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Collection efficiency for carbon and deuterium of different surfaces when exposed to the scrape-off plasma in tokamaks
593
Journal of Nuclear Materials 162-164 (1989) 593-597 North-Holland, Amsterdam
COLLECTION EFFICIENCY FOR CARBON AND DEUTERIUM OF DIFFERENT WHEN EXPOSED TO THE SCRAPE-OFF PLASMA IN TOKAMAKS H. BERGSAKER’,
M. RUBEL
2, B. EMMOTH
1 and P. WIENHOLD
SURFACES
3
’ Research Institute of Physics, Association Euratom-NFR,
S-104 05 Stockholm, Sweden ’ Space Research Centre, Ordona 21, 01-237 Warszawa, Poland 3 Institut fti Plasmaphysik, KFA, P.O. Box 1913, D-5170 Jiilich, Fed. Rep. Germany
Key words: collector probe, carbon, tokamak Carbon and deuterium have been collected on surface probes inserted in the limiter shadow of TEXTOR, with carbonized conditions, i.e. with carbonized graphite limiters and carbonized liner. The exposure time ranges from - 0.1 s to - 150 s, and a number of different collector materials have been used: Be, graphite, stainless steel, Al, Si, Ti and Au. The collection efficiency of these different surfaces for carbon and deuterium is compared. An experiment with secondary collectors has also been nerforrned in the scrape-off nlasma at TEXTOR, in order to estimate the combined effect of backscattering and erosion _ _ at the collector surface due to physical sputtering.
1. Introduction In carbonized tokamaks and in tokamaks with graphite limiters it has been shown that large amounts of carbon are eroded at points in the machine which are in intense contact with the plasma, and redeposited at other positions. Where carbon is deposited, hydrogen isotopes are also included, and thick layers of mainly carbon, saturated with hydrogen build up [1,2]. The physical structure of these layers has been identified as that of amorphous, hydrogenated carbon (a-C : D) [3]. This codeposition process is important to the tritium inventory in future experiments, and it also constitutes a pumping mechanism which may help to reduce recycling [4]. The use of surface collector probes is a well established technique to study impurity fluxes in the boundary plasma of tokamaks, and to investigate erosion and deposition of wall material [S]. When hydrogen is implanted into solid surfaces, only a certain, energy dependent saturated area1 density of hydrogen can be retained within the solid, even if it is exposed to large ion fluences. Consequently, both ion fluences and ion energy can in principle be determined by collecting hydrogen at surfaces in a controlled way [6]. When the hydrogen deposition is determined rather by the deposition of carbon, then the deposition of carbon on a graphite surface may be inferred from the amount of hydrogen which is retained [2].
0022-3115/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
The flux of carbon in the scrape-off plasma of tokamaks has recently been determined according to the deposition rate of carbon on surface probes of various shapes. The carbon collecting materials have been e.g. crystalline silicon [7], amorphous silicon deposited on aluminum substrate [8], or stainless steel [3], and the area1 density of collected carbon has been measured by enhanced proton backscattering [8], as well as by visual inspection of the interference fringes [3]. Earlier experiments with carbon collecting surface probes have been performed using stainless steel collectors and detecting carbon with AES [9], beryllium collectors and RBS analysis [lo], and using single crystal silicon collectors in combination with carbon analysis by channeling RBS
WI. In reality, the observed deposition rate is different from the flux, in that the sticking of carbon carrying species (atomic and molecular ions) at surfaces is not unity. The sticking probability may possibly depend on the physical structure and chemical composition of the collecting surface. The net deposition rate can also be further reduced by erosion of deposited carbon. Furthermore, if carbon which has been deposited at the surface is able to diffuse into the bulk material of the probe, it may be impossible to determine the deposition rate with surface analysis techniques. The present investigation was undertaken in order to know if there is any difference between different kinds of surfaces with respect to collection of carbon and B.V.
594
H. BergsBker et al. / Colleciion efficiency for carbon and deuterrum
hydrogen by co-deposition. Probes carrying arrays of many different materials were exposed to tokamak scrape-off plasmas (in TEXTOR and in JET [12]) and the amount of carbon and hydrogen which was retained was determined afterwards by ion beam analysis techniques. The influence of backscattering and/or re-erosion of deposited material at the primary collecting surface was investigated by collecting carbon and hydrogen on secondary surfaces.
