- Email: [email protected]
Probe measurements in the dite bundle divertor
383
Journal of Nuclear Materials 129 (1984) 383-389
PROBE MEASUREMENTS IN THE DITE BUNDLE DIVERTOR S.K. ERENTS, G.M. MCCRACKEN, J.W. PARTRIDGE
and D.H.J. GOODALL
Culham Laboratory, Abingdon, Oxon OX14 308,
Fusion Association)
UK (VKA EA/Euratom
Key words: DITE, bundle divertor, beat flux probe, impurity Measurements have been made of the plasma parameters in the DITE bundle divertor using an array of heat flux probes which can be scanned horizontally and vertically through the diverted flux bundle. Horizontal profiles of the saturation ion current, the electron temperature and the deposited power have been obtained for both ohmic and neutral beam heated discharges. Complete contours of these parameters have been obtained for the beam heated discharges. Maximum current densities of 7 A cme2, temperatures of 20-30 eV and deposited power of 25 MW mM2have been observed. In addition surface cokctor probes exposed for 163 discharges have been analysed and the main impurity shown to be stainless steel. The impurities were found to have penetrated - 100 nm into the carbon co&ctor and this is consistent with diffusion.
1. Intmduction The DITE tokamak has a major radius of 1.17 m, a minor radius of 0.26 m, a toroidal field of 2.7 T and can operate with plasma current of up to 250 kA [l]. The Mk.11 divertor [Z] has been operated at the full toroidal field of 2.7 T with plasma currents of 135 kA and densities of up to 4 x lOI mm3 in the present measurements. The separatrix is at 0.21 m and the flux bundle, which is extracted, enters the divertor chamber and strikes both sides of the neutralizer plate [3-S]. In order to investigate plasma- surface interaction in the divertor chamber a probe drive with an array of probes has been built. Both electrical probes and surface collector probes have been used but the present paper concentrates on the electrical probe measurements. These include profiles of ion saturation current, electron temperature and deposited power taken across the diverted flux bundle at various vertical positions using a heat flux probe array [6]. From these transverse profiles, contour plots of the above parameters have been constructed. In addition extensive measurements of the power deposited on the neutralizer plate have been made with an infra-red camera [7]. Some results from the heat flux probe array and the infra-red camera are compared.
(inset). A carbon shield 4 mm thick is placed on the probe drive facing the plasma flux. Behind this shield four tungsten probes each 5 x 3 x 1 mm3 are fitted into glass ceramic holders so that they are thermally and electrically isolated. A chromel-alumel thermocouple and a current carrying wire are spot-welded to each. The potential of each probe can be scanned using a fast scanning power amplifier with an output current capability of 2 A. The scanning voltage normally used is from - 100 V to + 10 V with respect to torus earth. The area of the probes has been reduced by a factor of 4 compared to the previous design in order to reduce the total current drawn. In addition to the heat flux probes there are four double Langmuir probes, two facing towards the neutralizer plate and two away from it. Measurements with these probes are described in a separate paper [9]. The probe array has been operated with the carbon shield and the neutralizer plate at floating potential.
2. Experimental arrangement A view of the divertor chamber with neutralizer plate and probe drive is shown in fig. 1. The probe drive is on the side facing the electron drift and can scan both horizontally and vertically across the flux bundle. A variety of probes have been mounted on the drive system. Some rn~~en~ of imp~ty fluxes, using surface collector probes, have already been reported [S]. The probes used in the present investigation are similar to the heat flux probes described by Stangeby et al. [6]. An array of four probes was used as shown in fig. 1 0022-3115/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics publishing Division)
A&A calibration
Fig. 1. Schematic of divertor chamber showing positions of heat flux probe and neutralizer plate. The heat flux probe array is shown inset.
5. DIVERTORS
S.K. Erents et al. / Probe measurements in the DITE bundle divertor
384
3. Results In the initial investigations it was found that there was a well-defined edge to the plasma on the inside edge of the flux bundle. This point corresponds to the first open field line outside the separatrix. Movement of this edge was observed as a function of time due to the ripple in the divertor coil power supply which is capacitor powered. On the outside of the flux bundle the probe current fell more slowly with increasing radius. The flux bundle is typically 50 mm wide. There was a large overlap in the positions available to adjacent probes obtained by moving the probe drive in successive discharges. It was established that the results from different probes were in good agreement, showing that the disturbing effect of the probe housing on the plasma is considered to be negligible. Measurements in a two-dimensional array of posi-
‘:
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3.1. Results with ohmic heating Plasma conditions for a series of discharges without neutral beam heating are shown in fig. 2. Conditions were Zp = 135 kA, ii, - 4 x 1019 rne3, Z?, = 2.6 T and T’(0) = 700 eV in a hydrogen plasma. The plasma was moved to the diverted position between 100 ms and 250 ms. Profiles of Z,, T, and P,, for one scan across the diverted plasma are shown in fig. 3. There is considerable scatter in the data and the lines have been drawn only to guide the eye. This scatter is to some extent due to real variations from one discharge to the next as shown by measurements with thermocouples on the neutralizer plate and data from the infra-red camera
In
400
200
tions were carried out over a series of similar discharges. Transverse profiles at different vertical positions were obtained and subsequently contours of parameters such as ion saturation current I,, electron temperature r,, power P, could be electron density n e, or deposited plotted. Measurements of this type have been made for a number of different operating conditions during the period in the middle of the pulse when the plasma is fully diverted by moving the outer flux surfaces [S].
lo!
