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Glow discharge carbonisation in JET
Journal of Nuclear Materials North-Holland, Amsterdam
145-147
(1987) 747-750 747
GLOW DISCHARGE
CARBONISATION
J.P. COAD, K.H. BEHRINGER
IN JET
and K.J. DIETZ
JET Joint Undertaking, Abingdon, Oxfordshire, United Kingdom
Key words:
carbonisation,
wall conditioning,
Many of the glow discharge
carbonisations
tokamaks.
plasma
impurities
of the JET vessel have been monitored
with removable
probes.
The probes
have
been analysed by Auger Spectroscopy, and the data correlated with subsequent impurity concentrations and radiated power levels in the plasma. Metal concentrations observed in the plasma are reduced for a time proportional to the amount of carbon deposited on the probes (and also on the limiters, presumably). has a significant effect on Z,,, for high power discharges
1. Introduction During operation the JET carbon limiters become contaminated with metals from the vessel wall etc [l], and from the limiters these contaminants can enter the plasma. Extensive work on TEXTOR [2-51 has suggested that covering the limiters and walls with a fresh layer of carbon (“carbonisation”) can reduce this ingress. The effect of carbonisation of the JET vessel was first studied in September 1984, and more exhaustive testing including the use of probes to monitor some of the treatments was carried out in the operating period from January to June 1985. The first few carbonisations in 1985 used a mixture of deuterium and a low concentration (2.15%) of methane (similar to the 3% used in September 1984) and the analysis of probes exposed in this mixture was briefly described at the Budapest Conference [l]. The results were completely in agreement with what may be expected from the work of the TEXTOR group [2-51, that is reduction of surface oxygen, and the formation of carbides within the surface without significant carbon overlayer formation. In the period from March 24th 1985 to the end of May 1985 attempts at heavier carbonisation were made, using mixtures of either 12 or 17% CH4 in D2, and probe measurements made during these experiments and their implications are reported here. 2. Experimental Samples were exposed to the carbonisation treatment using the Plasma Boundary Probe System “B” mounted into the top of the JET vessel in Octant V as described in ref. [l]. Eight samples (each 6 X 10 mm’) were loaded on each occasion, including silicon, Inconel and nickel samples: virgin as well as previously carbonised samples were used. The sample holder projected from 100 to 150 mm inside the vacuum vessel. The JET vessel was carbonised by running an RF-assisted DC glow discharge in a methane/deuterium gas mixture, using two electrodes recessed into the vessel wall (one in Octant III and one in Octant VI). Probes 0022-3115/87/$03.50 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
However,
since the beginning
of 1985 carbonisation
has not
exposed during three of the carbonisations at higher methane concentration were analysed, i.e. those on 24/3/85, 14/4/85 and 25-27/5/85, and compared with one exposed at lower concentration (3/2/85) and reported previously [l]. Details of the exposures are given in table 1: for each carbonisation the DC sheath potential was approximately 300 V and the gas pressure wasintherange4t08X10-3 mbar. The samples were analysed by Auger Electron Spectroscopy (AES) and Rutherford Backscattering (RBS). AES analyses the outermost 1 mn (approximately), and profiles of composition versus depth into the surface are achieved by successively removing a number of nanometres by argon ion bombardment and analysing at that depth. The removal of material by ion bombardment is not totally uniform over the area analysed, and the umount removed is also subject to error (by perhaps a factor of two), thus the depth scale in such profiles must be regarded as approximate. In RBS the sample surface is bombarded with protons or helium ions which have been accelerated to Table 1 Details
of sample exposures
Parameter Length baking
in the torus
3/2/85
24/3/85
14/4/85
25-27/5/85
>24h
>24h
-18h
of prior 21 h
Length of GDC in torus prior to carbonisation
6h
DC dischargecurrent
8A
5A
5A
Length of carbonisation
8h
6h
6h
48 h
12%
12%
17%
300°C
240°c
300°C
Percentage of methane in the gas mixture (balance is deuterium) Torus wall temperature
2.15%
300°C
- 24h
24 h
none (but vessel was cleaned for 24 h) 6A
- 0.8 to 3 MeV. RBS is particularly sensitive to the amount and distribution of heavy impurities in the outer - 1 pm of a light matrix, but can also be useful for (larger amounts of) light elements in a heavier matrix: it is based on the elastic scattering of a light nucleus by the Coulomb interaction with a nucleus in the sample surface. The spatial resolution is that of the particle beam, and in these experiments was 1 mm. Some information on the depth distribution of the analysed elements can be gleaned from the energy distribution of the detected particles, with a resolution of about 10 nm at best. Probe results were correlated with results from three of the JET diagnostics. Firstly, the power radiated from the JET plasma is measured using bolometers viewing along 34 chords in one poloidal location and by single bolometers in each o&ant. Secondly, impurity concentrations in the JET plasma are measured by the analysis of resonance line intensities in the VUV, using a McPherson Model 251 VUV broadband spectrometer, covering the wavelength lOt-1700 A [6]. Thirdly, spectral lines from hydrogen and low ionisation states of impurities are measured by means of visible spectroscopy [7]: the plasma light is collected from chords terminating on the wall at the top of the vessel, or on carbon limiters or an RF antenna.
