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Realization of a laboratory apparatus for plasma-wall interaction studies
212
Nuclear Instruments and Methods in Physics Research B53 (1991) 212-217 North-Holland
Realization of a laboratory apparatus for plasma-waft interaction studies E. Gauthier, J. Bardon and J.P. Palmari CRMCZ-CNRS, Campus Luminy, Case 913, 13288 Marseilk Cedex 09, France
A. Grosman CEN Cadarache, Associakm Euratom-C&4 SW la Fusion, 13118 Saint-Pnul-~z-Durance, France Received 17 October 1989 and in revised form 26 September 1990
‘Ibis paper describes SPISA (surface-plasma interaction simuiation apparatus), a new facility for studies on the interaction between iow temperature bigb density plasmas and graphite (or other) samples. The vahtes obtained for the temperature between 5 and 25 eV and the density between 10” mP3 and 1019 mm3 of the plasma simulate reasonably we11the edge plasma of the tokamaks.
The main features of this device are the versatility of the conditions for interaction, the possibility of sample analysis without breaking the vacuum and the careful design of the thermodesorption analysis aparatus. 1. Introduction The importance of the plasma-wall interaction was to be a matter of growing concern for the operation of large fusion devices during the last years (11. The recycling of hydrogen isotopes by the walls is of major importance for the total gas balance especially in the recent experiments with peaked density profiles [I]. It also determines the density and the transport properties in the scrape off layer (SOL). A striking example of the role of the walls is given by Esser et al. [2], who observed that 80% of the gas involved in a Textor plasma shot was provided by the outgassing of the carbon covering the walls. The interest of studying such processes to obtain predictive models has been clear for many years. However, in spite of numerous studies, reviewed e.g. by Wilson and Hsu 91, MMler ]4] or Ehrenberg [S], the recycling mecha~sms are not yet well understood and problems like the relative importance of hydrogen diffusion, detrapping and recombination in various materials are still to be solved. The physical and chemical sputtering and the codeposition phenomena play also a central role for the tokamaks operation. Routine replacement of eroded components can become a serious problem in future tritium fueling conditions. The codeposition of large amounts of hydrogen isotopes and sputtered carbon adds a new parameter in the recycling properties of the walls because codeposited hydrogen cannot reach any saturation. Sputtered carbon is also of importance for the radiative and transport properties in the SOL. shown
0168-583X/91/$03.50
The numerous studies on these phenomena made in the tokamaks suffer some drawbacks which lead these studies to be more descriptive than predictive. As can be seen in ref. [6] properties of the walls are largely history dependent and the study of the plasma-wall interaction cannot be the essential concern for determining the story of a sample. The number of parameters in fusion machines, such as the oxygen and oxygen compounds content, the sample location and temperature, etc. is so large that independent parameter variation which is necessary to the reasonable modeling of the phenomena occurring phenomena is unrealistic. Moreover, for obvious reasons, studies like the implantation profile of hydrogen or the chemical bonding states of the surface cannot be done in situ. Recent reports [7,8] have shown that the properties of the surface are greatly affected by the sample exposure to atmospheric conditions. Within this context, laboratory expe~ments must play an important role for the determination of the mechanisms such as the recycling phenomena or the erosion and codeposition processes. Until recently, these laboratory experiments have principally been done with ion beams in the keV energy range. Interesting results like absolute measurements of adsorbed hydrogen j7] and saturation effects [4] or ion induced desorption ([9] have been obtained by using these methods. But many results, out of the scope of these experiments, are needed like studies in the low-energy-~~-fl~ domain. Such conditions would be more relevant to the conditions of the plasma-wall interaction. To our knowledge, one experimental device exists at
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
213
E. Gauthier et al. / Plasma-wall interaction
first one contains the plasma source. It is pumped out by a 15001 s-’ turbomolecular pump. Before starting operation the vacuum is about lo-’ Pa and it remains below 10e3 Pa during plasma operation. The second chamber is a standard ultrahigh vacuum vessel used for the analysis of the plasma exposed samples. The two parts of the device are linked by a valve, allowing sample transfer without breaking the vacuum.
