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Study of first mirror exposure and protection in HL-2A tokamak
Fusion Engineering and Design 81 (2006) 2823–2826
Study of first mirror exposure and protection in HL-2A tokamak Y. Zhou ∗ , B.Y. Gao, Y.M. Jiao, Z.C. Deng, Y.W. Tang, J. Yi, C.L. Tian, X.T. Ding, Y. Liu Southwestern Institute of Physics, P.O. Box 432, No. 3 South Section 3, Circle Road 2, Chengdu 610041, Sichuan, China Available online 24 August 2006
Abstract Glass and copper mirrors were exposed inside the HL-2A vacuum chamber. Arc discharge mark has been found on the glass mirror. The optical transmission character of test mirrors has been measured. The main components of the deposited materials on the first mirror are carbon and iron analyzed by AES. The Monte-Carlo method has been used to simulate the defending effect of the shield for test mirror. After a 2 years tokamak experiment, one can find that the reflectivity of copper mirror decline slower than that of glass mirror. The first mirror is polluted easier during glow discharge and wall processing than omhic discharge. Using buffer and shutter can play down the pollution, especially during the glow discharge and wall processing. © 2006 Elsevier B.V. All rights reserved. Keywords: First mirror; Deposition; Protection method
1. Introduction First mirrors are the plasma facing components of plasma optical diagnostics system for superconductivity and other fusion devices. Due to a high level of a neutron and gamma radiation, the lifetime and optical state of the first mirror will face an enormous challenge. The degradation of the optical characters on the mirror surface mainly comes from the two opposite processes: one is erosion, and another is re-deposion [1]. Who plays a more important role needs test in present machines. Before 2001 the studies mainly put on material character simulation experiments in some ∗ Corresponding author. Tel.: +86 28 82850314; fax: +86 28 82850300. E-mail address: [email protected] (Y. Zhou).
0920-3796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2006.07.030
laboratory [1]. In recent years, the sample of the first mirror has been investigated in an actual fusion device, and modification of mirror optical properties was made [2–4]. But how to protect a mirror and make the lifetime longer will be an issue which needs to be solved urgently. The paper reports first results of different materials which a mirror exposed in HL-2A and defended effect of a buffer for the first mirror.
2. Experiments In order to investigate deposition and erosion behavior of the first mirror (FM) in tokamak plasma, some samples had been set in the HL-2A device. The mirrors were placed a high field side at 35 mm over the equatorial plan and kept a distance of 120 mm from
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500 mm and the height is 102 mm. The mirrors were exposed to 1100 pluses hydrogen discharge with varying the plasma current and density (Ip = 70–170 kA, ne = 0.5–1.6 × 1019 m−3 ) and a total exposure time of 165 s. In the third experiment we chose a copper mirror replaced a glass mirror to investigate because the ITER optical diagnostic mirror must be metal. The plasma discharge parameter is higher than last two experiments (Ip = 200–320 kA, ne = 1–4 × 1019 m−3 ). The sample mirror exposed to 1000 plasma pulses with total plasma exposed time of 260 s and half of hour siliconization.
3. Results Fig. 1. First mirrors experiment set up and buffer structure.
the LCFS which is a deposition dominated zone in HL-2A. The HL-2A device (with major radius R = 1.65 m and minor radius a = 0.4 m) is the close symmetric double-null divertor tokamak under construction at the SWIP new site [5] based on original ASDEX main components (magnet coils and vacuum vessel) and based on the experience from HL-1/1M. In the first experiment a glass mirror covered with the 2–3 m gold film was set facing the plasma. It was exposed to about 600 plasma pulses, the plasma current was 40–100 kA and the line averaged density was less than 0.5 × 1019 m−3 . For the second experiment a kind of buffer was placed beside the glass mirror, which has same parameter at the same position to test protected effect. Fig. 1 shows the position of the FM sample in HL-2A and the structure of the buffer. The buffer was fixed in the graphite limiter shadow; the length is
The samples appearance of a different FM material after plasma exposure shown in Fig. 2. The mirror (a), (b) and (c) indicate first, second and third experimental samples. The reflectivities were measured under almost normal incidence before and after exposure by means of a HCN laser (wavelength λ = 337 m). We can find that there was no gold film on sample (a) and the reflectivity decreased from 96 to 50%. Two different areas appeared on the second sample surface. One area is a deposition zone and the other is a gold film melted zone. And the reflectivities are 60 and 80%, respectively. The reflectivity of the deposition zone decreases faster than that of the gold film melted zone by absorption in the deposition material film. Some surface analyses had been done by scanning electron microscopy (SEM) for investigated what happened on the second test mirror. Fig. 3 shows the example for surface topography found on the gold film melted zone of second sample. It is known that the temperature of a gold melted point is about 1063 ◦ C, and the first wall temperature is not over 100 ◦ C during ohmic
Fig. 2. First mirror sample surface after plasma exposure.
