- Email: [email protected]
Measurement of fast particles in front of the ICRH-antenna of ASDEX
Fusion Engineering and Design 12 (1990) 193-196 North-Holland
MEASUREMENT
OF FAST PARTICLES
F. WESNER, V.M. PROZESKY Max
Planck
Inslilur
193
fir
Plasmaphysik,
IN FRONT OF THE ICRH-ANTENNA
*, R. BEHRISCH
Euratom
Association,
OF ASDEX
and G. STAUDENMAIER
D 8046 Garching,
Fed. Rep. Germany
Carbon probes were exposed in the center of the Faraday screen of the ASDEX ICRH antenna during single ICRH discharges. The subsequent analysis of the probes by ion beam techniques gave the depth, as a measure for the particle energy, and the total amount of the implanted hydrogen and deuterium. The results indicate, that during the RF fast ions hit both, the antenna surface and the wall around the torus. This can be an important reason for the production of heavy impurities by ICRH. A comparison with CX neutral fluxes shows, that these cannot explain the measured implantations. 1. Introduction The mechanism
of the impurity
production
duting
ICRH and the sources of these impurities are still unclear. Some experimental results indicate, that the Faraday screens of the antennae are the main source (e.g. in JET [l]), however in ASDEX there is some evidence that a major part of impurities comes from other areas [2]. Looking experimentally for the mechanism of the impurity production, one has to look for variations in the plasma boundary and in the wall bombardment processes during ICRH, and also for possible variations of these conditions along the toroidal circumference. The impurities can be produced by physical sputtering, if either the flux of ions or the flwc of CX neutrals, which hit the antennae or wall areas, is increased by the RF. Since such mechanisms should be concentrated on areas around the antennae, diagnostic methods should be applied looking into these areas. While the existence of fast ions in the boundary has already been demonstrated [3,4], the implanting and sputtering processesin front of the antennae have only been investigated by analysing the surface of dismantled antenna parts [5]. In these investigations the processeswere integrated over a long time. One possibility in the search for accelerated particles is the exposure and subsequent analysis of surface probes. Such a probe was inserted into the small hole in the centre of one ASDEX antenna, which is accessible from outside the torus. The aim of this experiment was to investigate the flux and the mean energy of hydrogen and deuterium * Guest scientist from Atomic Energy Corporation of South Africa, Pretoria.
0920-3796/90/$03.50
ions (or neutrals), which hit the antenna surface during ICRF heating, by measuring the mean depth and the total number of atoms implanted in the surface layer of the probe. 2. Experimental
setup and procedure
The experimental setup is shown in fig. 1. The carbon probe (12), 6 mm in diameter, is mounted on top of a small flexible teflon rod (7), which can be moved through
# Fig. 1. Section of the antenna centre area with the probe diagnostic setup (schematic): (1) Faraday screen; (2) centre conductor; (3) return conductor; (4) short circuit; (5) vacuum vessel wall and flange; (6) stainless steel pipe; (7) flexible teflon rod; (8) bellows; (9) flange connection to dismantle the whole bellows section including the retracted teflon rod with the probe; (10) vacuum valves; (11) tube to vacuum pump; (12) carbon probe.
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
194
F. Wesner et al. / Measurement O~J?.TIparticles
a stainless steel pipe (6) into the small hole in the antenna center. After retracting the probe by means of a long bellows (8) and closing a vacuum valve (lo), the probe can be exchanged without breaking the torus vacuum. The probe can be inserted or withdrawn within a few minutes between two discharges. Exchanging the probe and pumping out the system takes about two hours. Therefore only one probe per day can be installed. Before their installation, the probes were degassedin vacuum at a temperature above 1600° C to remove hydrocarbon and water contamination in the surface layers to values below 10” atoms/cm2. The amount and depth distribution of implanted H and D atoms were determined by “elastic recoil detection” (ERD) [6], using a helium beam of 2.6 MeV. The total number of atoms gives a lower limit for the fluxes of H and D (as long as the saturation of the carbon is not reached), and their mean depth allows the average energy of the particles, bombarding the surface, to be determined.