-
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.23716-34.135ms without
NBI
7 E 4
lo4 i
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103 I
50 1
-2
0
I 400
200 Time
0
(ms)
Fig. 2. Plasma parameters for discharges without neutral beam heating, recorded as a function of time. (a) Plasma current, Ir, and central density, ii,. (b) Power losses, P0 ohmic and PrBd radiated power in torus. (c) Position of plasma centre, Ag(mm). When Ag = 0 plasma is moved into divertor. Gas pressure in divertor as recorded on a Baratron gauge. (d) Plasma loop volts and H, radiation in divertor chamber.
:I!3 #f l
.“..‘:++,
20
30
40 Distance
50 frori
60
70
datum
(mm)
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80
90
Fig. 3. Profile of ion saturation current, ISAT. deposIted power, T, taken across the diverted flux bundle using the HFP, 10 mm below mid-plane. No neutral beam heating. The datum is the inner wall of the divertor chamber aa shown in fig. 1. ISAT and T, profiles measured at Pd and electron temperature,
135 ms into each discharge, when ne - 3 X lOI rne3.
S.K. Erents et al. / Probe measurements
looking at the ion drift side plate. The sharp cut off at the separatrix is clearly seen where 1, drops and r, rises, 35 mm from the inner wall of the BDII chamber. The high values of T, inboard of 30 mm are unreliable because of very low current signals in this region. Near the separatrix I, peaks at 2.5 A cm-* before falling exponentially with distance. The temperature is between 10 and 20 eV, rising to about 40 eV at the separatrix. The power profile appears to be a maximum about 15 mm from the separatrix position. Also the power does not fall off as rapidly at the separatrix - which is consistent with the rise in T,. The floating potential (not illustrated) is positive near the separatrix and falls to about -20 V in the body of the flux bundle. The positive potential is correlated with high T, and may be
* (a)
2 7
due to secondary electron emission. Full profiles at different vertical positions were not measured in these discharges. However measurements of the total current to four probes were made as a function of vertical position and showed that the effective height of the flux bundle was about 80 mm. This is confirmed by power contours from the infra-red camera and from high speed tine films taken looking edge on at the neutralizer plate. Since the width is about 50 mm the total area is estimated to be 4000 mm* on each plate. From this a rough estimate of the total current to one side of the divertor is 100 A and the total power is 8 kW. The latter is in reasonable agreement with the infra-red camera data which varied from 9 to 13 kW for this series of discharges with Ti, in the range 3 to 4 X 1019 rns3.
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Fig. 4. Heat flux probe measurements and plasma parameters for discharges with 1.6 h4W of neutral beam heating, recorded as a function of time. (a) Ion saturation current as measured by the HFP. (b) Electron temperature deduced from HFP current. (c) Deposited powers deduced from HFP temperature rise rate. (d) Plasma current, I,,, and central density, ii,. (e) Power losses, Pa ohmic and Prd radiated power in torus. (f) Radiated power in divertor chamber and neutral beam heating powers, (4 beam lines A, B, C and D). (g) As in fig. z(c). (h) As in fig. 2(d).
5. DIVERTORS
386
S.K. Erents et al. / Probe measurements
in the DITE bundle diverror
3.2. Results with neutral beam heating More detailed measurements of the diverted flux bundle have been made with neutral beam heating. The discharge conditions are shown in fig. 4 together with measurements from an individual probe. The toroidal field and plasma current are similar to the previous discharges, however deuterium was used as the working gas. The plasma was diverted from 100 ms to 200 ms. The density reached a m~mum of 3.8 x lOI me3 before diversion and fell to 2.6 X lOI9 me3 by the end of the diverted period. The neutraf beam heating is turned on from 80 to 180 ms. Gas puffing was into the divertor at a rate of 23 Torr 1 s-r both before and during the diverted period. The pressure rise was 6 mTorr in the divertor chamber. Profiles scanning approximately horizontally across the flux bundle have been measured at four vertical positions. Two profiles of I, are shown in fig. 5, along with similar profiles of T, and Pd. The values for Z, and c are taken in the period 140-160 ms. The power profiles are averaged over 135-190 ms. At y = 0 (10 mm below mid plane - see fig. 6) the current increases to 5 A cm-’ which compares with 2.5 A cm-* measured in
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’
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40 Distance
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60
70
60
90
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Fig. 5. Profiles of ion saturation current, IsAT, deposited power, Pd and electron temperature, T, taken across the diverted flux bundle using the HFP, 10 mm below mid-plane “0”; and at an angle some 35 mm below mid-plane “A”, “0” and “+“. Central density ii, falling 3.8 to 2.6X lOI m-3.
