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Erosion and redeposition at the JET limiters
356
Journalof
Nuclear
Materials
162-164
(19X9) 356
North-Holland.
EROSION AND REDEPOSITION
362
Amsterdam
AT THE JET LIMITERS
G.M. MCCRACKEN *, D.H.J. GOODALL B. DENNE ’ and R. BEHRISCH 4
*, P.C. STANGEBY
3, J.P. COAD
‘, J. ROTH
4,
’ JET Joint Undertaking, Abingdon, Oxon OX14 3EA. United Kingdom ’ UKA EA Culham Laboratocv (Culham /Euratom Fusion Associatirm). Abingdon. Oxon OX14 SDB, Umted Kingdom -’ Institute for Aerospace Studies, University of Toronto, Canada 4 Max-Planck-lnstitut ftir Plasmaphysik. D-8046 Garching bei Miinchen, Fed, Rep. German,,
Key words:
tokamak,
erosion,
redeposition,
JET. limiters
Physical measurements of the JET graphite limiter tiles, made before and after exposure to - 3000 tokamak discharges, have shown that there is erosion of the surface nearest the plasma and redeposition of the carbon radially further out. The spatial distribution of the erosion/redeposition agrees with a previously proposed analytical model. The sputter yield of the redeposited layer has been measured and found to be similar to bulk material. Metals from the antennae screens are injected into JET discharges during ICRF heating. The spatial redistribution of these metal impurities on the limiter has also been studied. The observed alternate layers of metal and redeposited carbon explains the decay of the metal concentration in the plasma in ohmic discharges following ICRF heated discharges.
1. Introduction
Impurities sputtered from limiters enter as neutrals into the plasma. They are then ionised and travel along magnetic field lines. On a slower time scale they diffuse across field lines both into the centre of the plasma and out again to be redeposited in the limiters. This general picture has been described in an earlier paper [l], where steady state equations were solved which predicted that near the last closed flux surface (LCFS) the limiters would suffer net erosion whereas at distances radially further out there would be net deposition. The model neglects chemical sputtering due to hydrogen and oxygen and considers physical sputtering only. In the present investigation we have measured erosion and deposition by careful physical measurements on some of the graphite limiter tiles in contact with the plasma in JET during the 1986 campaign. Direct experimental evidence of the erosion and redeposition has been obtained. In addition the redeposited layers have been examined by SIMS and sectioning. The redeposited layers show a layered structure. Using X-ray analysis, layers of nickel and chromium have been detected within the graphite deposit. These metallic layers are attributed to the metals released from the Faraday screens of the ICRF antennas [2]. The release of a metallic or other non intrinsic impurity into the plasma for a short burst is expected to
result in contamination of the limiters since the impurities will diffuse into the scrape off layer and be deposited on the limiter surface. From the limiters it is resputtered into the plasma and the cycle is repeated. A model is presented which describes the behaviour of such an impurity. This predicts that the impurity will gradually be transferred to regions of the limiter far from the LCFS and that it will be buried under the redeposited carbon layers.
2. Theory .?. 1. Steady
state
model
The plasma flux and the impurity flux to the limiter in the SOL are assumed to have exponential dependencies on radius with characteristic e-folding lengths A, and A,,,. Sputtering due to the plasma ions with yield Y,(r) and due to self sputtering by the impurities with yield Y,,,(u) is taken into account. The net erosion rate is given by [l] rsr=tn,(a)c,
- irz,(a)c,Y,(r)
[l-
Ym(r)]
expi-
exp(-7)
yj.
(If
357
GM. McCracken et al. / Erosion and redeposition at the JET limiters
We define an electron temperature at the last closed flux surface (LCFS) T,(a) and assume that the temperature also varies exponentially with radius, with an e-folding length X, [3]. The energy of the ion arriving at the surface Ei is the sum of the thermal energy and the acceleration due to the sheath potential. For an ion of charge Q E, = 2T, + 3QT,,
(2)
A value of Q = 4, based on recent experimental and theoretical evidence [4], is used throughout the present calculations, although the results are not very sensitive to the value of Q, at least at low temperatures. In order to evaluate the net erosion/redeposition we must know the impurity concentration in the plasma. We assume that there is no net loss of impurities from the limiter/plasma system. By integration of the total sputtered flux and the redeposited flux over the radial direction we can calculate the equilibrium impurity concentration nm/np [l]. We can then evaluate equation (1). Results for various values of edge parameters have been presented elsewhere 151.
