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Determination of carbon impurity fluxes in the limiter shadow of TEXTOR
Journal
of Nuclear
Materials
145-147
(1987) 6422644
North-Holland.
642
DETERMINATION OF TEXTOR
OF CARBON IMPURITY FLUXES
IN THE LIMITER SHADOW
P. WIENHOLD, J. WINTER, H.G. ESSER, F. WAELBROECK, B. EMMOTH * and H.E. SATHERBLOM * Institut ftir Pla.smuph.v.sik, Kernforschungsanlage POB 1913, D-51 70 Jiilich, Fed. Rep. German?,
Jiilich GmbH, Association
Key words:
probe,
impurity
fluxes, carbon,
collection
Amsterdam
H. BERGSAKER
*,
EURA TOM-KFA.
TEXTOR
The carbon fluxes in the limiter shadow of TEXTOR have been measured with time resolution using the Stockholm-TEXTOR collection probe. For this, a new collector foil made of pure aluminum covered with amorphous silicon has been used. The area density of the collected carbon was determined from the elastic backscattering of protons of 1.75 MeV energy. A preliminary value of 4 X lOI cmA2 s- ’ was found for the carbon flux 2.4 cm into the limiter shadow.
1. Introduction TEXTOR is presently routinely carbonized [l], i.e. its liner and limiters are covered with an amorphous carbon layer (a-C:H). This reduces drastically the metal impurities in the plasma ( nMe/iie ( 1 X lo- 5). Oxygen is mostly below 1% [2]. Carbon is considered to be the most prominent plasma contaminant under these conditions. Its concentration in the scrape-off layer (SOL) can, in principle, be measured by collector probes [3], but a problem is that most collector materials are illsuited to measure the carbon fluxes escaping from the plasma. Materials such as pure aluminium are ruled out because the carbon delivered by the plasma cannot be distinguished from the carbon components adsorbed during the transport from the tokamak to the analysis station. A new method has been developed in order to measure carbon fluxes in the limiter shadow of TEXTOR, time and space resolved by use of the StockholmTEXTOR probe. This probe has been used earlier for the collection of metal and oxygen impurities on graphite or papyex substrates which were subsequently analysed by the Rutherford backscattering technique at the analysis station in Stockholm [4-71. It is located in an equatorial position 157 o apart from the main limiters in ion drift direction.
foil was wound around the cylindrical probe head (r = 25 mm) which rotates synchronously with the tokamak discharge within a housing with a radial slit aperture (width = 1.9 mm). The carbon transported by the plasma in the toroidal direction is deposited time resolved (resolution 240 ms) on the circumference of the spinning cylinder. The radial distribution can also be analyzed. After exposure to the plasma, the probe is withdrawn, dismounted and transported to Stockholm. The carbon deposit has been studied by elastic backscattering of protons of 1.75 MeV energy where the scattering ‘*C(p, p)‘*C has its maximum cross section of about 1 bam/sr [8]. Fig. 1 shows (upper curve) a typical backscattering spectrum on an exposed area (number of backscattered protons versus their energy). The steep edge at the right hand side corresponds to the
2. Method For the collection of carbon, a pure aluminum foil (99.999%) was covered by, an amorphous silicon layer (a-Si:H) of roughly 2000 A thickness and used as substrate. This layer was deposited by use of a glow discharge in a silane atmosphere. The adsorption of oxygen and carbon oxides between exposure and analysis is negligible even when the transport is made in air. The * Research
Institute of Physics, Association S-104 05 Stockholm, Sweden.
EURATOM-EFN
0022-3115/87/$03.50 0 Elsevier Science publishers (North-Holland Physics publishing Division)
B.V.
8
1000
Energy
of scattered
1 1500
protons(keVl-
Fig. 1. Spectrum of protons with an incident energy of 1.75 MeV after their elastic backscattering under 170° as taken at the angular position of 240 grad of the probe cylinder. The steep edge and the modulation are caused by Si and Al, respectively, in the substrate. The peak area A, of the collected carbon is determined from the difference spectrum (see text). The collected deuterium can also be determined from the spectrum.
