- Email: [email protected]
Analysis of the local re-deposition behavior of carbon at the main walls in TEXTOR by CD4 gas injection and Quartz Microbalance techniques
Journal of Nuclear Materials 415 (2011) S246–S249
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Analysis of the local re-deposition behavior of carbon at the main walls in TEXTOR by CD4 gas injection and Quartz Microbalance techniques H.G. Esser a,⇑, A. Kirschner a, D. Borodin a, D. Matveev b, K. Ohya c, O. Schmitz a, V. Philipps a a
Institute of Energy and Climate Research – Plasma Physics, Forschungszentrum Jülich GmbH, Association EURATOM-FZJ, Partner in the Trilateral Euregio Cluster, Jülich, Germany Department of Applied Physics, Ghent University, Plateaustraat 22, B-9000 Ghent, Belgium c Institute of Technology and Science, The University of Tokushima, Tokushima 770-8506, Japan b
a r t i c l e
i n f o
Article history: Available online 21 February 2011
a b s t r a c t The local re-deposition of carbon that is chemically eroded from the main wall of fusion devices has been experimentally investigated using a specially designed Quartz Microbalance system. The system has been placed in the far SOL of TEXTOR with the front plate parallel to the field lines and CD4 has been injected in the vicinity. The total re-deposition efficiency on the front plate was about 1%. Modelling with ERO and the 3D-GAPS code reproduced the measured deposition efficiencies showing values of 2%. The remaining difference can be attributed to chemical re-erosion of the re-deposited C layer by the hydrogen atoms produced from the CD4 injection. The data show that re-deposition behavior on remote places does not suffer from enhanced erosion, as needed for the plasma wetted area. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Wall erosion and subsequent formation of amorphous hydrogenated carbon layers (a-C:H) are serious problems for future steady state fusion devices. They are precursors of fuel (Tritium) accumulation and dust formation and thus limit the life time of the facilities. A large experimental effort is ongoing to measure prominent parameters to improve understanding and predictability of erosion, transport and deposition processes. The understanding of layer formation on plasma-wetted areas has improved in the recent past based on measurements with QMB systems in various fusion devices [1–4] in connection with ERO modelling [5]. However the database for re-deposition for remote, plasma-shadowed areas is still insufficient and further data are required to validate modelling. The aim of the presented experiment was to gain quantitative data of hydrogenated carbon layer formation at such areas. This was simulated by the injection of CD4 through a carrier front plate (CFP) located in the far SOL of TEXTOR with the CFP surface oriented parallel to the magnetic field lines. A Quartz Microbalance system (QMB) located behind an orifice of the CFP provided in situ, online and shot-resolved data of carbon deposition resulting from the local CD4 injection. Experimental observations have been modeled with ERO and 3D-GAPS codes.
A cylindrical carrier system was designed for the limiter lock system in TEXTOR. It was equipped with a Quartz Microbalance system (QMB) next to a gas inlet system both behind apertures in the protective carrier front plate (CFP). The dimension of the CFP itself is 102 mm in diameter, see Fig. 1. The gas inlet, a tube with an inner diameter of 4 mm located at the rim was flush mounted with the front surface of the CFP. The distance of gas inlet to QMB aperture was 19 mm. More apertures in the CFP allowed the exposure of Si samples for post-mortem analysis. The key item of the QMB is a disk like quartz-crystal integrated in an electrical circuit with a mass sensitivity of 7.5 10 9 g/Hz and a resolution of 3 Hz (1 monolayer at densities of 1 g/cm3). It was placed 8 mm recessed behind a circular aperture of 0.5 cm2 within the CFP. The principle of the QMB measuring method is explained in more detail in [6]. The gold plated surface of the QMB-crystal with a sensitive area of 1 cm2 was oriented parallel to the CFP surface and to the magnetic field lines so that its normal pointed towards the plasma centre. Thus, it is expected that predominantly neutrals pass the QMB aperture forming hydrogenated carbon layers on the crystal surface. The system was exposed to the far SOL of TEXTOR from the top of the vacuum vessel and the CFP was placed at well defined radial distances with respect to the LCFS of the plasma as shown in Fig. 2. CD4 was injected during the flat top phases of the plasma discharges and the carbon deposition efficiency Rdep on the QMB, defined as the number of carbon atoms deposited on the QMB relative to the amount of injected C atoms, has been studied in two parameters scans. At first, the dependence of the deposition efficiency Rdep was studied depending on the