l3EI)
2jkJ!J;$_. ,, IO
20
30
40 Depth
Fig. 1. Carbon
50
60
70
After the exposure on 24/3/85 and 14/4/85 the probe samples still appeared clean and bright, though the end cap in each case was slightly discoloured (brownish). After the 48 h exposure at the end of May
on Inconel samples after carbonisation.
70 1 60
50 Atomk % ‘I 40 i I,
X
24/3/65
0
3/2/65
A
14/4/W
.
20
30
40
50
60 Depth
Fig. 2. Carbon
1000
the end cap and indeed all of the probe projecting into the torus was covered with a powdery black deposit which was easily brushed off: the Si and half the Inconel samples were coated with a dense black layer, whilst the coating appeared to have spalled from the Ni and remaining Inconel samples. The carbon levels obtained by AES for the carboni-
3. Probe results
10
80
(nm)
70
00
(2NIl (4NIl
25-27/5/05 (5NI) spdled oreo
SO
(nml
on nickel samples
(3NIl
after carbonisation.
100
110
120
130
J.P. Coad et al. / Glow discharge carbonisation in JET
sations 24/3/85 and 14/4/85 are plotted for Inconel substrates in fig. 1 and nickel in fig. 2, and are compared with results for a carbonisation using 2.12% CH, (on 3/2/85). After the heavy carbonisation of 25-27/5/85, the coating thickness was derived by comparing AES and RBS from intact areas of coating, and step height measurements at the edge of spalled areas of coating. In fig. 1 this mean determination of thickness is used in the profile obtained from a coated Inconel sample exposed on 25-27/5/85, combined with the initial composition of the carbon coated area obtained by AES. The profile obtained from a spalled area is also given to show the limited penetration of carbon into the substrate. In fig. 2, only the AES information from the spalled area of a nickel sample exposed on 25-27/5/85 is included, which again shows the lack of carbon penetration into the matrix: there is no comparable information from areas of intact carbon coating, since no large enough areas remained on the surface. Hydrogen and deuterium contents of coatings deposited on 25-27/5/85 have been estimated by a combination of particle beam techniques including RBS, and the results give approximately equal hydrogen and deuterium concentrations. The total of hydrogen plus deuterium is about half the amount of carbon in the film (in at. %). 4. Effect on plasma parameters There was a marked reduction in the metal impurity levels in the plasma following the early carbomsations in 1984. There were also progressive decreases in the oxygen level, to around 1% of the electron density at the centre of the plasma. The radiated power eventually L,Ni XXV 117.93A o 0 V 629.73 A - C IV 312.43A
r
749
,
0
I
chlaine
PLASMA
Fig.
4.