2.2. Time dependent model The steady state model is a reasonable approximation if the surface concentration of the sputtered atoms is constant, i.e. if the only impurity considered is the limiter material e.g. carbon. In such a case the sputter yields at any radius will depend only on the local electron temperature. However, if another impurity is introduced into the plasma and deposited on the limiter surface, the surface concentration and hence the sputter yield will vary with time. Let the surface concentration be @(r, t). The local particle balance can be written
g=r,,p, t)-ITm(r, t)Y,(r)fy -r,(r)Y,(r)y,
diffusion and the gain due to plasma sputtering self-sputtering
and
where L is the poloidal length of the limiter. Integrating eq. (4) over time we obtain n,(t)
=n,(O)
- $iIS(,.
t) dr,
(5)
where n,(O) is the initial impurity concentration in the plasma. To solve eqs. (4) and (5) we neglect the time required to reach equilibrium radial distribution of impurities in the plasma compared witht the time scale for erosion and redeposition. In order to solve these equations we must have a relationship between nm and r,,,. A simple minded approach is to say that the impurity content of the plasma is characterized by a decay time 7,. Assuming equilibrium between plasma outflow from the main plasma and parallel transport in the SOL, we may write V~,/T,,,
= ~~~~~(
r) dr = LX&,.,(
a).
We estimate r, = a2/6D, (- 0.3 s for JET), the decay time in cylindrical geometry. At long times recycling causes 7, to go over to the confinement or replacement time which is shorter than 7,; this more complex case will be treated elsewhere. Eq. (5) can now be solved numerically using eq. (3) to obtain the time dependence of n,(t) and 8(x, t). It is noted that we make the simplifying assumption that all sputtered atoms enter the plasma and are ionised within the LCFS. No ionisation in the SOL is considered. This is a reasonable assumption for the glancing incidence geometry of the JET limiters and at low edge densities. A Monte Carlo model has also been used to relax this assumption and to examine other effects such as wall limiter exchange [6].
0
where the first term on the right-hand side is the incident flux of impurities, the second is the flux removed by self sputter and the third the flux removed by plasma sputtering. We have assumed that the sputter removal rate is proportional to the fractional coverage of the surface for B s @a,where 0, is the number of atoms in a monolayer. For 8 > $ we take S/8, = 1. The rate of change of the impurity concentration in the plasma, n m is determined by the loss to the SOL due to cross field
3. Experimental measurements 3. I. Erosion and redeposition of carbon
A number of experimental approaches have been made to measure erosion and redeposition, including the implanting of isotopic markers [7] and thin film activation [8]. In the present investigation in JET the erosion was expected to be large and macroscopic mea-
358
GM. McCracken
x
et al. / Erosion and redeposition at the JET limiten
Measurement Mechanical
by section
after
measurement
before
exposure 8 after
exposure
/ 200
Ion side
Electronside
Fig. 1. Experimental measurements of erosion and redeposition of carbon on the JET limiter tile 4, octant 4 exposed in 1986.
surements of JET limiter tiles were made using a commercial coordinate measuring machine. The tiles were measured prior to installation in JET in 1986. A 13C marker was also implanted at a depth of 2 pm. The tiles were removed after 3200 discharges and remeasured. It was found that changes in the dimensions of the tiles were easily detected. The result from one tile is shown in fig. 1. The measurements are recorded normal to the base of the limiter. A clear pattern is observed With the erosion being evident on the surface closest to the plasma and deposition occurring further away, as predicted by the modelling. However, in order to compare the experimental data with the model we must take into account the angle of the magnetic field lines with the limiter surface. The erosion in the direction of the field lines has been calculated as a function of radius by dividing the measured erosion or deposition by sin 6 where B is the angle between the surface and the field at that radius. The results are compared to the steady state model in fig. 2. Good agreement between the shapes of the calculated and measured distributions are obtained with the transition between erosion and deposition being accurately predicted. Measurements made on other limiter tiles gave qualitativeIy similar results, but the pattern of erosion and redeposition was not as clear. However, the general observation of erosion near the last closed flux surface and deposition further out is almost universally observed occurring on the protection tiles of the RF antennae as well as on the limiter tiles. Difficulties have been encountered when making measurements on the
RF protection tiles due to the tile shape changing, probably due to thermal cycling. A change in shape was noted after two months operation in 1987 which did not change further in the foilowing four months operation. This problem did not appear with the more massive limiter tiies used in the discrete limiters during 1986. No
DISTANCE
FROM
LCFS
imml
Fig. 2. Comparison of theoretical erosion rate calculated from eq. (1) with experimental erosion measured in the direction of the field tines.