2. Experimental As a part of the preliminary collector probe experiments at JET [7], cylindrical probe shields of graphite, Inconel and a shield which was made partly of graphite and partly of Inconel were introduced in the limiter shadow using the Fast Transfer and Plasma Boundary Probe Systems, and were exposed to one or more complete discharges. At TEXTOR, fixed or rotating surface probes have been exposed to the scrape-off using the Stockholm-TEXTOR probe manipulator [13]. A probe carrying two arrays of collector pieces of Be, Al, Si, Ti, Ni and various kinds of graphite @chunk, polished and non-polished, Papyex) was exposed to 5 discharges in the limiter shadow in TEXTOR. Strips of different materials were mounted on cylindrical supports and exposed to the plasma in a time resolved manner by rotating the cylinders behind a slit aperture. All samples were exposed to deuterium plasmas. samples were brought to After being exposed, Stockholm for analysis. The area1 density of deuterium which had been retained was determined using the D( 3He, p)4He nuclear reaction with 0.8 MeV 3He. The amount of collected carbon was measured, when possible (little carbon in the substrate bulk material, sufficiently much carbon collected at the surface), by means of the non-Rutherford l2 C(p, p)12 C elastic backscattering, of 1.735 and/or 1.5 MeV protons. In the case of graphite collectors, carbon is deposited at the surface together with oxygen, metals and deuterium. If it can be established that these occluded species do not diffuse, and that ion energies are sufficiently low that ions are not able to penetrate substantially below the surface, then in principle it is possible to infer the amount of carbon which has been deposited from depth profiles of deuterium, oxygen and metals
[W Since impurity fluxes and other relevant plasma parameters are known to vary smoothly in the machine, any observed discontinuity in the deposition rate for carbon and deuterium when going from one substrate to
an adjacent different one must be attributed to properties of the collecting surface. To study such discontinuities consequently gives a possibility to compare relative collection efficiencies of different substrates. In a different type of experiment, carbon and deuterium were collected on rotating cylinders, carrying an aluminum foil covered with amorphous, hydrogenated silicon [8] and a stainless steel foil, respectively. Backscattered or re-eroded carbon and deuterium were collected on secondary collectors, in the first case on narrow strips of Al with a-Si: H which had been mounted across the slit aperture, in the second case on the backside of the stainless steel slits.
3. Results and discussion Fig. 1 shows the area1 density of deuterium and carbon deposited on a probe shield which has been exposed to seven discharges in JET (using the FTS [7]), three of which were disruptive. The surface is partly graphite and partly inconel. The general, more or less exponential decrease is just a radial decrease in deposition rate which is usually observed with collector probes, and which can be attributed to cross field diffusion of ions from the confined plasma. A discontinuity is observed, suggesting that up to twice as much deuterium has been retained at the graphite surface, compared to the inconel. On the inconel substrate, the areal density of carbon was determined directly by enhanced proton
slj
1
Q
DISTANCE
FROM
PLASMA
(mm)
Fig. 1. Area1 concentrations of deuterium and carbon collected on a probe shield, made of graphite and partly of inconel, exposed to the scrape-off plasma in JET (# 11143-49). Carbon is determined directly on the inconel surface while on the graphite this amount can be determined indirectly 1121.
595
ii Bergsaker et al. / Colleciiorzefficiencyfor carbon and deuterium 100
a I-1
T
E 0) zl c
z0 5
1 i
Ti’ A, 0.1 3s
DISTANCE
FROM
PLASMA
1
(mm)
Fig. 2. Area1 concentration of deuteriwn and carbon collected on different metal substrates exposed to the edge plasma in TEXTOR: (a) P31663 and 64; (b) #31809 and IO.