UCLA [lo] which simulates realistic conditions of the plasma-surface interactions. The PISCES apparatus uses a duoplasmatron as plasma source and includes many analytic tools. Interesting results [11,12] have been published in respect to the erosion, redeposition and codeposition phenomena. In this paper we describe the realization and the characteristics of SPISA, a new laboratory apparatus for the simulation of the plasma-wall interaction conditions. The source is a duopigatron giving roughly the same plasma electron temperature and density as in PISCES, i.e. T, -C 25 eV; N, < 1019 rnm3. The sample can be biased to change the energy of impinging ions and it can be transferred into an analysis vessel without breaking the vacuum. First results on recycling mechanisms obtained with this apparatus [8,13] have shown differences to those obtained in ion beam experiments. The versatility of our device can give interesting results in an extended range of plasma wall interaction studies although some differences with tokamaks experiments are inherent to such a device like the low ionic temperature (0.05 eV), the relatively high fraction of molecular ions H; and Hl and the large ion Larmor radii.
2.1. The plasma source The main feature of the duopigatron plasma source is the presence of an anticathode which is approximately at the same potential as the cathode, providing longitudinal oscillations of the hot electrons resulting in a better ionization efficiency than in conventional sources [14,15]. A schematic of the source is given in fig. 2. It is symmetric around the Z-axis. The cathode is a 1 mm in diameter tungsten wire which is supported by two water-cooled copper rods. This cathode is separated from the anodic chamber by an intermediate electrode (IE) of soft iron with a duct 12.5 mm in diameter. After the IE is the stainless steel anode which is cylindrical 150 mm in length and 50 mm in diameter. The anticathode is generally a graphite disk 70 mm in diameter set 30 mm after the anode. One water-cooled copper coil is located outside the vacuum chamber. Its magnetic field lines are guided by the IE. Another coil is inside the vacuum chamber and provides a good uni-
2. Experimental device A schematic of the experimental setup is shown in fig. 1. It is made up of two stainless steel vessels. The
Plasma Source
Sample
I lolder
Optical Visible Spectrometer Double
Langmuir
Probe
I.R. Camera
/
Mass Spectrometer
Pumping
170 I/s
Pumping
1500 I/S
500 l/s
Fig. 1. Scheme of the experimental apparatus.
driven Furnace
214
E. Gauthier et al. / Plasma-wall Antothode
sheath
3
I
I AK
-_
-t Z
P
interaction
drical Langmuir probe by varying the primary parameters. The electrical measurements are done with insulation amplifiers and lowpass filters due to the frequent plasma instabilities. These instabilities are in fact 70 kHz oscillations which occur in about one out of three plasma shots. We did not observe any influence of these instabilities on the hydrogen implanted quantities. In the domain shown in fig. 3, the reproductibility of the filtered values of the arc current and the electron temperature and density is better than 5%. The density and the electron temperature are nearly constant along the Z-axis. A typical radial density profile is given in fig. 4. The ionic composition of the plasma had been measured previously [16] when the source was used as a part of neutral beam injector in TFR by using a specially built mass spectrometer. The ions were extracted from the plasma across a molybdenum grid which played the role of anticathode. Within the typical plasma conditions described above, the proportions of H+, Hz and
Fig. 2. a) Approximate distribution of the potential in the plasma source. b) Scheme of the source: K - cathode, A anode, IE - intermediate electrode, a.k - anticathode, Ssample.
of the magnetic field inside the anode and the anticathode
formity
20
the anode and with a value of
between 0.005 T. A 300 D resistor between the anode and IE enables a self-bias of the IE which allows the initiation of the plasma in the cathode chamber. Subsequently, this plasma diffuses in the anode chamber, the intermediary electrode voltage drops to its floating potential giving the potential difference necessary to the existence of the anodic plasma between the anode and IE. The parameters which can be adjusted are: the arc potential between anode and cathode. v,, Pa, the pressure in the cathodic chamber without the plasma. the heating current of the cathode. If, ZC’ the intensity in the coils giving the magnetic field B.
the potential of the anticathode. Typical values of these parameters are V, = 200 V; P, = 0.1 Pa; I, = 70 A; B = 0.005 T; V, floating. With these standard conditions, the arc current is about 50 A, the electron density 5 X 1Or8 mm3 and the electron temperature 15 eV. With these parameters the power flux on the anticathode or the sample is about 400 W cm-*. The attainable values for the electron parameters is given in fig. 3. They are measured with a double cylin-
15
7 2 F 10
5
V k,
1
0
1
2
3
4
5
6
ne (10”
7
8
9
10
mm31
Fig. 3. Domain of the source operation measured between the anode and the anticathode with a double Langmuir probe.