Y. Zhou et al. / Fusion Engineering and Design 81 (2006) 2823–2826
Fig. 3. Surface topography of the gold film melted area on glass mirrors.
discharge on HL-2A. Therefore, maybe arc discharge mark appeared on the second sample, because sometime arc discharge has been observed on HL-2A. On the other hand, the second sample is glass mirror which makes the first wall and gold film not in same potential. Some electriferous particles accumulate on the gold film surface and make arc discharge with plasma easily. On the third experiment, a copper mirror was set on the same place, but the phenomenon of melted gold film has not appeared any more. In order to study interaction between plasma and FM material, the composition of the contaminating film on the second sample were measured by Auger electron spectroscopy (AES) and shown in Fig. 4. The layer deposited on the surface of deposition zone was found to consist mainly of carbon (about 40%), iron (about 30%), and a few nickel, chromium, oxygen, etc. According to the AES estimation, the thickness of the layer deposited was about 40nm. The deposition
Fig. 4. Atomic concentration on the sample (b) (deposition area).
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is possibly come from high energy particles sputtered with the first wall (stainless steel), limiter (graphite) and water cool pipe. The process of deposition on the FM is very complex in a fusion device. Plasma discharge, glow discharge and wall processing all could make the material redeposition on the first mirror. A special contrast experiment was performed to demonstrate which one plays an important role in re-deposition on the FM. At the same position, one crystal window is with one shutter and another window is without a shutter. The shutter was opened during plasma discharge and shuted during the wall processing (glow discharge, siliconization). After 2 years experiments, a contaminating film with yellow color has been found on the window (no shutter) surface. The transmission of protected window and no-protected one changed from about 90% (new window) to about 80 and 23%, respectively (measured by 632.8 nm laser). The signal from the plasma could not pass through the window that was not protected by shutter but the window, which was protected by shutter, works well. These results show that wall processing is easier to case of the first mirror pollution than the omhic discharge under the HL-2A experimental parameter. On the third experiment, the glass mirror has been replaced by a copper mirror. After plasma exposure, the contaminating films formed on the sample surface, which could be clearly seen in Fig. 2(c). One can find that the film of deposition is not uniformity on the surface obviously, the central area is thick, the edge area is thin, and the reflectivity decreased less. Compared second test mirror with first test mirror we can see that about 2/3 gold film has been saved by use of shield, in despite of arc discharge destroy the deposition profile on the second mirror surface. The exposure experiments proved that a buffer is a useful way to protect a mirror in the fusion device. Shield structure optimization could reduce degradation rate of the optical characteristics and make the FM lifetime longer. In order to simulate the shield effect of a buffer, we have done simple modeling (the Monte-Carlo method). Here we assume: plasma ions are no collision, an injecting particle profile is uniform (injection angle from 0◦ to 180◦ ), and a particle and buffer wall is elasticity collision (no absorbing). Fig. 5 shows the deposition result of numerical simulation under the buffer protecting. The deposition profile by calculation coincides well with that by experiment.
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pollute first mirror and diagnostic window. Buffer and shutter could protect mirror or window from polluted obviously. The simulation and experiment results of a double plan buffer show that the deposition film of the mirror in a fringe is much thinner than that in the middle.
Acknowledgements
Fig. 5. The profile of deposition material by simulation for sample (c).