P. Wienhold et al. / Carbon impurit_vfluxes
collisions with the heavy silicon nuclei. The irregular background in the central part originates from nonRutherford scattering in the aluminum substrate and is also present on areas where practically no carbon was deposited. The small peak at the far left is due to the collected deuterium [9] which was used as fueling gas for the plasma. The carbon peak occurs on top of the modulated curve. Its area A, is evaluated from the difference between the spectra obtained on exposed and unexposed areas of the foil (lower curve). The areal density of the collected carbon atoms can be determined from A, taking into account the number of incident protons, the reaction cross section and the solid angle of the detector. This has been translated into carbon fluxes assuming a sticking coefficient of unity and taking the time resolution into account. The method will be described in more detail in a forthcoming paper [lo]. 3. Results and discussion After a fresh carbonization of the liner and the stainless steel limiters (June 1985) and about 100 discharges, a sample was exposed to a series of shots in deuterium with nearly identical plasma parameters: Ir, = 340 kA, a = 46 cm, Br(0) = 2 T, duration = 3.5 s. The line averaged density was 6, = 2.5 X 10” cme3 for all exposures except the second where it was 3 x 1013 cm -3. Fig. 2 shows the flux density of carbon 2.4 cm into the limiter shadow (r = 48.4 cm), measured on the circumference of the probe cylinder (0 to 400 grad). For the first exposure (#16136), the-cylinder was brought to the angular position of 20 grad and then turned by 70 grad synchronously during the 3.5 s long
# 16157
in the limiter shadow
643
discharge. A new angular position 120 grad was chosen for the beginning of the next exposure and the procedure was iterated. The four exposed areas are separated by unexposed areas of 30 grad (shadowed). A higher plasma density, Z,(O) = 3.0 X lOi cme3 was chosen for the second exposure (# 16157) to assess the influence of iI,. The tendency to saturation was studied by overlaying two identical discharges (# 16165, 16166) in the third exposure, and nine (# 16167-16175) in the last. The radial distribution is not discussed here. The figure shows that at this depth of 2.4 cm in the SOL, the unexposed areas also carry carbon the origin of which is presently being examined. Its fluctuating amount corresponds to a,flux density about 4-5 x 1016 cm-* s-l (indicated as dashed line). This level is exceeded by the carbon fluxes observed during the plasma exposures. The time evolution is best seen on the third and fourth exposures. The fluxes increase to a maximum value within less than one second and already decrease during the flat top phase (0.3-2.2 s) of density and current. Increasing the electron density (second exposure) seems to decrease the carbon impurity production. Such a behaviour had been seen on the metallic impurity fluxes in the limiter shadow before carbonization (4-71. Overlaying two discharges (third exposure) approximately doubles the value of the carbon flux, whereas after nine discharges it has increased by a factor of about six only (fourth exposure). This indicates that there might be a tendency to saturate at the higher fluences. However, the lower flux value of about 4 X 1016 cme2 s-l obtained with one single discharge (difference between the maximum and the background level) should not be influenced by saturation. The flux of carbon impurities is larger by a factor of more than ten than the fluxes of metal impurities measured in the presence of metal limiters and liner before carbonization (51. In the plasma core the metal was then of the order of parts per thousand whereas the carbon concentration after carbonization is of the order of a per cent. 4. Conclusions
#16165
1’ /
# 16166
F
grad of crcumference
-
Fig. 2. Carbon flux @Joas determined on the circumference of the probe cylinder 2.4 cm in the limiter shadow of TEXTOR. Four exposed areas of 70 grad each, corresponding to the duration of 3.5 s of the plasma discharges, are separated from four unexposed areas (shadowed). The dashed line indicates a background. The inner walls of the machine were carbonized.
The new technique for the collection of carbon on amorphous silicon substrates (a-Si:H) and its analysis by the elastic backscattering of protons seems to be a promising method in order to determine carbon impurity fluxes time resolved in the limiter shadow of tokamaks. Still here are open questions, like the unknown sticking coefficient of C-atoms on such substrates and the degree of erosion by e.g. the hydrogen fluxes. Admittedly, the presented figure of 4 x 1016 crnm2 s-l for the carbon flux is very preliminary, but certainly not far from being realistic. The authors are very thankful to the TEXTOR-team and many colleagues for their help, in particular to W.