⇑ Corresponding and presenting author. Address: Forschungszentrum Jülich, IEF4, 52425 Jülich, Germany. E-mail address: [email protected] (H.G. Esser). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.10.091
H.G. Esser et al. / Journal of Nuclear Materials 415 (2011) S246–S249
field BT = 2.5 T, line averaged density ne = 2.5 1019 m 3, minor plasma radius ra = 46 cm. The CD4 injection was solely applied during the 4 s flat top phases controlled by spectroscopic observation of CD and Da lines from top and horizontal as shown in Fig. 2. ERO modelling for ohmic discharges has been carried out for which the local electron density ne = 1 1017 m 3 and electron temperature Te = 4 eV in front of the CFP were extrapolated from values measured at 3.5 cm outside the LCFS by the He–beam diagnostic [7]. They were assumed to be constant further away from the LCFS.
19 mm QMB crystal behind orifice behindorifice
(a)
S247
(b) CD4 gas inlet
3. Results Fig. 1. Schematic: arrangement of QMB + gas inlet on carrier. (a) Carrier section through QMB crystal and CD4 gas inlet hole. (b) View from plasma centre toward the carrier front plate (CFP).
Carrier Carrier with withQMB QMB on limiter limiterlock lock
CD4 gas gas inlet inlet CD4
Last Last closed closedflux flux surface
Spectroscopical Spectroscopical observations observationsof of C C
Scrape Scrapeoff off layer layer (SOL)
Vacuum chamber
Fig. 2. Poloidal cross section of TEXTOR showing set up of carrier with QMB + gas inlet on limiter lock.
amount of injected CD4, with variations of the amount of puffed CD4 between 1.23 1019 and 3.32 1020 (±3%) molecules per discharge while the radial position of the CFP in the SOL was kept fixed at a minor radius of ra = 52 cm, i.e. 60 mm behind the LCFS at 46 cm. In a second scan the dependence of Rdep on the distance of the CPF to the LCFS was studied, which was altered between 50 mm and 80 mm. For this scan the amount of injected CD4 was kept constant at 2.48 1020 molecules per discharge. The majority of plasmas were beam heated with 800 kW for 4 s except a few ohmic ones carried out for reference. All NBI-heated discharges had the same plasma parameters: plasma current IP = 350 kA, magnetic
-2
areal C density [cm ]
16
3.1.1. Sweep of injected number of CD4 molecules In a first parameter study, the dependence of Rdep on the QMBcrystal was measured as function of the amount of injected CD4. The radial position of the CFP in the SOL was kept at a distance of 60 mm to the LCFS. The number of C atoms injected per discharge in form of CD4 was changed by more than a factor 10 in the range from 1.23 1019 to 3.32 1020. It has been found that the number of C atoms deposited on the QMB–crystal is proportional to the amount of injected gas, as shown in Fig. 3. This results in a constant Rdep on the QMB for the gas scan of 0.0058% ± 0.002% indicating, that the injected CD4 molecules do not change the local plasma significantly. No profound difference between ohmic and NBI-heated discharges was observed. 3.1.2. Sweep of radial distance of the QMB In a second parameter scan the amount of injected CD4 per discharge was kept fixed to 2.48 1020 molecules and the distance of the CFP with respect to the LCFS was varied between 50 mm and 80 mm. The C deposition per discharge was increasing with decreasing distance to the LCFS. Again no profound difference was observed between ohmic and beam heated shots. At the closest position of the CFP to LCFS, a maximum deposition of 1.9 1016 C/cm2 was measured whereas at the outermost position of 80 mm an areal C density [6] of 6.8 1015 C/cm2 was observed. This is shown in Fig. 4. The Rdep drops by about a factor 3 for moving the CFP from 50 mm to 80 mm. The integrated areal C density on the QMB crystal at the end of the experiment is equivalent to 4.16 1017 C atoms resulting in a Rdep = 0.005% on the QMB. 3.2. Local carbon deposition efficiencies measured by post-mortem analysis The carrier was removed after the experiment and the CFP surface showed characteristic interference colours of an inhomoge-
16
3.0x10
2.5x10
3.1. Local carbon deposition efficiencies
NBI shots ohmic shots
16
2.0x10
16
1.5x10
16
1.0x10
15
5.0x10
0.0 0.0
20
1.0x10
2.0x10
20
3.0x10
20
no of injected C atoms Fig. 3. Areal density of C deposited on QMB crystal vs. amount of CD4 injected into the SOL of TEXTOR.
