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Friction and wear in the hostile environment of a tokamak
Fusion Engineering North-Holland
and Design
13 (1990)
307-311
307
Friction and wear in the hostile environment of a tokarnak P. Marmy,
D. Ringer
Paul Scherrer
Institute
Submitted Handling
(PSI)
and U. Stiefel [J ormerly
28 March 1988; accepted Editor: Robert W. Conn
EIR, 26 May
Eidg. Insrilur
fir
ReakrorJorschungl.
CH-5303
Wiirenlingen,
Switzerland
1988
The tribology knowledge reported in this paper was obtained during the construction of a sample manipulator for the Textor tokamak and during tests made in relation with the development of a drive mechanism for the ALT II limiter. After some year of operation, the combination steel-bronze coated with MoS, dry lubrication has proven to be very reliable. The plasma induced chemical vapour deposition of thin films on MoS, may increase drastically the friction coefficient and lead to seizure effects. Nevertheless, for typical films as used in today’s tokamaks ( cl00 nm) only a small increase of the friction coefficient could be detected. The paper deals with two specific applications for the tokamak TEXTOR; it should however be noted, that severe tribology problems occur generally when movable parts are used in plasma devices for diagnostic or other purposes.
1. Introduction The Swiss activities at the TEXTOR project are based on the IAEA umbrella agreement on Plasma Walls Interaction. It was soon realized, that a thin film coating with a “low Z” material would reduce the impurity level of the plasma considerably [1,2]. When the idea of adding a new divertor concept, namely a toroidal belt limiter called ALT II to the TEXTOR tokamak came to realization [3], it was obvious that the tribology problems in combination with the wall coating had to be reinvestigated very carefully. The drive mechanisms serve for the remote positioning of the limiter blades with respect to the plasma radius. A detailed description of ALT II and of it’s significance is given elsewhere in this issue. Fig. 1 shows a view of the Swiss manipulator extracting a specimen holder from a receiving cup mounted on the first wall of TEXTOR [4]. The ALT II limiter blade with its two drive jacks can just be seen behind the manipulator. Eight such blades are mounted all around the torus. To have complicated drive mechanisms doing ‘well in the hostile environment of a tokamak is not an obvious task. Some knowledge of tribology is necessary to master the problems and find a good solution. In short, tribology is the science which describes the interactions between mechanical parts moving one relative to the other. The tribosystem can be divided into 4 0920-3796/90/$03.50
0 1990 - Elsevier
Science
elements: the first body, the partner, the interface, and the environment. The interface has in general three different duties: - to lower the friction forces between the mechanical parts, - to extract the heat produced by the friction, - to remove the particles worn off. In most tribosystems the interface is found to be oil. The particular environment of a tokamak consists of ultra high vacuum, high temperatures, and possibly the presence of a plasma from a P-CVD (Plasma Chemical Vapor Deposition) process. Liquid lubrication and grease must obviously be avoided and the first question is to know whether exotic combinations of materials would allow no lubrication at all. In that case the interface of the tribosystem would be the oxide layers or the core material itself.
2. The wear mechanism Dealing with the more simple case of steels and considering dry friction, the wear mechanism taking place is adhesive wear. Most metals are partially protected against this type of wear by oxide layers which form naturally on their surface. These oxide layers may be removed mechanically by friction allowing the metallic substrates to come into contact. These contacts
Publishers
B.V. (North-Holland)
produce a surface deterioration and thus enhance the friction coefficient. Wear becomes larger and may produce in certain geometrical conditions the seizing of the parts in contact. However the oxide layers are renewed instantly by This diminishes the wear rate. On the oxygen reactions. contrary. in a high vacuum this natural defence dbes not exist anymore because the oxide layers need time to be renewed [S]. In a vacuum of lo-’ mbar. the time to produce one oxide layer is 30 s. It should be pointed out that a full protection consists of some 50 layers. dry friction between metal In an ultra high vacuum parts will always lead to failure of the surface. It is only a question of time [6]. The measurements of fig. 2 show that a non lubricated test leads to a friction increase by a factor of ten. The treatment of the plate with MoSz spray or powder gives a very good protection against adhesive wear. Molybdenum disulfide is a layer lattice material with very good lubricant characteristics when applicated as a solid film.