The author would like to thanks HL-2A team for running tokamak device and plasma diagnostics. This work is performed with the support of National Natural Science Foundation of China under No. 10575030.
4. Conclusions
References
Glass and copper test mirrors have been irradiated in HL-2A chamber. The experiments show that the reflectivity of a glass and copper mirror has decreased after the exposure to plasma in HL-2A deposition zone. Copper mirror is better than glass mirror. Arc discharge mark is observed on a glass mirror, but not observed on a copper mirror. It seems that insulated glass makes the gold film in the different potential with the first wall, the electric particle should accumulate on the gold film surface, and an electric arc should happen. Carbon, iron, nickel, chromium and oxygen materials were found on a deposition area of the FM surface. These materials absorb optical energy and reduce the reflectivity. Wall processing plays an important role to
[1] V. Voitsenya, A.E. Costley, V. Bandurko, A. Bardamid, V. Bondarenko, Y. Hirooka, et al., Diagnostic first mirrors for buring plasma experiments, Rev. Sci. Instrum. 72 (1) (2001) 475–482. [2] A. Litnovsky, G. De Temmerman, V. Philipps, P. Wienhold, G. Sergienko, A. Kreter, et al., Investigations of metal diagnostic mirrors in TEXTOR, in: Eighth ITPA Topical Group on Diagnostics Meeting, 2005. [3] A. Sagara, S. Masuzaki, T. Morisaki, S. Morita, H. Funaba, M. Goto, et al., Helical divertor operation and erosion/deposition at target surfaces in LHD, J. Nucl. Mater. 1 (2003) 313–316. [4] V.S. Voitsenya, A. Sagara, A.I. Belyaeva, V.N. Bondarenko, V.G. Konovalov, A.D. Kudlenko, et al., Effect of exposure inside the large helical device vessel on the optical properties of stainless steel mirrors, Plasma Devices Operations 13 (4) (2005) 291–300. [5] Y. Liu, HL-2A Team, Recent experimental results from the HL1M tokamak and progress in the HL-2A project, Nucl. Fusion 44 (2004) 372–375.
Study of first mirror exposure and protection in HL-2A tokamak Y. Zhou ∗ , B.Y. Gao, Y.M. Jiao, Z.C. Deng, Y.W. Tang, J. Yi, C.L. Tian, X.T. Ding, Y. Liu Southwestern Institute of Physics, P.O. Box 432, No. 3 South Section 3, Circle Road 2, Chengdu 610041, Sichuan, China Available online 24 August 2006
Abstract Glass and copper mirrors were exposed inside the HL-2A vacuum chamber. Arc discharge mark has been found on the glass mirror. The optical transmission character of test mirrors has been measured. The main components of the deposited materials on the first mirror are carbon and iron analyzed by AES. The Monte-Carlo method has been used to simulate the defending effect of the shield for test mirror. After a 2 years tokamak experiment, one can find that the reflectivity of copper mirror decline slower than that of glass mirror. The first mirror is polluted easier during glow discharge and wall processing than omhic discharge. Using buffer and shutter can play down the pollution, especially during the glow discharge and wall processing. © 2006 Elsevier B.V. All rights reserved. Keywords: First mirror; Deposition; Protection method
1. Introduction First mirrors are the plasma facing components of plasma optical diagnostics system for superconductivity and other fusion devices. Due to a high level of a neutron and gamma radiation, the lifetime and optical state of the first mirror will face an enormous challenge. The degradation of the optical characters on the mirror surface mainly comes from the two opposite processes: one is erosion, and another is re-deposion [1]. Who plays a more important role needs test in present machines. Before 2001 the studies mainly put on material character simulation experiments in some ∗ Corresponding author. Tel.: +86 28 82850314; fax: +86 28 82850300. E-mail address: [email protected] (Y. Zhou).
0920-3796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2006.07.030
laboratory [1]. In recent years, the sample of the first mirror has been investigated in an actual fusion device, and modification of mirror optical properties was made [2–4]. But how to protect a mirror and make the lifetime longer will be an issue which needs to be solved urgently. The paper reports first results of different materials which a mirror exposed in HL-2A and defended effect of a buffer for the first mirror.