359
G.M. McCracken et al. / Erosion and redeposition at the JET limiters
Table 1 Sputter yield for redeposited carbon 1 keV H+
1 keV DC
100°C
0.011 0.0093
0.0155 0.0183
530°c
0.083
0.085
3-
-6
./-\
detailed measurements have yet been made on the tiles from the belt limiters used in 1987/K
,--.-_
1-1 6
12
I
I
16
20
Depth
3.2. Properties of the redeposited layers
In order to study the deposited layers in more detail samples were sectioned and polished. Examinations were carried out in the optical microscope and the scanning electron microscope (SEM). It was found that the deposition occurred in layers. These layers were in places distorted and broken with parts of the deposited layer clearly having been lost or flaked away. The missing
Y-.. ,
I
24
“0 5 x
0
26
(pm)
Fig. 3. SIMS depth profiles of nickel and hydrogen in a 20 pm thick layer of redeposited carbon from JET limiter tile 4. strata are on the electron drift side where the carbon thickness is - 130 pm. On the ion drift side where the carbon has a maximum thickness of 40 pm the strata are continuous. By examining where the layer appeared to be complete, measurements of the thickness of the
n
JET
LIMITER
GEOMETRY
18
I
7
/:
I I
6: (a) 5
FROM
LCFS
50 RADIAL
h-d
\
I 0
DISTANCE
\
(b)
\’
0.01s
100 DISTANCE
’
‘, \ \ 150 FROM
LCFS
\\ 200
lmml
Fig. 4. (a) Spatial distributions of nickel on the surface of JET carbon limiter tiles exposed in 1985 plotted as a function of distance from the LCFS. (b) Calculation of the spatial distribution of nickel at various times on a limiter using eq. (5) and the parameters noted in the figure.
360
G.M. McCracken 1L
I
I
I
et al. / Erosion and redeposition at the JET limiters
I
I
I
I
I
I
I
1
-7-
T, lo1 : 50eV
12“e
Spectroscopic I =lL971-72
lnormallzed I------0
at
2x10
(01:
19
-3
m
fntenslty
I
t =O)
1 st discharge
y
2nd discharge
I
1
I
I
I
II
I
I
1
I
2
L
6
0
10
12
1L
16
18
20
I
22
21
TIME (s)
Fig. 5. Comparison
of the experimentally
measured
decay of Ni XXVI in an ohmically calculated using eq. (5).
deposit were made. These measurements are included in the results shown in fig. 1. Where they overlap, the measurements obtained by sectioning are in good agreement with the mechanical measurements. From their appearance in the SEM the layers seem to have a density lower than the bulk graphite material but no direct measurements have yet been made. The sputter yield of a 20 pm thick layer of the deposited material has been measured using the weight loss technique. The results for 1 keV proton and deuteron bombardment are shown in table 1. The sputter yields for pyrolytic graphite measured under the same conditions are 30% less at room temperature and 25% higher at 570 o C. Thus the difference between the redeposited layer and normal graphite can probably be neglected. This is an important conclusion for assessing the net erosion to be expected in the next generation devices. Examination with the SEM showed more detail than the optical microscope with indications of at least 300 layers. Using electron induced X-ray analysis it was found that some of these layers were due to nickel and chromium. These metallic layers were not unexpected as the ICRH antennas had Faraday screens made from nickel and chromium. Spectroscopic observations have shown that the nickel and chromium are released from these Faraday screens when the antennas are energized
heated
JET
plasma
Ni concentration
[2]. The depth distribution of the impurities in the carbon has also been investigated using SIMS. Results for nickel and ‘jC in a - 20 pm thick carbon layer from the ion drift side of the limiter are shown in fig. 3. Very marked peaks in the nickel concentration are observed at depths of approximately 5, 10, 15 and 20 pm. The position of the substrate is clearly demonstrated by the 13C peak. This was the marker implanted at a depth of 2 pm in the limiter surface before exposure to the plasma. The deuterium and hydrogen concentrations were typically 5% of the carbon concentration i.e. much below the saturation level of - 0.4 H/C. 3.3. Spatial and time distribution
of metals
Extensive measurements of the spatial distribution of metals on the limiter have been made after JET campaigns in 1983, 1984 and 1985 [9]. Typical results for tiles from octant 1 and octant 5 limiters after the 1985 campaign are shown in fig. 4a. The distribution of nickel is qualitatively similar to that shown in the calculation, fig. 4b. The decay of the nickel concentration has an e-folding length of roughly 20 mm, similar to that obtained from the plasma flux measurements and used in the calculation for both plasma and impurities. However the maximum in the distribution is much closer to the LCFS (25 mm versus 70 mm) than in the
G.ii4, McCracken et al. / Erosion and redeposition at the JET limiters
model. This is almost certainly due to deposition of carbon on top of the nickel, as observed in the SEM, thus preventing the nickel from being sputtered. The model which has been used to calculate the spatial distribution also predicts the decay of the impurity concentration in the plasma. This decay has been well documented in JET for a number of cases [1,2]. A comparison of the spectroscopic measurements of Ni XXVI with the modelling is shown in fig. 5. The nickel was introduced by ICRH in the previous discharge. Then there were 3 ohmically heated discharges in which the nickel concentration steadily decayed. The initial value of the model has been normalized, but other parameters are the same as those used for the carbon modelling. Although the experimental data show a slower initial decay than the model the agreement is remarkably good. This strongly supports the view that the metallic impurity concentration in the plasma is determined by the surface concentration on the limiters, and that the decay is determined by the rate of transfer of the metal to large minor radius, where it is buried by carbon.