3. Experimental
results
For minority heating (H-minority in He), 3 probes were exposed in the antenna at different ICRH power levels and different pulse lengths. One additional probe was used in a discharge with comparable plasma parameters, but with neutral injection heating and without ICRH; and for one discharge a “control probe” was exposed with a manipulator at a toroidal distance of about 1.5 m from one and about 3 m from the other antenna. For all these probes both antennae were operated simultaneously with equal power and pulse length. Depth profiles of hydrogen and deuterium, measured in an antenna probe and in the control probe distant from the antenna, both exposed during the same discharge, are shown in fig. 2. The mean energies corresponding to the depth values are also given in the figure. Since the deuterium content in the He-plasma was quite small and uncontrolled, no conclusion should be drawn from the H/D ratio. The total implanted amount of the H and D and its dependence on the product, power times pulse length, is shown in fig. 3. The equivalent values of the control probe without ICRH and of the probe installed far from the antenna are also plotted. The implanted amounts increase with increasing power times pulse length. This is more pronounced for hydrogen, which finally shows a saturation for larger amounts. Indeed,
0
1000
2000
+
H In anlenna probe
*
H in probe outside
3000
40bo
5’ IO
dspoalllon depth I Angalr6m ,
I
6.3
0
I
I
I
1
10.4
11.2
12.4
16,6
palilcle enetgy
Fig. 2. Depth profiles of implanted hydrogen and deuterium atoms in the antenna probe and in the control probe at a toroidal distance of 1.5 m from the antenna (probes Nr. 7 and 7s ASDEX shot Nr. 29133). In the lower scale mean energy values are given, corresponding to the depth [7].
the maximum values for H correspond to the known saturation concentration of hydrogen in carbon 181. For the second harmonic heating of hydrogen with > 30% of hydrogen in a deuterium plasma, 2 probes
15 -
6 0 -0.0
0.2
0.4
0.6
0.6
1.0
1.2
1.4
6
Fig. 3. Implanted hydrogen and deuterium versus RF power times pulse length for minority heating (H in He). 0 hydrogen 6 : antenna probe A
without
deuterlum
5-8: antenna probes exposed
during RF
ICRH
7s : probe exposed far from antenna same shot as nr.7
F. Wesner
et al. / Measurement
of H in a H/D mixture plasma. The average energies of these particles are in the range of 1 to 2 keV, if they reach the probe at normal incidence, otherwise the energies are higher. These fluxes are larger and more energetic than the charge exchange neutral particle fluxes as measured with a neutral particle detector at the midplane of
O-------O I
I -1
0
Power
0
Hydrogen
A
Deuterium
ASDEX
1
2
x pulselength
3
4
195
of fast particles
5
I MWs
Fig. 4. Implanted hydrogen and deuterium versus the product of RF-power and pulselength for two discharges with second harmonic heating. The values of the probe without ICRH are also included. Saturation is probably reached in the carbon for the hydrogen implantation.
been exposed and analyzed. The depth distribution showed a peak at 230 A for H and at 200 A for D, corresponding to a mean energy of 1.6 keV and 1.4 keV, respectively. The total amount is shown in fig. 4 for both probes. The hydrogen is implanted up to saturation in both cases, while the amount of deuterium increases again with P x 1. In both cases, second harmonic and minority heating, no clear dependence of the depth of the implanted particles, corresponding to their average energy, on the RF power could be found, probably due to the relatively large error bars on the results of depth measurements. The results for the implanted amounts of H and D in the exposed carbon probes show a clear tendency to increase with the product of power and pulselength, as long as saturation of hydrogen in the carbon is not yet reached. To avoid saturation (for hydrogen, especially in case of second harmonic heating), shorter ICRH pulses should be applied.
have
during
ICRH
in the same discharge.
