- Email: [email protected]
Limiter viewing on JET
Journal
440
of Nuclear
Materials
128 & 129 (1984) 440- 444
LIMITER VIEWING ON JET M.A. PICK * and D. SUMMERS JET Joint Undertaking, A bingdon, Oxon, UK
Key words:
limiter observation,
limiter heat load, energy
scrape-off
layer
JET is equipped with a limiter viewing system which consists of several CCD (charge-coupled device) cameras. The system allows us to record images of the limiters in the wavelength region from - 400 to - 1100 nm at a rate of 50 images per second and with a spatial resolution of 3 mm. By introducing suitable wavelength filters, we can measure the thermal light emission from the limiter and thereby deduce the surface temperature distribution, or observe spectral emission lines (e.g. H, at 656.3 nm or CI at - 910 nm). We report in this paper on the behaviour of the carbon limiters during the initial operation of JET. Arcing on the limiter surface is observed under certain discharge conditions, as well as the occasional localised hot-spot due to non-thermal electrons. From the temperature distribution across the limiters, we are able to deduce the thickness A of the energy scrape-off Layer as a function of time throughout discharges with plasma currents exceeding 1 MA. It was found that A was generally on the order of 15 mm during the flat-top of most dischargesand tendedto increaseby a factor of two or threetowards the end of the discharge. The limiters reached temperatures of - 1000 o C in the hottest regions at the highest plasma currents achieved so far ( - 3 MA). Preliminary estimates indicate that 25% of the ohmic input power reaches the limiters.
1. Introduction During the initial operation of JET four inertially cooled graphite limiters were used. The use of these components requires that the heat flux be limited so that the surface temperature of the limiters remain below the design value of about 2000 OC. fn order to measure the surface temperature, we designed a system to image the limiters in the wavelength region from the visible to the near infra-red. The thermal image of the limiter also enables us to assess the effective utilisation of the avaiiable limiter surface area. From the temperature profile, across the limiter and the known physical shape of the limiter, we can deduce the thickness of the energy scrape-off layer. The total power deposition can be deduced from the temperature distribution across the limiters and, in addition, the limiter viewing system can be used to monitor the occurrence of hot spots on the limiters due to runaway electron beams. Finally, the limiter viewing can be used to image light emission from the recycling cloud around the limiter giving an indication of its intensity, size and distribution during a discharge.
2. The limiters and the liitar
straight in poloidal direction and curved in the toroidal direction, as shown in fig. 1. The curvature is a compromise between achieving an evenly distributed power load for the assumed thickness of the energy scrape-off
viewing system
The graphite limiters in JET are situated at the mid-plane of the vacuum vessel at its outer circumference on the left hand side of octants 2, 4, 6 and 8. The limiters are each 80 cm high and 40 cm wide; * On leave from Brookhaven USA.
National
Laboratory,
Upton,
NY
~22-3115/84/$03.~ 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Fig. 1. A graphite limiter installed in JET. The liter is 40 cm wide and each of the eight tiles is 10 cm high. A cross-section of the limiter in toroidal direction is shown schematically.
