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Measurements of impurity fluxes in the ASDEX plasma boundary
142
Journal
MEASUREMENTS E. TAGLAUER,
OF IMPURITY FLUXES
B.M.U.
SCHERZER,
of Nuclear
Materials 1 I I North-Holland
& I 12 (1982) 142- 146 Publishing
Company
IN THE ASDEX PLASMA BOUNDARY
P. VARGA
*, R. BEHRISCH,
CHEN
CHENG
KAI **
and ASDEX-TEAM Max-Planck-Institut
fiir Plasmaph.vsik,
EURA TOM-Associution,
D-8046 Garching/Miinchen.
Fe&d
Republic of German,
Impurity fluxes parallel to the magnetic field in the plasma boundary of ASDEX were measured with a collector probe. The probe allows simultaneous measurements of the radial distribution and can be rotated in order to obtain time resolution. The amount of impurities deposited on the graphite collector stripes is subsequently analysed by Rutherford backscattering. The predominant species detected are oxygen and stainless steel components, other impurities such as Ti, Si, MO, and Au are also found. For typical divertor discharges the average Fe flux near the outer wall is about 1.3X lOI Fe-atoms/cm*,s with an e-folding length of 1.1 cm. The oxygen flux is about one order of magnitude higher. Time-resolved measurements show a maximum deposition rate at the beginning of the discharge. Impurity fluxes for discharges with various parameters including limiter and divertor operation are discussed.
1. Introduction
2. Experiment
Perturbing impurity elements have to be overcome by purgatorial procedures before we can activate the energy which excites the sun [ 11. Studies of the production and behaviour of impurities and efforts for their elimination in present day fusion experiments such as the Garching tokamak ASDEX include the action of a magnetic divertor [2] or limiters with various surface materials [3]. Impurity fluxes in the plasma boundary, i.e. the region between the separatrix or the limiter and the walls of the vacuum vessel, can be measured using collector probes [4]. Analysing the surface of the collectors after exposure to the plasma yields information about the impurity species and their radial and time distributions under varying discharge conditions. Thus, if proper analysis is possible, production and confinement of impurities in the boundary layer can be studied. In the present paper we report on measurements of impurity fluxes in the ASDEX plasma boundary during limiter and divertor discharges, with and without neu-
The probe basically consists of a stainless steel cylinder (diameter 50 mm) which holds six collector stripes around its circumference. There are six 4 mm diameter apertures in the MO-shield in front of the stripes positioned normal to the magnetic field lines on opposite sides of the cap. Time resolution can be obtained by rotation of the cylinder around its axis, a value of about 0.5 s was chosen for the present work. All data given in this paper were taken on the ion drift side of the probe. The position of the probe relative to the plasma is controlled by axial movement. Some details of the probe are given in a separate paper on hydrogen flux measurements [5]. Collector stripe material was in most cases carbon foil, in a few cases when carbon was to be detected. aluminum foil. After the exposure the foils were analysed with Rutherford backscattering using a 2.5 MeV accelerator. For this purpose the foils had to be taken out of the vacuum and transported in air. However, this does not necessarily affect the possibility to detect trapped oxygen gas is not readily oxygen, since molecular adsorbed on graphite. Data were taken at ASDEX in various modes of operation (see also fig. 1): divertor discharges, (a) double null divertor operation (both divertors activated); (b) single null divertor operation (only the upper divertor activated, in the presence of the toroidal limiter, but the plasma shifted towards the upper divertor) and toroidal limiter operation. Neutral particle injection of
tral
particle
injection.
* Guest scientist from Technical University of Vienna, Austria. ** Guest scientist from Institute of Atomic Energy, Academia Sinica, Beijing, People’s Republic of China.
0022-3115/82/0000-0000/$02.75
0 1982 North-Holland
E. Taglauer et al. / Measurements
DIVERTOR
ENTRANCE
FIXED
SLIT
LIMITER
MWABLE
UMKER
FIXED LIMITER
143
of impurity jibes
converted into an impurity flwc as a first approximation. However, the transmission of apertures for plasma particles in a magnetic field depends on the energy and charge state of the ions, as was pointed out by Staudenmaier et al. [6]. In addition, the sheath potential between the plasma and the probe can accelerate ions (plasma particles and impurities) so that they hit the collector surface with an energy well above the ion temperature in the plasma boundary [7]. The probe was in all cases at the potential of the vacuum vessel. No attempt was made in the present work to estimate the size of these effects in our experimental situation because of lack of the necessary data.