This is
shown in fig. 5, where the calculated depth profile due to the CX neutral bombardment is compared with the flux measured by the probes. The total fluence of the CX neutrals is about a factor 20 smaller and the average energy is considerably lower. This indicates that the particles measured by the probes during ICRH are probably energetic ions, not neutrals. The fast particles are created during ICRH either in front of the antennae moving around the torus and sputter wherever they hit metal parts, or they are created all around the torus e.g. by waves, the maximum of this process also being in the antenna region. This is indicated by the depth distribution of the implanted H and D in the control probe distant from the antenna, which shows a similar function, but a smaller fluence (fig. 5). For the measured fluence of about 1016 D/cm2 s and 10” H/cm2 s, a sputtering yield of 2 X 10v2 for D and 8 X lo-’ for H and an antenna area of about 0.2 m2 (ASDEX), a flux of about 2 x 10” atoms s-I
-
antenna probe
-c-r
Cmllmlpmb
4. Discussion
The small number of probes exposed does not allow detailed conclusions. The measured dependencies as shown in figs. 2-4, however, can give some indication about the impurity production mechanism during ICRH. The measured depth profiles of H and D indicate a fluence of about 1016 D per cm2 and MWs and of > 10” H per cm2 and MWs for both, minority (H in He, containing some D) and second harmonic heating
00 0
1000
2000
3000
4000
5
IO
UeposlUon depth I AngsWm
Fig. 5. Comparison between the deposition profile of implanted hydrogen measured at the antenna probe and at the control probe with a profile calculated from the measured energy spectrum of charge exchange neutrals (solid line).
196
F. Wesner
et al. / Measurement
sputtered from each antenna can be estimated. If the area, at which these particles could hit the wall is about 10 cm broad all around the torus (e.g., at the entrance of the two ASDEX divertors), this number would be further increased by a factor of 10. If only a few per cent of these sputtered particles reach the plasma core, they would explain the impurity production during ICRH.
Acknowledgement The authors thank Dr H.-U. Fahrbach and Dr W. Herrmann for the evaluation of the relevant charge exchange diagnostic data and for fruitful discussions.
References [l] K.H. Behringer et al., Metal sources and general impurity behaviour in JET plasmas during ICRH, Proc. 131h European Conf. on Controlled Fusion and Plasma Heating, Schliersee, part 1 (1986) pp. 176-179.
o//art
particles
[2] J.-M. Noterdaeme et al.. Exnerimental results on edee . effects during ICRF heating of ASDEX plasmas, Fusion Engrg. Des. 12 (1990) 127-137, in these Proceedings. 131 G. Janeschitz et al., Impurity production during ICRF heating, Proc. 13th European Conf. on Controlled Fusion and Plasma Heating, Schliersee, part 1 (1986) pp. 407-410. [41 F. Ryter et al., Comparison of ICRH and LH accelerated hydrogen ions in NI heated ASDEX plasmas Proc. 13th European Conf. on Controlled Fusion and Plasma Heating, Schliersee, part 1 (1986) pp. 101-104. PI R. Behrisch et al., Deposition and erosion at the open and closed ICRH antennae of ASDEX, Proc. 14th European Conf. on Controlled Fusion and Plasma Physics, Madrid 1987, pp. 778-781. 161G. Staudenmaier et al., Trapping of deuterium implanted in carbon and silicon: A calibration for particle energy measurements in the plasma boundary of tokamaks, J. Nucl. Mater. 84 (1979) 149-156. 171 H.H. Andersen and J.F. Ziegler, Hydrogen, stopping powers and ranges in all elements, The Stopping and Ranges of Ions in Matter, Vol. 3 (Pergamon Press, New York, 1977). 181G. Staudenmaier et al., Deuterium ion fluxes, temperatures and densities in the scrape-off layer of TFR 600, Nucl. Fus. 20 (1) (1980) 96-101.