3. Rolling
Fig. 1. Internal view of the TEXTOR torus limiter blade under assembly and the EIR in its inserted position.
showing an ALT probr mnnipulator
with I IW Fig. 2. Measurements
II
and
sliding
friction
In a tokamak environment, sliding friction is more of a problem than rolling friction. The reason is that in general the geometric constraints are more severe and the removal of worn off particles is more difficult through linear movement. loadings rolling fricHowever at very high surface tion can also lead to severe difficulties. The manipulator
MoSz lubricotlon I 203
300
.
.
..I.,,, 4w
50
of the coefficient of friction as 3 function of the number of cycles for 3 cylinder sliding mbar, T= 300’ C. polished surfaces. plate ETGlOO. cylinder hard metal.
on plate.
Vacuum:
lo-’
I? Marny,
D. Ringer,
II. Sliefel
/ Friction
309
and wear
that was built in PSI is raised vertically by means of a ball screw. In the first tests a machined ball screw with MoS, lubrication was ued. It failed after some weeks of operation [4]. The problems disappeared with the use of a rolled ball screw lubricated with boron nitride grease. The use of ball bearings is no problem if they are chosen with more play than standard. They should be treated on a mill with MoS, powder. Another solution consists of depositing WS, compound by sputtering with the result of improving the adhesion of the lubricant. However at very high loadings the use of vacuum oil is recommended.
wear characteristics. The Poro Bronze MN is a soft bronze containing up to 25% lead. One of both partners is coated with MoS, spray. This combination has very good emergency running characteristics when all the lubricant is worn off. Because of more stringent environment, the ALT II drive mechanisms use a slightly different combination [7]. The steel is a high strength ferritic steel named NITRONIC 60 and the bronze a NSB-bronze with 2% Ni and 0.8% Mn. The combination was successfully tested at PSI. However, the emergency running characteristics were poor due to the lack of lead [8]. Another argument for using bronze is its high thermal conductivity which enhances the heat removal.
4. Journal
oxide: The steel used was again ETGlOO. This combination was the only combination found to run without any lubrication [9]. The wear mechanism taking place is abrasive wear (CH-Patent No. 664427). Because of the high cost and the difficulty of manufacturing, this combination should be used only for extreme cases.
bearings
The choice of the material combination used should take into account the following points: - The materials should be compatible with the environment of a tokamak which means low magnetic permeability and if possible no traces of volatile alloying elements, which could contaminate the vacuum. The vapor pressure of a metal is a function of pressure and temperature. The figures should be checked for operating conditions. - The coefficients of expansion of the partners should be compatible, to keep the play of the assembly in a known tolerance over the whole temperature range. - High contact pressure between the sliding parts should be avoided. The contact pressure can be checked with Hertz’s formula:
Steel-Zirconium
6. Use of tungsten in tokamak applications The use to tungsten or tungsten-rhenium alloy for tribology or shielding applications should be made with caution. In the presence of oxygen and high temperatures, tungsten is forming tungsten oxides which are unstable and may be released in powder form. Through the intense irradiation by 14 MeV neutrons, hafnium oxide and other impurities may be formed and contaminate the whole machine very badly.