PULSES
(In date order)
Results of impurity analyses before and during the period of RF heating for some selected pulses.
decreased from - 80% PJa to 40% Pn at moderate electron densities after several carbonisations [8]. Metal impurity and relative radiated power levels recovered however in - 20 plasma pulses. In 1985 with the addition of carbon tiles covering the inner wall and clean limiters, initial metal levels were very low, and Prad/Po was typically 40%: carbonisations at similar levels to those in 1984 were carried out with little further benefit. At the end of February 1985, when (among other factors) the carbon limiters were partially retracted for RF heating experiments, nickel levels suddenly became significantly higher (of the order of 0.2% in the plasma) and P,,/P, values were also higher. Subsequent carbonisation at 12% CH, reduced both these variables, but only for - 20 pulses as shown in
^ C;arDonlsaIlon r Carbonisation
Heavy carbonisation r
Shot #
Fig. 3. Intensities of selected VUV impurity lines (6, = 2~ 1Ol9 mm3) demonstrating the respective impurity behaviour after light and heavy carbonisation.
750
J.P. Coder
al. /
Glow discharge
fig. 3 for nickel VUV line intensities. Carbon and oxygen levels were more or less unaffected by the carbonisations. After the 48 h carbonisation using a methane concentration of 17% on 25-27/5/86 the nickel concentration in the plasma was reduced by X 100 and P,,,/Po also reduced and the effect persisted until near the end of the operating period (for - 200 pulses) (fig. 3). The effects on the Ni and Cl concentrations of the heavy carbonisation can be seen in fig. 4. The carbon and oxygen concentrations were once more not affected by the carbonisation, at least not immediately. The oxygen level did start to decrease about 200 pulses after the carbonisation (see also fig. 3), but the timing of this decrease correlates with the increase in the nickel levels, so may be due to gettering by the re-exposed metal. An important consideration in the use of carbonisation is the effect on the RF heating antennae. The Faraday screens of the antennae in use in 1985 were nickel, and the absolute signal from the nickel impurities rose during RF-heated discharges. This production of additional Ni was only prevented by the heavy carbonisation (fig. 4). 5. Discussion and conclusions Increasing the methane content during GDC to 12% from 2.12% did provide a slight increase in the amount of carbon deposited on nickel and Inconel samples, when the vessel wall was at 300°C. However, only for a clean Inconel sample was a thin layer of pure carbon actually achieved, and the silicon samples did not show any increased carbon coverage at all. With the wall at 240°C (14/4/85), the carbon levels observed on the probes were similar to those obtained using 2.12% CH4 on 3/2/85. When the methane content was increased to 17% deposition occurred on all surfaces. The carbon was powdery and poorly adhered to the stainless steel holder (which had been used on several previous occasions), but the coating on the samples was clearly more dense and structured since there was evidence of internal stress (leading to some spalling) and it was resistant to rubbing with a tissue. The spalling suggests that the adhesion of the carbon coating to the samples was inadequate, and this is probably because the samples were not glow-discharge cleaned prior to the carbonisation to remove the oxide film: the presence of the oxide film would also explain the negligible carbon diffusion into the substrate during the carbonisation. The silicon samples were covered in a similar way to the Inconel, and it appears that a good quality coating was also deposited over the vessel wall [9]. Clearly the deposition is not now influenced by the substrate, as the carbon arrival rate at the surface exceeds the removal rate by - 1 pm was deposited on the all mechanisms. Although probe samples other measurements suggest the average coating over the vessel was - 0.1 pm [9]: the difference may be that since the probe projects into the vessel it becomes a primary electrode for the discharge. It should
curhomatlotl
/n JET