ASDEX D -Discharges 8
TOROIDAL LIMITER
2920 - 2930 At : r0 2s
IL
I,
Fig. 1. Cross-section of the ASDEX tokamak in single null divertor configuration. The upper divertor is in operation, the lower divertor chamber is partly covered by the toroidal limiter plates. The plasma diameter is approimately 80 cm. The lower collector
probe was used for the present
study.
H was applied occassionally in all of the modes. Fig. 1 shows that the lower collector probe used in these experiments is geometrically closer to the plasma than the fixed and the movable limiter. Considering the magnetic field lines shows that the flux tube for this probe (i.e. the foremost aperture) extends to almost a complete large circumference of the torus and intercepts the entrance structure of the upper divertor. It should be noted that the position of the probe relative to the plasma (separatrix) is not very well defined for divertor discharges. That is, the plasma position may change between shots or during the discharge or during neutral injection by an amount of the order of 1 cm, which is the order of magnitude of a decay length. Therefore the probe position relative to the wall can be exactly determined, but relative to the plasma it is sometimes uncertain. Rutherford backscattering analysis of the collector stripes yields absolute numbers for the collected fluence, i.e. the number of atoms per cm2. For oxygen ions the trapping efficiency of the probe may be c 1 so that the measured oxygen flux is only a ‘lower limit. Knowing the probe geometry and speed of rotation this can be
2*~hl 0
1
2
Fig. 2. Time-resolved impurity fluxes for double null divertor operation, probe position 11.7 cm outside the separatrix.
144
E. Taglauer et al. / Measurements of impurity fluxes
3. Resuks and discussion
An example for the detected impurity species and their time distributions is given in fig. 2 for divertor discharges. It shows that the most prominent impurities are oxygen and iron but others are also present. (With our scattering geometry the Rutherford backscattering technique is not able to distinguish between iron and the other metal constituents of stainless steel. Similarly Au stands for all elements between Ta and Pb.) Oxygen and iron are also found as impurities in many other tokamak experiments [8]. Fig. 2 also shows another feature generally seen in all our measurements: the time distribution of the impurity flux has a high maximum at the beginning of the discharge, it decreases during the plateau phase to a value lower by about a factor of 5 and shows a second maximum towards the end of the discharge. Thus it has to be concluded that the highest impurity production occurs by intense interaction of the plasma with wall structures during the periods of poor confinement. For probe measurements it is important to check whether the deposited material is directly proportional to the fluence, i.e. that the actual surface concentration is not influenced by erosion, diffusion, nonunity stick-
ing etc. This was generally found for the iron flux but not in all cases for oxygen as shown in fig. 3 for surface concentrations integrated over different numbers of shots. It should be noted that the lowest data point for 0 is still well above the noise level. A series of measurements in the single null divertor mode is presented in fig. 4 which also contains data of deuterium fluxes [5] for comparison. In these discharges the stainless steel toroidal limiter was mounted in ASDEX. but the plasma was vertically shifted by about 3 cm (see fig. 1). It can be seen that the Fe fluxes are higher compared to pure divertor discharges. In fig.4 discharges with neutral particle injection are compared with otherwise identical discharges without neutral injection. The measurements show a third peak in the impurity flux during the neutral injection period. During this time the major radius of the plasma is increased by an amount up to 40 mm. It is assumed that the increased impurity flux is a consequence of reduced confinement during this period, which can also be seen in the dip in the electron density distribution shown in fig. 4. ASDEX SD - DISCHARGES
#L673-7L Jon
# L675
ASDEX 3857 - 3865 D - DISCHARGES LOO kA
l!!!!? -At
0
0
0
2
Fig. 3. Time-integrated of discharges.
6
1.