MEASUREMENT
OF FAST PARTICLES
F. WESNER, V.M. PROZESKY Max
Planck
Inslilur
193
fir
Plasmaphysik,
IN FRONT OF THE ICRH-ANTENNA
*, R. BEHRISCH
Euratom
Association,
OF ASDEX
and G. STAUDENMAIER
D 8046 Garching,
Fed. Rep. Germany
Carbon probes were exposed in the center of the Faraday screen of the ASDEX ICRH antenna during single ICRH discharges. The subsequent analysis of the probes by ion beam techniques gave the depth, as a measure for the particle energy, and the total amount of the implanted hydrogen and deuterium. The results indicate, that during the RF fast ions hit both, the antenna surface and the wall around the torus. This can be an important reason for the production of heavy impurities by ICRH. A comparison with CX neutral fluxes shows, that these cannot explain the measured implantations. 1. Introduction The mechanism
of the impurity
production
duting
ICRH and the sources of these impurities are still unclear. Some experimental results indicate, that the Faraday screens of the antennae are the main source (e.g. in JET [l]), however in ASDEX there is some evidence that a major part of impurities comes from other areas [2]. Looking experimentally for the mechanism of the impurity production, one has to look for variations in the plasma boundary and in the wall bombardment processes during ICRH, and also for possible variations of these conditions along the toroidal circumference. The impurities can be produced by physical sputtering, if either the flux of ions or the flwc of CX neutrals, which hit the antennae or wall areas, is increased by the RF. Since such mechanisms should be concentrated on areas around the antennae, diagnostic methods should be applied looking into these areas. While the existence of fast ions in the boundary has already been demonstrated [3,4], the implanting and sputtering processesin front of the antennae have only been investigated by analysing the surface of dismantled antenna parts [5]. In these investigations the processeswere integrated over a long time. One possibility in the search for accelerated particles is the exposure and subsequent analysis of surface probes. Such a probe was inserted into the small hole in the centre of one ASDEX antenna, which is accessible from outside the torus. The aim of this experiment was to investigate the flux and the mean energy of hydrogen and deuterium * Guest scientist from Atomic Energy Corporation of South Africa, Pretoria.
0920-3796/90/$03.50
ions (or neutrals), which hit the antenna surface during ICRF heating, by measuring the mean depth and the total number of atoms implanted in the surface layer of the probe. 2. Experimental
setup and procedure
The experimental setup is shown in fig. 1. The carbon probe (12), 6 mm in diameter, is mounted on top of a small flexible teflon rod (7), which can be moved through
# Fig. 1. Section of the antenna centre area with the probe diagnostic setup (schematic): (1) Faraday screen; (2) centre conductor; (3) return conductor; (4) short circuit; (5) vacuum vessel wall and flange; (6) stainless steel pipe; (7) flexible teflon rod; (8) bellows; (9) flange connection to dismantle the whole bellows section including the retracted teflon rod with the probe; (10) vacuum valves; (11) tube to vacuum pump; (12) carbon probe.
0 1990 - Elsevier Science Publishers B.V. (North-Holland)
194
F. Wesner et al. / Measurement O~J?.TIparticles
a stainless steel pipe (6) into the small hole in the antenna center. After retracting the probe by means of a long bellows (8) and closing a vacuum valve (lo), the probe can be exchanged without breaking the torus vacuum. The probe can be inserted or withdrawn within a few minutes between two discharges. Exchanging the probe and pumping out the system takes about two hours. Therefore only one probe per day can be installed. Before their installation, the probes were degassedin vacuum at a temperature above 1600° C to remove hydrocarbon and water contamination in the surface layers to values below 10” atoms/cm2. The amount and depth distribution of implanted H and D atoms were determined by “elastic recoil detection” (ERD) [6], using a helium beam of 2.6 MeV. The total number of atoms gives a lower limit for the fluxes of H and D (as long as the saturation of the carbon is not reached), and their mean depth allows the average energy of the particles, bombarding the surface, to be determined.