be noted that this may also apply to the limiters which, though larger (0.8 m x 0.4 m), project further into the vessel (- 260 mm). The change in the power balance after carbonisation in 1985 shows that there is a relevance in reducing the metal content by carbonisation. Clearly the heavier treatment gave a more prolonged effect, by approximately the ratio of the amounts of carbon deposited on the probes (which we assume will also reflect the relative amounts deposited over the contaminants on the limiters). However, the plasma is very difficult to control for a few pulses after carbonisation due to gas release from the wall, and the values of Z,,, are not significantly improved for high density discharges, since these are dominated by the contributions of oxygen and carbon. In 1986, as at the start of 1985, metal levels are generally very low. Furthermore, although significant Ni levels have built up an occasions following periods of RF heating experiments, the level has rapidly reduced in subsequent discharges [lo]. It may be therefore, that the persistent source of Ni in 1985 has been eliminated: the removal of the nickel-limiters and the covering of the octant joints with carbon tiles prior to 1986 operation may be significant. As it stands there is thus little point in carbonising the JET vessel in 1986, especially as line radiation from metals only accounts for about 20% of Prad. However, Ni and Cr levels do increase when the RF antennae (one of which is now Cr-plated) are in use. There is thus a case for heavily carbonising the antennae screens, but the treatment, as presently contrived, would also cover the vessel and cause significant disruption of the programme due to the treatment and recovery times involved. The AES analyses were carried out by Loughborough Consultants (P. Weedon, B. Davies, D. Hall). The par title beam analysis experiments were carried out at IPP, Garching (W. Mijller, J. Roth, P. Martinelli, H. Wielunski) and at Culham (J. Partridge, K. Erents). The authors are indebted to Dr J. von Seggem for help in the preparation of this manuscript. References [l] J.P. Coad et al., Proc. 12th Eur. Conf. on Contr. Fus. and Plasma Phys., Budapest, Part II (1985) p. 571. [2] F. Waelbroeck and the TEXTOR team, J. Nucl. Mater. 121 (1984) 378. [3] J. Winter et al., .I. Nucl. Mater. 122 & 123 (1984) 1187. [4] J. Winter et al., J. Nucl. Mater. 128 & 129 (1984) 841. [5] K.H. Besocke et al., J. Nucl. Mater. 136 (1985) 124. [6] B. Denne et al., Proc. 12th Eur. Conf. on Contr. Fus. and Plasma Phys., Budapest, Part I (1985) p. 379. [7] M. Stamp et al., Proc. 12th Eur. Conf. on Contr. Fus. and Plasma Phys., Budapest, Part II (1985) p. 539. [8] K.H. Behringer et al., Nucl. Fusion 26 (1986) 751. [9] J.P. Coad et al, these Proc. (PSI-VII), J. Nucl. Mater. 145-147 (1987). [lo] K.H. Behringer et al., Proc. 13th Eur. Conf. on Plasma Heating, Schliersee (1986).
145-147
(1987) 747-750 747
GLOW DISCHARGE
CARBONISATION
J.P. COAD, K.H. BEHRINGER
IN JET
and K.J. DIETZ
JET Joint Undertaking, Abingdon, Oxfordshire, United Kingdom
Key words:
carbonisation,
wall conditioning,
Many of the glow discharge
carbonisations
tokamaks.
plasma
impurities
of the JET vessel have been monitored
with removable
probes.
The probes
have
been analysed by Auger Spectroscopy, and the data correlated with subsequent impurity concentrations and radiated power levels in the plasma. Metal concentrations observed in the plasma are reduced for a time proportional to the amount of carbon deposited on the probes (and also on the limiters, presumably). has a significant effect on Z,,, for high power discharges
1. Introduction During operation the JET carbon limiters become contaminated with metals from the vessel wall etc [l], and from the limiters these contaminants can enter the plasma. Extensive work on TEXTOR [2-51 has suggested that covering the limiters and walls with a fresh layer of carbon (“carbonisation”) can reduce this ingress. The effect of carbonisation of the JET vessel was first studied in September 1984, and more exhaustive testing including the use of probes to monitor some of the treatments was carried out in the operating period from January to June 1985. The first few carbonisations in 1985 used a mixture of deuterium and a low concentration (2.15%) of methane (similar to the 3% used in September 1984) and the analysis of probes exposed in this mixture was briefly described at the Budapest Conference [l]. The results were completely in agreement with what may be expected from the work of the TEXTOR group [2-51, that is reduction of surface oxygen, and the formation of carbides within the surface without significant carbon overlayer formation. In the period from March 24th 1985 to the end of May 1985 attempts at heavier carbonisation were made, using mixtures of either 12 or 17% CH4 in D2, and probe measurements made during these