NO. OF
TIME [sl
8
SHOTS
deposition
as a function
,]‘TAt,
1
of the number
2
3
0
1
,
1
2
3
TIMElsl
Fig. 4. Time-resolved deuterium and impurity fluxes for single null divertor operation with (left) and without (right) neutral hydrogen injection (3 X 0.6 MW. 200 ms).
E. Taglauer et al. / Measurements
A
ASDEX
ASDEX 1d7x
D - DISCHARGES 3857 10
145
of impurity fluxes
b
SD - DISCHARGES
KL
F
SL
- 3865
LOO kA - Fe 6 SHOTS
1
[cm]
POSITION 0.’
POSITION Fig. 5. Radial distribution numbers of shots.
[cm I
of the deposited
iron
for varius
Examples for the radial decrease of the impurity fluxes are given in figs. 5 and 6. Fig. 5 shows the radial decay of time-integrated fluxes in divertor discharges. A characteristic decay length of about 1.1 cm is found. The data given in fig. 6 for deuterium, oxygen and iron fluxes in SD-discharges show a more complicated be-
Table 1 Measured
impurity
Type of discharge
flux in ASDEX
’ Jp Ml
for various
;& cmp3]
Double
null divertor
discharge
Fig. 6. Radial distribution of deuterium and impurity fluxes for single null divertor discharges, a stands for the initial, e for the final peak, NI refers to neutral injection. The inserts show the geometrical position of the movable (KL) and fixed (SL) poloidal limiters.
haviour. Different slopes are found for geometrical positions in front and behind the movable (KL) and the fixed (SL) limiters. Oxygen again shows a somewhat different distribution. The deuterium fluxes level off towards the plasma centre, this was interpreted as a saturation effect leading to an estimate of the ion tem-
conditions
Distance from separatrix
Fe-flux [1O’4 atoms/cm2.s]
[cm1
Initial peak
o-flux [1O’5 atoms/cm2.s]
Plateau
Neutral injection
Initial peak
Plateau
Neutral injection
250 400
1.5 2.4
11.7 10.1
0.62 3.5
0.2 0.5
no 2.0
1.8 _
0.5 _
no
Single null divertor
325
1.5
12.6
8.0
2.0
6.0
3.8
2
2.8
Toroidal SS-limiter AZ =4.5 cm Toroidal graphite limiter
325 230
1.5 1.5
12.0 7.1
11.0 11.0
0.4 1.2
no no
7.7
1.5
no
(9.9)
(3.0)
146
E. Taglauer et al. / Measurements
perature [5]. The radial decay of the iron flux can be used to estimate the diffusion constant D, perpendicular to the magnetic field in the plasma boundary [9]. In the flux tube model the decay length X is connected to the length of the flux tube L by [lo] X2= D, .4L/v where u is the thermal velocity of the ions parallel to the magnetic field lines. No detailed analysis of this kind was made in this work since the temperature of the impurity ions is not known. Using an ion temperature of 30 to 50 eV as found for deuterium [5] D, values of the order of lo3 to lo4 cm*/s are found. Typical values of the impurity fluxes at various phases of the discharge for the different modes of operation in ASDEX are given in table 1. The probe positions are not exactly the same but comparisons can be made using fig. 6. The table shows that particularly the iron, but also the oxygen flux is much higher in the presence of the toroidal stainless steel limiter. This is most pronounced for the initial peak in the impurity flux for which the experimental errors in the analysis are lowest. It should be noted that this strong increase in the impurity flux occurs although the centre of the plasma is shifted vertically 4.5 cm away from the limiter. This observation is in agreement with other plasma diagnostics for this type of discharge [2]. For toroidal graphite limiter discharges impurity fluxes are found which are comparable to normal divertor operation. For the graphite limiter discharges aluminum was used as probe material in order to be able to analyse carbon deposition, but no carbon could be detected above the background level of 1.3 X lOI C-atoms/cm2. (Several hours of glow discharge cleaning before tokamak operation resulted in the deposition of about 10 monolayers of carbon on the cylindrical cap of the probe.)
4. Conclusions Time-resolved measurements of impurity fluxes (Fe. 0) in the ASDEX plasma boundary were made using collector probes. The time distributions show in all cases an initial peak which is a factor of 4 to 10 higher than the flux in the plateau phase of the discharge.
of impurity fluxes
Another, the
somewhat
discharge.