4. EXiussion and conclusions The simple modelling of steady state limiter sputtering gives results in good agreement with the experimental data from the limiters exposed in 1986. The spatial distribution of the erosion and redeposition is well reproduced, while the absolute yield is about a factor of 4 lower experimentally than predicted by the sputtering calculations. With exposures integrated over a large number of discharges, including disruptions and vessel conditioning it is unrealistic to hope to get better agreement in absolute numbers. Further experimental work using exposures to a small number of well-defined discharges is required in order to make more detailed comparisons between theory and experiment. The measurements of the sputtering yields of the redeposited carbon gives results in quite good agreement with bulk pyrolytic graphite. Thus erosion rates from redeposited material will be similar to the bulk material. This is an important result for the design of limiters and divertors. Two major approximations have been made in order to obtain the steady state analytic solution. The first is that no ionisation of impurities occurs in the SOL. This is only true if the mean free path for ionisation is long compared with the distance from the limiter surface to the LCFS. Simple estimates show that for the JET glancing-incidence geometry, using typical measured densities and temperatures in the SOL, this approxima-
361
tion is justified. However, if operation is extended to higher densities, such an approximation no longer holds. More complex numerical calculations [6,10] will then be required. The second simplification is the assumption that the e-folding length for the plasma ions and impu~ties is the same. The pattern of erosion and redeposition is very sensitive to the ratio of X,/h,. The good agreement with experiment in the present case indicates that they must be equal to within 10-208. This is a rather interesting result of general importance in the investigation of impurity transport in the SOL. The results of the model for metal impurity sputtering show an interesting pattern of behaviour in general agreement with experimentally measured spatial distributions of metallic impurities on the limiter. This data also indicates that X, = hr. Further simplifications have had to be made for the solution of these time dependent equations. In particular it has been assumed that there is a single constant global impurity decay time and that for surface concentrations less than a monolayer the sputtering yield is proportional to the surface concentration. A more detailed model taking into account 1D impurity transport in the plasma has still to be considered. However the general model gives a clear picture of the time dependence of the impurities in the plasma, showing that the long decay time of the impurity concentration is controlled by the resputtering that occurs from the limiter and that the eventual loss is due to gradual deposition of the metals at radial points far from the plasma, where the sputter yield is zero. Metals deposited at large minor radii will be buried by subsequent deposition of carbon. This explains the frequently observed decay and disappearance of impurities introduced into the plasma for a short period [l]. This clean up process is a rather fortunate consequence of the sputtering process. It can clearly be relied on in future operations. The accidental introduction of impurities into the tokamak is thus not nearly so serious an effect as might previously have been supposed.
References [l] G.M. McCracken, J. Ehrenberg, P.E. Stott, R. Behrisch and L. de Kock, J. Nucl. Mater. 145-147 (1987) 621. [2] K.H. Behringer, J. Nucl. Mater. 145-147 (1987) 145. [3] S.K. Erents, J.A. Tagle, G.M. McCracken, P.C. Stangeby and L. de Kock, Nucl. Fusion 26 (19%) 1591. [4] G.F. Matthews, in: these Proc. (PSI-S), J. Nucl. Mater. 162-164 (1989) 38.
362
G.M. McCracken
et al. / Erosion and redeposition at the JET limiters
[5] G.M. McCracken, P.C. Stangeby and C.S. Pitcher, Int. Workshop on Plasma Edge Theory, Augustusburg, April 1988, to be published in Contributions to Plasma Physics. [6] P.C. Stangeby, ibid. ref. [5]. [7] J.B. Roberto, J. Roth, E. Taglauer and O.W. Holland, J. Nucl. Mater. 128 & 129 (1984) 244.