experiments and their implications are reported here. 2. Experimental Samples were exposed to the carbonisation treatment using the Plasma Boundary Probe System “B” mounted into the top of the JET vessel in Octant V as described in ref. [l]. Eight samples (each 6 X 10 mm’) were loaded on each occasion, including silicon, Inconel and nickel samples: virgin as well as previously carbonised samples were used. The sample holder projected from 100 to 150 mm inside the vacuum vessel. The JET vessel was carbonised by running an RF-assisted DC glow discharge in a methane/deuterium gas mixture, using two electrodes recessed into the vessel wall (one in Octant III and one in Octant VI). Probes 0022-3115/87/$03.50 Q Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
However,
since the beginning
of 1985 carbonisation
has not
exposed during three of the carbonisations at higher methane concentration were analysed, i.e. those on 24/3/85, 14/4/85 and 25-27/5/85, and compared with one exposed at lower concentration (3/2/85) and reported previously [l]. Details of the exposures are given in table 1: for each carbonisation the DC sheath potential was approximately 300 V and the gas pressure wasintherange4t08X10-3 mbar. The samples were analysed by Auger Electron Spectroscopy (AES) and Rutherford Backscattering (RBS). AES analyses the outermost 1 mn (approximately), and profiles of composition versus depth into the surface are achieved by successively removing a number of nanometres by argon ion bombardment and analysing at that depth. The removal of material by ion bombardment is not totally uniform over the area analysed, and the umount removed is also subject to error (by perhaps a factor of two), thus the depth scale in such profiles must be regarded as approximate. In RBS the sample surface is bombarded with protons or helium ions which have been accelerated to Table 1 Details
of sample exposures
Parameter Length baking
in the torus
3/2/85
24/3/85
14/4/85
25-27/5/85
>24h
>24h
-18h
of prior 21 h
Length of GDC in torus prior to carbonisation
6h
DC dischargecurrent
8A
5A
5A
Length of carbonisation
8h
6h
6h
48 h
12%
12%
17%
300°C
240°c
300°C
Percentage of methane in the gas mixture (balance is deuterium) Torus wall temperature
2.15%
300°C
- 24h
24 h
none (but vessel was cleaned for 24 h) 6A
- 0.8 to 3 MeV. RBS is particularly sensitive to the amount and distribution of heavy impurities in the outer - 1 pm of a light matrix, but can also be useful for (larger amounts of) light elements in a heavier matrix: it is based on the elastic scattering of a light nucleus by the Coulomb interaction with a nucleus in the sample surface. The spatial resolution is that of the particle beam, and in these experiments was 1 mm. Some information on the depth distribution of the analysed elements can be gleaned from the energy distribution of the detected particles, with a resolution of about 10 nm at best. Probe results were correlated with results from three of the JET diagnostics. Firstly, the power radiated from the JET plasma is measured using bolometers viewing along 34 chords in one poloidal location and by single bolometers in each o&ant. Secondly, impurity concentrations in the JET plasma are measured by the analysis of resonance line intensities in the VUV, using a McPherson Model 251 VUV broadband spectrometer, covering the wavelength lOt-1700 A [6]. Thirdly, spectral lines from hydrogen and low ionisation states of impurities are measured by means of visible spectroscopy [7]: the plasma light is collected from chords terminating on the wall at the top of the vessel, or on carbon limiters or an RF antenna.
l3EI)
2jkJ!J;$_. ,, IO
20
30
40 Depth
Fig. 1. Carbon
50
60
70
After the exposure on 24/3/85 and 14/4/85 the probe samples still appeared clean and bright, though the end cap in each case was slightly discoloured (brownish). After the 48 h exposure at the end of May
on Inconel samples after carbonisation.
70 1 60
50 Atomk % ‘I 40 i I,
X
24/3/65
0
3/2/65
A
14/4/W
.
20
30
40
50
60 Depth
Fig. 2. Carbon
1000
the end cap and indeed all of the probe projecting into the torus was covered with a powdery black deposit which was easily brushed off: the Si and half the Inconel samples were coated with a dense black layer, whilst the coating appeared to have spalled from the Ni and remaining Inconel samples. The carbon levels obtained by AES for the carboni-
3. Probe results
10
80
(nm)
70
00
(2NIl (4NIl
25-27/5/05 (5NI) spdled oreo
SO
(nml
on nickel samples
(3NIl
after carbonisation.