During
smaller neutral
peak occurs hydrogen
at the
end
injection
of into
plasmas also an increase in the impurity fluxes is found. These distributions are similar to those found for hydrogen and deuterium [5]. Comparisons of divertor and limiter discharges reveal a strongly increased impurity production for the toroidal stainlesss steel limiter, whereas the graphite limiter shows similar behaviour as divertor discharges without toroidal limiter. A more detailed analysis to obtain impurity density profiles and transport data in the boundary layer requires additional measurements of charge states and energies. The dependence of the measured oxygen fluxes on radial position and fluence is in some cases different from that found for Fe or deuterium. deuterium
Acknowledgement The valuable technical assistance of G. Nagleder. H. Schmidl, K. Gehringer and H. Wacker is gratefully acknowledged.
References [II Dante Alighieri, Divina Commedia (Verona. 13 11). PI W. Engelhardt and ASDEX team, these proceedings. 131 H. Vernickel and ASDEX team, these proceedings, [41 Ph. Staib, these proceedings. [51 J. Roth, P. Varga, A.P. Martinelli, B.M.U. Scherzer, Chen Cheng-Kai, W.R. Wampler. E. Taglauer and ASDEX-team. these proceedings. P. Staib and W. Poschenrieder, J. Nucl. [61 G. Staudenmaier, Mater. 93/94 (1980) 121. and V.I. Tereshin. Kh [71 VS. Voitsenya, S.I. Solodovchenko FTI 81-24 (Kharkov. USSR 1981). in PI Proc. 4th Intern. Conf. Plasma Surface Interactions Controlled Fusion Devices, Eds. H. Vernickel. R. Behrisch, B.M.U. Schemer and F. Wagner, J. Nucl. Mater. 93/94 ( 1980). P. Staib and K. Ertl. these proceedings. [91 G. Staudenmaier. P. Staib. G. Venus. and TFR-team J. [lOI G. Staudenmaier. Nucl. Mater. 76/77 (1978) 445.
Journal
MEASUREMENTS E. TAGLAUER,
OF IMPURITY FLUXES
B.M.U.
SCHERZER,
of Nuclear
Materials 1 I I North-Holland
& I 12 (1982) 142- 146 Publishing
Company
IN THE ASDEX PLASMA BOUNDARY
P. VARGA
*, R. BEHRISCH,
CHEN
CHENG
KAI **
and ASDEX-TEAM Max-Planck-Institut
fiir Plasmaph.vsik,
EURA TOM-Associution,
D-8046 Garching/Miinchen.
Fe&d
Republic of German,
Impurity fluxes parallel to the magnetic field in the plasma boundary of ASDEX were measured with a collector probe. The probe allows simultaneous measurements of the radial distribution and can be rotated in order to obtain time resolution. The amount of impurities deposited on the graphite collector stripes is subsequently analysed by Rutherford backscattering. The predominant species detected are oxygen and stainless steel components, other impurities such as Ti, Si, MO, and Au are also found. For typical divertor discharges the average Fe flux near the outer wall is about 1.3X lOI Fe-atoms/cm*,s with an e-folding length of 1.1 cm. The oxygen flux is about one order of magnitude higher. Time-resolved measurements show a maximum deposition rate at the beginning of the discharge. Impurity fluxes for discharges with various parameters including limiter and divertor operation are discussed.