7. Influence of MoS, = load, E = Young’s modulus, r, = shaft radius, = bearing radius and b = bearing length. - It is an advantage to choose partners with different hardness and if possible make the bearing from the softer material. The reason is a better removal of worn off particles. P r,
5. Two tested solutions Steel-Bronze with MoS, dry lubricant: The Swiss manipulator performs successfully for several years with an ETGlOO-Poro Bronze MN combination. The ETGlOO is a high strength ferritic steel with very good
of B/C
films on the lubrication
behaviour
In order to reduce the metallic contamination of the plasma, a new technique of low Z protective coatings has been developed [lo]. This technique involves the in situ deposition of amorphous B.$ : H films on the inside of the tokamak using a Plasma Chemical Vapour Deposition process (P-CVD). The advantage of B$: H coating over the carbonisation which is already used at TEXTOR [11,12] is its reduced chemical reactivity towards hydrogen. Before applying the new coating, it had to be experimentally verified that no damage would result to the ALT II drive mechanisms. For this reason the influence of this new environment on tribology was carefully
310
Fig. 3. Bronze-steel combination vacuum and at 3OO’C. Below, surface pressure
Table
checked in an experiment scaled 1 to 1 [13]. The experiments were conducted in the facility that was used in previous tests (according to fig. 2) but in the presence of the new environment. The plate was DIN 1.4311 stainless steel, coated with MoS, and the cylinder a WChardmetal coated with TiN. It should be noted, that the drive mechanism is relatively well shielded behind the liner and therefore only sees the gas mixture but not the glow discharge under ordinary conditions. However the possibility of actually coating the mechanisms can not be totally excluded. Both conditions were tested therefore. All tests were conducted in high vacuum at 300 o C and after the respective exposure. Out of numerous tests some typical results are summarized in table 1. They clearly indicate that the P-CVD environment has some influence on the lubricating behaviour of MO%. The galling observed in the third test was due to the high surface roughness and to a too thin MO& layer thickness. Before this test the measured friction coefficient in vacuum without P-CVD environment was already 0.16. In the presence of the reactive gas, there is no deposition when the P-CVD process is switched off. Test#2 indicates a modification of the friction coefficient by a factor of two as compared to test#l. This effect could not be seen in test#4, where the surface pressure and the roughness were reduced. After glow discharge a thin BC film was deposited on the MO&. Test#5 showes no increase of the friction coefficient. Test #6 indicates a clear increase of the friction coefficient. After observation of the surface, it was seen that the hard BC film had broken into small fragments. The soft Mo!$ layer underneath was not
successfully tested in high the weight which produced a of 10 N/mm’.
1
Measurement Plate roughness
of friction
coefficient
(v)
Surface (N/mm2)
6
65
in the presence pressure
of a P-CVD Test
environment Environment
before
No.
test
DC-Film thickness
Friction coefficient
(tinI)
2
40
65
1
vacuum
2
CH,:5% B2H, in Hc=l:lO
none
0.04-0.11
3
same + glow
0.11
0.2-0.6
4
CH,:5% B,H,inHe=l:lO
none
0.05
0.02-0.05
discharge
5
same + glow
discharge
0.02
0.05
6
same + glow
discharge
0.09
0.07-0.09
7
H,
none
0.08-0.09
+ glow
discharge
P. Marmy,
removed fragments gen leads indicated 0.1 should
D. Rhger,
and took up its lubricating action when the where worn off. A glow discharge into hydroto a change of the properties of MoS, as in test#7. However, a friction coefficient of not be considered as a failure.
8. Summary
Through the development of the Swiss manipulator and the ALT II drive mechanisms, the tribology knowledge in tokamak conditions could be improved. A reliable metal-bronze combination has been found to work during a long period and under severe conditions. The environment of a P-CVD process impairs the lubrication properties of MoS,. Sliding parts and bearings in the vicinity of the plasma should be shielded to preclude the deposition of amorphous boron-carbon, when technologicaly feasible. In very severe conditions, the use of dry lubrication is not reliable anymore and special oil and grease must be employed when possible.