MA. Pick. 0. Smers
layer, limitations due to the width of the main ports of JET through which the limiters are introduced into the’ machine, and the restrictions due to the manufactu~ng process f&2]. During the initial operation of JET, the graphite hmiters were positioned at a distance of 212 mm from the wall (i.e. the rigid sections of the octants). For the initial operation of JET in June 19S3, eight nickel limiters were installed in the torus in addition to the four graphite ones, During subsequent operations the nickel hmiters were kept fully retracted behind the graphite limiters. In January 1984 four of the nickel limiters were removed, resulting in the configuration shown in fig. 2. The limiters can be viewed through four sapphire windows attached to JET. These limiter viewing ports are situated on either side of the pumping boxes at octants 1 and 5, as shown in fig. 2. Two additional windows are attached to octants 3 and 7. Ah four carbon limiters (and 2 nickel limiters) can thus be viewed simultaneously. The cameras are commercially available CCIRstandard (50 fields/s) CCD (charge-coupled device) video cameras (English Electric Valve Co Model 4310). These CCD cameras are we11 suited for the purpose because they are small (the head containing the CCD circuit measures - 40 X 35 X 50 mm, this box is separated from the rest of the electronics by a 4 m cable) and they can be operated in very strong magnetic fields (we have operated such a camera in a field of 1.3 Teslaf. A mirror Ioeated behind the sapphire window reflects the image vertically down to the camera head located about 1 m below the mid-plane of the torus. The camera head is at the centre of a lead sphere of - 50 cm diameter. This arrangement places the camera in a PRESENT: AFTER
Carbon Umitcrs at 23,48,68.80 Nickel Limiters at 24 40.60. BD,
SEPTEMBER
441
f Limiter viewing on JET
position where the mechanioal structure of the machine, together with the lead sphere, provide an effective shield against X-ray irradiation due to runaway electron beams. During the course of the observations, it became &ear that runaway electron beams and consequently high X-ray irradiation was not a serious problem in the present JET operating conditions. For that reason the above mentioned mirror was chosen to be transparent below about 700 nm, which alfowcd a second camera to be attached horiaontaRy so as to aRow the same Limiter to be observed at two different wavelengths simultaneously, or to attach an J-J, monitor or a spectrometer at that position. The CCD sensor has a resolution of 576 x 380 pixels and is sensitive from the visible to the near i&a-red (- 1100 run). The system permits the remote introduction of four different optical filters. Depending upon which filter is used, the system provides a thermal image of the limiter at any temperature above - 500°C. The temperature range with one particular filter and fixed aperture is Limited by the dynamic range of the combined camera and video recording system @NY High Band Umatic BVU g20) to arrogated 250 o C. Each camera was calibrated for every filter (10 nm wide centered at 800,900 or lOO0 nm) and Jl apertures using a black-body calibration source. Assuming the ernissivity is homogeneous over the observed surface the relal&e error in the temperature measurements is quite small as indicated in the figures. However, due to unknown variations in the emissivity of the limiter as well as possible contamination of the optical window, it is estimated that the absolute temperatures can be up to SO0 higher than stated.
3. Observations tith limiter viedg
system
:
A0 Antennae Ot 28 and 60. Carbon LimilerSa( ZQ,LB,6D,BB. Nickel Limiters at 40,80. KU. 316
KU.712
Fig. 2. A view of JET from the top showing the positions of the limiters and camera positions.
During the initial operation of JET between 2 and 4 of the graphite limiters were observed each with one or two cameras. The cameras were equipped with - 10 nm wide filters at either 800, %JQ 1ooO nm or at the H, wavelength. The latter allowed one to observe the hydrogen recycling behaviour at and in the vicinity of the limiters. The 900 nm filter was particularty useful for the observation of arcing on the graphite limiters due to its coincidence with a carbon emission line. The following points fist the main observations: - The very earIy discharges in JET were accompanied by numerous arcs on the surface and the edges of the limiters. Later, arcing virtually disappeared but disruptive discharges were usually associated with its appearance. Fig. 3 shows an example of arcing on the limiter during the early period. - At higher plasma currents ( Z=1 MA), thermal loading of the limiters was observed with temperatures reaching 800-1000°C in the hottest regions at the highest currents achieved so far ( - 3 MA). The temperature distri-
6. LIMITERS;
EXPERIMENT
AND THEORY
442
M.A. Pick, D. Summers / Limiter uiewing on JET
l
l
***
l
l
l l .
. . l
l
Fig. 5. Temperature distribution in toroidal direction across the graphite limiter in JET showing the typical temperature difference between the ion side and electron side. The ion side is hotter. Discharge No. 1307.