1. Introduction
2. Experiment
Perturbing impurity elements have to be overcome by purgatorial procedures before we can activate the energy which excites the sun [ 11. Studies of the production and behaviour of impurities and efforts for their elimination in present day fusion experiments such as the Garching tokamak ASDEX include the action of a magnetic divertor [2] or limiters with various surface materials [3]. Impurity fluxes in the plasma boundary, i.e. the region between the separatrix or the limiter and the walls of the vacuum vessel, can be measured using collector probes [4]. Analysing the surface of the collectors after exposure to the plasma yields information about the impurity species and their radial and time distributions under varying discharge conditions. Thus, if proper analysis is possible, production and confinement of impurities in the boundary layer can be studied. In the present paper we report on measurements of impurity fluxes in the ASDEX plasma boundary during limiter and divertor discharges, with and without neu-
The probe basically consists of a stainless steel cylinder (diameter 50 mm) which holds six collector stripes around its circumference. There are six 4 mm diameter apertures in the MO-shield in front of the stripes positioned normal to the magnetic field lines on opposite sides of the cap. Time resolution can be obtained by rotation of the cylinder around its axis, a value of about 0.5 s was chosen for the present work. All data given in this paper were taken on the ion drift side of the probe. The position of the probe relative to the plasma is controlled by axial movement. Some details of the probe are given in a separate paper on hydrogen flux measurements [5]. Collector stripe material was in most cases carbon foil, in a few cases when carbon was to be detected. aluminum foil. After the exposure the foils were analysed with Rutherford backscattering using a 2.5 MeV accelerator. For this purpose the foils had to be taken out of the vacuum and transported in air. However, this does not necessarily affect the possibility to detect trapped oxygen gas is not readily oxygen, since molecular adsorbed on graphite. Data were taken at ASDEX in various modes of operation (see also fig. 1): divertor discharges, (a) double null divertor operation (both divertors activated); (b) single null divertor operation (only the upper divertor activated, in the presence of the toroidal limiter, but the plasma shifted towards the upper divertor) and toroidal limiter operation. Neutral particle injection of
tral
particle
injection.
* Guest scientist from Technical University of Vienna, Austria. ** Guest scientist from Institute of Atomic Energy, Academia Sinica, Beijing, People’s Republic of China.
0022-3115/82/0000-0000/$02.75
0 1982 North-Holland
E. Taglauer et al. / Measurements
DIVERTOR
ENTRANCE
FIXED
SLIT
LIMITER
MWABLE
UMKER
FIXED LIMITER
143
of impurity jibes
converted into an impurity flwc as a first approximation. However, the transmission of apertures for plasma particles in a magnetic field depends on the energy and charge state of the ions, as was pointed out by Staudenmaier et al. [6]. In addition, the sheath potential between the plasma and the probe can accelerate ions (plasma particles and impurities) so that they hit the collector surface with an energy well above the ion temperature in the plasma boundary [7]. The probe was in all cases at the potential of the vacuum vessel. No attempt was made in the present work to estimate the size of these effects in our experimental situation because of lack of the necessary data.
ASDEX D -Discharges 8
TOROIDAL LIMITER
2920 - 2930 At : r0 2s
IL
I,
Fig. 1. Cross-section of the ASDEX tokamak in single null divertor configuration. The upper divertor is in operation, the lower divertor chamber is partly covered by the toroidal limiter plates. The plasma diameter is approimately 80 cm. The lower collector
probe was used for the present
study.
H was applied occassionally in all of the modes. Fig. 1 shows that the lower collector probe used in these experiments is geometrically closer to the plasma than the fixed and the movable limiter. Considering the magnetic field lines shows that the flux tube for this probe (i.e. the foremost aperture) extends to almost a complete large circumference of the torus and intercepts the entrance structure of the upper divertor. It should be noted that the position of the probe relative to the plasma (separatrix) is not very well defined for divertor discharges. That is, the plasma position may change between shots or during the discharge or during neutral injection by an amount of the order of 1 cm, which is the order of magnitude of a decay length. Therefore the probe position relative to the wall can be exactly determined, but relative to the plasma it is sometimes uncertain. Rutherford backscattering analysis of the collector stripes yields absolute numbers for the collected fluence, i.e. the number of atoms per cm2. For oxygen ions the trapping efficiency of the probe may be c 1 so that the measured oxygen flux is only a ‘lower limit. Knowing the probe geometry and speed of rotation this can be
2*~hl 0
1
2
Fig. 2. Time-resolved impurity fluxes for double null divertor operation, probe position 11.7 cm outside the separatrix.
