Y. Li, J. Liu, G. Zhang, R. Ma, et al., J. Nucl. Mater. 169 (1989) 167. [16] A.A. Pisarev, B.A. Gurovich, Nucl. Instrum. Methods Phys. Res., Sect. B 115 (1996) 540. [17] O. Motojima, A. Komori, N. Ohyabu, A. Sagara, T. Hino, et al., Nucl. Fusion 43 (2003) 1674. [18] A. Sagara, N. Noda, N. Ashikawa, A. Komori, et al., J. Nucl. Mater. 313–316 (2003) 1. [19] T. Hino, T. Ohuchi, Y. Yamauchi, Y. Hirohata, et al., J. Nucl. Mater. 290–293 (2001) 1176.