Isat
(a)
40
contours
I”
A
rn-* x104
_,
/Y--60
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-50
-60
1
IO
-
-.____--to
30 Distance
40
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60
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80
the ohmically heated discharges described above. At a vertical position y = - 30 mm in the flux bundle the current density is higher and the profile slightly more peaked. The data at + 30 mm are lower and much more scattered. The deposited power reaches a maximum of 4 kW cme2 at the centre of the scan at y = -30 mm. T, profiles show considerable scatter in data with values of T, generally in the range 10 to 30 eV. The four profiles which have been measured have been combined to produce contour maps of I,, T, and Pd. These are shown in fig. 6. Because of insufficient data some of the contours have been extrapolated these are shown as dotted lines. In the case of the P,, results there is some uncertainty in the shape at the lower inside edge because there is no one complete contour. The curves have been drawn consistent with the I, measurements. A ripple in the bundle divertor capacitor bank driven current causes an oscillation in the position of the diverted flux bundle. The amplitude of this oscillation is - 10 mm, based on the measured variation in 1, with divertor current when the probe is placed at the edge of the flux bundle. The apparent shift in position of maximum deposited power, compared with maximum T,, can be ascribed partially to oscillation of the flux bundle and partially to real differences in parameters between one discharge and the next. 3.3. Sputter Auger analysis of heat flux probe shield A carbon shield exposed to 163 various discharges in the DITE bundle divertor was later analysed by Sputter
38-l
SK. Erenis et al. / Probe meawrements in the DITE bundle divertor Pd contours
(b)
40
135
in
W
k*
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-169ms
30
Te contours
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20
30
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Distance
50 from
60
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70
60 Distance
(mm)
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Fig. 6. Contour plots of ion saturation current, I,,,, deposited power, P., and electron temperature, T, for the diverted flux bundle using the HFP. Measurements taken with 1.6 MW neutral beam heating. The shift in the maximum for TCcompared with I,, and P,, is probably due to plasma oscillation, (see text).
Auger Spectroscopy to obtain impurity depth distributions. The major impurity detected was iron, with small traces of oxygen, sulphur and nitrogen. The iron concentrations are plotted in fig. 7 in per cent by weight as a function of depth. Generally for curve 1 the sample was positioned at the outer edges of the flux bundle; for curve 2 in the region of high flux gradient and for curve 3 near to the centre of the bundle. The surface tempera-
ture of the carbon shield is estimated to reach 1300 K during some high power discharges, from infra-red camera and heat flux observations. Assuming the highest deposited power observed in this series of discharges (4 kW cm-*), and applying theory for pulse heating of a semi-infinite solid, a surface temperature rise of just over 1100 K is calculated. The heat diffusion length is 2.5 mm, compared with the shield thickness of 4 mm. The bulk temperature after the heat pulse will be < 400 K.
4. Discussion
0 0
20
40
60
60 Depth
100
120
140
160
(nM)
Fig. 7. The concentration of Fe on the HFP shield after exposure to 163 discharges in the divertor chamber. Measurements made using sputter Auger spectroriiky. (1) Sample at outer edge of flux bundle, (2) !Sample at position of high flux gradient, (3) Sample in centre of flux tkmdle.
It is interesting to compare the contour maps of the three parameters Z,, T, and Pd since each is derived from independent measurements. They are clearly in general agreement showing the centre of the flwr bundle about 35 mm below the mid plane, (the probe elements are 1 cm below mid plane), in agreement with observations of the tine film. The effect is predicted, being due to the action of the vertical field. The centre of the flux bundle on the ion drift side is deflected upwards also by 35 mm in similar discharges, as is confirmed by the film and by the infra-red camera data. The general shapes and size of the different contour plots also agree. In the previous study of heat flux probes we have 5. DIVERTORS
388
S.K. Erents et al. / Probe measurements