References
[l] C. Braganza, S. Vepiek and protective coatings prepared CVD, J. Nucl. Mater. 85&86 [2] P. Groner. J.K. Ginuewski
P. Groner. Boron compound by means of low pressure (1979) 1133-1137. and S. Vepiek, Boron and
U. Stir/e1
/ Fricrion
and wenr
311
doped boron first wall coatings by plasma CVD, J. Nucl. Mater. 103&104 (1981) 257-260. 131 R.W. Conn et al., An advanced limiter experiment of large toroidal extent - ALT II, J. Nucl. Mater. 121 (1984). 141 P. Marmy and U. Stiefel. A first wall manipulator for the Textor Tokamak, Proceedings of the Robotics and Remote Handling in Hostile Environments, April 1984, Gatlinburg (ANS). PI The Welding Institute 3517/2/77 (August 1977). im PI P. Marmy, Reibung and Verschleisserscheinungen UHV, Vakuum Technik 6 (1983). 171 G.W. Brown, Development of a high vacuum high temperature movable limiter support, Fusion Technology (November 1986). PI D. Ringer, TEXTOR-Materialversuche fur Gleitlager, PSI Technical Notice TM-22-86-15 (August 1986). PSI Technical [91 P. Marmy, TEXTOR-Materialversuche, Notice TM-22-82-55 (November 1982). 1101S. Veptek, U. Stiefel and D. Ringer, Development of in situ deposition of protective coatings for fusion devices, ENC’86 Transactions, Geneva, June 1-6, 1986. 1111P. Wienhold, F. Waelbroeck, H. Bergsaker, B. Schweer, H.G. Esser and J. Winter, Determination of carbon fluxes in the limiter shadow of TEXTOR by analysis of carbon deposits on steel targets, Proc. 14th Europ. Conf. on Contr. Fusion and Plasma Physics, Madrid, 22-26 June 1987. PI J. Winter, Surface conditioning of fusion devices by carbonization: Hydrogen recycling and wall pumping, J. Vat. Sci. Technol. A5 (4) (Juli/Aug. 1987) 2286-2292. 1131 D. Ringer, Einwirkung von a-B,vC:H Schichten auf die Tribologie von MO&. PSI Technical Notice AN-22-88-01 (February 1988).
and Design
13 (1990)
307-311
307
Friction and wear in the hostile environment of a tokarnak P. Marmy,
D. Ringer
Paul Scherrer
Institute
Submitted Handling
(PSI)
and U. Stiefel [J ormerly
28 March 1988; accepted Editor: Robert W. Conn
EIR, 26 May
Eidg. Insrilur
fir
ReakrorJorschungl.
CH-5303
Wiirenlingen,
Switzerland
1988
The tribology knowledge reported in this paper was obtained during the construction of a sample manipulator for the Textor tokamak and during tests made in relation with the development of a drive mechanism for the ALT II limiter. After some year of operation, the combination steel-bronze coated with MoS, dry lubrication has proven to be very reliable. The plasma induced chemical vapour deposition of thin films on MoS, may increase drastically the friction coefficient and lead to seizure effects. Nevertheless, for typical films as used in today’s tokamaks ( cl00 nm) only a small increase of the friction coefficient could be detected. The paper deals with two specific applications for the tokamak TEXTOR; it should however be noted, that severe tribology problems occur generally when movable parts are used in plasma devices for diagnostic or other purposes.