Fig. 4. Local C deposition efficiency vs. distance of carrier front plate (CFP) to last closed flux surface (LCFS).
S248
H.G. Esser et al. / Journal of Nuclear Materials 415 (2011) S246–S249 Table 1 Areal density of deposited C on carrier front plate and QMB-crystal. Location on CFP
Fig. 5. Inhomogeneous colourful a–C:H layer on the carrier front plate (CFP) due to CD4 gas injection; locations of C analysis by EPMA indicated.
neous thin transparent a-C:D layer, see Fig. 5. The integral amount of C-atoms on the CFP was determined by colorimetry, calibrated with electron probe micro analysis (EPMA). EPMA was carried out at certain locations on the CFP and on Si samples marked with numbers in Fig. 5. In total, 8.2 1019 C-atoms were determined on the CFP. With a total gas injection of 8.05 1021 CD4 molecules, this corresponds to an integral deposition efficiency of 1% for the whole CFP area. The areal C density along an EPMA line scan, indicated as arrow in Fig. 5, starting at the rim of the CFP and crossing the gas inlet hole and the QMB crystal aperture is plotted in Fig. 6. About 1.70 1018 C/cm2 were found at maximum next to the QMB aperture. This value decreases towards the gas injection and reaches 8.0 1017 C/cm2 at its rim, a factor of 2 reduced. The values of the areal C densities measured by EPMA at the marked positions in Fig. 5 are listed in Table 1.
4. Modelling of carbon re-deposition with the ERO code The 3D impurity transport code ERO [5] has been used to model carbon flux from gas injection re-deposited to the QMB quartz within ohmic discharges. Necessary plasma parameters are taken from He beam measurements [7] of discharge 106958. This diagnostic delivers data up to about 30 mm outside the LCFS,
180
2
]
Basis material
16
1 2 3 4 5 7 6 8 9 10 11
83 10 161 1016 35 1016 170 1016 134 1016 250 1016 20 1016 274 1016 222 1016 180 1016 123 1016
SS SS QMB crystal SS SS SS SS Silicon Silicon Silicon Silicon
R CFP
8232 1016
SS
whereas the CFP was located further outside. Therefore electron density and temperature were assumed to be constant in the SOL with. Te = 4 eV, ne = 1 1011 cm3 as measured at the most outward position. The magnetic field is oriented parallel to the CFP surface and located deeply in the SOL with low plasma temperature and density, resulting in a very small plasma flux to the CFP surface. Therefore erosion and deposition due to background plasma is neglected in the modelling. Fig. 7 shows the observed and modelled normalised radial profiles of CD emission during the CD4 injection, with the CFP located 60 mm outside the LCFS [8]. Very good agreement is obtained between the modelled and observed pattern of the CD emission. For the modelling, 100% sticking is assumed in a first step for all returning species. The modelled number of carbon atoms to the entrance aperture of the QMB related to the amount of injected carbon is plotted in Fig. 4 for various locations of the CFP together with deposition efficiencies on the QMB. The observed radial dependence is well reproduced by the modelling, but the measured deposition efficiencies are about a factor of 20 smaller than the modelled ones. However, this calculation is based on the whole carbon flux reaching the QMB aperture, while the QMB is 8 mm recessed behind it. Modelling with the 3D-GAPS code [9] shows that this geometry with recessed QMB leads to reduction of carbon deposition on the QMB quartz by about a factor of 6, leaving a factor of 3–4 between modelling assuming sticking coefficient of 1 and the experiment. The modelling shows that the returning carbon and hydrocarbon species have low energies (<3 eV) and experiments and molecular dynamic (MD) simulations show large reflection coefficients of such low-energetic species. With this we can estimate decrease of the deposition on the QMB by a factor of about 2. ERO modelling for the CFP at 60 mm under ohmic conditions with reflection coefficients according to MD calculations show a deposition efficiency on the whole CFP surface of 2%, which comes close to the measured value of 1%. Moreover, the possible re-erosion of deposited carbon due to D atoms injected with CD4 and returning to the surface is not taken
160 1
140
CD light (a.u.)