interpreted the deposited energy to be mainly due to incident ions and have made corrections for the energy reflection coefficient of the ions at the tungsten probe. However we now have evidence that the tungsten is in most cases covered with a layer of carbon so that the correction for backscattering is rather small (3% to 16% for ions in the range 300 eV and 30 eV). Away from the separatrix the data is consistent with T, = Ti and using this assumption we have calculated the plasma density. The maximum density in the flux bundle is 9 X 1018 rnp3. Taking the contours from fig. 6 for the beam heated discharges we can integrate them to find the total power and particle current flowing into the divertor. The integrated power is 36 kW which is in general agreement with the results from the infra-red camera in similar discharges, (20-80 kW). If the probes are coated with carbon the numbers should be directly comparable since the neutralizer plate is also carbon and both techniques measure deposited power. The total ion saturation current is 142 A to each side, = 284 A. It is interesting to compare this with the rate of change of density shown in fig. 4. Before diversion the density is rising at 4.2 X 10” rnw3 s-’ and immediately after diversion the density is decreasing at a rate of 2.5 X 102’ rn-’ s-‘. Since the gas puffing rate has been kept constant, taking the volume of the plasma into account, we find that this density change corresponds to a change in loss rate of 1.0 X 102’ ions s-’ or 600 A equivalent. The gas puffing rate into the divertor is 230 A equivalent. In the ohmic discharge the change in loss rate is much smaller ,< 10 A equivalent. This is attributed to a lower edge temperature. The high current at the divertor neutralizer plate can be expected to result in high erosion rates. This is observed macroscopically from layers of carbon being deposited on the tungsten probes and their surroundings. Measurements on the carbon shield indicated a change of thickness of 14 pm after exposure to 335 discharges under various conditions. Taking an average current density of 5 A cmm2 for 100 ms/discharge we estimate an erosion rate of - 0.24 atoms/ion which is considerably higher than that for chemical sputtering by 100 eV deuterons on carbon at - 600°C, (3 x 1O-2 atoms/ion). More detailed measurements are planned. The iron penetration depth of 120 nm (curve 3, fig. 7) is thought to be due to diffusion of iron in carbon at the high surface temperatures attained in the divertor, (- 1300 K). The apparent diffusion coefficient, D, of metals in graphite varies with time for short times (t less than 60 min), but the product Dt is constant if there is a constant penetration depth [ll]. Data for iron and nickel are similar at temperatures above 1900 K. At lower temperatures only data for nickel is available but this gives Dt = 3.6 X lo-’ cm2 indicating a penetration depth of 120 microns, in good agreement with the results of fig. 7. The penetration increases by an order
in the DITE bundle divertor
of magnitude at 1900 K for Ni and at 2100 K for Fe. The diagnosis of the bundle divertor shows it to have one of the highest low energy plasma fluxes available anywhere. It also has the attraction of being readily accessible for insertion of different materials and diagnostics. One interesting application of such a high intensity plasma source would be the investigation of hydrogen isotope implantation and release under very high flux conditions. This is of considerable interest for the estimation of tritium inventory in limiter and wall materials for DT burning devices.
5. Conclusions Detailed studies of the plasma on the electron dift side of the bundle divertor have been carried out. These show plasma currents up to 7 A cmm2 and power densities above 25 MW me2 during gas puffing into the divertor. The integrated power is - 36 kW and the integrated current is - 140 A during beam heated discharges at densities 9 x 1018 rnw3. Less detailed data are available for ohmically heated discharges but the integrated power and the ion current are probably lower by a factor of 50 and 6 respectively. The conditions investigated correspond to relatively high operating densities. At lower densities I 2 X 1019 mP3 much higher power levels and much higher fractions of the heating power are transferred to the neutralizer plate. However, even at the power levels found in this investigation the diverted plasma with densities of 9 x 10” mP3 at 25 eV and power levels of 25 MW me2 is one of the most intense available for studying plasma surface interactions. Future plans include measurements of erosion of materials and of hydrogen isotope inventories under a range of operating conditions.
Acknowledgements We are grateful to W. Dearing for the design and construction of the probes and to the DITE physics and operating teams for their cooperation in these experiments.
References (11 J.W.M. Paul et al., Plasma Physics and Controlled Nuclear Fusion Research Proc. 6th Int. Conf. Berchtesgaden, Vol. 2 (IAEA, Vienna, 1977) p. 269. 121 P.E. Stott, C.M. Wilson and A. Gibson, Nucl. Fusion 17 (1977) 48.
/31 S.J. Fielding et al., in: Controlled Nuclear Fusion and Plasma Physics, Proc. 8th Eur. Conf. Prague 1977, Vol. I p. 36. 141 K.B. Axon et al., in: Plasma Physics and Controlled Nuclear Fusion Research, Proc. 9th Int. Conf. Baltimore, 1982, Vol. III (IAEA, Vienna, 1983).
S. K. Erents et rri. / Probe measurements in the DITE bundle divertor [SJ S.J. Fielding, D.H.J. Goodall, J. Hugill and P.C. Johnson, Proc. 11th European Conf. on Controlled Fusion and Plasma Physics, Aachen, 1983 (European Physical Society) p. 455. [6] P.C. Stangeby, G.M. McCracken and SK. Erents, J. Vat. Sci. Tech. 1 (1983) No. 2. [7] D.H.J. Goodall, Nucl. Technol./Fusion, to be published.
389
[8] G. Mezey, G.M. McCracken and J.W. Partridge, Nucl. Technol./Fusion, to be published. 191 G. Proudfoot and P.J. Harbour, these Proceedings. [lOI B.A. Powell, private ~~u~~tion. [ll] W. Weisweiler and G.D. Nageshwar, Carbon 13 (1975) 175.