[8] D.H.J. Goodall, T.W. Conlon, C. Sofield and G.M. McCracken, J. Nucl. Mater. 76 & 77 (1978) 492. [9] R. Behrisch et al. J. Nucl. Mater. 145-147 (1987) 731. [lo] J.N. Brooks, Nucl. Technol. Fusion 4 (1983) 33.
Journalof
Nuclear
Materials
162-164
(19X9) 356
North-Holland.
EROSION AND REDEPOSITION
362
Amsterdam
AT THE JET LIMITERS
G.M. MCCRACKEN *, D.H.J. GOODALL B. DENNE ’ and R. BEHRISCH 4
*, P.C. STANGEBY
3, J.P. COAD
‘, J. ROTH
4,
’ JET Joint Undertaking, Abingdon, Oxon OX14 3EA. United Kingdom ’ UKA EA Culham Laboratocv (Culham /Euratom Fusion Associatirm). Abingdon. Oxon OX14 SDB, Umted Kingdom -’ Institute for Aerospace Studies, University of Toronto, Canada 4 Max-Planck-lnstitut ftir Plasmaphysik. D-8046 Garching bei Miinchen, Fed, Rep. German,,
Key words:
tokamak,
erosion,
redeposition,
JET. limiters
Physical measurements of the JET graphite limiter tiles, made before and after exposure to - 3000 tokamak discharges, have shown that there is erosion of the surface nearest the plasma and redeposition of the carbon radially further out. The spatial distribution of the erosion/redeposition agrees with a previously proposed analytical model. The sputter yield of the redeposited layer has been measured and found to be similar to bulk material. Metals from the antennae screens are injected into JET discharges during ICRF heating. The spatial redistribution of these metal impurities on the limiter has also been studied. The observed alternate layers of metal and redeposited carbon explains the decay of the metal concentration in the plasma in ohmic discharges following ICRF heated discharges.
1. Introduction
Impurities sputtered from limiters enter as neutrals into the plasma. They are then ionised and travel along magnetic field lines. On a slower time scale they diffuse across field lines both into the centre of the plasma and out again to be redeposited in the limiters. This general picture has been described in an earlier paper [l], where steady state equations were solved which predicted that near the last closed flux surface (LCFS) the limiters would suffer net erosion whereas at distances radially further out there would be net deposition. The model neglects chemical sputtering due to hydrogen and oxygen and considers physical sputtering only. In the present investigation we have measured erosion and deposition by careful physical measurements on some of the graphite limiter tiles in contact with the plasma in JET during the 1986 campaign. Direct experimental evidence of the erosion and redeposition has been obtained. In addition the redeposited layers have been examined by SIMS and sectioning. The redeposited layers show a layered structure. Using X-ray analysis, layers of nickel and chromium have been detected within the graphite deposit. These metallic layers are attributed to the metals released from the Faraday screens of the ICRF antennas [2]. The release of a metallic or other non intrinsic impurity into the plasma for a short burst is expected to
result in contamination of the limiters since the impurities will diffuse into the scrape off layer and be deposited on the limiter surface. From the limiters it is resputtered into the plasma and the cycle is repeated. A model is presented which describes the behaviour of such an impurity. This predicts that the impurity will gradually be transferred to regions of the limiter far from the LCFS and that it will be buried under the redeposited carbon layers.
2. Theory .?. 1. Steady
state
model
The plasma flux and the impurity flux to the limiter in the SOL are assumed to have exponential dependencies on radius with characteristic e-folding lengths A, and A,,,. Sputtering due to the plasma ions with yield Y,(r) and due to self sputtering by the impurities with yield Y,,,(u) is taken into account. The net erosion rate is given by [l] rsr=tn,(a)c,
- irz,(a)c,Y,(r)
[l-
Ym(r)]
expi-
exp(-7)
yj.
(If
357
GM. McCracken et al. / Erosion and redeposition at the JET limiters
We define an electron temperature at the last closed flux surface (LCFS) T,(a) and assume that the temperature also varies exponentially with radius, with an e-folding length X, [3]. The energy of the ion arriving at the surface Ei is the sum of the thermal energy and the acceleration due to the sheath potential. For an ion of charge Q E, = 2T, + 3QT,,
(2)
A value of Q = 4, based on recent experimental and theoretical evidence [4], is used throughout the present calculations, although the results are not very sensitive to the value of Q, at least at low temperatures. In order to evaluate the net erosion/redeposition we must know the impurity concentration in the plasma. We assume that there is no net loss of impurities from the limiter/plasma system. By integration of the total sputtered flux and the redeposited flux over the radial direction we can calculate the equilibrium impurity concentration nm/np [l]. We can then evaluate equation (1). Results for various values of edge parameters have been presented elsewhere 151.