E. Gauthier et al. / Pl~~-wa~i
215
i~tgra~iion
+
l
l +
*
18.0 +
+
I
L + l
Fig. 4. Typical radial profiles of a) the electron temperature, b) the electron density.
HT were about 60, 30 and 10% for hydrogen and respectively 70, 25 and 50% for deuterium. A slight increase for H” and the corresponding decrease for Hl and HT were observed when the arc current was increased. These results may be compared to the 80% of D+ estimated for the deuterium plasma of PISCES [17]. The plasma duration is minimum 100 ms to obtain stationary conditions. The maximum duration depends on the arc power and is limited by the anode heating. It is typically equal to 10 s, giving a m~mum fluence of 1019 s-l for one shot. The minimum duration between the shots is 60 s. A quadrupole mass spectrometer gives the gas composition of the main chamber by differential pumping. An optical visible spectrometer is connected to the plasma by optical fibers. In particular, it enables the molecular bands of carbon compounds with a Z-resolution of two millimeters to be observed. It is also used to check the Langmuir probe measurements of T, by the ratio of the intensity of Dar and Dy lines. The sample can be the anticath~e itself when large
areas of exposed graphite are needed. It can be rotated perpendicularly to its axis to expose the two faces of the graphite disk to the plasma. Generally the samples are slabs of graphite of 11 X 11 x 2 mm. They can be heated by a quartz lamp to temperatures up to 800°C. The temperature measurements are made by a thermocouple inserted into a 1 mm hole in the bulk of the slab. The true surface temperature during the plasma exposure cannot be measured but we have obtained a temperature increase of 30 K during an exposure of 0.5 s to the plasma for the thermocouple, hence for the whole sample. From this value, a numerical simulation gives a surface temperature increase lower than 80 K. The sample is insulated electrically and thermally from its molybdenum holder. It can be biased at a given voltage by a moveable molybdenum contact. We have checked by Auger surface analysis that no perceptible amount of any metal was deposited onto the surface of the sample when the cathode heating current was set at reasonable values (lower than 90 A).
216
E. Gauthier et al. / Plasma-wall
3. Analysis apparatus The sample can be transferred without breaking the vacuum from the plasma source to the analysis vessel. It is a conventional UHV one with a background pressure of lo-* Pa and a nominal pumping speed of 500 Is-‘. The diameter of the valve is such that the total pressure increase in the analysis vessel is less than 5 x lo-’ Pa during the opening of the valve. The transfer duration is about 60 s giving less than half a monolayer possibly adsorbed on the walls. It is fitted with a thermodesorption device and the adjunction of an EELS (electron energy loss spectroscopy) apparatus is planned. The thermodesorption analysis is a powerful tool [13,18,19] as it theoretically allows to identify many features of the limiting mechanisms of the hydrogen desorption from the sample by studying the kinetic of this desorption when a ramp of temperature is applied to this sample. The heating ramp was checked to be rectilinear within 1 K accuracy. It allows heating rates between 0 and 4 KS-’ between 300 and 1200 K. The uniformity of the sample temperature checked by an infrared camera was better than 10 K across the whole implanted region. The influence of the sample holder was found to be negligible by recording the deuterium signal of a nonexposed sample mounted on a deuterium plasma exposed holder. Hydrogen is the main component of the residual gas. Therefore the walls of the vessel are covered by a monolayer of hydrogen and cannot adsorb deuterium in significant quantity. This was checked by baking the vessel after a desorption. No deuterium was observed during this baking.
4. Discussion The plasma source simulates reasonably well the scrape off layer in a tokamak. But care must be taken before to extend the obtained results to real conditions. The maximum electron temperature (25 eV) is too low [20] to simulate all the limiter conditions (100 eV). The ion temperature is very low and approximately equal to 500 K. Therefore the ion bombardment energy is not about 5 kT, as in the SOL [21] but about 3 kT, the sheath potential. This difference can be avoided by biasing the sample, but in this case, the number of incoming electrons would be lowered preventing the study of synergetic effects. Another point must be kept in mind. When a thermodesorption is proceeded at elevated temperature, the sample surface which became amorphous during the plasma exposure, is partly regraphitised above 500°C [22]. This phenomenon was observed by the progressive
interaction
lowering of the sp3 band and band in EELS preliminary successive plasma exposures sorptions may give different plasma exposures for the state
the appearance of the sp2 experiments. Therefore, separated by thermoderesults from consecutive of the surface.