F
2.0-
Fig. 3. Video image of arcing on the graphite limiter (using 900 nm filter).
in
l*O-
JET
i /
t
o)7x)-
bution showed two peaks and fig. 4 shows an example of the video image of the thermal loading. The analysis of a video line across the limiter is shown in fig. 5 where the temperatures reached are indicated. Fig. 6b shows
(b)
6%
6CQ.
SM28 .
(c)
24. 20. 16-
. : .. . . . . ,. .. . .. . .. .. . .. . . . . ... .. . . . . . l
l284.
TIME Is1
Fig. 4. Video image of thermal loading of graphite limiter JET (using 800 nm filter).
in
Fig. 6. Results of a typical 2 MA plasma discharge in JET (Discharge No. 1353): (a) the plasma current as a function of time; (b) the maxims temperature of the graphite limiter; (c) the distance between the two temperature maxima and the calculated thickness of the energy scrape-off layer during the discharge.
443
M.A. Pick, D. Summers / Limiter viewing on JET
the time dependence of the peak temperature during a typical discharge. The associated plasma current is depicted in fig. 6a. One can estimate the temperature rise, AT, of the limiter surface by applying the formula for the surface temperature increase of a flat semi-infinite slab under a constant heat load: AT=
2 pt”2 ( *xpc)“2
where P is the heat load, h the heat conductivity, p the density and c the specific heat. The values for the limiter material are X=l&tW/cm.K,
c=1.1.f/g~Kandp=1.81g/cm3.
Taking the rise in temperature of the hottest areas to be - 300° we can deduce that the local heat load is - 260 W/cm*. Under constant heat load one would expect the surface temperature to increase art’/2. However, it was generally found that the temperature of the limiters tended to remain constant or even decrease slightly during the flat top of the discharges. This can only be explained by a reduction of the power loading of the limiters caused by a increase of the radiated power or a decrease in the ohmic input power. At present a detailed quantitative analysis of this effect cannot be made. - The distance between the two peaks in the temperature distribution is a direct measure of the thickness of the energy scarpe-off layer [3]. During the course of the experiments, it was possible to measure this distance during a discharge, to extract the time dependence of the energy scrape-off layer thickness. This is shown in fig. 6c for the pulse #1353. The behaviour of the scrape-off layer (as shown in the figure) appears to be typical of most discharges so far. Very similar curves result for all of the investigated discharges with peak plasma currents between 1 and 3 MA and densities between about 0.8 and 2.5 X lOI rne3. The thickness of the energy scrape-off layer reaches a minimum of approximately 15 mm during the flat-top period and then generally increases by a factor of two or three towards the end of the discharge. - The thermal load on the limiters is not completely symmetrical as can be seen in fig. 5. The ion side of the limiters viewed was slightly hotter than the electron side. This observation is apparently associated with the observation that imaging the limiters at the H, wavelength showed that the hydrogen recycling is also asymmetrical and that it occurs primarily on the ion side of the limiters. The observation that the ion side of the limiters carry a greater thermal load and the corresponding difference in the intensity of the recycling cloud at the limiters is not understood, as yet. Differences in the thermal load of the ion side compared to the electron side of a spherical test-limiter in TEXTOR were re-
ported by Samm [4]. In this case, the electron side was hotter than the ion side and this was explained as due to differences in the connection lengths. Similar observations were made by Taylor et al. [5] in Doublet III, who reported that the asymmetry in the temperature profile was due to the oblique viewing angle. However, if the emissivity of the limiter is close to unity, as it is in our case, then the viewing angle should not influence the temperature measurement. From the measured temperature dist~bution across the limiter we can estimate that about 25% of the total ohmic input power is deposited on the limiters. This value has been calculated using a simple one dimensional heat flux model devised by Gondhalekar and Dietz (61. The calculation also includes an assumption for the temperature of those areas of the limiter not visible in the infra-red image i.e. below - 400°C. We assumed these areas of the limiters to be uniformly 3OO’C; above the wall temperature but below the detection limit. The value for the power deposition derived in this way does not contradict the measured total radiated power obtained from the bolometer array, i.e. about 70% of the total ohmic power input. However, this estimate is obviously still approximate as firstly the absolute temperature of the whole area of the limiter at the beginning and during the discharges is not well known and secondly the measurements are hampered at high power loading by the occurrence of rapid fluctuations in emission. We attribute these fluctuations to spectral emission from gaseous hydrocarbons emanating from the limiters.