Beyer (IGV, JiiIich) who prepared his apparatus.
the substrate
foils in
References [l] J. Winter, these Proc. (PSI-VII),
J. Nucl. Mater. 145-147 (1987). [2] J. Schliiter, E. Graffmann, L. KBnen, F. Waelbroeck, G. Waidmann, J. Winter and TEXTOR team, 12th Eur. Conf. on Controlled Fusion and Plasma Physics, September 2-6. 1985, Budapest, Hungary. 133 G. Staudenmaier, J. Vat. Sci. Technol. A3 (1985) 1091. [4] H.E. Sltherblom, M. Braun, B. Emmoth. T. Fried, J. Hilke, P.A. Holmstriim, F. Waelbroeck, P.Wienhold and J. Winter, Nucl. Instr. and Meth. A240 (1985) 171.
J. Winter, Nucl. Instr. and Meth. A240 (1985) 171. [5] B. Emmoth, M. Braun. H.E. Satherblom, P. Wienhold. J. Winter and F. Waelbroeck, J. Nucl. Mater. 12X & 129 (1984) 1193. [6] B. Emmoth, S. Nagata, H.Bergs%ker, H.E. stherblom, P. Wienhold. J. Winter and F. Waelbroeck. these Proc. (PSIVII), J. Nucl. Mater. 145-147 (1987). [7] P. Wienhold, J. Winter, H.G. Esser, F. Waelbreock, H.E. Sltherblom, B. Emmoth and M. Braun, these Proc. (PSIVII), J. Nucl. Mater. 145-147 (1987). [S] E. Rauhala, Nucl. Instr. and Meth. B12 (1985) 447. [9] R.A. Langley, Proc. Int. Conf. on Radiation Effects and Tritium Technology for Fusion Reactors, Vol. IV, 1976, Springfield, USA (CONF-750989) p. 158. [lo] H. Bergs&er et al., to be published.
of Nuclear
Materials
145-147
(1987) 6422644
North-Holland.
642
DETERMINATION OF TEXTOR
OF CARBON IMPURITY FLUXES
IN THE LIMITER SHADOW
P. WIENHOLD, J. WINTER, H.G. ESSER, F. WAELBROECK, B. EMMOTH * and H.E. SATHERBLOM * Institut ftir Pla.smuph.v.sik, Kernforschungsanlage POB 1913, D-51 70 Jiilich, Fed. Rep. German?,
Jiilich GmbH, Association
Key words:
probe,
impurity
fluxes, carbon,
collection
Amsterdam
H. BERGSAKER
*,
EURA TOM-KFA.
TEXTOR
The carbon fluxes in the limiter shadow of TEXTOR have been measured with time resolution using the Stockholm-TEXTOR collection probe. For this, a new collector foil made of pure aluminum covered with amorphous silicon has been used. The area density of the collected carbon was determined from the elastic backscattering of protons of 1.75 MeV energy. A preliminary value of 4 X lOI cmA2 s- ’ was found for the carbon flux 2.4 cm into the limiter shadow.
1. Introduction TEXTOR is presently routinely carbonized [l], i.e. its liner and limiters are covered with an amorphous carbon layer (a-C:H). This reduces drastically the metal impurities in the plasma ( nMe/iie ( 1 X lo- 5). Oxygen is mostly below 1% [2]. Carbon is considered to be the most prominent plasma contaminant under these conditions. Its concentration in the scrape-off layer (SOL) can, in principle, be measured by collector probes [3], but a problem is that most collector materials are illsuited to measure the carbon fluxes escaping from the plasma. Materials such as pure aluminium are ruled out because the carbon delivered by the plasma cannot be distinguished from the carbon components adsorbed during the transport from the tokamak to the analysis station. A new method has been developed in order to measure carbon fluxes in the limiter shadow of TEXTOR, time and space resolved by use of the StockholmTEXTOR probe. This probe has been used earlier for the collection of metal and oxygen impurities on graphite or papyex substrates which were subsequently analysed by the Rutherford backscattering technique at the analysis station in Stockholm [4-71. It is located in an equatorial position 157 o apart from the main limiters in ion drift direction.