backscattering. On the graphite surface, the thickness of the deposited layer was determined from depth distributions of oxygen and metals [12]. The procedure is valid only if the impurities do not penetrate substantiaUy below the original surface. This condition is likely to be fulfilled, since the implantation range in graphite of ions with energies which can be expected (in the lower 100 eV range) is small compared to the observed depth profiles. The argument gains support from the fact that the calculated areal density of deposited carbon implies the same deuterium to carbon ratio (about 20%) as in the layer which has been deposited on inconel substrate. It thus appears that the deposition rate of carbon and deuterium is lower on inconel substrate than on graphite. Fig. 2 shows examples where carbon and deuterium have been collected on strips of various materials in the limiter shadow of TEXTOR. Discontinuities occur occasionally, particularly in the areal density of deuterium. There is a tendency, although not very consistent, to have larger areaI densities of deuterium on substrates of low-2 elements. Since the collected areal density is low in these cases, in contrast to fig. 1, such excursions may be expected due to differences in implantation range and saturation concentration of deu-
terium in the substrates. No drastic differences in the collection rate of carbon were observed, neither in the examples shown in fig. 2, nor in a number of similar cases. Fig. 3 shows two examples where a rough graphite surface appears to behave differently, compared to other surfaces, including mechanically polished graphite. Thus, it may be tentatively concluded that the collection rate of carbon and deuterium does generally not depend on the collector material, but that there may be an influence of the surface structure. Given that any difference at all is observed this is reasonable from the point of view that, once the surface has been covered with a hydrogenated carbon layer, the chemical composition at the surface should be the same, regardless of the substrate material. However, the number of observations are yet very limited, and more experiments would be necessary to make definite conclusions, except that the differences in collection rate are generally a factor two or less. Figs. 4 and 5 shows a somewhat different attempt to estimate how the deposition rate of carbon at a probe surface relates to the flux in the plasma. Carbon and deuterium species are allowed to hit a collector surface in the scrape-off plasma. The backside of the aperture
100
0.59
_a
DISTANCE
FROM
PLASMA
(mm)
Fig. 3. Area1 concen~ation of deuterium coliected on different carbon substrates and amorphous hydrogenated silicon exposed to edge plasma in TEXTOR: (a) #29996-98; (b) #29185, 86,94-98.
596
H. Bergsdker
<
+---i$ f w
et al. / Collection efficiency for carbon and deuterium
surface
Fig. 4. Aperture slits and a probe surface; a schematic illustration of the basis for calculations of deposition/re-erosion effects. The collecting surfaces are orthogonal to the magnetic field direction. slits acts as secondary collector, which presumably collects a fraction of the carbon and deuterium which is reflected or eroded at the primary collector surface. The ratio of deposits on the secondary collector to those on the primary is particularly interesting, since it may give information which is independent of the incident flux. The experiment was carried out with stainless steel slits and stainless steel foil collector which were exposed to roughly 150 s of TEXTOR discharges. The slit dimensions were w = 4.5 mm and d = 1.5 mm. The deuterium content on the backside carbon layer is generally lower than that on the front side. The ratio of carbon concentrations on the back and on the front side is more or less constant, in the range 0.12-0.20. at the distance 30-50 mm from the plasma. However, it is clear that at the radial position 20-25 mm the carbon concentration detected on the primary collector decreases sharply.
We first attempt to interpret the observations in terms of reflection and/or self sputtering at the primary collector surface. Carbonaceous ions stream along the magnetic field lines and impinge with flux density @c at a collector probe surface through a long aperture of width w. Backscattered or sputtered atoms may be deposited on the backside of the slits which define the aperture, and which act as secondary collectors. Carbon is also collected on the front side surface of the slits. Suppose now that carbon is ejected with cosine angular distribution from the primary collector surface within the slit image (0 < .$ < w) at rate r@c (r may be the probability of reflection, or the self-sputtering yield). The flux density of carbon incident at the back side of the slit will then be @pk(X)
_
r@cd2 /= 77
=- r
&,j‘“’
” [(E+x)‘+q’+d’]’
Pm
@c 2 [
d5
w+x
(w~-&F.
1
(1)
If carbon is deposited on the front side of the slit at a rate (1 - r)@c, and if carbon which has been reflected or sputtered at the probe surface has a high probability to stick when it hits the secondary collector, then the ratio of the area1 density collected on the back side of the slit at x = 0 to the area1 density on the front side will be cb to)
p
front
ICf7
.-
c
E
z0 5
D 10
z a1
side
back side
/
l 0
n
l
3 I
b
IIJ1l”““’ 20
DISTANCE
Fig. 5. Area1 concentrations
60
40 FROM
PLASMA
(mm)
of deuterium and carbon mea-
sured on the both sides of the aperture slits exposed TEXTOR discharges (X29486-29548).