3. Experimental
results
For minority heating (H-minority in He), 3 probes were exposed in the antenna at different ICRH power levels and different pulse lengths. One additional probe was used in a discharge with comparable plasma parameters, but with neutral injection heating and without ICRH; and for one discharge a “control probe” was exposed with a manipulator at a toroidal distance of about 1.5 m from one and about 3 m from the other antenna. For all these probes both antennae were operated simultaneously with equal power and pulse length. Depth profiles of hydrogen and deuterium, measured in an antenna probe and in the control probe distant from the antenna, both exposed during the same discharge, are shown in fig. 2. The mean energies corresponding to the depth values are also given in the figure. Since the deuterium content in the He-plasma was quite small and uncontrolled, no conclusion should be drawn from the H/D ratio. The total implanted amount of the H and D and its dependence on the product, power times pulse length, is shown in fig. 3. The equivalent values of the control probe without ICRH and of the probe installed far from the antenna are also plotted. The implanted amounts increase with increasing power times pulse length. This is more pronounced for hydrogen, which finally shows a saturation for larger amounts. Indeed,
0
1000
2000
+
H In anlenna probe
*
H in probe outside
3000
40bo
5’ IO
dspoalllon depth I Angalr6m ,
I
6.3
0
I
I
I
1
10.4
11.2
12.4
16,6
palilcle enetgy
Fig. 2. Depth profiles of implanted hydrogen and deuterium atoms in the antenna probe and in the control probe at a toroidal distance of 1.5 m from the antenna (probes Nr. 7 and 7s ASDEX shot Nr. 29133). In the lower scale mean energy values are given, corresponding to the depth [7].
the maximum values for H correspond to the known saturation concentration of hydrogen in carbon 181. For the second harmonic heating of hydrogen with > 30% of hydrogen in a deuterium plasma, 2 probes
15 -
6 0 -0.0
0.2
0.4
0.6
0.6
1.0
1.2
1.4
6
Fig. 3. Implanted hydrogen and deuterium versus RF power times pulse length for minority heating (H in He). 0 hydrogen 6 : antenna probe A
without
deuterlum
5-8: antenna probes exposed
during RF
ICRH
7s : probe exposed far from antenna same shot as nr.7
F. Wesner
et al. / Measurement
of H in a H/D mixture plasma. The average energies of these particles are in the range of 1 to 2 keV, if they reach the probe at normal incidence, otherwise the energies are higher. These fluxes are larger and more energetic than the charge exchange neutral particle fluxes as measured with a neutral particle detector at the midplane of
O-------O I
I -1
0
Power
0
Hydrogen
A
Deuterium
ASDEX
1
2
x pulselength
3
4
195
of fast particles
5
I MWs
Fig. 4. Implanted hydrogen and deuterium versus the product of RF-power and pulselength for two discharges with second harmonic heating. The values of the probe without ICRH are also included. Saturation is probably reached in the carbon for the hydrogen implantation.
been exposed and analyzed. The depth distribution showed a peak at 230 A for H and at 200 A for D, corresponding to a mean energy of 1.6 keV and 1.4 keV, respectively. The total amount is shown in fig. 4 for both probes. The hydrogen is implanted up to saturation in both cases, while the amount of deuterium increases again with P x 1. In both cases, second harmonic and minority heating, no clear dependence of the depth of the implanted particles, corresponding to their average energy, on the RF power could be found, probably due to the relatively large error bars on the results of depth measurements. The results for the implanted amounts of H and D in the exposed carbon probes show a clear tendency to increase with the product of power and pulselength, as long as saturation of hydrogen in the carbon is not yet reached. To avoid saturation (for hydrogen, especially in case of second harmonic heating), shorter ICRH pulses should be applied.
have
during
ICRH
in the same discharge.