5. Conclusion Notwithstanding the limitations pointed above, the simulation apparatus which is described is very versatile and reasonably simple to operate. It is actually principally aimed at the interpretation of the recycling phenomena but other aspects of plasma-surface interactions can be studied. The first results [8,13] show differences in the implanted sites with energetic ion beam experiments and demonstrate the interest of such studies.
References
PI For example see P.H. Rebut, K.J. Dietz and P.P. Lallia J. Nucl. Mater.
162-164
(1989) 172.
121 H.G. Esser et al., 16th Eur. Conf. on Controlled
Fusion and Plasma Physics, Venice (1989), Post deadline paper. [31 K.L. Wilson and W.L. Hsu, J. Nucl. Mater. 145/147 (1987) 121. 141 W. Moller J. Nucl. Mater. 162/164 (1989) 138. [51 J. Ehrenberg, J. Nucl. Mater. 162/164 (1989) 63. T. Banno, H.G. Esser, L. KBnen, V. 161 F. Walbroeck, Philipps, P. Wienhold and J. Winter, J. Nucl. Mater. 162/164 (1989) 496. 171 G.M. McCracken, R. Behrisch, J.P. Coad, D.H.J. Goodall, P. Harbour, L. de Kock, M.A. Pick, C.S. Pitcher, J. Roth and P.C. Strangeby, Europhysics Conf. Abstracts 13 B III (1989) 947. 181 E. Gauthier, These Marseille (1989). 191 W. Mijller and B.M.V. Scherzer, Appl. Phys. Lett. 50 (1987) 1870. WI D.M. Goebel and R.W. Conn, J. Nucl. Mater. 128/129 (1984) 249. [ll] D.M. Goebel et al., J. Nucl. Mater. 145/147 (1987) 61. [12] Y. Hirooka, R.W. Conn, D.M. Goebel, B. LaBombard, R. Lehmer, W.K. Leung, R.E. Nygren and Y. Ra, J. Nucl. Mater. 162/164 (1989) 1004. [13] E. Gauthier, J. Bardon, J.P. Palmari and A. Grosman, Eur. Conf. Abst. 13 B 111 (1989) 1065. [14] R.A. Demirkbanov, U.V. Kursanov and V.M. Blagovechchensky, Prib. Tekh. Eksp. 1 (1964) 30. [15] J.P. Grandchamp, These, Orsay, France (1982) No. 2619. [16] J.F. Bonnal, J. Druaux and R. Oberson, Rapport Euratom CEA FC 791 (1975). [17] D.M. Goebel, J. Bohdansky, R.W. Conn, Y. Hirooka, B. LaBombard, W.K. Leung, R.E. Nygren, J. Roth and G.R. Tynan, Nuclear Fusion 28 (6) (1988) 1041. [18] G. Ehrlich, J. Appl. Phys. 32 (1961) 4. [19] L.A. Petermann, Prog. in Surf. Sci. 3(l) (1972) 2.
E. Gauthier et al. / Plasma-wall (201 SK. Erents, P.J. Harbour, S. Clement, D.D.R. Summers, G.M. McCracken, J.A. TagIe and L. de Kock, Eur. Conf. Abst. 13 B III (1989) 939. [21] R.A. Pitts, G.M. McCracken, G.F. Matthews and S.J. Fielding, Eur. Conf. Abst. 13 B III (1989) 955.
interaction
[22] J. Fink, T. Miiller-Heinzerling Commun. 47 (9) (1983) 697.
21-l and
J. Pfliiger,
Sol St.
Nuclear Instruments and Methods in Physics Research B53 (1991) 212-217 North-Holland
Realization of a laboratory apparatus for plasma-waft interaction studies E. Gauthier, J. Bardon and J.P. Palmari CRMCZ-CNRS, Campus Luminy, Case 913, 13288 Marseilk Cedex 09, France
A. Grosman CEN Cadarache, Associakm Euratom-C&4 SW la Fusion, 13118 Saint-Pnul-~z-Durance, France Received 17 October 1989 and in revised form 26 September 1990
‘Ibis paper describes SPISA (surface-plasma interaction simuiation apparatus), a new facility for studies on the interaction between iow temperature bigb density plasmas and graphite (or other) samples. The vahtes obtained for the temperature between 5 and 25 eV and the density between 10” mP3 and 1019 mm3 of the plasma simulate reasonably we11the edge plasma of the tokamaks.