to 150 s of
--=qik+$ Cf
(2)
Inserting the ratio of unity in eq. (2) gives r = 0.67, whereas the ratio 0.2 gives r = 0.3. Alternatively, the observed redeposition may be explained with hydrogen sputtering. For example, if the deuterium flux at 30 mm outside the last closed flux surface is taken to be @p = 2 x lo*’ m-* s-l [14] and the electron temperature T’ = 7.5 eV [15], then yield for physical sputtering by deuterium is 2.5 X lop3 [16], and consequently 7.5 X lo*’ C atoms me2 should have been sputtered at the primary collector during 150 s. Provided that a large fraction of the physically sputtered atoms stick when they reach the secondary collector then, by the geometrical argument of eq. (l), roughly 3 X 10” C atoms carbon should have been m -* of physically sputtered collected at the back side of the slit. This is about one order of magnitude less than what is found at that radial position and, moreover, with a scrape-off layer width of 10 mm for particle flux and 35 mm for
H. Bergsdker et al. / Collection efficiency for carbon and deuterium
temperature [15], physical sputtering would be expected to fall off much more rapidly with increasing radius than the modest decrease in the thickness of the redeposition layer on the secondary collector. The chemical erosion yield is not so well known at high fluxes [16], but if the yield is as high as 3 X lo-* [16], the expected erosion rate at 30 mm from the plasma would have been 6 x 1019 m-* s-l, i.e. 9 x lo*’ m-* during 150 s of the exposure. This value is almost equal to the carbon concentration measured at the primary collector. At 20 mm from the confined plasma, the areal density of carbon on the primary collector drops as the plasma is approached, without an associated drop in the amount of carbon on the secondary collector. The drop at the primary collector can not be due only to an increase in erosion by physical sputtering, since it would then be accompanied by an increase at the secondary collector. It may be due to chemical erosion, since carbon which is released in molecular form should have low probability of sticking at the secondary collector. The drop can also be attributed to a genuine decrease of the incident carbon flux. Such a radial distribution is expected if the carbon originates mainly at the liner (wall), and becomes ionized before reaching the confined plasma [17]. The absence of a corresponding decrease at the secondary collector could in this case be due to a simultaneously increasing rate of physical sputtering, or to an increasing reflection probability. Clearly new experiments of the described type, under various conditions and preferably with ion saturation current and electron temperature being measured in immidiate vicinity to the collector surfaces, should help to clarify this point. From the large amounts of carbon found on the secondary collectors it is clear that, due to reflection, re-erosion or both, the deposition rate of carbon on surface probes which are exposed to plasma under conditions similar to those given above can not immediately be translated into carbon fluxes in the scrapeoff plasma.
The observed differences are typically a factor two or less. The net deposition rate is determined by incident flux, sticking probability and the rate of re-erosion, and the observed differences in collection efficiency may be due to differences either in sticking or in erosion rate; it is difficult at present to propose specific mechanism. The influence of reflection and/or re-erosion at the collector surfaces was investigated in an experiment with secondary collectors. It is shown that the combined effect of reflection and re-erosion is not negligible in experiments where carbon is collected on surfaces which are introduced in the scrape-off plasma of tokamaks, and that consequently great care has to be taken in converting deposition rates at surfaces to flwces in the plasma.
References [l] W.R. Wampler,
[2] [3]
[4] [5] [6] [7]
[8] [9] [lo] [ll] [12]
[13] 4. Conclusions The rate of deuterium and carbon deposition on collector surfaces which are exposed to the scrape-off plasma in tokamaks did not in general show a strong dependence on substrate material, but evidence suggests that there is probably an influence of surface roughness.
597
B.L. Doyle and A.E. Pontau, J. Nucl. Mater. 145-147 (1987) 353. H. Bergs%ker et al., J. Nucl. Mater. 145-147 (1987) 727. P. Wienhold et al., Prcc. 14th Eur. Conf. on Controlled Fusion and Plasma Physics, Madrid, Spain, 1987, Part II, p. 782. W.L. Hsu and R.A. Causey, J. Vat. Sci. Technol. A5 (1987) 2768. G. Staudenmaier, J. Vat. Sci. Technol. A3 (1985) 1091. W.R. Wampler, D.K. Brice and C.W. Magee, J. Nucl. Mater. 102 (1982) 304. H. Bergs&er et al., Proc. 14th Eur. Conf on Controlled Fusion and Plasma Physics, Madrid, Spain, 1987, Part II, p. 732. P. Wienhold et al., J. Nucl. Mater. 145-147 (1987) 642. S.A. Cohen and H.F. Dylla, J. Nucl. Mater. 76 L 77 (1978) 425. Y. Hori et al. J. Nucl. Mater. 111 & 112 (1982) 137. R.A. Zuhr, S.P. Withrow and J.P. Roberto, J. Nucl. Mater. 93 & 94 (1980) 127. H. Bergsaker et al., Proc. 14th Eur. Conf. on Controlled Fusion and Plasma Physics, Madrid, Spain, 1987, Part II, p. 728. H.E. Satherblom et al., Nucl. Instr. and Meth. A240
(1985) 171. 1141U. Shm et al., in: these Proc. (PSI-8) J. Nucl. Mater.