2.2. Time dependent model The steady state model is a reasonable approximation if the surface concentration of the sputtered atoms is constant, i.e. if the only impurity considered is the limiter material e.g. carbon. In such a case the sputter yields at any radius will depend only on the local electron temperature. However, if another impurity is introduced into the plasma and deposited on the limiter surface, the surface concentration and hence the sputter yield will vary with time. Let the surface concentration be @(r, t). The local particle balance can be written
g=r,,p, t)-ITm(r, t)Y,(r)fy -r,(r)Y,(r)y,
diffusion and the gain due to plasma sputtering self-sputtering
and
where L is the poloidal length of the limiter. Integrating eq. (4) over time we obtain n,(t)
=n,(O)
- $iIS(,.
t) dr,
(5)
where n,(O) is the initial impurity concentration in the plasma. To solve eqs. (4) and (5) we neglect the time required to reach equilibrium radial distribution of impurities in the plasma compared witht the time scale for erosion and redeposition. In order to solve these equations we must have a relationship between nm and r,,,. A simple minded approach is to say that the impurity content of the plasma is characterized by a decay time 7,. Assuming equilibrium between plasma outflow from the main plasma and parallel transport in the SOL, we may write V~,/T,,,
= ~~~~~(
r) dr = LX&,.,(
a).
We estimate r, = a2/6D, (- 0.3 s for JET), the decay time in cylindrical geometry. At long times recycling causes 7, to go over to the confinement or replacement time which is shorter than 7,; this more complex case will be treated elsewhere. Eq. (5) can now be solved numerically using eq. (3) to obtain the time dependence of n,(t) and 8(x, t). It is noted that we make the simplifying assumption that all sputtered atoms enter the plasma and are ionised within the LCFS. No ionisation in the SOL is considered. This is a reasonable assumption for the glancing incidence geometry of the JET limiters and at low edge densities. A Monte Carlo model has also been used to relax this assumption and to examine other effects such as wall limiter exchange [6].
0
where the first term on the right-hand side is the incident flux of impurities, the second is the flux removed by self sputter and the third the flux removed by plasma sputtering. We have assumed that the sputter removal rate is proportional to the fractional coverage of the surface for B s @a,where 0, is the number of atoms in a monolayer. For 8 > $ we take S/8, = 1. The rate of change of the impurity concentration in the plasma, n m is determined by the loss to the SOL due to cross field
3. Experimental measurements 3. I. Erosion and redeposition of carbon
A number of experimental approaches have been made to measure erosion and redeposition, including the implanting of isotopic markers [7] and thin film activation [8]. In the present investigation in JET the erosion was expected to be large and macroscopic mea-
358
GM. McCracken
x
et al. / Erosion and redeposition at the JET limiten
Measurement Mechanical
by section
after
measurement
before
exposure 8 after
exposure
/ 200
Ion side
Electronside
Fig. 1. Experimental measurements of erosion and redeposition of carbon on the JET limiter tile 4, octant 4 exposed in 1986.
surements of JET limiter tiles were made using a commercial coordinate measuring machine. The tiles were measured prior to installation in JET in 1986. A 13C marker was also implanted at a depth of 2 pm. The tiles were removed after 3200 discharges and remeasured. It was found that changes in the dimensions of the tiles were easily detected. The result from one tile is shown in fig. 1. The measurements are recorded normal to the base of the limiter. A clear pattern is observed With the erosion being evident on the surface closest to the plasma and deposition occurring further away, as predicted by the modelling. However, in order to compare the experimental data with the model we must take into account the angle of the magnetic field lines with the limiter surface. The erosion in the direction of the field lines has been calculated as a function of radius by dividing the measured erosion or deposition by sin 6 where B is the angle between the surface and the field at that radius. The results are compared to the steady state model in fig. 2. Good agreement between the shapes of the calculated and measured distributions are obtained with the transition between erosion and deposition being accurately predicted. Measurements made on other limiter tiles gave qualitativeIy similar results, but the pattern of erosion and redeposition was not as clear. However, the general observation of erosion near the last closed flux surface and deposition further out is almost universally observed occurring on the protection tiles of the RF antennae as well as on the limiter tiles. Difficulties have been encountered when making measurements on the
RF protection tiles due to the tile shape changing, probably due to thermal cycling. A change in shape was noted after two months operation in 1987 which did not change further in the foilowing four months operation. This problem did not appear with the more massive limiter tiies used in the discrete limiters during 1986. No
DISTANCE
FROM
LCFS
imml
Fig. 2. Comparison of theoretical erosion rate calculated from eq. (1) with experimental erosion measured in the direction of the field tines.