2. Experiments In order to investigate deposition and erosion behavior of the first mirror (FM) in tokamak plasma, some samples had been set in the HL-2A device. The mirrors were placed a high field side at 35 mm over the equatorial plan and kept a distance of 120 mm from
2824
Y. Zhou et al. / Fusion Engineering and Design 81 (2006) 2823–2826
500 mm and the height is 102 mm. The mirrors were exposed to 1100 pluses hydrogen discharge with varying the plasma current and density (Ip = 70–170 kA, ne = 0.5–1.6 × 1019 m−3 ) and a total exposure time of 165 s. In the third experiment we chose a copper mirror replaced a glass mirror to investigate because the ITER optical diagnostic mirror must be metal. The plasma discharge parameter is higher than last two experiments (Ip = 200–320 kA, ne = 1–4 × 1019 m−3 ). The sample mirror exposed to 1000 plasma pulses with total plasma exposed time of 260 s and half of hour siliconization.
3. Results Fig. 1. First mirrors experiment set up and buffer structure.
the LCFS which is a deposition dominated zone in HL-2A. The HL-2A device (with major radius R = 1.65 m and minor radius a = 0.4 m) is the close symmetric double-null divertor tokamak under construction at the SWIP new site [5] based on original ASDEX main components (magnet coils and vacuum vessel) and based on the experience from HL-1/1M. In the first experiment a glass mirror covered with the 2–3 m gold film was set facing the plasma. It was exposed to about 600 plasma pulses, the plasma current was 40–100 kA and the line averaged density was less than 0.5 × 1019 m−3 . For the second experiment a kind of buffer was placed beside the glass mirror, which has same parameter at the same position to test protected effect. Fig. 1 shows the position of the FM sample in HL-2A and the structure of the buffer. The buffer was fixed in the graphite limiter shadow; the length is
The samples appearance of a different FM material after plasma exposure shown in Fig. 2. The mirror (a), (b) and (c) indicate first, second and third experimental samples. The reflectivities were measured under almost normal incidence before and after exposure by means of a HCN laser (wavelength λ = 337 m). We can find that there was no gold film on sample (a) and the reflectivity decreased from 96 to 50%. Two different areas appeared on the second sample surface. One area is a deposition zone and the other is a gold film melted zone. And the reflectivities are 60 and 80%, respectively. The reflectivity of the deposition zone decreases faster than that of the gold film melted zone by absorption in the deposition material film. Some surface analyses had been done by scanning electron microscopy (SEM) for investigated what happened on the second test mirror. Fig. 3 shows the example for surface topography found on the gold film melted zone of second sample. It is known that the temperature of a gold melted point is about 1063 ◦ C, and the first wall temperature is not over 100 ◦ C during ohmic
Fig. 2. First mirror sample surface after plasma exposure.
Y. Zhou et al. / Fusion Engineering and Design 81 (2006) 2823–2826
Fig. 3. Surface topography of the gold film melted area on glass mirrors.
discharge on HL-2A. Therefore, maybe arc discharge mark appeared on the second sample, because sometime arc discharge has been observed on HL-2A. On the other hand, the second sample is glass mirror which makes the first wall and gold film not in same potential. Some electriferous particles accumulate on the gold film surface and make arc discharge with plasma easily. On the third experiment, a copper mirror was set on the same place, but the phenomenon of melted gold film has not appeared any more. In order to study interaction between plasma and FM material, the composition of the contaminating film on the second sample were measured by Auger electron spectroscopy (AES) and shown in Fig. 4. The layer deposited on the surface of deposition zone was found to consist mainly of carbon (about 40%), iron (about 30%), and a few nickel, chromium, oxygen, etc. According to the AES estimation, the thickness of the layer deposited was about 40nm. The deposition
Fig. 4. Atomic concentration on the sample (b) (deposition area).