5. DIVERTORS
Journal of Nuclear Materials 129 (1984) 383-389
PROBE MEASUREMENTS IN THE DITE BUNDLE DIVERTOR S.K. ERENTS, G.M. MCCRACKEN, J.W. PARTRIDGE
and D.H.J. GOODALL
Culham Laboratory, Abingdon, Oxon OX14 308,
Fusion Association)
UK (VKA EA/Euratom
Key words: DITE, bundle divertor, beat flux probe, impurity Measurements have been made of the plasma parameters in the DITE bundle divertor using an array of heat flux probes which can be scanned horizontally and vertically through the diverted flux bundle. Horizontal profiles of the saturation ion current, the electron temperature and the deposited power have been obtained for both ohmic and neutral beam heated discharges. Complete contours of these parameters have been obtained for the beam heated discharges. Maximum current densities of 7 A cme2, temperatures of 20-30 eV and deposited power of 25 MW mM2have been observed. In addition surface cokctor probes exposed for 163 discharges have been analysed and the main impurity shown to be stainless steel. The impurities were found to have penetrated - 100 nm into the carbon co&ctor and this is consistent with diffusion.
1. Intmduction The DITE tokamak has a major radius of 1.17 m, a minor radius of 0.26 m, a toroidal field of 2.7 T and can operate with plasma current of up to 250 kA [l]. The Mk.11 divertor [Z] has been operated at the full toroidal field of 2.7 T with plasma currents of 135 kA and densities of up to 4 x lOI mm3 in the present measurements. The separatrix is at 0.21 m and the flux bundle, which is extracted, enters the divertor chamber and strikes both sides of the neutralizer plate [3-S]. In order to investigate plasma- surface interaction in the divertor chamber a probe drive with an array of probes has been built. Both electrical probes and surface collector probes have been used but the present paper concentrates on the electrical probe measurements. These include profiles of ion saturation current, electron temperature and deposited power taken across the diverted flux bundle at various vertical positions using a heat flux probe array [6]. From these transverse profiles, contour plots of the above parameters have been constructed. In addition extensive measurements of the power deposited on the neutralizer plate have been made with an infra-red camera [7]. Some results from the heat flux probe array and the infra-red camera are compared.
(inset). A carbon shield 4 mm thick is placed on the probe drive facing the plasma flux. Behind this shield four tungsten probes each 5 x 3 x 1 mm3 are fitted into glass ceramic holders so that they are thermally and electrically isolated. A chromel-alumel thermocouple and a current carrying wire are spot-welded to each. The potential of each probe can be scanned using a fast scanning power amplifier with an output current capability of 2 A. The scanning voltage normally used is from - 100 V to + 10 V with respect to torus earth. The area of the probes has been reduced by a factor of 4 compared to the previous design in order to reduce the total current drawn. In addition to the heat flux probes there are four double Langmuir probes, two facing towards the neutralizer plate and two away from it. Measurements with these probes are described in a separate paper [9]. The probe array has been operated with the carbon shield and the neutralizer plate at floating potential.
2. Experimental arrangement A view of the divertor chamber with neutralizer plate and probe drive is shown in fig. 1. The probe drive is on the side facing the electron drift and can scan both horizontally and vertically across the flux bundle. A variety of probes have been mounted on the drive system. Some rn~~en~ of imp~ty fluxes, using surface collector probes, have already been reported [S]. The probes used in the present investigation are similar to the heat flux probes described by Stangeby et al. [6]. An array of four probes was used as shown in fig. 1 0022-3115/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics publishing Division)
A&A calibration
Fig. 1. Schematic of divertor chamber showing positions of heat flux probe and neutralizer plate. The heat flux probe array is shown inset.
5. DIVERTORS
S.K. Erents et al. / Probe measurements in the DITE bundle divertor
384
3. Results In the initial investigations it was found that there was a well-defined edge to the plasma on the inside edge of the flux bundle. This point corresponds to the first open field line outside the separatrix. Movement of this edge was observed as a function of time due to the ripple in the divertor coil power supply which is capacitor powered. On the outside of the flux bundle the probe current fell more slowly with increasing radius. The flux bundle is typically 50 mm wide. There was a large overlap in the positions available to adjacent probes obtained by moving the probe drive in successive discharges. It was established that the results from different probes were in good agreement, showing that the disturbing effect of the probe housing on the plasma is considered to be negligible. Measurements in a two-dimensional array of posi-
‘:
E
c
,o -i IC
01 -0
1
4
\I
3.1. Results with ohmic heating Plasma conditions for a series of discharges without neutral beam heating are shown in fig. 2. Conditions were Zp = 135 kA, ii, - 4 x 1019 rne3, Z?, = 2.6 T and T’(0) = 700 eV in a hydrogen plasma. The plasma was moved to the diverted position between 100 ms and 250 ms. Profiles of Z,, T, and P,, for one scan across the diverted plasma are shown in fig. 3. There is considerable scatter in the data and the lines have been drawn only to guide the eye. This scatter is to some extent due to real variations from one discharge to the next as shown by measurements with thermocouples on the neutralizer plate and data from the infra-red camera
In
400
200
tions were carried out over a series of similar discharges. Transverse profiles at different vertical positions were obtained and subsequently contours of parameters such as ion saturation current I,, electron temperature r,, power P, could be electron density n e, or deposited plotted. Measurements of this type have been made for a number of different operating conditions during the period in the middle of the pulse when the plasma is fully diverted by moving the outer flux surfaces [S].
lo!