The main features of this device are the versatility of the conditions for interaction, the possibility of sample analysis without breaking the vacuum and the careful design of the thermodesorption analysis aparatus. 1. Introduction The importance of the plasma-wall interaction was to be a matter of growing concern for the operation of large fusion devices during the last years (11. The recycling of hydrogen isotopes by the walls is of major importance for the total gas balance especially in the recent experiments with peaked density profiles [I]. It also determines the density and the transport properties in the scrape off layer (SOL). A striking example of the role of the walls is given by Esser et al. [2], who observed that 80% of the gas involved in a Textor plasma shot was provided by the outgassing of the carbon covering the walls. The interest of studying such processes to obtain predictive models has been clear for many years. However, in spite of numerous studies, reviewed e.g. by Wilson and Hsu 91, MMler ]4] or Ehrenberg [S], the recycling mecha~sms are not yet well understood and problems like the relative importance of hydrogen diffusion, detrapping and recombination in various materials are still to be solved. The physical and chemical sputtering and the codeposition phenomena play also a central role for the tokamaks operation. Routine replacement of eroded components can become a serious problem in future tritium fueling conditions. The codeposition of large amounts of hydrogen isotopes and sputtered carbon adds a new parameter in the recycling properties of the walls because codeposited hydrogen cannot reach any saturation. Sputtered carbon is also of importance for the radiative and transport properties in the SOL. shown
0168-583X/91/$03.50
The numerous studies on these phenomena made in the tokamaks suffer some drawbacks which lead these studies to be more descriptive than predictive. As can be seen in ref. [6] properties of the walls are largely history dependent and the study of the plasma-wall interaction cannot be the essential concern for determining the story of a sample. The number of parameters in fusion machines, such as the oxygen and oxygen compounds content, the sample location and temperature, etc. is so large that independent parameter variation which is necessary to the reasonable modeling of the phenomena occurring phenomena is unrealistic. Moreover, for obvious reasons, studies like the implantation profile of hydrogen or the chemical bonding states of the surface cannot be done in situ. Recent reports [7,8] have shown that the properties of the surface are greatly affected by the sample exposure to atmospheric conditions. Within this context, laboratory expe~ments must play an important role for the determination of the mechanisms such as the recycling phenomena or the erosion and codeposition processes. Until recently, these laboratory experiments have principally been done with ion beams in the keV energy range. Interesting results like absolute measurements of adsorbed hydrogen j7] and saturation effects [4] or ion induced desorption ([9] have been obtained by using these methods. But many results, out of the scope of these experiments, are needed like studies in the low-energy-~~-fl~ domain. Such conditions would be more relevant to the conditions of the plasma-wall interaction. To our knowledge, one experimental device exists at
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
213
E. Gauthier et al. / Plasma-wall interaction
first one contains the plasma source. It is pumped out by a 15001 s-’ turbomolecular pump. Before starting operation the vacuum is about lo-’ Pa and it remains below 10e3 Pa during plasma operation. The second chamber is a standard ultrahigh vacuum vessel used for the analysis of the plasma exposed samples. The two parts of the device are linked by a valve, allowing sample transfer without breaking the vacuum.
UCLA [lo] which simulates realistic conditions of the plasma-surface interactions. The PISCES apparatus uses a duoplasmatron as plasma source and includes many analytic tools. Interesting results [11,12] have been published in respect to the erosion, redeposition and codeposition phenomena. In this paper we describe the realization and the characteristics of SPISA, a new laboratory apparatus for the simulation of the plasma-wall interaction conditions. The source is a duopigatron giving roughly the same plasma electron temperature and density as in PISCES, i.e. T, -C 25 eV; N, < 1019 rnm3. The sample can be biased to change the energy of impinging ions and it can be transferred into an analysis vessel without breaking the vacuum. First results on recycling mechanisms obtained with this apparatus [8,13] have shown differences to those obtained in ion beam experiments. The versatility of our device can give interesting results in an extended range of plasma wall interaction studies although some differences with tokamaks experiments are inherent to such a device like the low ionic temperature (0.05 eV), the relatively high fraction of molecular ions H; and Hl and the large ion Larmor radii.