162-164 (1989) 24. P51 W.J. Corbett et al., in: these Proc. (PSI-8), J. Nucl. Mater.
162-164 (1989) 221. WI J. Roth, J. Nucl. Mater. 145-147 (1987) 87. 1171P. Wienhold, F. Waelbroeck, H. Bergs&r, J. Winter and H.G. Esser, in: these Proc. (PSI-I), J. Nucl. Mater. 162-164 (1989) 369.
Journal of Nuclear Materials 162-164 (1989) 593-597 North-Holland, Amsterdam
COLLECTION EFFICIENCY FOR CARBON AND DEUTERIUM OF DIFFERENT WHEN EXPOSED TO THE SCRAPE-OFF PLASMA IN TOKAMAKS H. BERGSAKER’,
M. RUBEL
2, B. EMMOTH
1 and P. WIENHOLD
SURFACES
3
’ Research Institute of Physics, Association Euratom-NFR,
S-104 05 Stockholm, Sweden ’ Space Research Centre, Ordona 21, 01-237 Warszawa, Poland 3 Institut fti Plasmaphysik, KFA, P.O. Box 1913, D-5170 Jiilich, Fed. Rep. Germany
Key words: collector probe, carbon, tokamak Carbon and deuterium have been collected on surface probes inserted in the limiter shadow of TEXTOR, with carbonized conditions, i.e. with carbonized graphite limiters and carbonized liner. The exposure time ranges from - 0.1 s to - 150 s, and a number of different collector materials have been used: Be, graphite, stainless steel, Al, Si, Ti and Au. The collection efficiency of these different surfaces for carbon and deuterium is compared. An experiment with secondary collectors has also been nerforrned in the scrape-off nlasma at TEXTOR, in order to estimate the combined effect of backscattering and erosion _ _ at the collector surface due to physical sputtering.
1. Introduction In carbonized tokamaks and in tokamaks with graphite limiters it has been shown that large amounts of carbon are eroded at points in the machine which are in intense contact with the plasma, and redeposited at other positions. Where carbon is deposited, hydrogen isotopes are also included, and thick layers of mainly carbon, saturated with hydrogen build up [1,2]. The physical structure of these layers has been identified as that of amorphous, hydrogenated carbon (a-C : D) [3]. This codeposition process is important to the tritium inventory in future experiments, and it also constitutes a pumping mechanism which may help to reduce recycling [4]. The use of surface collector probes is a well established technique to study impurity fluxes in the boundary plasma of tokamaks, and to investigate erosion and deposition of wall material [S]. When hydrogen is implanted into solid surfaces, only a certain, energy dependent saturated area1 density of hydrogen can be retained within the solid, even if it is exposed to large ion fluences. Consequently, both ion fluences and ion energy can in principle be determined by collecting hydrogen at surfaces in a controlled way [6]. When the hydrogen deposition is determined rather by the deposition of carbon, then the deposition of carbon on a graphite surface may be inferred from the amount of hydrogen which is retained [2].
0022-3115/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
The flux of carbon in the scrape-off plasma of tokamaks has recently been determined according to the deposition rate of carbon on surface probes of various shapes. The carbon collecting materials have been e.g. crystalline silicon [7], amorphous silicon deposited on aluminum substrate [8], or stainless steel [3], and the area1 density of collected carbon has been measured by enhanced proton backscattering [8], as well as by visual inspection of the interference fringes [3]. Earlier experiments with carbon collecting surface probes have been performed using stainless steel collectors and detecting carbon with AES [9], beryllium collectors and RBS analysis [lo], and using single crystal silicon collectors in combination with carbon analysis by channeling RBS
WI. In reality, the observed deposition rate is different from the flux, in that the sticking of carbon carrying species (atomic and molecular ions) at surfaces is not unity. The sticking probability may possibly depend on the physical structure and chemical composition of the collecting surface. The net deposition rate can also be further reduced by erosion of deposited carbon. Furthermore, if carbon which has been deposited at the surface is able to diffuse into the bulk material of the probe, it may be impossible to determine the deposition rate with surface analysis techniques. The present investigation was undertaken in order to know if there is any difference between different kinds of surfaces with respect to collection of carbon and B.V.