120 100 80 60
C on QMB crystal 35 X 1016 [cm-2]
CD4 injection
areal carbon density [1016/cm 2 ]
200
Areal C density [cm
40 20 0 0
QMB crystal
10
20
EXP
0.8
ERO
0.6 Carrier front plate
0.4 0.2 0
30
40
50
distance from rim of carrier front plate (CFP) [mm] Fig. 6. Line scan of areal C density on the carrier front plate (CFP) crossing CD4 gas inlet and QMB aperture.
-2
-1
0
1
2
3
4
5
6
Distance from LCFS (cm) Fig. 7. Modelled and measured distribution of CD light in front of the carrier front plate (CFP) during CD4 puffing into the scrape off layer (SOL).
H.G. Esser et al. / Journal of Nuclear Materials 415 (2011) S246–S249
into account and could well explain the remaining difference. It can thus be concluded that the deposition at the remote, plasmashadowed areas of the CFP surface can be understood on the basis of existing knowledge on hydrocarbon transport and sticking physics without the need to assume a noticeable enhanced re-erosion of re-deposited carbon. We conclude again, that enhanced re-erosion of re-deposits seems to be specific for layers re-deposited under simultaneous impact of larger fluxes of energetic particles [11]. 5. Summary In-situ and shot-resolved parameter studies of carbon layer formation in remote areas (far SOL) of TEXTOR have been carried out using a QMB system combined with an adjacent gas inlet on a movable carrier system. Local re-deposition was measured for a variation of the amount of injected CD4 at fixed radial location in the SOL and a radial scan in the SOL with the amount of injected CD4 kept constant. A local C deposition efficiency Rdep of 1% on the whole CFP surface was deduced from post-mortem analysis. It represents deposition integrated over all discharges including the gas and the radial scan as well as ohmic and NBI-heated conditions. Modelling of the carbon re-deposition for ohmic discharges with
S249
ERO and 3D-GAPS (transport inside the QMB housing) was done under the assumption of negligible plasma flux at these remote areas and therefore without re-erosion. It resulted in a re-deposition efficiency of 2% and is thus in good agreement with the measured data both in the absolute re-deposition as well as the radial dependence. It can be concluded that observed re-deposition from CD4 injection at remote surface in far SOL of TEXTOR can be understood without assumption of enhanced re-erosion of re-deposits, which typically is required to reproduce layer formation at plasmawetted areas. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [11]
C.H. Skinner et al., J. Nucl. Mater. 363–365 (2007) 247. V. Rohde et al., J. Nucl. Mater. 290–293 (2001) 317. H.G. Esser et al., J. Nucl. Mater. 390–391 (2009) 148. H.G. Esser et al., J. Nucl. Mater. 363–365 (2007) 146. A. Kirschner et al., Nucl. Fusion 40 (2000) 989. H.G. Esser et al., Fusion Eng. Des. 66–68 (2003) 855. O. Schmitz et al., Plasma Phys. Control. Fusion 50 (2008) 115004. R. Ding et al., Plasma Phys. Control. Fusion 51 (2009) 055019. D. Matveev et al., Plasma Phys. Control. Fusion 52 (2010) 075007. A. Kirschner et al., J. Nucl. Mater. 328 (2004) 62.