-
1
y.0
.23716-34.135ms without
NBI
7 E 4
lo4 i
7
103 I
50 1
-2
0
I 400
200 Time
0
(ms)
Fig. 2. Plasma parameters for discharges without neutral beam heating, recorded as a function of time. (a) Plasma current, Ir, and central density, ii,. (b) Power losses, P0 ohmic and PrBd radiated power in torus. (c) Position of plasma centre, Ag(mm). When Ag = 0 plasma is moved into divertor. Gas pressure in divertor as recorded on a Baratron gauge. (d) Plasma loop volts and H, radiation in divertor chamber.
:I!3 #f l
.“..‘:++,
20
30
40 Distance
50 frori
60
70
datum
(mm)
+:*,*
80
90
Fig. 3. Profile of ion saturation current, ISAT. deposIted power, T, taken across the diverted flux bundle using the HFP, 10 mm below mid-plane. No neutral beam heating. The datum is the inner wall of the divertor chamber aa shown in fig. 1. ISAT and T, profiles measured at Pd and electron temperature,
135 ms into each discharge, when ne - 3 X lOI rne3.
S.K. Erents et al. / Probe measurements
looking at the ion drift side plate. The sharp cut off at the separatrix is clearly seen where 1, drops and r, rises, 35 mm from the inner wall of the BDII chamber. The high values of T, inboard of 30 mm are unreliable because of very low current signals in this region. Near the separatrix I, peaks at 2.5 A cm-* before falling exponentially with distance. The temperature is between 10 and 20 eV, rising to about 40 eV at the separatrix. The power profile appears to be a maximum about 15 mm from the separatrix position. Also the power does not fall off as rapidly at the separatrix - which is consistent with the rise in T,. The floating potential (not illustrated) is positive near the separatrix and falls to about -20 V in the body of the flux bundle. The positive potential is correlated with high T, and may be
* (a)
2 7
due to secondary electron emission. Full profiles at different vertical positions were not measured in these discharges. However measurements of the total current to four probes were made as a function of vertical position and showed that the effective height of the flux bundle was about 80 mm. This is confirmed by power contours from the infra-red camera and from high speed tine films taken looking edge on at the neutralizer plate. Since the width is about 50 mm the total area is estimated to be 4000 mm* on each plate. From this a rough estimate of the total current to one side of the divertor is 100 A and the total power is 8 kW. The latter is in reasonable agreement with the infra-red camera data which varied from 9 to 13 kW for this series of discharges with Ti, in the range 3 to 4 X 1019 rns3.
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;
-Time (ms)
Fig. 4. Heat flux probe measurements and plasma parameters for discharges with 1.6 h4W of neutral beam heating, recorded as a function of time. (a) Ion saturation current as measured by the HFP. (b) Electron temperature deduced from HFP current. (c) Deposited powers deduced from HFP temperature rise rate. (d) Plasma current, I,,, and central density, ii,. (e) Power losses, Pa ohmic and Prd radiated power in torus. (f) Radiated power in divertor chamber and neutral beam heating powers, (4 beam lines A, B, C and D). (g) As in fig. z(c). (h) As in fig. 2(d).
5. DIVERTORS
386
S.K. Erents et al. / Probe measurements
in the DITE bundle diverror
3.2. Results with neutral beam heating More detailed measurements of the diverted flux bundle have been made with neutral beam heating. The discharge conditions are shown in fig. 4 together with measurements from an individual probe. The toroidal field and plasma current are similar to the previous discharges, however deuterium was used as the working gas. The plasma was diverted from 100 ms to 200 ms. The density reached a m~mum of 3.8 x lOI me3 before diversion and fell to 2.6 X lOI9 me3 by the end of the diverted period. The neutraf beam heating is turned on from 80 to 180 ms. Gas puffing was into the divertor at a rate of 23 Torr 1 s-r both before and during the diverted period. The pressure rise was 6 mTorr in the divertor chamber. Profiles scanning approximately horizontally across the flux bundle have been measured at four vertical positions. Two profiles of I, are shown in fig. 5, along with similar profiles of T, and Pd. The values for Z, and c are taken in the period 140-160 ms. The power profiles are averaged over 135-190 ms. At y = 0 (10 mm below mid plane - see fig. 6) the current increases to 5 A cm-’ which compares with 2.5 A cm-* measured in
‘“+-----
L ,i
A,O,+
Y=-30mm
with
I i
10
105
’
A
20
NBI
.b
I
4
~~~~,~
30
40 Distance
50 from
60
70
60
90
doWm(mm)
Fig. 5. Profiles of ion saturation current, IsAT, deposited power, Pd and electron temperature, T, taken across the diverted flux bundle using the HFP, 10 mm below mid-plane “0”; and at an angle some 35 mm below mid-plane “A”, “0” and “+“. Central density ii, falling 3.8 to 2.6X lOI m-3.