1. Introduction The Swiss activities at the TEXTOR project are based on the IAEA umbrella agreement on Plasma Walls Interaction. It was soon realized, that a thin film coating with a “low Z” material would reduce the impurity level of the plasma considerably [1,2]. When the idea of adding a new divertor concept, namely a toroidal belt limiter called ALT II to the TEXTOR tokamak came to realization [3], it was obvious that the tribology problems in combination with the wall coating had to be reinvestigated very carefully. The drive mechanisms serve for the remote positioning of the limiter blades with respect to the plasma radius. A detailed description of ALT II and of it’s significance is given elsewhere in this issue. Fig. 1 shows a view of the Swiss manipulator extracting a specimen holder from a receiving cup mounted on the first wall of TEXTOR [4]. The ALT II limiter blade with its two drive jacks can just be seen behind the manipulator. Eight such blades are mounted all around the torus. To have complicated drive mechanisms doing ‘well in the hostile environment of a tokamak is not an obvious task. Some knowledge of tribology is necessary to master the problems and find a good solution. In short, tribology is the science which describes the interactions between mechanical parts moving one relative to the other. The tribosystem can be divided into 4 0920-3796/90/$03.50
0 1990 - Elsevier
Science
elements: the first body, the partner, the interface, and the environment. The interface has in general three different duties: - to lower the friction forces between the mechanical parts, - to extract the heat produced by the friction, - to remove the particles worn off. In most tribosystems the interface is found to be oil. The particular environment of a tokamak consists of ultra high vacuum, high temperatures, and possibly the presence of a plasma from a P-CVD (Plasma Chemical Vapor Deposition) process. Liquid lubrication and grease must obviously be avoided and the first question is to know whether exotic combinations of materials would allow no lubrication at all. In that case the interface of the tribosystem would be the oxide layers or the core material itself.
2. The wear mechanism Dealing with the more simple case of steels and considering dry friction, the wear mechanism taking place is adhesive wear. Most metals are partially protected against this type of wear by oxide layers which form naturally on their surface. These oxide layers may be removed mechanically by friction allowing the metallic substrates to come into contact. These contacts
Publishers
B.V. (North-Holland)
produce a surface deterioration and thus enhance the friction coefficient. Wear becomes larger and may produce in certain geometrical conditions the seizing of the parts in contact. However the oxide layers are renewed instantly by This diminishes the wear rate. On the oxygen reactions. contrary. in a high vacuum this natural defence dbes not exist anymore because the oxide layers need time to be renewed [S]. In a vacuum of lo-’ mbar. the time to produce one oxide layer is 30 s. It should be pointed out that a full protection consists of some 50 layers. dry friction between metal In an ultra high vacuum parts will always lead to failure of the surface. It is only a question of time [6]. The measurements of fig. 2 show that a non lubricated test leads to a friction increase by a factor of ten. The treatment of the plate with MoSz spray or powder gives a very good protection against adhesive wear. Molybdenum disulfide is a layer lattice material with very good lubricant characteristics when applicated as a solid film.
3. Rolling
Fig. 1. Internal view of the TEXTOR torus limiter blade under assembly and the EIR in its inserted position.
showing an ALT probr mnnipulator
with I IW Fig. 2. Measurements
II
and
sliding
friction
In a tokamak environment, sliding friction is more of a problem than rolling friction. The reason is that in general the geometric constraints are more severe and the removal of worn off particles is more difficult through linear movement. loadings rolling fricHowever at very high surface tion can also lead to severe difficulties. The manipulator
MoSz lubricotlon I 203
300
.
.
..I.,,, 4w
50
of the coefficient of friction as 3 function of the number of cycles for 3 cylinder sliding mbar, T= 300’ C. polished surfaces. plate ETGlOO. cylinder hard metal.
on plate.
Vacuum:
lo-’
I? Marny,
D. Ringer,
II. Sliefel
/ Friction
309
and wear
that was built in PSI is raised vertically by means of a ball screw. In the first tests a machined ball screw with MoS, lubrication was ued. It failed after some weeks of operation [4]. The problems disappeared with the use of a rolled ball screw lubricated with boron nitride grease. The use of ball bearings is no problem if they are chosen with more play than standard. They should be treated on a mill with MoS, powder. Another solution consists of depositing WS, compound by sputtering with the result of improving the adhesion of the lubricant. However at very high loadings the use of vacuum oil is recommended.
wear characteristics. The Poro Bronze MN is a soft bronze containing up to 25% lead. One of both partners is coated with MoS, spray. This combination has very good emergency running characteristics when all the lubricant is worn off. Because of more stringent environment, the ALT II drive mechanisms use a slightly different combination [7]. The steel is a high strength ferritic steel named NITRONIC 60 and the bronze a NSB-bronze with 2% Ni and 0.8% Mn. The combination was successfully tested at PSI. However, the emergency running characteristics were poor due to the lack of lead [8]. Another argument for using bronze is its high thermal conductivity which enhances the heat removal.