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Analysis of the local re-deposition behavior of carbon at the main walls in TEXTOR by CD4 gas injection and Quartz Microbalance techniques H.G. Esser a,⇑, A. Kirschner a, D. Borodin a, D. Matveev b, K. Ohya c, O. Schmitz a, V. Philipps a a
Institute of Energy and Climate Research – Plasma Physics, Forschungszentrum Jülich GmbH, Association EURATOM-FZJ, Partner in the Trilateral Euregio Cluster, Jülich, Germany Department of Applied Physics, Ghent University, Plateaustraat 22, B-9000 Ghent, Belgium c Institute of Technology and Science, The University of Tokushima, Tokushima 770-8506, Japan b
a r t i c l e
i n f o
Article history: Available online 21 February 2011
a b s t r a c t The local re-deposition of carbon that is chemically eroded from the main wall of fusion devices has been experimentally investigated using a specially designed Quartz Microbalance system. The system has been placed in the far SOL of TEXTOR with the front plate parallel to the field lines and CD4 has been injected in the vicinity. The total re-deposition efficiency on the front plate was about 1%. Modelling with ERO and the 3D-GAPS code reproduced the measured deposition efficiencies showing values of 2%. The remaining difference can be attributed to chemical re-erosion of the re-deposited C layer by the hydrogen atoms produced from the CD4 injection. The data show that re-deposition behavior on remote places does not suffer from enhanced erosion, as needed for the plasma wetted area. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Wall erosion and subsequent formation of amorphous hydrogenated carbon layers (a-C:H) are serious problems for future steady state fusion devices. They are precursors of fuel (Tritium) accumulation and dust formation and thus limit the life time of the facilities. A large experimental effort is ongoing to measure prominent parameters to improve understanding and predictability of erosion, transport and deposition processes. The understanding of layer formation on plasma-wetted areas has improved in the recent past based on measurements with QMB systems in various fusion devices [1–4] in connection with ERO modelling [5]. However the database for re-deposition for remote, plasma-shadowed areas is still insufficient and further data are required to validate modelling. The aim of the presented experiment was to gain quantitative data of hydrogenated carbon layer formation at such areas. This was simulated by the injection of CD4 through a carrier front plate (CFP) located in the far SOL of TEXTOR with the CFP surface oriented parallel to the magnetic field lines. A Quartz Microbalance system (QMB) located behind an orifice of the CFP provided in situ, online and shot-resolved data of carbon deposition resulting from the local CD4 injection. Experimental observations have been modeled with ERO and 3D-GAPS codes.
A cylindrical carrier system was designed for the limiter lock system in TEXTOR. It was equipped with a Quartz Microbalance system (QMB) next to a gas inlet system both behind apertures in the protective carrier front plate (CFP). The dimension of the CFP itself is 102 mm in diameter, see Fig. 1. The gas inlet, a tube with an inner diameter of 4 mm located at the rim was flush mounted with the front surface of the CFP. The distance of gas inlet to QMB aperture was 19 mm. More apertures in the CFP allowed the exposure of Si samples for post-mortem analysis. The key item of the QMB is a disk like quartz-crystal integrated in an electrical circuit with a mass sensitivity of 7.5 10 9 g/Hz and a resolution of 3 Hz (1 monolayer at densities of 1 g/cm3). It was placed 8 mm recessed behind a circular aperture of 0.5 cm2 within the CFP. The principle of the QMB measuring method is explained in more detail in [6]. The gold plated surface of the QMB-crystal with a sensitive area of 1 cm2 was oriented parallel to the CFP surface and to the magnetic field lines so that its normal pointed towards the plasma centre. Thus, it is expected that predominantly neutrals pass the QMB aperture forming hydrogenated carbon layers on the crystal surface. The system was exposed to the far SOL of TEXTOR from the top of the vacuum vessel and the CFP was placed at well defined radial distances with respect to the LCFS of the plasma as shown in Fig. 2. CD4 was injected during the flat top phases of the plasma discharges and the carbon deposition efficiency Rdep on the QMB, defined as the number of carbon atoms deposited on the QMB relative to the amount of injected C atoms, has been studied in two parameters scans. At first, the dependence of the deposition efficiency Rdep was studied depending on the
⇑ Corresponding and presenting author. Address: Forschungszentrum Jülich, IEF4, 52425 Jülich, Germany. E-mail address: [email protected] (H.G. Esser). 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.10.091
H.G. Esser et al. / Journal of Nuclear Materials 415 (2011) S246–S249
field BT = 2.5 T, line averaged density ne = 2.5 1019 m 3, minor plasma radius ra = 46 cm. The CD4 injection was solely applied during the 4 s flat top phases controlled by spectroscopic observation of CD and Da lines from top and horizontal as shown in Fig. 2. ERO modelling for ohmic discharges has been carried out for which the local electron density ne = 1 1017 m 3 and electron temperature Te = 4 eV in front of the CFP were extrapolated from values measured at 3.5 cm outside the LCFS by the He–beam diagnostic [7]. They were assumed to be constant further away from the LCFS.