Isat
(a)
40
contours
I”
A
rn-* x104
_,
/Y--60
135m5
t
-50
-60
1
IO
-
-.____--to
30 Distance
40
-~ from
50
60
datum
(mm)
70
80
the ohmically heated discharges described above. At a vertical position y = - 30 mm in the flux bundle the current density is higher and the profile slightly more peaked. The data at + 30 mm are lower and much more scattered. The deposited power reaches a maximum of 4 kW cme2 at the centre of the scan at y = -30 mm. T, profiles show considerable scatter in data with values of T, generally in the range 10 to 30 eV. The four profiles which have been measured have been combined to produce contour maps of I,, T, and Pd. These are shown in fig. 6. Because of insufficient data some of the contours have been extrapolated these are shown as dotted lines. In the case of the P,, results there is some uncertainty in the shape at the lower inside edge because there is no one complete contour. The curves have been drawn consistent with the I, measurements. A ripple in the bundle divertor capacitor bank driven current causes an oscillation in the position of the diverted flux bundle. The amplitude of this oscillation is - 10 mm, based on the measured variation in 1, with divertor current when the probe is placed at the edge of the flux bundle. The apparent shift in position of maximum deposited power, compared with maximum T,, can be ascribed partially to oscillation of the flux bundle and partially to real differences in parameters between one discharge and the next. 3.3. Sputter Auger analysis of heat flux probe shield A carbon shield exposed to 163 various discharges in the DITE bundle divertor was later analysed by Sputter
38-l
SK. Erenis et al. / Probe meawrements in the DITE bundle divertor Pd contours
(b)
40
135
in
W
k*
x10’
-169ms
30
Te contours
(cl
40
t
30
in tV
135-162ms
t
20 -
s -
10 10 -
0
ii
.-6
5
.z f
-10 -
3 2
-zo-
P
t >
o-
.-s .= g
-lO-
a f
-2o-
f
-3O-
-30 -40
-
-50
-
-60
-60
-
I
lo
20
30
LO
Distance
50 from
60
datum
70
60 Distance
(mm)
from
doturn
Fig. 6. Contour plots of ion saturation current, I,,,, deposited power, P., and electron temperature, T, for the diverted flux bundle using the HFP. Measurements taken with 1.6 MW neutral beam heating. The shift in the maximum for TCcompared with I,, and P,, is probably due to plasma oscillation, (see text).
Auger Spectroscopy to obtain impurity depth distributions. The major impurity detected was iron, with small traces of oxygen, sulphur and nitrogen. The iron concentrations are plotted in fig. 7 in per cent by weight as a function of depth. Generally for curve 1 the sample was positioned at the outer edges of the flux bundle; for curve 2 in the region of high flux gradient and for curve 3 near to the centre of the bundle. The surface tempera-
ture of the carbon shield is estimated to reach 1300 K during some high power discharges, from infra-red camera and heat flux observations. Assuming the highest deposited power observed in this series of discharges (4 kW cm-*), and applying theory for pulse heating of a semi-infinite solid, a surface temperature rise of just over 1100 K is calculated. The heat diffusion length is 2.5 mm, compared with the shield thickness of 4 mm. The bulk temperature after the heat pulse will be < 400 K.
4. Discussion
0 0
20
40
60
60 Depth
100
120
140
160
(nM)
Fig. 7. The concentration of Fe on the HFP shield after exposure to 163 discharges in the divertor chamber. Measurements made using sputter Auger spectroriiky. (1) Sample at outer edge of flux bundle, (2) !Sample at position of high flux gradient, (3) Sample in centre of flux tkmdle.