2.1. The plasma source The main feature of the duopigatron plasma source is the presence of an anticathode which is approximately at the same potential as the cathode, providing longitudinal oscillations of the hot electrons resulting in a better ionization efficiency than in conventional sources [14,15]. A schematic of the source is given in fig. 2. It is symmetric around the Z-axis. The cathode is a 1 mm in diameter tungsten wire which is supported by two water-cooled copper rods. This cathode is separated from the anodic chamber by an intermediate electrode (IE) of soft iron with a duct 12.5 mm in diameter. After the IE is the stainless steel anode which is cylindrical 150 mm in length and 50 mm in diameter. The anticathode is generally a graphite disk 70 mm in diameter set 30 mm after the anode. One water-cooled copper coil is located outside the vacuum chamber. Its magnetic field lines are guided by the IE. Another coil is inside the vacuum chamber and provides a good uni-
2. Experimental device A schematic of the experimental setup is shown in fig. 1. It is made up of two stainless steel vessels. The
Plasma Source
Sample
I lolder
Optical Visible Spectrometer Double
Langmuir
Probe
I.R. Camera
/
Mass Spectrometer
Pumping
170 I/s
Pumping
1500 I/S
500 l/s
Fig. 1. Scheme of the experimental apparatus.
driven Furnace
214
E. Gauthier et al. / Plasma-wall Antothode
sheath
3
I
I AK
-_
-t Z
P
interaction
drical Langmuir probe by varying the primary parameters. The electrical measurements are done with insulation amplifiers and lowpass filters due to the frequent plasma instabilities. These instabilities are in fact 70 kHz oscillations which occur in about one out of three plasma shots. We did not observe any influence of these instabilities on the hydrogen implanted quantities. In the domain shown in fig. 3, the reproductibility of the filtered values of the arc current and the electron temperature and density is better than 5%. The density and the electron temperature are nearly constant along the Z-axis. A typical radial density profile is given in fig. 4. The ionic composition of the plasma had been measured previously [16] when the source was used as a part of neutral beam injector in TFR by using a specially built mass spectrometer. The ions were extracted from the plasma across a molybdenum grid which played the role of anticathode. Within the typical plasma conditions described above, the proportions of H+, Hz and
Fig. 2. a) Approximate distribution of the potential in the plasma source. b) Scheme of the source: K - cathode, A anode, IE - intermediate electrode, a.k - anticathode, Ssample.
of the magnetic field inside the anode and the anticathode
formity
20
the anode and with a value of
between 0.005 T. A 300 D resistor between the anode and IE enables a self-bias of the IE which allows the initiation of the plasma in the cathode chamber. Subsequently, this plasma diffuses in the anode chamber, the intermediary electrode voltage drops to its floating potential giving the potential difference necessary to the existence of the anodic plasma between the anode and IE. The parameters which can be adjusted are: the arc potential between anode and cathode. v,, Pa, the pressure in the cathodic chamber without the plasma. the heating current of the cathode. If, ZC’ the intensity in the coils giving the magnetic field B.
the potential of the anticathode. Typical values of these parameters are V, = 200 V; P, = 0.1 Pa; I, = 70 A; B = 0.005 T; V, floating. With these standard conditions, the arc current is about 50 A, the electron density 5 X 1Or8 mm3 and the electron temperature 15 eV. With these parameters the power flux on the anticathode or the sample is about 400 W cm-*. The attainable values for the electron parameters is given in fig. 3. They are measured with a double cylin-
15
7 2 F 10
5
V k,
1
0
1
2
3
4
5
6
ne (10”
7
8
9
10
mm31
Fig. 3. Domain of the source operation measured between the anode and the anticathode with a double Langmuir probe.
E. Gauthier et al. / Pl~~-wa~i
215
i~tgra~iion
+
l
l +
*
18.0 +
+
I
L + l
Fig. 4. Typical radial profiles of a) the electron temperature, b) the electron density.