foil was wound around the cylindrical probe head (r = 25 mm) which rotates synchronously with the tokamak discharge within a housing with a radial slit aperture (width = 1.9 mm). The carbon transported by the plasma in the toroidal direction is deposited time resolved (resolution 240 ms) on the circumference of the spinning cylinder. The radial distribution can also be analyzed. After exposure to the plasma, the probe is withdrawn, dismounted and transported to Stockholm. The carbon deposit has been studied by elastic backscattering of protons of 1.75 MeV energy where the scattering ‘*C(p, p)‘*C has its maximum cross section of about 1 bam/sr [8]. Fig. 1 shows (upper curve) a typical backscattering spectrum on an exposed area (number of backscattered protons versus their energy). The steep edge at the right hand side corresponds to the
2. Method For the collection of carbon, a pure aluminum foil (99.999%) was covered by, an amorphous silicon layer (a-Si:H) of roughly 2000 A thickness and used as substrate. This layer was deposited by use of a glow discharge in a silane atmosphere. The adsorption of oxygen and carbon oxides between exposure and analysis is negligible even when the transport is made in air. The * Research
Institute of Physics, Association S-104 05 Stockholm, Sweden.
EURATOM-EFN
0022-3115/87/$03.50 0 Elsevier Science publishers (North-Holland Physics publishing Division)
B.V.
8
1000
Energy
of scattered
1 1500
protons(keVl-
Fig. 1. Spectrum of protons with an incident energy of 1.75 MeV after their elastic backscattering under 170° as taken at the angular position of 240 grad of the probe cylinder. The steep edge and the modulation are caused by Si and Al, respectively, in the substrate. The peak area A, of the collected carbon is determined from the difference spectrum (see text). The collected deuterium can also be determined from the spectrum.
P. Wienhold et al. / Carbon impurit_vfluxes
collisions with the heavy silicon nuclei. The irregular background in the central part originates from nonRutherford scattering in the aluminum substrate and is also present on areas where practically no carbon was deposited. The small peak at the far left is due to the collected deuterium [9] which was used as fueling gas for the plasma. The carbon peak occurs on top of the modulated curve. Its area A, is evaluated from the difference between the spectra obtained on exposed and unexposed areas of the foil (lower curve). The areal density of the collected carbon atoms can be determined from A, taking into account the number of incident protons, the reaction cross section and the solid angle of the detector. This has been translated into carbon fluxes assuming a sticking coefficient of unity and taking the time resolution into account. The method will be described in more detail in a forthcoming paper [lo]. 3. Results and discussion After a fresh carbonization of the liner and the stainless steel limiters (June 1985) and about 100 discharges, a sample was exposed to a series of shots in deuterium with nearly identical plasma parameters: Ir, = 340 kA, a = 46 cm, Br(0) = 2 T, duration = 3.5 s. The line averaged density was 6, = 2.5 X 10” cme3 for all exposures except the second where it was 3 x 1013 cm -3. Fig. 2 shows the flux density of carbon 2.4 cm into the limiter shadow (r = 48.4 cm), measured on the circumference of the probe cylinder (0 to 400 grad). For the first exposure (#16136), the-cylinder was brought to the angular position of 20 grad and then turned by 70 grad synchronously during the 3.5 s long
# 16157
in the limiter shadow
643