144
E. Taglauer et al. / Measurements of impurity fluxes
3. Resuks and discussion
An example for the detected impurity species and their time distributions is given in fig. 2 for divertor discharges. It shows that the most prominent impurities are oxygen and iron but others are also present. (With our scattering geometry the Rutherford backscattering technique is not able to distinguish between iron and the other metal constituents of stainless steel. Similarly Au stands for all elements between Ta and Pb.) Oxygen and iron are also found as impurities in many other tokamak experiments [8]. Fig. 2 also shows another feature generally seen in all our measurements: the time distribution of the impurity flux has a high maximum at the beginning of the discharge, it decreases during the plateau phase to a value lower by about a factor of 5 and shows a second maximum towards the end of the discharge. Thus it has to be concluded that the highest impurity production occurs by intense interaction of the plasma with wall structures during the periods of poor confinement. For probe measurements it is important to check whether the deposited material is directly proportional to the fluence, i.e. that the actual surface concentration is not influenced by erosion, diffusion, nonunity stick-
ing etc. This was generally found for the iron flux but not in all cases for oxygen as shown in fig. 3 for surface concentrations integrated over different numbers of shots. It should be noted that the lowest data point for 0 is still well above the noise level. A series of measurements in the single null divertor mode is presented in fig. 4 which also contains data of deuterium fluxes [5] for comparison. In these discharges the stainless steel toroidal limiter was mounted in ASDEX. but the plasma was vertically shifted by about 3 cm (see fig. 1). It can be seen that the Fe fluxes are higher compared to pure divertor discharges. In fig.4 discharges with neutral particle injection are compared with otherwise identical discharges without neutral injection. The measurements show a third peak in the impurity flux during the neutral injection period. During this time the major radius of the plasma is increased by an amount up to 40 mm. It is assumed that the increased impurity flux is a consequence of reduced confinement during this period, which can also be seen in the dip in the electron density distribution shown in fig. 4. ASDEX SD - DISCHARGES
#L673-7L Jon
# L675
ASDEX 3857 - 3865 D - DISCHARGES LOO kA
l!!!!? -At
0
0
0
2
Fig. 3. Time-integrated of discharges.
6
1.
NO. OF
TIME [sl
8
SHOTS
deposition
as a function
,]‘TAt,
1
of the number
2
3
0
1
,
1
2
3
TIMElsl
Fig. 4. Time-resolved deuterium and impurity fluxes for single null divertor operation with (left) and without (right) neutral hydrogen injection (3 X 0.6 MW. 200 ms).
E. Taglauer et al. / Measurements
A
ASDEX
ASDEX 1d7x
D - DISCHARGES 3857 10
145
of impurity fluxes
b
SD - DISCHARGES
KL
F
SL
- 3865
LOO kA - Fe 6 SHOTS
1
[cm]
POSITION 0.’
POSITION Fig. 5. Radial distribution numbers of shots.
[cm I
of the deposited
iron
for varius
Examples for the radial decrease of the impurity fluxes are given in figs. 5 and 6. Fig. 5 shows the radial decay of time-integrated fluxes in divertor discharges. A characteristic decay length of about 1.1 cm is found. The data given in fig. 6 for deuterium, oxygen and iron fluxes in SD-discharges show a more complicated be-
Table 1 Measured
impurity
Type of discharge
flux in ASDEX
’ Jp Ml
for various
;& cmp3]
Double
null divertor
discharge
Fig. 6. Radial distribution of deuterium and impurity fluxes for single null divertor discharges, a stands for the initial, e for the final peak, NI refers to neutral injection. The inserts show the geometrical position of the movable (KL) and fixed (SL) poloidal limiters.