594
H. BergsBker et al. / Colleciion efficiency for carbon and deuterrum
hydrogen by co-deposition. Probes carrying arrays of many different materials were exposed to tokamak scrape-off plasmas (in TEXTOR and in JET [12]) and the amount of carbon and hydrogen which was retained was determined afterwards by ion beam analysis techniques. The influence of backscattering and/or re-erosion of deposited material at the primary collecting surface was investigated by collecting carbon and hydrogen on secondary surfaces.
2. Experimental As a part of the preliminary collector probe experiments at JET [7], cylindrical probe shields of graphite, Inconel and a shield which was made partly of graphite and partly of Inconel were introduced in the limiter shadow using the Fast Transfer and Plasma Boundary Probe Systems, and were exposed to one or more complete discharges. At TEXTOR, fixed or rotating surface probes have been exposed to the scrape-off using the Stockholm-TEXTOR probe manipulator [13]. A probe carrying two arrays of collector pieces of Be, Al, Si, Ti, Ni and various kinds of graphite @chunk, polished and non-polished, Papyex) was exposed to 5 discharges in the limiter shadow in TEXTOR. Strips of different materials were mounted on cylindrical supports and exposed to the plasma in a time resolved manner by rotating the cylinders behind a slit aperture. All samples were exposed to deuterium plasmas. samples were brought to After being exposed, Stockholm for analysis. The area1 density of deuterium which had been retained was determined using the D( 3He, p)4He nuclear reaction with 0.8 MeV 3He. The amount of collected carbon was measured, when possible (little carbon in the substrate bulk material, sufficiently much carbon collected at the surface), by means of the non-Rutherford l2 C(p, p)12 C elastic backscattering, of 1.735 and/or 1.5 MeV protons. In the case of graphite collectors, carbon is deposited at the surface together with oxygen, metals and deuterium. If it can be established that these occluded species do not diffuse, and that ion energies are sufficiently low that ions are not able to penetrate substantially below the surface, then in principle it is possible to infer the amount of carbon which has been deposited from depth profiles of deuterium, oxygen and metals
[W Since impurity fluxes and other relevant plasma parameters are known to vary smoothly in the machine, any observed discontinuity in the deposition rate for carbon and deuterium when going from one substrate to
an adjacent different one must be attributed to properties of the collecting surface. To study such discontinuities consequently gives a possibility to compare relative collection efficiencies of different substrates. In a different type of experiment, carbon and deuterium were collected on rotating cylinders, carrying an aluminum foil covered with amorphous, hydrogenated silicon [8] and a stainless steel foil, respectively. Backscattered or re-eroded carbon and deuterium were collected on secondary collectors, in the first case on narrow strips of Al with a-Si: H which had been mounted across the slit aperture, in the second case on the backside of the stainless steel slits.
3. Results and discussion Fig. 1 shows the area1 density of deuterium and carbon deposited on a probe shield which has been exposed to seven discharges in JET (using the FTS [7]), three of which were disruptive. The surface is partly graphite and partly inconel. The general, more or less exponential decrease is just a radial decrease in deposition rate which is usually observed with collector probes, and which can be attributed to cross field diffusion of ions from the confined plasma. A discontinuity is observed, suggesting that up to twice as much deuterium has been retained at the graphite surface, compared to the inconel. On the inconel substrate, the areal density of carbon was determined directly by enhanced proton
slj
1
Q
DISTANCE
FROM
PLASMA
(mm)
Fig. 1. Area1 concentrations of deuterium and carbon collected on a probe shield, made of graphite and partly of inconel, exposed to the scrape-off plasma in JET (# 11143-49). Carbon is determined directly on the inconel surface while on the graphite this amount can be determined indirectly 1121.