4. Journal
oxide: The steel used was again ETGlOO. This combination was the only combination found to run without any lubrication [9]. The wear mechanism taking place is abrasive wear (CH-Patent No. 664427). Because of the high cost and the difficulty of manufacturing, this combination should be used only for extreme cases.
bearings
The choice of the material combination used should take into account the following points: - The materials should be compatible with the environment of a tokamak which means low magnetic permeability and if possible no traces of volatile alloying elements, which could contaminate the vacuum. The vapor pressure of a metal is a function of pressure and temperature. The figures should be checked for operating conditions. - The coefficients of expansion of the partners should be compatible, to keep the play of the assembly in a known tolerance over the whole temperature range. - High contact pressure between the sliding parts should be avoided. The contact pressure can be checked with Hertz’s formula:
Steel-Zirconium
6. Use of tungsten in tokamak applications The use to tungsten or tungsten-rhenium alloy for tribology or shielding applications should be made with caution. In the presence of oxygen and high temperatures, tungsten is forming tungsten oxides which are unstable and may be released in powder form. Through the intense irradiation by 14 MeV neutrons, hafnium oxide and other impurities may be formed and contaminate the whole machine very badly.
7. Influence of MoS, = load, E = Young’s modulus, r, = shaft radius, = bearing radius and b = bearing length. - It is an advantage to choose partners with different hardness and if possible make the bearing from the softer material. The reason is a better removal of worn off particles. P r,
5. Two tested solutions Steel-Bronze with MoS, dry lubricant: The Swiss manipulator performs successfully for several years with an ETGlOO-Poro Bronze MN combination. The ETGlOO is a high strength ferritic steel with very good
of B/C
films on the lubrication
behaviour
In order to reduce the metallic contamination of the plasma, a new technique of low Z protective coatings has been developed [lo]. This technique involves the in situ deposition of amorphous B.$ : H films on the inside of the tokamak using a Plasma Chemical Vapour Deposition process (P-CVD). The advantage of B$: H coating over the carbonisation which is already used at TEXTOR [11,12] is its reduced chemical reactivity towards hydrogen. Before applying the new coating, it had to be experimentally verified that no damage would result to the ALT II drive mechanisms. For this reason the influence of this new environment on tribology was carefully
310
Fig. 3. Bronze-steel combination vacuum and at 3OO’C. Below, surface pressure
Table
checked in an experiment scaled 1 to 1 [13]. The experiments were conducted in the facility that was used in previous tests (according to fig. 2) but in the presence of the new environment. The plate was DIN 1.4311 stainless steel, coated with MoS, and the cylinder a WChardmetal coated with TiN. It should be noted, that the drive mechanism is relatively well shielded behind the liner and therefore only sees the gas mixture but not the glow discharge under ordinary conditions. However the possibility of actually coating the mechanisms can not be totally excluded. Both conditions were tested therefore. All tests were conducted in high vacuum at 300 o C and after the respective exposure. Out of numerous tests some typical results are summarized in table 1. They clearly indicate that the P-CVD environment has some influence on the lubricating behaviour of MO%. The galling observed in the third test was due to the high surface roughness and to a too thin MO& layer thickness. Before this test the measured friction coefficient in vacuum without P-CVD environment was already 0.16. In the presence of the reactive gas, there is no deposition when the P-CVD process is switched off. Test#2 indicates a modification of the friction coefficient by a factor of two as compared to test#l. This effect could not be seen in test#4, where the surface pressure and the roughness were reduced. After glow discharge a thin BC film was deposited on the MO&. Test#5 showes no increase of the friction coefficient. Test #6 indicates a clear increase of the friction coefficient. After observation of the surface, it was seen that the hard BC film had broken into small fragments. The soft Mo!$ layer underneath was not
successfully tested in high the weight which produced a of 10 N/mm’.