4. Conduaions The limiter viewing system utilising CCD cameras was found to be well suited for the intended purpose and can provide valuable real-time information on the behaviour of the limiters during plasma discharges. It is planned to enhance the capabilities of the system by incorporating an infra-red array camera with which the limiters can be imaged at longer wavelengths, i.e. lower temperatures, so as to be able to measure the temperature areas of the whole limiter. A digital image analysis system will enable the rapid quantitative analysis of the available data.
Acknowledgements We acknowledge the assistance of and helpful discussions with many members of the JET team, especially, D. Cowell, K.J. Dietz, M. Watkins, A. Gondhalekar, L. de Kock, P. Stott, K. Behringer and P. Morgan.
6. LIMITERS;
EXPERIMENT
AND THEORY
444
MA.
Pick, D. Summers / Limiter viewing on JET
References [l] K.J. Dietz, Proc. 11 SOFT, Vol. II (1980) 1053. (21 P.H. Rebut and K.J. Die@ Proc. 12 SOFT, Vol. I (1982) 85. [3] K.J. Dietz, JET Internal Report FlO(D)84.
[4] U. Samm, Proc. 11th and Plasma Physics, [S] T. Taylor, N. Brooks (1982) 569. [6] A. Gondhalekar and
European Conf. on Controlled Fusion Aachen 1983. Vol. JD p. 413. and K. Ioki, J. Nucl. Mater. 111/112 K.J. Dietz, private
communication.
440
of Nuclear
Materials
128 & 129 (1984) 440- 444
LIMITER VIEWING ON JET M.A. PICK * and D. SUMMERS JET Joint Undertaking, A bingdon, Oxon, UK
Key words:
limiter observation,
limiter heat load, energy
scrape-off
layer
JET is equipped with a limiter viewing system which consists of several CCD (charge-coupled device) cameras. The system allows us to record images of the limiters in the wavelength region from - 400 to - 1100 nm at a rate of 50 images per second and with a spatial resolution of 3 mm. By introducing suitable wavelength filters, we can measure the thermal light emission from the limiter and thereby deduce the surface temperature distribution, or observe spectral emission lines (e.g. H, at 656.3 nm or CI at - 910 nm). We report in this paper on the behaviour of the carbon limiters during the initial operation of JET. Arcing on the limiter surface is observed under certain discharge conditions, as well as the occasional localised hot-spot due to non-thermal electrons. From the temperature distribution across the limiters, we are able to deduce the thickness A of the energy scrape-off Layer as a function of time throughout discharges with plasma currents exceeding 1 MA. It was found that A was generally on the order of 15 mm during the flat-top of most dischargesand tendedto increaseby a factor of two or threetowards the end of the discharge. The limiters reached temperatures of - 1000 o C in the hottest regions at the highest plasma currents achieved so far ( - 3 MA). Preliminary estimates indicate that 25% of the ohmic input power reaches the limiters.
1. Introduction During the initial operation of JET four inertially cooled graphite limiters were used. The use of these components requires that the heat flux be limited so that the surface temperature of the limiters remain below the design value of about 2000 OC. fn order to measure the surface temperature, we designed a system to image the limiters in the wavelength region from the visible to the near infra-red. The thermal image of the limiter also enables us to assess the effective utilisation of the avaiiable limiter surface area. From the temperature profile, across the limiter and the known physical shape of the limiter, we can deduce the thickness of the energy scrape-off layer. The total power deposition can be deduced from the temperature distribution across the limiters and, in addition, the limiter viewing system can be used to monitor the occurrence of hot spots on the limiters due to runaway electron beams. Finally, the limiter viewing can be used to image light emission from the recycling cloud around the limiter giving an indication of its intensity, size and distribution during a discharge.