100
110
120
130
J.P. Coad et al. / Glow discharge carbonisation in JET
sations 24/3/85 and 14/4/85 are plotted for Inconel substrates in fig. 1 and nickel in fig. 2, and are compared with results for a carbonisation using 2.12% CH, (on 3/2/85). After the heavy carbonisation of 25-27/5/85, the coating thickness was derived by comparing AES and RBS from intact areas of coating, and step height measurements at the edge of spalled areas of coating. In fig. 1 this mean determination of thickness is used in the profile obtained from a coated Inconel sample exposed on 25-27/5/85, combined with the initial composition of the carbon coated area obtained by AES. The profile obtained from a spalled area is also given to show the limited penetration of carbon into the substrate. In fig. 2, only the AES information from the spalled area of a nickel sample exposed on 25-27/5/85 is included, which again shows the lack of carbon penetration into the matrix: there is no comparable information from areas of intact carbon coating, since no large enough areas remained on the surface. Hydrogen and deuterium contents of coatings deposited on 25-27/5/85 have been estimated by a combination of particle beam techniques including RBS, and the results give approximately equal hydrogen and deuterium concentrations. The total of hydrogen plus deuterium is about half the amount of carbon in the film (in at. %). 4. Effect on plasma parameters There was a marked reduction in the metal impurity levels in the plasma following the early carbomsations in 1984. There were also progressive decreases in the oxygen level, to around 1% of the electron density at the centre of the plasma. The radiated power eventually L,Ni XXV 117.93A o 0 V 629.73 A - C IV 312.43A
r
749
,
0
I
chlaine
PLASMA
Fig.
4.
PULSES
(In date order)
Results of impurity analyses before and during the period of RF heating for some selected pulses.
decreased from - 80% PJa to 40% Pn at moderate electron densities after several carbonisations [8]. Metal impurity and relative radiated power levels recovered however in - 20 plasma pulses. In 1985 with the addition of carbon tiles covering the inner wall and clean limiters, initial metal levels were very low, and Prad/Po was typically 40%: carbonisations at similar levels to those in 1984 were carried out with little further benefit. At the end of February 1985, when (among other factors) the carbon limiters were partially retracted for RF heating experiments, nickel levels suddenly became significantly higher (of the order of 0.2% in the plasma) and P,,/P, values were also higher. Subsequent carbonisation at 12% CH, reduced both these variables, but only for - 20 pulses as shown in
^ C;arDonlsaIlon r Carbonisation
Heavy carbonisation r
Shot #
Fig. 3. Intensities of selected VUV impurity lines (6, = 2~ 1Ol9 mm3) demonstrating the respective impurity behaviour after light and heavy carbonisation.
750
J.P. Coder
al. /
Glow discharge
fig. 3 for nickel VUV line intensities. Carbon and oxygen levels were more or less unaffected by the carbonisations. After the 48 h carbonisation using a methane concentration of 17% on 25-27/5/86 the nickel concentration in the plasma was reduced by X 100 and P,,,/Po also reduced and the effect persisted until near the end of the operating period (for - 200 pulses) (fig. 3). The effects on the Ni and Cl concentrations of the heavy carbonisation can be seen in fig. 4. The carbon and oxygen concentrations were once more not affected by the carbonisation, at least not immediately. The oxygen level did start to decrease about 200 pulses after the carbonisation (see also fig. 3), but the timing of this decrease correlates with the increase in the nickel levels, so may be due to gettering by the re-exposed metal. An important consideration in the use of carbonisation is the effect on the RF heating antennae. The Faraday screens of the antennae in use in 1985 were nickel, and the absolute signal from the nickel impurities rose during RF-heated discharges. This production of additional Ni was only prevented by the heavy carbonisation (fig. 4). 