1
Measurement Plate roughness
of friction
coefficient
(v)
Surface (N/mm2)
6
65
in the presence pressure
of a P-CVD Test
environment Environment
before
No.
test
DC-Film thickness
Friction coefficient
(tinI)
2
40
65
1
vacuum
2
CH,:5% B2H, in Hc=l:lO
none
0.04-0.11
3
same + glow
0.11
0.2-0.6
4
CH,:5% B,H,inHe=l:lO
none
0.05
0.02-0.05
discharge
5
same + glow
discharge
0.02
0.05
6
same + glow
discharge
0.09
0.07-0.09
7
H,
none
0.08-0.09
+ glow
discharge
P. Marmy,
removed fragments gen leads indicated 0.1 should
D. Rhger,
and took up its lubricating action when the where worn off. A glow discharge into hydroto a change of the properties of MoS, as in test#7. However, a friction coefficient of not be considered as a failure.
8. Summary
Through the development of the Swiss manipulator and the ALT II drive mechanisms, the tribology knowledge in tokamak conditions could be improved. A reliable metal-bronze combination has been found to work during a long period and under severe conditions. The environment of a P-CVD process impairs the lubrication properties of MoS,. Sliding parts and bearings in the vicinity of the plasma should be shielded to preclude the deposition of amorphous boron-carbon, when technologicaly feasible. In very severe conditions, the use of dry lubrication is not reliable anymore and special oil and grease must be employed when possible.
References
[l] C. Braganza, S. Vepiek and protective coatings prepared CVD, J. Nucl. Mater. 85&86 [2] P. Groner. J.K. Ginuewski
P. Groner. Boron compound by means of low pressure (1979) 1133-1137. and S. Vepiek, Boron and
U. Stir/e1
/ Fricrion
and wenr
311
doped boron first wall coatings by plasma CVD, J. Nucl. Mater. 103&104 (1981) 257-260. 131 R.W. Conn et al., An advanced limiter experiment of large toroidal extent - ALT II, J. Nucl. Mater. 121 (1984). 141 P. Marmy and U. Stiefel. A first wall manipulator for the Textor Tokamak, Proceedings of the Robotics and Remote Handling in Hostile Environments, April 1984, Gatlinburg (ANS). PI The Welding Institute 3517/2/77 (August 1977). im PI P. Marmy, Reibung and Verschleisserscheinungen UHV, Vakuum Technik 6 (1983). 171 G.W. Brown, Development of a high vacuum high temperature movable limiter support, Fusion Technology (November 1986). PI D. Ringer, TEXTOR-Materialversuche fur Gleitlager, PSI Technical Notice TM-22-86-15 (August 1986). PSI Technical [91 P. Marmy, TEXTOR-Materialversuche, Notice TM-22-82-55 (November 1982). 1101S. Veptek, U. Stiefel and D. Ringer, Development of in situ deposition of protective coatings for fusion devices, ENC’86 Transactions, Geneva, June 1-6, 1986. 1111P. Wienhold, F. Waelbroeck, H. Bergsaker, B. Schweer, H.G. Esser and J. Winter, Determination of carbon fluxes in the limiter shadow of TEXTOR by analysis of carbon deposits on steel targets, Proc. 14th Europ. Conf. on Contr. Fusion and Plasma Physics, Madrid, 22-26 June 1987. PI J. Winter, Surface conditioning of fusion devices by carbonization: Hydrogen recycling and wall pumping, J. Vat. Sci. Technol. A5 (4) (Juli/Aug. 1987) 2286-2292. 1131 D. Ringer, Einwirkung von a-B,vC:H Schichten auf die Tribologie von MO&. PSI Technical Notice AN-22-88-01 (February 1988).