2. The limiters and the liitar
straight in poloidal direction and curved in the toroidal direction, as shown in fig. 1. The curvature is a compromise between achieving an evenly distributed power load for the assumed thickness of the energy scrape-off
viewing system
The graphite limiters in JET are situated at the mid-plane of the vacuum vessel at its outer circumference on the left hand side of octants 2, 4, 6 and 8. The limiters are each 80 cm high and 40 cm wide; * On leave from Brookhaven USA.
National
Laboratory,
Upton,
NY
~22-3115/84/$03.~ 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Fig. 1. A graphite limiter installed in JET. The liter is 40 cm wide and each of the eight tiles is 10 cm high. A cross-section of the limiter in toroidal direction is shown schematically.
MA. Pick. 0. Smers
layer, limitations due to the width of the main ports of JET through which the limiters are introduced into the’ machine, and the restrictions due to the manufactu~ng process f&2]. During the initial operation of JET, the graphite hmiters were positioned at a distance of 212 mm from the wall (i.e. the rigid sections of the octants). For the initial operation of JET in June 19S3, eight nickel limiters were installed in the torus in addition to the four graphite ones, During subsequent operations the nickel hmiters were kept fully retracted behind the graphite limiters. In January 1984 four of the nickel limiters were removed, resulting in the configuration shown in fig. 2. The limiters can be viewed through four sapphire windows attached to JET. These limiter viewing ports are situated on either side of the pumping boxes at octants 1 and 5, as shown in fig. 2. Two additional windows are attached to octants 3 and 7. Ah four carbon limiters (and 2 nickel limiters) can thus be viewed simultaneously. The cameras are commercially available CCIRstandard (50 fields/s) CCD (charge-coupled device) video cameras (English Electric Valve Co Model 4310). These CCD cameras are we11 suited for the purpose because they are small (the head containing the CCD circuit measures - 40 X 35 X 50 mm, this box is separated from the rest of the electronics by a 4 m cable) and they can be operated in very strong magnetic fields (we have operated such a camera in a field of 1.3 Teslaf. A mirror Ioeated behind the sapphire window reflects the image vertically down to the camera head located about 1 m below the mid-plane of the torus. The camera head is at the centre of a lead sphere of - 50 cm diameter. This arrangement places the camera in a PRESENT: AFTER
Carbon Umitcrs at 23,48,68.80 Nickel Limiters at 24 40.60. BD,
SEPTEMBER
441
f Limiter viewing on JET
position where the mechanioal structure of the machine, together with the lead sphere, provide an effective shield against X-ray irradiation due to runaway electron beams. During the course of the observations, it became &ear that runaway electron beams and consequently high X-ray irradiation was not a serious problem in the present JET operating conditions. For that reason the above mentioned mirror was chosen to be transparent below about 700 nm, which alfowcd a second camera to be attached horiaontaRy so as to aRow the same Limiter to be observed at two different wavelengths simultaneously, or to attach an J-J, monitor or a spectrometer at that position. The CCD sensor has a resolution of 576 x 380 pixels and is sensitive from the visible to the near i&a-red (- 1100 run). The system permits the remote introduction of four different optical filters. Depending upon which filter is used, the system provides a thermal image of the limiter at any temperature above - 500°C. The temperature range with one particular filter and fixed aperture is Limited by the dynamic range of the combined camera and video recording system @NY High Band Umatic BVU g20) to arrogated 250 o C. Each camera was calibrated for every filter (10 nm wide centered at 800,900 or lOO0 nm) and Jl apertures using a black-body calibration source. Assuming the ernissivity is homogeneous over the observed surface the relal&e error in the temperature measurements is quite small as indicated in the figures. However, due to unknown variations in the emissivity of the limiter as well as possible contamination of the optical window, it is estimated that the absolute temperatures can be up to SO0 higher than stated.