HT were about 60, 30 and 10% for hydrogen and respectively 70, 25 and 50% for deuterium. A slight increase for H” and the corresponding decrease for Hl and HT were observed when the arc current was increased. These results may be compared to the 80% of D+ estimated for the deuterium plasma of PISCES [17]. The plasma duration is minimum 100 ms to obtain stationary conditions. The maximum duration depends on the arc power and is limited by the anode heating. It is typically equal to 10 s, giving a m~mum fluence of 1019 s-l for one shot. The minimum duration between the shots is 60 s. A quadrupole mass spectrometer gives the gas composition of the main chamber by differential pumping. An optical visible spectrometer is connected to the plasma by optical fibers. In particular, it enables the molecular bands of carbon compounds with a Z-resolution of two millimeters to be observed. It is also used to check the Langmuir probe measurements of T, by the ratio of the intensity of Dar and Dy lines. The sample can be the anticath~e itself when large
areas of exposed graphite are needed. It can be rotated perpendicularly to its axis to expose the two faces of the graphite disk to the plasma. Generally the samples are slabs of graphite of 11 X 11 x 2 mm. They can be heated by a quartz lamp to temperatures up to 800°C. The temperature measurements are made by a thermocouple inserted into a 1 mm hole in the bulk of the slab. The true surface temperature during the plasma exposure cannot be measured but we have obtained a temperature increase of 30 K during an exposure of 0.5 s to the plasma for the thermocouple, hence for the whole sample. From this value, a numerical simulation gives a surface temperature increase lower than 80 K. The sample is insulated electrically and thermally from its molybdenum holder. It can be biased at a given voltage by a moveable molybdenum contact. We have checked by Auger surface analysis that no perceptible amount of any metal was deposited onto the surface of the sample when the cathode heating current was set at reasonable values (lower than 90 A).
216
E. Gauthier et al. / Plasma-wall
3. Analysis apparatus The sample can be transferred without breaking the vacuum from the plasma source to the analysis vessel. It is a conventional UHV one with a background pressure of lo-* Pa and a nominal pumping speed of 500 Is-‘. The diameter of the valve is such that the total pressure increase in the analysis vessel is less than 5 x lo-’ Pa during the opening of the valve. The transfer duration is about 60 s giving less than half a monolayer possibly adsorbed on the walls. It is fitted with a thermodesorption device and the adjunction of an EELS (electron energy loss spectroscopy) apparatus is planned. The thermodesorption analysis is a powerful tool [13,18,19] as it theoretically allows to identify many features of the limiting mechanisms of the hydrogen desorption from the sample by studying the kinetic of this desorption when a ramp of temperature is applied to this sample. The heating ramp was checked to be rectilinear within 1 K accuracy. It allows heating rates between 0 and 4 KS-’ between 300 and 1200 K. The uniformity of the sample temperature checked by an infrared camera was better than 10 K across the whole implanted region. The influence of the sample holder was found to be negligible by recording the deuterium signal of a nonexposed sample mounted on a deuterium plasma exposed holder. Hydrogen is the main component of the residual gas. Therefore the walls of the vessel are covered by a monolayer of hydrogen and cannot adsorb deuterium in significant quantity. This was checked by baking the vessel after a desorption. No deuterium was observed during this baking.
4. Discussion The plasma source simulates reasonably well the scrape off layer in a tokamak. But care must be taken before to extend the obtained results to real conditions. The maximum electron temperature (25 eV) is too low [20] to simulate all the limiter conditions (100 eV). The ion temperature is very low and approximately equal to 500 K. Therefore the ion bombardment energy is not about 5 kT, as in the SOL [21] but about 3 kT, the sheath potential. This difference can be avoided by biasing the sample, but in this case, the number of incoming electrons would be lowered preventing the study of synergetic effects. Another point must be kept in mind. When a thermodesorption is proceeded at elevated temperature, the sample surface which became amorphous during the plasma exposure, is partly regraphitised above 500°C [22]. This phenomenon was observed by the progressive
interaction
lowering of the sp3 band and band in EELS preliminary successive plasma exposures sorptions may give different plasma exposures for the state
the appearance of the sp2 experiments. Therefore, separated by thermoderesults from consecutive of the surface.
5. Conclusion Notwithstanding the limitations pointed above, the simulation apparatus which is described is very versatile and reasonably simple to operate. It is actually principally aimed at the interpretation of the recycling phenomena but other aspects of plasma-surface interactions can be studied. The first results [8,13] show differences in the implanted sites with energetic ion beam experiments and demonstrate the interest of such studies.
References
PI For example see P.H. Rebut, K.J. Dietz and P.P. Lallia J. Nucl. Mater.
162-164
(1989) 172.
121 H.G. Esser et al., 16th Eur. Conf. on Controlled
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J. Pfliiger,
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