595
ii Bergsaker et al. / Colleciiorzefficiencyfor carbon and deuterium 100
a I-1
T
E 0) zl c
z0 5
1 i
Ti’ A, 0.1 3s
DISTANCE
FROM
PLASMA
1
(mm)
Fig. 2. Area1 concentration of deuteriwn and carbon collected on different metal substrates exposed to the edge plasma in TEXTOR: (a) P31663 and 64; (b) #31809 and IO.
backscattering. On the graphite surface, the thickness of the deposited layer was determined from depth distributions of oxygen and metals [12]. The procedure is valid only if the impurities do not penetrate substantiaUy below the original surface. This condition is likely to be fulfilled, since the implantation range in graphite of ions with energies which can be expected (in the lower 100 eV range) is small compared to the observed depth profiles. The argument gains support from the fact that the calculated areal density of deposited carbon implies the same deuterium to carbon ratio (about 20%) as in the layer which has been deposited on inconel substrate. It thus appears that the deposition rate of carbon and deuterium is lower on inconel substrate than on graphite. Fig. 2 shows examples where carbon and deuterium have been collected on strips of various materials in the limiter shadow of TEXTOR. Discontinuities occur occasionally, particularly in the areal density of deuterium. There is a tendency, although not very consistent, to have larger areaI densities of deuterium on substrates of low-2 elements. Since the collected areal density is low in these cases, in contrast to fig. 1, such excursions may be expected due to differences in implantation range and saturation concentration of deu-
terium in the substrates. No drastic differences in the collection rate of carbon were observed, neither in the examples shown in fig. 2, nor in a number of similar cases. Fig. 3 shows two examples where a rough graphite surface appears to behave differently, compared to other surfaces, including mechanically polished graphite. Thus, it may be tentatively concluded that the collection rate of carbon and deuterium does generally not depend on the collector material, but that there may be an influence of the surface structure. Given that any difference at all is observed this is reasonable from the point of view that, once the surface has been covered with a hydrogenated carbon layer, the chemical composition at the surface should be the same, regardless of the substrate material. However, the number of observations are yet very limited, and more experiments would be necessary to make definite conclusions, except that the differences in collection rate are generally a factor two or less. Figs. 4 and 5 shows a somewhat different attempt to estimate how the deposition rate of carbon at a probe surface relates to the flux in the plasma. Carbon and deuterium species are allowed to hit a collector surface in the scrape-off plasma. The backside of the aperture
100
0.59
_a
DISTANCE
FROM
PLASMA
(mm)
Fig. 3. Area1 concen~ation of deuterium coliected on different carbon substrates and amorphous hydrogenated silicon exposed to edge plasma in TEXTOR: (a) #29996-98; (b) #29185, 86,94-98.
596
H. Bergsdker
<
+---i$ f w
et al. / Collection efficiency for carbon and deuterium
surface
Fig. 4. Aperture slits and a probe surface; a schematic illustration of the basis for calculations of deposition/re-erosion effects. The collecting surfaces are orthogonal to the magnetic field direction. slits acts as secondary collector, which presumably collects a fraction of the carbon and deuterium which is reflected or eroded at the primary collector surface. The ratio of deposits on the secondary collector to those on the primary is particularly interesting, since it may give information which is independent of the incident flux. The experiment was carried out with stainless steel slits and stainless steel foil collector which were exposed to roughly 150 s of TEXTOR discharges. The slit dimensions were w = 4.5 mm and d = 1.5 mm. The deuterium content on the backside carbon layer is generally lower than that on the front side. The ratio of carbon concentrations on the back and on the front side is more or less constant, in the range 0.12-0.20. at the distance 30-50 mm from the plasma. However, it is clear that at the radial position 20-25 mm the carbon concentration detected on the primary collector decreases sharply.
We first attempt to interpret the observations in terms of reflection and/or self sputtering at the primary collector surface. Carbonaceous ions stream along the magnetic field lines and impinge with flux density @c at a collector probe surface through a long aperture of width w. Backscattered or sputtered atoms may be deposited on the backside of the slits which define the aperture, and which act as secondary collectors. Carbon is also collected on the front side surface of the slits. Suppose now that carbon is ejected with cosine angular distribution from the primary collector surface within the slit image (0 < .$ < w) at rate r@c (r may be the probability of reflection, or the self-sputtering yield). The flux density of carbon incident at the back side of the slit will then be @pk(X)
_
r@cd2 /= 77
=- r
&,j‘“’
” [(E+x)‘+q’+d’]’
Pm
@c 2 [
d5
w+x
(w~-&F.