haviour. Different slopes are found for geometrical positions in front and behind the movable (KL) and the fixed (SL) limiters. Oxygen again shows a somewhat different distribution. The deuterium fluxes level off towards the plasma centre, this was interpreted as a saturation effect leading to an estimate of the ion tem-
conditions
Distance from separatrix
Fe-flux [1O’4 atoms/cm2.s]
[cm1
Initial peak
o-flux [1O’5 atoms/cm2.s]
Plateau
Neutral injection
Initial peak
Plateau
Neutral injection
250 400
1.5 2.4
11.7 10.1
0.62 3.5
0.2 0.5
no 2.0
1.8 _
0.5 _
no
Single null divertor
325
1.5
12.6
8.0
2.0
6.0
3.8
2
2.8
Toroidal SS-limiter AZ =4.5 cm Toroidal graphite limiter
325 230
1.5 1.5
12.0 7.1
11.0 11.0
0.4 1.2
no no
7.7
1.5
no
(9.9)
(3.0)
146
E. Taglauer et al. / Measurements
perature [5]. The radial decay of the iron flux can be used to estimate the diffusion constant D, perpendicular to the magnetic field in the plasma boundary [9]. In the flux tube model the decay length X is connected to the length of the flux tube L by [lo] X2= D, .4L/v where u is the thermal velocity of the ions parallel to the magnetic field lines. No detailed analysis of this kind was made in this work since the temperature of the impurity ions is not known. Using an ion temperature of 30 to 50 eV as found for deuterium [5] D, values of the order of lo3 to lo4 cm*/s are found. Typical values of the impurity fluxes at various phases of the discharge for the different modes of operation in ASDEX are given in table 1. The probe positions are not exactly the same but comparisons can be made using fig. 6. The table shows that particularly the iron, but also the oxygen flux is much higher in the presence of the toroidal stainless steel limiter. This is most pronounced for the initial peak in the impurity flux for which the experimental errors in the analysis are lowest. It should be noted that this strong increase in the impurity flux occurs although the centre of the plasma is shifted vertically 4.5 cm away from the limiter. This observation is in agreement with other plasma diagnostics for this type of discharge [2]. For toroidal graphite limiter discharges impurity fluxes are found which are comparable to normal divertor operation. For the graphite limiter discharges aluminum was used as probe material in order to be able to analyse carbon deposition, but no carbon could be detected above the background level of 1.3 X lOI C-atoms/cm2. (Several hours of glow discharge cleaning before tokamak operation resulted in the deposition of about 10 monolayers of carbon on the cylindrical cap of the probe.)
4. Conclusions Time-resolved measurements of impurity fluxes (Fe. 0) in the ASDEX plasma boundary were made using collector probes. The time distributions show in all cases an initial peak which is a factor of 4 to 10 higher than the flux in the plateau phase of the discharge.
of impurity fluxes
Another, the
somewhat
discharge.
During
smaller neutral
peak occurs hydrogen
at the
end
injection
of into
plasmas also an increase in the impurity fluxes is found. These distributions are similar to those found for hydrogen and deuterium [5]. Comparisons of divertor and limiter discharges reveal a strongly increased impurity production for the toroidal stainlesss steel limiter, whereas the graphite limiter shows similar behaviour as divertor discharges without toroidal limiter. A more detailed analysis to obtain impurity density profiles and transport data in the boundary layer requires additional measurements of charge states and energies. The dependence of the measured oxygen fluxes on radial position and fluence is in some cases different from that found for Fe or deuterium. deuterium
Acknowledgement The valuable technical assistance of G. Nagleder. H. Schmidl, K. Gehringer and H. Wacker is gratefully acknowledged.
References [II Dante Alighieri, Divina Commedia (Verona. 13 11). PI W. Engelhardt and ASDEX team, these proceedings. 131 H. Vernickel and ASDEX team, these proceedings, [41 Ph. Staib, these proceedings. [51 J. Roth, P. Varga, A.P. Martinelli, B.M.U. Scherzer, Chen Cheng-Kai, W.R. Wampler. E. Taglauer and ASDEX-team. these proceedings. P. Staib and W. Poschenrieder, J. Nucl. [61 G. Staudenmaier, Mater. 93/94 (1980) 121. and V.I. Tereshin. Kh [71 VS. Voitsenya, S.I. Solodovchenko FTI 81-24 (Kharkov. USSR 1981). in PI Proc. 4th Intern. Conf. Plasma Surface Interactions Controlled Fusion Devices, Eds. H. Vernickel. R. Behrisch, B.M.U. Schemer and F. Wagner, J. Nucl. Mater. 93/94 ( 1980). P. Staib and K. Ertl. these proceedings. [91 G. Staudenmaier. P. Staib. G. Venus. and TFR-team J. [lOI G. Staudenmaier. Nucl. Mater. 76/77 (1978) 445.