discharge. A new angular position 120 grad was chosen for the beginning of the next exposure and the procedure was iterated. The four exposed areas are separated by unexposed areas of 30 grad (shadowed). A higher plasma density, Z,(O) = 3.0 X lOi cme3 was chosen for the second exposure (# 16157) to assess the influence of iI,. The tendency to saturation was studied by overlaying two identical discharges (# 16165, 16166) in the third exposure, and nine (# 16167-16175) in the last. The radial distribution is not discussed here. The figure shows that at this depth of 2.4 cm in the SOL, the unexposed areas also carry carbon the origin of which is presently being examined. Its fluctuating amount corresponds to a,flux density about 4-5 x 1016 cm-* s-l (indicated as dashed line). This level is exceeded by the carbon fluxes observed during the plasma exposures. The time evolution is best seen on the third and fourth exposures. The fluxes increase to a maximum value within less than one second and already decrease during the flat top phase (0.3-2.2 s) of density and current. Increasing the electron density (second exposure) seems to decrease the carbon impurity production. Such a behaviour had been seen on the metallic impurity fluxes in the limiter shadow before carbonization (4-71. Overlaying two discharges (third exposure) approximately doubles the value of the carbon flux, whereas after nine discharges it has increased by a factor of about six only (fourth exposure). This indicates that there might be a tendency to saturate at the higher fluences. However, the lower flux value of about 4 X 1016 cme2 s-l obtained with one single discharge (difference between the maximum and the background level) should not be influenced by saturation. The flux of carbon impurities is larger by a factor of more than ten than the fluxes of metal impurities measured in the presence of metal limiters and liner before carbonization (51. In the plasma core the metal was then of the order of parts per thousand whereas the carbon concentration after carbonization is of the order of a per cent. 4. Conclusions
#16165
1’ /
# 16166
F
grad of crcumference
-
Fig. 2. Carbon flux @Joas determined on the circumference of the probe cylinder 2.4 cm in the limiter shadow of TEXTOR. Four exposed areas of 70 grad each, corresponding to the duration of 3.5 s of the plasma discharges, are separated from four unexposed areas (shadowed). The dashed line indicates a background. The inner walls of the machine were carbonized.
The new technique for the collection of carbon on amorphous silicon substrates (a-Si:H) and its analysis by the elastic backscattering of protons seems to be a promising method in order to determine carbon impurity fluxes time resolved in the limiter shadow of tokamaks. Still here are open questions, like the unknown sticking coefficient of C-atoms on such substrates and the degree of erosion by e.g. the hydrogen fluxes. Admittedly, the presented figure of 4 x 1016 crnm2 s-l for the carbon flux is very preliminary, but certainly not far from being realistic. The authors are very thankful to the TEXTOR-team and many colleagues for their help, in particular to W.
Beyer (IGV, JiiIich) who prepared his apparatus.
the substrate
foils in
References [l] J. Winter, these Proc. (PSI-VII),
J. Nucl. Mater. 145-147 (1987). [2] J. Schliiter, E. Graffmann, L. KBnen, F. Waelbroeck, G. Waidmann, J. Winter and TEXTOR team, 12th Eur. Conf. on Controlled Fusion and Plasma Physics, September 2-6. 1985, Budapest, Hungary. 133 G. Staudenmaier, J. Vat. Sci. Technol. A3 (1985) 1091. [4] H.E. Sltherblom, M. Braun, B. Emmoth. T. Fried, J. Hilke, P.A. Holmstriim, F. Waelbroeck, P.Wienhold and J. Winter, Nucl. Instr. and Meth. A240 (1985) 171.
J. Winter, Nucl. Instr. and Meth. A240 (1985) 171. [5] B. Emmoth, M. Braun. H.E. Satherblom, P. Wienhold. J. Winter and F. Waelbroeck, J. Nucl. Mater. 12X & 129 (1984) 1193. [6] B. Emmoth, S. Nagata, H.Bergs%ker, H.E. stherblom, P. Wienhold. J. Winter and F. Waelbroeck. these Proc. (PSIVII), J. Nucl. Mater. 145-147 (1987). [7] P. Wienhold, J. Winter, H.G. Esser, F. Waelbreock, H.E. Sltherblom, B. Emmoth and M. Braun, these Proc. (PSIVII), J. Nucl. Mater. 145-147 (1987). [S] E. Rauhala, Nucl. Instr. and Meth. B12 (1985) 447. [9] R.A. Langley, Proc. Int. Conf. on Radiation Effects and Tritium Technology for Fusion Reactors, Vol. IV, 1976, Springfield, USA (CONF-750989) p. 158. [lo] H. Bergs&er et al., to be published.