3. Observations tith limiter viedg
system
:
A0 Antennae Ot 28 and 60. Carbon LimilerSa( ZQ,LB,6D,BB. Nickel Limiters at 40,80. KU. 316
KU.712
Fig. 2. A view of JET from the top showing the positions of the limiters and camera positions.
During the initial operation of JET between 2 and 4 of the graphite limiters were observed each with one or two cameras. The cameras were equipped with - 10 nm wide filters at either 800, %JQ 1ooO nm or at the H, wavelength. The latter allowed one to observe the hydrogen recycling behaviour at and in the vicinity of the limiters. The 900 nm filter was particularty useful for the observation of arcing on the graphite limiters due to its coincidence with a carbon emission line. The following points fist the main observations: - The very earIy discharges in JET were accompanied by numerous arcs on the surface and the edges of the limiters. Later, arcing virtually disappeared but disruptive discharges were usually associated with its appearance. Fig. 3 shows an example of arcing on the limiter during the early period. - At higher plasma currents ( Z=1 MA), thermal loading of the limiters was observed with temperatures reaching 800-1000°C in the hottest regions at the highest currents achieved so far ( - 3 MA). The temperature distri-
6. LIMITERS;
EXPERIMENT
AND THEORY
442
M.A. Pick, D. Summers / Limiter uiewing on JET
l
l
***
l
l
l l .
. . l
l
Fig. 5. Temperature distribution in toroidal direction across the graphite limiter in JET showing the typical temperature difference between the ion side and electron side. The ion side is hotter. Discharge No. 1307.
F
2.0-
Fig. 3. Video image of arcing on the graphite limiter (using 900 nm filter).
in
l*O-
JET
i /
t
o)7x)-
bution showed two peaks and fig. 4 shows an example of the video image of the thermal loading. The analysis of a video line across the limiter is shown in fig. 5 where the temperatures reached are indicated. Fig. 6b shows
(b)
6%
6CQ.
SM28 .
(c)
24. 20. 16-
. : .. . . . . ,. .. . .. . .. .. . .. . . . . ... .. . . . . . l
l284.
TIME Is1
Fig. 4. Video image of thermal loading of graphite limiter JET (using 800 nm filter).
in
Fig. 6. Results of a typical 2 MA plasma discharge in JET (Discharge No. 1353): (a) the plasma current as a function of time; (b) the maxims temperature of the graphite limiter; (c) the distance between the two temperature maxima and the calculated thickness of the energy scrape-off layer during the discharge.
443
M.A. Pick, D. Summers / Limiter viewing on JET
the time dependence of the peak temperature during a typical discharge. The associated plasma current is depicted in fig. 6a. One can estimate the temperature rise, AT, of the limiter surface by applying the formula for the surface temperature increase of a flat semi-infinite slab under a constant heat load: AT=
2 pt”2 ( *xpc)“2
where P is the heat load, h the heat conductivity, p the density and c the specific heat. The values for the limiter material are X=l&tW/cm.K,
c=1.1.f/g~Kandp=1.81g/cm3.