19 mm QMB crystal behind orifice behindorifice
(a)
S247
(b) CD4 gas inlet
3. Results Fig. 1. Schematic: arrangement of QMB + gas inlet on carrier. (a) Carrier section through QMB crystal and CD4 gas inlet hole. (b) View from plasma centre toward the carrier front plate (CFP).
Carrier Carrier with withQMB QMB on limiter limiterlock lock
CD4 gas gas inlet inlet CD4
Last Last closed closedflux flux surface
Spectroscopical Spectroscopical observations observationsof of C C
Scrape Scrapeoff off layer layer (SOL)
Vacuum chamber
Fig. 2. Poloidal cross section of TEXTOR showing set up of carrier with QMB + gas inlet on limiter lock.
amount of injected CD4, with variations of the amount of puffed CD4 between 1.23 1019 and 3.32 1020 (±3%) molecules per discharge while the radial position of the CFP in the SOL was kept fixed at a minor radius of ra = 52 cm, i.e. 60 mm behind the LCFS at 46 cm. In a second scan the dependence of Rdep on the distance of the CPF to the LCFS was studied, which was altered between 50 mm and 80 mm. For this scan the amount of injected CD4 was kept constant at 2.48 1020 molecules per discharge. The majority of plasmas were beam heated with 800 kW for 4 s except a few ohmic ones carried out for reference. All NBI-heated discharges had the same plasma parameters: plasma current IP = 350 kA, magnetic
-2
areal C density [cm ]
16
3.1.1. Sweep of injected number of CD4 molecules In a first parameter study, the dependence of Rdep on the QMBcrystal was measured as function of the amount of injected CD4. The radial position of the CFP in the SOL was kept at a distance of 60 mm to the LCFS. The number of C atoms injected per discharge in form of CD4 was changed by more than a factor 10 in the range from 1.23 1019 to 3.32 1020. It has been found that the number of C atoms deposited on the QMB–crystal is proportional to the amount of injected gas, as shown in Fig. 3. This results in a constant Rdep on the QMB for the gas scan of 0.0058% ± 0.002% indicating, that the injected CD4 molecules do not change the local plasma significantly. No profound difference between ohmic and NBI-heated discharges was observed. 3.1.2. Sweep of radial distance of the QMB In a second parameter scan the amount of injected CD4 per discharge was kept fixed to 2.48 1020 molecules and the distance of the CFP with respect to the LCFS was varied between 50 mm and 80 mm. The C deposition per discharge was increasing with decreasing distance to the LCFS. Again no profound difference was observed between ohmic and beam heated shots. At the closest position of the CFP to LCFS, a maximum deposition of 1.9 1016 C/cm2 was measured whereas at the outermost position of 80 mm an areal C density [6] of 6.8 1015 C/cm2 was observed. This is shown in Fig. 4. The Rdep drops by about a factor 3 for moving the CFP from 50 mm to 80 mm. The integrated areal C density on the QMB crystal at the end of the experiment is equivalent to 4.16 1017 C atoms resulting in a Rdep = 0.005% on the QMB. 3.2. Local carbon deposition efficiencies measured by post-mortem analysis The carrier was removed after the experiment and the CFP surface showed characteristic interference colours of an inhomoge-
16
3.0x10
2.5x10
3.1. Local carbon deposition efficiencies
NBI shots ohmic shots
16
2.0x10
16
1.5x10
16
1.0x10
15
5.0x10
0.0 0.0
20
1.0x10
2.0x10
20
3.0x10
20
no of injected C atoms Fig. 3. Areal density of C deposited on QMB crystal vs. amount of CD4 injected into the SOL of TEXTOR.