This is
shown in fig. 5, where the calculated depth profile due to the CX neutral bombardment is compared with the flux measured by the probes. The total fluence of the CX neutrals is about a factor 20 smaller and the average energy is considerably lower. This indicates that the particles measured by the probes during ICRH are probably energetic ions, not neutrals. The fast particles are created during ICRH either in front of the antennae moving around the torus and sputter wherever they hit metal parts, or they are created all around the torus e.g. by waves, the maximum of this process also being in the antenna region. This is indicated by the depth distribution of the implanted H and D in the control probe distant from the antenna, which shows a similar function, but a smaller fluence (fig. 5). For the measured fluence of about 1016 D/cm2 s and 10” H/cm2 s, a sputtering yield of 2 X 10v2 for D and 8 X lo-’ for H and an antenna area of about 0.2 m2 (ASDEX), a flux of about 2 x 10” atoms s-I
-
antenna probe
-c-r
Cmllmlpmb
4. Discussion
The small number of probes exposed does not allow detailed conclusions. The measured dependencies as shown in figs. 2-4, however, can give some indication about the impurity production mechanism during ICRH. The measured depth profiles of H and D indicate a fluence of about 1016 D per cm2 and MWs and of > 10” H per cm2 and MWs for both, minority (H in He, containing some D) and second harmonic heating
00 0
1000
2000
3000
4000
5
IO
UeposlUon depth I AngsWm
Fig. 5. Comparison between the deposition profile of implanted hydrogen measured at the antenna probe and at the control probe with a profile calculated from the measured energy spectrum of charge exchange neutrals (solid line).
196
F. Wesner
et al. / Measurement
sputtered from each antenna can be estimated. If the area, at which these particles could hit the wall is about 10 cm broad all around the torus (e.g., at the entrance of the two ASDEX divertors), this number would be further increased by a factor of 10. If only a few per cent of these sputtered particles reach the plasma core, they would explain the impurity production during ICRH.
Acknowledgement The authors thank Dr H.-U. Fahrbach and Dr W. Herrmann for the evaluation of the relevant charge exchange diagnostic data and for fruitful discussions.
References [l] K.H. Behringer et al., Metal sources and general impurity behaviour in JET plasmas during ICRH, Proc. 131h European Conf. on Controlled Fusion and Plasma Heating, Schliersee, part 1 (1986) pp. 176-179.
o//art
particles
[2] J.-M. Noterdaeme et al.. Exnerimental results on edee . effects during ICRF heating of ASDEX plasmas, Fusion Engrg. Des. 12 (1990) 127-137, in these Proceedings. 131 G. Janeschitz et al., Impurity production during ICRF heating, Proc. 13th European Conf. on Controlled Fusion and Plasma Heating, Schliersee, part 1 (1986) pp. 407-410. [41 F. Ryter et al., Comparison of ICRH and LH accelerated hydrogen ions in NI heated ASDEX plasmas Proc. 13th European Conf. on Controlled Fusion and Plasma Heating, Schliersee, part 1 (1986) pp. 101-104. PI R. Behrisch et al., Deposition and erosion at the open and closed ICRH antennae of ASDEX, Proc. 14th European Conf. on Controlled Fusion and Plasma Physics, Madrid 1987, pp. 778-781. 161G. Staudenmaier et al., Trapping of deuterium implanted in carbon and silicon: A calibration for particle energy measurements in the plasma boundary of tokamaks, J. Nucl. Mater. 84 (1979) 149-156. 171 H.H. Andersen and J.F. Ziegler, Hydrogen, stopping powers and ranges in all elements, The Stopping and Ranges of Ions in Matter, Vol. 3 (Pergamon Press, New York, 1977). 181G. Staudenmaier et al., Deuterium ion fluxes, temperatures and densities in the scrape-off layer of TFR 600, Nucl. Fus. 20 (1) (1980) 96-101.