1
(1)
If carbon is deposited on the front side of the slit at a rate (1 - r)@c, and if carbon which has been reflected or sputtered at the probe surface has a high probability to stick when it hits the secondary collector, then the ratio of the area1 density collected on the back side of the slit at x = 0 to the area1 density on the front side will be cb to)
p
front
ICf7
.-
c
E
z0 5
D 10
z a1
side
back side
/
l 0
n
l
3 I
b
IIJ1l”““’ 20
DISTANCE
Fig. 5. Area1 concentrations
60
40 FROM
PLASMA
(mm)
of deuterium and carbon mea-
sured on the both sides of the aperture slits exposed TEXTOR discharges (X29486-29548).
to 150 s of
--=qik+$ Cf
(2)
Inserting the ratio of unity in eq. (2) gives r = 0.67, whereas the ratio 0.2 gives r = 0.3. Alternatively, the observed redeposition may be explained with hydrogen sputtering. For example, if the deuterium flux at 30 mm outside the last closed flux surface is taken to be @p = 2 x lo*’ m-* s-l [14] and the electron temperature T’ = 7.5 eV [15], then yield for physical sputtering by deuterium is 2.5 X lop3 [16], and consequently 7.5 X lo*’ C atoms me2 should have been sputtered at the primary collector during 150 s. Provided that a large fraction of the physically sputtered atoms stick when they reach the secondary collector then, by the geometrical argument of eq. (l), roughly 3 X 10” C atoms carbon should have been m -* of physically sputtered collected at the back side of the slit. This is about one order of magnitude less than what is found at that radial position and, moreover, with a scrape-off layer width of 10 mm for particle flux and 35 mm for
H. Bergsdker et al. / Collection efficiency for carbon and deuterium
temperature [15], physical sputtering would be expected to fall off much more rapidly with increasing radius than the modest decrease in the thickness of the redeposition layer on the secondary collector. The chemical erosion yield is not so well known at high fluxes [16], but if the yield is as high as 3 X lo-* [16], the expected erosion rate at 30 mm from the plasma would have been 6 x 1019 m-* s-l, i.e. 9 x lo*’ m-* during 150 s of the exposure. This value is almost equal to the carbon concentration measured at the primary collector. At 20 mm from the confined plasma, the areal density of carbon on the primary collector drops as the plasma is approached, without an associated drop in the amount of carbon on the secondary collector. The drop at the primary collector can not be due only to an increase in erosion by physical sputtering, since it would then be accompanied by an increase at the secondary collector. It may be due to chemical erosion, since carbon which is released in molecular form should have low probability of sticking at the secondary collector. The drop can also be attributed to a genuine decrease of the incident carbon flux. Such a radial distribution is expected if the carbon originates mainly at the liner (wall), and becomes ionized before reaching the confined plasma [17]. The absence of a corresponding decrease at the secondary collector could in this case be due to a simultaneously increasing rate of physical sputtering, or to an increasing reflection probability. Clearly new experiments of the described type, under various conditions and preferably with ion saturation current and electron temperature being measured in immidiate vicinity to the collector surfaces, should help to clarify this point. From the large amounts of carbon found on the secondary collectors it is clear that, due to reflection, re-erosion or both, the deposition rate of carbon on surface probes which are exposed to plasma under conditions similar to those given above can not immediately be translated into carbon fluxes in the scrapeoff plasma.
The observed differences are typically a factor two or less. The net deposition rate is determined by incident flux, sticking probability and the rate of re-erosion, and the observed differences in collection efficiency may be due to differences either in sticking or in erosion rate; it is difficult at present to propose specific mechanism. The influence of reflection and/or re-erosion at the collector surfaces was investigated in an experiment with secondary collectors. It is shown that the combined effect of reflection and re-erosion is not negligible in experiments where carbon is collected on surfaces which are introduced in the scrape-off plasma of tokamaks, and that consequently great care has to be taken in converting deposition rates at surfaces to flwces in the plasma.
References [l] W.R. Wampler,
[2] [3]
[4] [5] [6] [7]
[8] [9] [lo] [ll] [12]
[13] 4. Conclusions The rate of deuterium and carbon deposition on collector surfaces which are exposed to the scrape-off plasma in tokamaks did not in general show a strong dependence on substrate material, but evidence suggests that there is probably an influence of surface roughness.
597
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