2825
is possibly come from high energy particles sputtered with the first wall (stainless steel), limiter (graphite) and water cool pipe. The process of deposition on the FM is very complex in a fusion device. Plasma discharge, glow discharge and wall processing all could make the material redeposition on the first mirror. A special contrast experiment was performed to demonstrate which one plays an important role in re-deposition on the FM. At the same position, one crystal window is with one shutter and another window is without a shutter. The shutter was opened during plasma discharge and shuted during the wall processing (glow discharge, siliconization). After 2 years experiments, a contaminating film with yellow color has been found on the window (no shutter) surface. The transmission of protected window and no-protected one changed from about 90% (new window) to about 80 and 23%, respectively (measured by 632.8 nm laser). The signal from the plasma could not pass through the window that was not protected by shutter but the window, which was protected by shutter, works well. These results show that wall processing is easier to case of the first mirror pollution than the omhic discharge under the HL-2A experimental parameter. On the third experiment, the glass mirror has been replaced by a copper mirror. After plasma exposure, the contaminating films formed on the sample surface, which could be clearly seen in Fig. 2(c). One can find that the film of deposition is not uniformity on the surface obviously, the central area is thick, the edge area is thin, and the reflectivity decreased less. Compared second test mirror with first test mirror we can see that about 2/3 gold film has been saved by use of shield, in despite of arc discharge destroy the deposition profile on the second mirror surface. The exposure experiments proved that a buffer is a useful way to protect a mirror in the fusion device. Shield structure optimization could reduce degradation rate of the optical characteristics and make the FM lifetime longer. In order to simulate the shield effect of a buffer, we have done simple modeling (the Monte-Carlo method). Here we assume: plasma ions are no collision, an injecting particle profile is uniform (injection angle from 0◦ to 180◦ ), and a particle and buffer wall is elasticity collision (no absorbing). Fig. 5 shows the deposition result of numerical simulation under the buffer protecting. The deposition profile by calculation coincides well with that by experiment.
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Y. Zhou et al. / Fusion Engineering and Design 81 (2006) 2823–2826
pollute first mirror and diagnostic window. Buffer and shutter could protect mirror or window from polluted obviously. The simulation and experiment results of a double plan buffer show that the deposition film of the mirror in a fringe is much thinner than that in the middle.
Acknowledgements
Fig. 5. The profile of deposition material by simulation for sample (c).
The author would like to thanks HL-2A team for running tokamak device and plasma diagnostics. This work is performed with the support of National Natural Science Foundation of China under No. 10575030.
4. Conclusions
References
Glass and copper test mirrors have been irradiated in HL-2A chamber. The experiments show that the reflectivity of a glass and copper mirror has decreased after the exposure to plasma in HL-2A deposition zone. Copper mirror is better than glass mirror. Arc discharge mark is observed on a glass mirror, but not observed on a copper mirror. It seems that insulated glass makes the gold film in the different potential with the first wall, the electric particle should accumulate on the gold film surface, and an electric arc should happen. Carbon, iron, nickel, chromium and oxygen materials were found on a deposition area of the FM surface. These materials absorb optical energy and reduce the reflectivity. Wall processing plays an important role to
[1] V. Voitsenya, A.E. Costley, V. Bandurko, A. Bardamid, V. Bondarenko, Y. Hirooka, et al., Diagnostic first mirrors for buring plasma experiments, Rev. Sci. Instrum. 72 (1) (2001) 475–482. [2] A. Litnovsky, G. De Temmerman, V. Philipps, P. Wienhold, G. Sergienko, A. Kreter, et al., Investigations of metal diagnostic mirrors in TEXTOR, in: Eighth ITPA Topical Group on Diagnostics Meeting, 2005. [3] A. Sagara, S. Masuzaki, T. Morisaki, S. Morita, H. Funaba, M. Goto, et al., Helical divertor operation and erosion/deposition at target surfaces in LHD, J. Nucl. Mater. 1 (2003) 313–316. [4] V.S. Voitsenya, A. Sagara, A.I. Belyaeva, V.N. Bondarenko, V.G. Konovalov, A.D. Kudlenko, et al., Effect of exposure inside the large helical device vessel on the optical properties of stainless steel mirrors, Plasma Devices Operations 13 (4) (2005) 291–300. [5] Y. Liu, HL-2A Team, Recent experimental results from the HL1M tokamak and progress in the HL-2A project, Nucl. Fusion 44 (2004) 372–375.