Fig. 4. Local C deposition efficiency vs. distance of carrier front plate (CFP) to last closed flux surface (LCFS).
S248
H.G. Esser et al. / Journal of Nuclear Materials 415 (2011) S246–S249 Table 1 Areal density of deposited C on carrier front plate and QMB-crystal. Location on CFP
Fig. 5. Inhomogeneous colourful a–C:H layer on the carrier front plate (CFP) due to CD4 gas injection; locations of C analysis by EPMA indicated.
neous thin transparent a-C:D layer, see Fig. 5. The integral amount of C-atoms on the CFP was determined by colorimetry, calibrated with electron probe micro analysis (EPMA). EPMA was carried out at certain locations on the CFP and on Si samples marked with numbers in Fig. 5. In total, 8.2 1019 C-atoms were determined on the CFP. With a total gas injection of 8.05 1021 CD4 molecules, this corresponds to an integral deposition efficiency of 1% for the whole CFP area. The areal C density along an EPMA line scan, indicated as arrow in Fig. 5, starting at the rim of the CFP and crossing the gas inlet hole and the QMB crystal aperture is plotted in Fig. 6. About 1.70 1018 C/cm2 were found at maximum next to the QMB aperture. This value decreases towards the gas injection and reaches 8.0 1017 C/cm2 at its rim, a factor of 2 reduced. The values of the areal C densities measured by EPMA at the marked positions in Fig. 5 are listed in Table 1.
4. Modelling of carbon re-deposition with the ERO code The 3D impurity transport code ERO [5] has been used to model carbon flux from gas injection re-deposited to the QMB quartz within ohmic discharges. Necessary plasma parameters are taken from He beam measurements [7] of discharge 106958. This diagnostic delivers data up to about 30 mm outside the LCFS,
180
2
]
Basis material
16
1 2 3 4 5 7 6 8 9 10 11
83 10 161 1016 35 1016 170 1016 134 1016 250 1016 20 1016 274 1016 222 1016 180 1016 123 1016
SS SS QMB crystal SS SS SS SS Silicon Silicon Silicon Silicon
R CFP
8232 1016
SS
whereas the CFP was located further outside. Therefore electron density and temperature were assumed to be constant in the SOL with. Te = 4 eV, ne = 1 1011 cm3 as measured at the most outward position. The magnetic field is oriented parallel to the CFP surface and located deeply in the SOL with low plasma temperature and density, resulting in a very small plasma flux to the CFP surface. Therefore erosion and deposition due to background plasma is neglected in the modelling. Fig. 7 shows the observed and modelled normalised radial profiles of CD emission during the CD4 injection, with the CFP located 60 mm outside the LCFS [8]. Very good agreement is obtained between the modelled and observed pattern of the CD emission. For the modelling, 100% sticking is assumed in a first step for all returning species. The modelled number of carbon atoms to the entrance aperture of the QMB related to the amount of injected carbon is plotted in Fig. 4 for various locations of the CFP together with deposition efficiencies on the QMB. The observed radial dependence is well reproduced by the modelling, but the measured deposition efficiencies are about a factor of 20 smaller than the modelled ones. However, this calculation is based on the whole carbon flux reaching the QMB aperture, while the QMB is 8 mm recessed behind it. Modelling with the 3D-GAPS code [9] shows that this geometry with recessed QMB leads to reduction of carbon deposition on the QMB quartz by about a factor of 6, leaving a factor of 3–4 between modelling assuming sticking coefficient of 1 and the experiment. The modelling shows that the returning carbon and hydrocarbon species have low energies (<3 eV) and experiments and molecular dynamic (MD) simulations show large reflection coefficients of such low-energetic species. With this we can estimate decrease of the deposition on the QMB by a factor of about 2. ERO modelling for the CFP at 60 mm under ohmic conditions with reflection coefficients according to MD calculations show a deposition efficiency on the whole CFP surface of 2%, which comes close to the measured value of 1%. Moreover, the possible re-erosion of deposited carbon due to D atoms injected with CD4 and returning to the surface is not taken
160 1
140
CD light (a.u.)