359
G.M. McCracken et al. / Erosion and redeposition at the JET limiters
Table 1 Sputter yield for redeposited carbon 1 keV H+
1 keV DC
100°C
0.011 0.0093
0.0155 0.0183
530°c
0.083
0.085
3-
-6
./-\
detailed measurements have yet been made on the tiles from the belt limiters used in 1987/K
,--.-_
1-1 6
12
I
I
16
20
Depth
3.2. Properties of the redeposited layers
In order to study the deposited layers in more detail samples were sectioned and polished. Examinations were carried out in the optical microscope and the scanning electron microscope (SEM). It was found that the deposition occurred in layers. These layers were in places distorted and broken with parts of the deposited layer clearly having been lost or flaked away. The missing
Y-.. ,
I
24
“0 5 x
0
26
(pm)
Fig. 3. SIMS depth profiles of nickel and hydrogen in a 20 pm thick layer of redeposited carbon from JET limiter tile 4. strata are on the electron drift side where the carbon thickness is - 130 pm. On the ion drift side where the carbon has a maximum thickness of 40 pm the strata are continuous. By examining where the layer appeared to be complete, measurements of the thickness of the
n
JET
LIMITER
GEOMETRY
18
I
7
/:
I I
6: (a) 5
FROM
LCFS
50 RADIAL
h-d
\
I 0
DISTANCE
\
(b)
\’
0.01s
100 DISTANCE
’
‘, \ \ 150 FROM
LCFS
\\ 200
lmml
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deposit were made. These measurements are included in the results shown in fig. 1. Where they overlap, the measurements obtained by sectioning are in good agreement with the mechanical measurements. From their appearance in the SEM the layers seem to have a density lower than the bulk graphite material but no direct measurements have yet been made. The sputter yield of a 20 pm thick layer of the deposited material has been measured using the weight loss technique. The results for 1 keV proton and deuteron bombardment are shown in table 1. The sputter yields for pyrolytic graphite measured under the same conditions are 30% less at room temperature and 25% higher at 570 o C. Thus the difference between the redeposited layer and normal graphite can probably be neglected. This is an important conclusion for assessing the net erosion to be expected in the next generation devices. Examination with the SEM showed more detail than the optical microscope with indications of at least 300 layers. Using electron induced X-ray analysis it was found that some of these layers were due to nickel and chromium. These metallic layers were not unexpected as the ICRH antennas had Faraday screens made from nickel and chromium. Spectroscopic observations have shown that the nickel and chromium are released from these Faraday screens when the antennas are energized
heated
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Ni concentration
[2]. The depth distribution of the impurities in the carbon has also been investigated using SIMS. Results for nickel and ‘jC in a - 20 pm thick carbon layer from the ion drift side of the limiter are shown in fig. 3. Very marked peaks in the nickel concentration are observed at depths of approximately 5, 10, 15 and 20 pm. The position of the substrate is clearly demonstrated by the 13C peak. This was the marker implanted at a depth of 2 pm in the limiter surface before exposure to the plasma. The deuterium and hydrogen concentrations were typically 5% of the carbon concentration i.e. much below the saturation level of - 0.4 H/C. 3.3. Spatial and time distribution
of metals
Extensive measurements of the spatial distribution of metals on the limiter have been made after JET campaigns in 1983, 1984 and 1985 [9]. Typical results for tiles from octant 1 and octant 5 limiters after the 1985 campaign are shown in fig. 4a. The distribution of nickel is qualitatively similar to that shown in the calculation, fig. 4b. The decay of the nickel concentration has an e-folding length of roughly 20 mm, similar to that obtained from the plasma flux measurements and used in the calculation for both plasma and impurities. However the maximum in the distribution is much closer to the LCFS (25 mm versus 70 mm) than in the
G.ii4, McCracken et al. / Erosion and redeposition at the JET limiters
model. This is almost certainly due to deposition of carbon on top of the nickel, as observed in the SEM, thus preventing the nickel from being sputtered. The model which has been used to calculate the spatial distribution also predicts the decay of the impurity concentration in the plasma. This decay has been well documented in JET for a number of cases [1,2]. A comparison of the spectroscopic measurements of Ni XXVI with the modelling is shown in fig. 5. The nickel was introduced by ICRH in the previous discharge. Then there were 3 ohmically heated discharges in which the nickel concentration steadily decayed. The initial value of the model has been normalized, but other parameters are the same as those used for the carbon modelling. Although the experimental data show a slower initial decay than the model the agreement is remarkably good. This strongly supports the view that the metallic impurity concentration in the plasma is determined by the surface concentration on the limiters, and that the decay is determined by the rate of transfer of the metal to large minor radius, where it is buried by carbon.