Taking the rise in temperature of the hottest areas to be - 300° we can deduce that the local heat load is - 260 W/cm*. Under constant heat load one would expect the surface temperature to increase art’/2. However, it was generally found that the temperature of the limiters tended to remain constant or even decrease slightly during the flat top of the discharges. This can only be explained by a reduction of the power loading of the limiters caused by a increase of the radiated power or a decrease in the ohmic input power. At present a detailed quantitative analysis of this effect cannot be made. - The distance between the two peaks in the temperature distribution is a direct measure of the thickness of the energy scarpe-off layer [3]. During the course of the experiments, it was possible to measure this distance during a discharge, to extract the time dependence of the energy scrape-off layer thickness. This is shown in fig. 6c for the pulse #1353. The behaviour of the scrape-off layer (as shown in the figure) appears to be typical of most discharges so far. Very similar curves result for all of the investigated discharges with peak plasma currents between 1 and 3 MA and densities between about 0.8 and 2.5 X lOI rne3. The thickness of the energy scrape-off layer reaches a minimum of approximately 15 mm during the flat-top period and then generally increases by a factor of two or three towards the end of the discharge. - The thermal load on the limiters is not completely symmetrical as can be seen in fig. 5. The ion side of the limiters viewed was slightly hotter than the electron side. This observation is apparently associated with the observation that imaging the limiters at the H, wavelength showed that the hydrogen recycling is also asymmetrical and that it occurs primarily on the ion side of the limiters. The observation that the ion side of the limiters carry a greater thermal load and the corresponding difference in the intensity of the recycling cloud at the limiters is not understood, as yet. Differences in the thermal load of the ion side compared to the electron side of a spherical test-limiter in TEXTOR were re-
ported by Samm [4]. In this case, the electron side was hotter than the ion side and this was explained as due to differences in the connection lengths. Similar observations were made by Taylor et al. [5] in Doublet III, who reported that the asymmetry in the temperature profile was due to the oblique viewing angle. However, if the emissivity of the limiter is close to unity, as it is in our case, then the viewing angle should not influence the temperature measurement. From the measured temperature dist~bution across the limiter we can estimate that about 25% of the total ohmic input power is deposited on the limiters. This value has been calculated using a simple one dimensional heat flux model devised by Gondhalekar and Dietz (61. The calculation also includes an assumption for the temperature of those areas of the limiter not visible in the infra-red image i.e. below - 400°C. We assumed these areas of the limiters to be uniformly 3OO’C; above the wall temperature but below the detection limit. The value for the power deposition derived in this way does not contradict the measured total radiated power obtained from the bolometer array, i.e. about 70% of the total ohmic power input. However, this estimate is obviously still approximate as firstly the absolute temperature of the whole area of the limiter at the beginning and during the discharges is not well known and secondly the measurements are hampered at high power loading by the occurrence of rapid fluctuations in emission. We attribute these fluctuations to spectral emission from gaseous hydrocarbons emanating from the limiters.
4. Conduaions The limiter viewing system utilising CCD cameras was found to be well suited for the intended purpose and can provide valuable real-time information on the behaviour of the limiters during plasma discharges. It is planned to enhance the capabilities of the system by incorporating an infra-red array camera with which the limiters can be imaged at longer wavelengths, i.e. lower temperatures, so as to be able to measure the temperature areas of the whole limiter. A digital image analysis system will enable the rapid quantitative analysis of the available data.
Acknowledgements We acknowledge the assistance of and helpful discussions with many members of the JET team, especially, D. Cowell, K.J. Dietz, M. Watkins, A. Gondhalekar, L. de Kock, P. Stott, K. Behringer and P. Morgan.
6. LIMITERS;
EXPERIMENT
AND THEORY
444
MA.
Pick, D. Summers / Limiter viewing on JET
References [l] K.J. Dietz, Proc. 11 SOFT, Vol. II (1980) 1053. (21 P.H. Rebut and K.J. Die@ Proc. 12 SOFT, Vol. I (1982) 85. [3] K.J. Dietz, JET Internal Report FlO(D)84.
[4] U. Samm, Proc. 11th and Plasma Physics, [S] T. Taylor, N. Brooks (1982) 569. [6] A. Gondhalekar and
European Conf. on Controlled Fusion Aachen 1983. Vol. JD p. 413. and K. Ioki, J. Nucl. Mater. 111/112 K.J. Dietz, private
communication.