5. Discussion and conclusions Increasing the methane content during GDC to 12% from 2.12% did provide a slight increase in the amount of carbon deposited on nickel and Inconel samples, when the vessel wall was at 300°C. However, only for a clean Inconel sample was a thin layer of pure carbon actually achieved, and the silicon samples did not show any increased carbon coverage at all. With the wall at 240°C (14/4/85), the carbon levels observed on the probes were similar to those obtained using 2.12% CH4 on 3/2/85. When the methane content was increased to 17% deposition occurred on all surfaces. The carbon was powdery and poorly adhered to the stainless steel holder (which had been used on several previous occasions), but the coating on the samples was clearly more dense and structured since there was evidence of internal stress (leading to some spalling) and it was resistant to rubbing with a tissue. The spalling suggests that the adhesion of the carbon coating to the samples was inadequate, and this is probably because the samples were not glow-discharge cleaned prior to the carbonisation to remove the oxide film: the presence of the oxide film would also explain the negligible carbon diffusion into the substrate during the carbonisation. The silicon samples were covered in a similar way to the Inconel, and it appears that a good quality coating was also deposited over the vessel wall [9]. Clearly the deposition is not now influenced by the substrate, as the carbon arrival rate at the surface exceeds the removal rate by - 1 pm was deposited on the all mechanisms. Although probe samples other measurements suggest the average coating over the vessel was - 0.1 pm [9]: the difference may be that since the probe projects into the vessel it becomes a primary electrode for the discharge. It should
curhomatlotl
/n JET
be noted that this may also apply to the limiters which, though larger (0.8 m x 0.4 m), project further into the vessel (- 260 mm). The change in the power balance after carbonisation in 1985 shows that there is a relevance in reducing the metal content by carbonisation. Clearly the heavier treatment gave a more prolonged effect, by approximately the ratio of the amounts of carbon deposited on the probes (which we assume will also reflect the relative amounts deposited over the contaminants on the limiters). However, the plasma is very difficult to control for a few pulses after carbonisation due to gas release from the wall, and the values of Z,,, are not significantly improved for high density discharges, since these are dominated by the contributions of oxygen and carbon. In 1986, as at the start of 1985, metal levels are generally very low. Furthermore, although significant Ni levels have built up an occasions following periods of RF heating experiments, the level has rapidly reduced in subsequent discharges [lo]. It may be therefore, that the persistent source of Ni in 1985 has been eliminated: the removal of the nickel-limiters and the covering of the octant joints with carbon tiles prior to 1986 operation may be significant. As it stands there is thus little point in carbonising the JET vessel in 1986, especially as line radiation from metals only accounts for about 20% of Prad. However, Ni and Cr levels do increase when the RF antennae (one of which is now Cr-plated) are in use. There is thus a case for heavily carbonising the antennae screens, but the treatment, as presently contrived, would also cover the vessel and cause significant disruption of the programme due to the treatment and recovery times involved. The AES analyses were carried out by Loughborough Consultants (P. Weedon, B. Davies, D. Hall). The par title beam analysis experiments were carried out at IPP, Garching (W. Mijller, J. Roth, P. Martinelli, H. Wielunski) and at Culham (J. Partridge, K. Erents). The authors are indebted to Dr J. von Seggem for help in the preparation of this manuscript. References [l] J.P. Coad et al., Proc. 12th Eur. Conf. on Contr. Fus. and Plasma Phys., Budapest, Part II (1985) p. 571. [2] F. Waelbroeck and the TEXTOR team, J. Nucl. Mater. 121 (1984) 378. [3] J. Winter et al., .I. Nucl. Mater. 122 & 123 (1984) 1187. [4] J. Winter et al., J. Nucl. Mater. 128 & 129 (1984) 841. [5] K.H. Besocke et al., J. Nucl. Mater. 136 (1985) 124. [6] B. Denne et al., Proc. 12th Eur. Conf. on Contr. Fus. and Plasma Phys., Budapest, Part I (1985) p. 379. [7] M. Stamp et al., Proc. 12th Eur. Conf. on Contr. Fus. and Plasma Phys., Budapest, Part II (1985) p. 539. [8] K.H. Behringer et al., Nucl. Fusion 26 (1986) 751. [9] J.P. Coad et al, these Proc. (PSI-VII), J. Nucl. Mater. 145-147 (1987). [lo] K.H. Behringer et al., Proc. 13th Eur. Conf. on Plasma Heating, Schliersee (1986).