It is interesting to compare the contour maps of the three parameters Z,, T, and Pd since each is derived from independent measurements. They are clearly in general agreement showing the centre of the flwr bundle about 35 mm below the mid plane, (the probe elements are 1 cm below mid plane), in agreement with observations of the tine film. The effect is predicted, being due to the action of the vertical field. The centre of the flux bundle on the ion drift side is deflected upwards also by 35 mm in similar discharges, as is confirmed by the film and by the infra-red camera data. The general shapes and size of the different contour plots also agree. In the previous study of heat flux probes we have 5. DIVERTORS
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S.K. Erents et al. / Probe measurements
interpreted the deposited energy to be mainly due to incident ions and have made corrections for the energy reflection coefficient of the ions at the tungsten probe. However we now have evidence that the tungsten is in most cases covered with a layer of carbon so that the correction for backscattering is rather small (3% to 16% for ions in the range 300 eV and 30 eV). Away from the separatrix the data is consistent with T, = Ti and using this assumption we have calculated the plasma density. The maximum density in the flux bundle is 9 X 1018 rnp3. Taking the contours from fig. 6 for the beam heated discharges we can integrate them to find the total power and particle current flowing into the divertor. The integrated power is 36 kW which is in general agreement with the results from the infra-red camera in similar discharges, (20-80 kW). If the probes are coated with carbon the numbers should be directly comparable since the neutralizer plate is also carbon and both techniques measure deposited power. The total ion saturation current is 142 A to each side, = 284 A. It is interesting to compare this with the rate of change of density shown in fig. 4. Before diversion the density is rising at 4.2 X 10” rnw3 s-’ and immediately after diversion the density is decreasing at a rate of 2.5 X 102’ rn-’ s-‘. Since the gas puffing rate has been kept constant, taking the volume of the plasma into account, we find that this density change corresponds to a change in loss rate of 1.0 X 102’ ions s-’ or 600 A equivalent. The gas puffing rate into the divertor is 230 A equivalent. In the ohmic discharge the change in loss rate is much smaller ,< 10 A equivalent. This is attributed to a lower edge temperature. The high current at the divertor neutralizer plate can be expected to result in high erosion rates. This is observed macroscopically from layers of carbon being deposited on the tungsten probes and their surroundings. Measurements on the carbon shield indicated a change of thickness of 14 pm after exposure to 335 discharges under various conditions. Taking an average current density of 5 A cmm2 for 100 ms/discharge we estimate an erosion rate of - 0.24 atoms/ion which is considerably higher than that for chemical sputtering by 100 eV deuterons on carbon at - 600°C, (3 x 1O-2 atoms/ion). More detailed measurements are planned. The iron penetration depth of 120 nm (curve 3, fig. 7) is thought to be due to diffusion of iron in carbon at the high surface temperatures attained in the divertor, (- 1300 K). The apparent diffusion coefficient, D, of metals in graphite varies with time for short times (t less than 60 min), but the product Dt is constant if there is a constant penetration depth [ll]. Data for iron and nickel are similar at temperatures above 1900 K. At lower temperatures only data for nickel is available but this gives Dt = 3.6 X lo-’ cm2 indicating a penetration depth of 120 microns, in good agreement with the results of fig. 7. The penetration increases by an order
in the DITE bundle divertor
of magnitude at 1900 K for Ni and at 2100 K for Fe. The diagnosis of the bundle divertor shows it to have one of the highest low energy plasma fluxes available anywhere. It also has the attraction of being readily accessible for insertion of different materials and diagnostics. One interesting application of such a high intensity plasma source would be the investigation of hydrogen isotope implantation and release under very high flux conditions. This is of considerable interest for the estimation of tritium inventory in limiter and wall materials for DT burning devices.
5. Conclusions Detailed studies of the plasma on the electron dift side of the bundle divertor have been carried out. These show plasma currents up to 7 A cmm2 and power densities above 25 MW me2 during gas puffing into the divertor. The integrated power is - 36 kW and the integrated current is - 140 A during beam heated discharges at densities 9 x 1018 rnw3. Less detailed data are available for ohmically heated discharges but the integrated power and the ion current are probably lower by a factor of 50 and 6 respectively. The conditions investigated correspond to relatively high operating densities. At lower densities I 2 X 1019 mP3 much higher power levels and much higher fractions of the heating power are transferred to the neutralizer plate. However, even at the power levels found in this investigation the diverted plasma with densities of 9 x 10” mP3 at 25 eV and power levels of 25 MW me2 is one of the most intense available for studying plasma surface interactions. Future plans include measurements of erosion of materials and of hydrogen isotope inventories under a range of operating conditions.
Acknowledgements We are grateful to W. Dearing for the design and construction of the probes and to the DITE physics and operating teams for their cooperation in these experiments.
References (11 J.W.M. Paul et al., Plasma Physics and Controlled Nuclear Fusion Research Proc. 6th Int. Conf. Berchtesgaden, Vol. 2 (IAEA, Vienna, 1977) p. 269. 121 P.E. Stott, C.M. Wilson and A. Gibson, Nucl. Fusion 17 (1977) 48.
/31 S.J. Fielding et al., in: Controlled Nuclear Fusion and Plasma Physics, Proc. 8th Eur. Conf. Prague 1977, Vol. I p. 36. 141 K.B. Axon et al., in: Plasma Physics and Controlled Nuclear Fusion Research, Proc. 9th Int. Conf. Baltimore, 1982, Vol. III (IAEA, Vienna, 1983).
S. K. Erents et rri. / Probe measurements in the DITE bundle divertor [SJ S.J. Fielding, D.H.J. Goodall, J. Hugill and P.C. Johnson, Proc. 11th European Conf. on Controlled Fusion and Plasma Physics, Aachen, 1983 (European Physical Society) p. 455. [6] P.C. Stangeby, G.M. McCracken and SK. Erents, J. Vat. Sci. Tech. 1 (1983) No. 2. [7] D.H.J. Goodall, Nucl. Technol./Fusion, to be published.
389
[8] G. Mezey, G.M. McCracken and J.W. Partridge, Nucl. Technol./Fusion, to be published. 191 G. Proudfoot and P.J. Harbour, these Proceedings. [lOI B.A. Powell, private ~~u~~tion. [ll] W. Weisweiler and G.D. Nageshwar, Carbon 13 (1975) 175.
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