120 100 80 60
C on QMB crystal 35 X 1016 [cm-2]
CD4 injection
areal carbon density [1016/cm 2 ]
200
Areal C density [cm
40 20 0 0
QMB crystal
10
20
EXP
0.8
ERO
0.6 Carrier front plate
0.4 0.2 0
30
40
50
distance from rim of carrier front plate (CFP) [mm] Fig. 6. Line scan of areal C density on the carrier front plate (CFP) crossing CD4 gas inlet and QMB aperture.
-2
-1
0
1
2
3
4
5
6
Distance from LCFS (cm) Fig. 7. Modelled and measured distribution of CD light in front of the carrier front plate (CFP) during CD4 puffing into the scrape off layer (SOL).
H.G. Esser et al. / Journal of Nuclear Materials 415 (2011) S246–S249
into account and could well explain the remaining difference. It can thus be concluded that the deposition at the remote, plasmashadowed areas of the CFP surface can be understood on the basis of existing knowledge on hydrocarbon transport and sticking physics without the need to assume a noticeable enhanced re-erosion of re-deposited carbon. We conclude again, that enhanced re-erosion of re-deposits seems to be specific for layers re-deposited under simultaneous impact of larger fluxes of energetic particles [11]. 5. Summary In-situ and shot-resolved parameter studies of carbon layer formation in remote areas (far SOL) of TEXTOR have been carried out using a QMB system combined with an adjacent gas inlet on a movable carrier system. Local re-deposition was measured for a variation of the amount of injected CD4 at fixed radial location in the SOL and a radial scan in the SOL with the amount of injected CD4 kept constant. A local C deposition efficiency Rdep of 1% on the whole CFP surface was deduced from post-mortem analysis. It represents deposition integrated over all discharges including the gas and the radial scan as well as ohmic and NBI-heated conditions. Modelling of the carbon re-deposition for ohmic discharges with
S249
ERO and 3D-GAPS (transport inside the QMB housing) was done under the assumption of negligible plasma flux at these remote areas and therefore without re-erosion. It resulted in a re-deposition efficiency of 2% and is thus in good agreement with the measured data both in the absolute re-deposition as well as the radial dependence. It can be concluded that observed re-deposition from CD4 injection at remote surface in far SOL of TEXTOR can be understood without assumption of enhanced re-erosion of re-deposits, which typically is required to reproduce layer formation at plasmawetted areas. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [11]
C.H. Skinner et al., J. Nucl. Mater. 363–365 (2007) 247. V. Rohde et al., J. Nucl. Mater. 290–293 (2001) 317. H.G. Esser et al., J. Nucl. Mater. 390–391 (2009) 148. H.G. Esser et al., J. Nucl. Mater. 363–365 (2007) 146. A. Kirschner et al., Nucl. Fusion 40 (2000) 989. H.G. Esser et al., Fusion Eng. Des. 66–68 (2003) 855. O. Schmitz et al., Plasma Phys. Control. Fusion 50 (2008) 115004. R. Ding et al., Plasma Phys. Control. Fusion 51 (2009) 055019. D. Matveev et al., Plasma Phys. Control. Fusion 52 (2010) 075007. A. Kirschner et al., J. Nucl. Mater. 328 (2004) 62.