4. EXiussion and conclusions The simple modelling of steady state limiter sputtering gives results in good agreement with the experimental data from the limiters exposed in 1986. The spatial distribution of the erosion and redeposition is well reproduced, while the absolute yield is about a factor of 4 lower experimentally than predicted by the sputtering calculations. With exposures integrated over a large number of discharges, including disruptions and vessel conditioning it is unrealistic to hope to get better agreement in absolute numbers. Further experimental work using exposures to a small number of well-defined discharges is required in order to make more detailed comparisons between theory and experiment. The measurements of the sputtering yields of the redeposited carbon gives results in quite good agreement with bulk pyrolytic graphite. Thus erosion rates from redeposited material will be similar to the bulk material. This is an important result for the design of limiters and divertors. Two major approximations have been made in order to obtain the steady state analytic solution. The first is that no ionisation of impurities occurs in the SOL. This is only true if the mean free path for ionisation is long compared with the distance from the limiter surface to the LCFS. Simple estimates show that for the JET glancing-incidence geometry, using typical measured densities and temperatures in the SOL, this approxima-
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tion is justified. However, if operation is extended to higher densities, such an approximation no longer holds. More complex numerical calculations [6,10] will then be required. The second simplification is the assumption that the e-folding length for the plasma ions and impu~ties is the same. The pattern of erosion and redeposition is very sensitive to the ratio of X,/h,. The good agreement with experiment in the present case indicates that they must be equal to within 10-208. This is a rather interesting result of general importance in the investigation of impurity transport in the SOL. The results of the model for metal impurity sputtering show an interesting pattern of behaviour in general agreement with experimentally measured spatial distributions of metallic impurities on the limiter. This data also indicates that X, = hr. Further simplifications have had to be made for the solution of these time dependent equations. In particular it has been assumed that there is a single constant global impurity decay time and that for surface concentrations less than a monolayer the sputtering yield is proportional to the surface concentration. A more detailed model taking into account 1D impurity transport in the plasma has still to be considered. However the general model gives a clear picture of the time dependence of the impurities in the plasma, showing that the long decay time of the impurity concentration is controlled by the resputtering that occurs from the limiter and that the eventual loss is due to gradual deposition of the metals at radial points far from the plasma, where the sputter yield is zero. Metals deposited at large minor radii will be buried by subsequent deposition of carbon. This explains the frequently observed decay and disappearance of impurities introduced into the plasma for a short period [l]. This clean up process is a rather fortunate consequence of the sputtering process. It can clearly be relied on in future operations. The accidental introduction of impurities into the tokamak is thus not nearly so serious an effect as might previously have been supposed.
References [l] G.M. McCracken, J. Ehrenberg, P.E. Stott, R. Behrisch and L. de Kock, J. Nucl. Mater. 145-147 (1987) 621. [2] K.H. Behringer, J. Nucl. Mater. 145-147 (1987) 145. [3] S.K. Erents, J.A. Tagle, G.M. McCracken, P.C. Stangeby and L. de Kock, Nucl. Fusion 26 (19%) 1591. [4] G.F. Matthews, in: these Proc. (PSI-S), J. Nucl. Mater. 162-164 (1989) 38.
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[5] G.M. McCracken, P.C. Stangeby and C.S. Pitcher, Int. Workshop on Plasma Edge Theory, Augustusburg, April 1988, to be published in Contributions to Plasma Physics. [6] P.C. Stangeby, ibid. ref. [5]. [7] J.B. Roberto, J. Roth, E. Taglauer and O.W. Holland, J. Nucl. Mater. 128 & 129 (1984) 244.
[8] D.H.J. Goodall, T.W. Conlon, C. Sofield and G.M. McCracken, J. Nucl. Mater. 76 & 77 (1978) 492. [9] R. Behrisch et al. J. Nucl. Mater. 145-147 (1987) 731. [lo] J.N. Brooks, Nucl. Technol. Fusion 4 (1983) 33.