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Impurity investigations in the boundary layer of the DITE tokamak
159
Journal of Nuclear Materials 111& 112 (1982) 159-161 North-Holland Publishing Company
IMPURITY INVESTIGATIONS TOKAMAK G.M. MCCRACKEN, Culham Laboratory,
IN THE BOUNDARY
J.W. PARTRIDGE,
Abingdon,
LAYER OF THE DITE
S.K. ERENTS
OX14 3DB, UK (Euraiom /UKAEA
Associatroni
and C.J. SOFIELD, AERE
SM. FERGUSON
Harwell, Oxon, UK
1. Introduction The mechanisms responsible for the production of impurities and the behaviour of the impurities in the boundary layers of tokamaks are still not well understood. Previous investigations have shown that a valuable technique for studying impurity fluxes is the use of collecting surfaces followed by surface analysis [l]. In the present study rotating carbon discs and cylinders have been used as collectors. After exposure the samples have been transferred in atmosphere to a chamber for surface analysis. Analytical techniques used have included Auger spectroscopy, Rutherford Backscattering, 2H( 3He, p) 4He nuclear reaction analysis and ion induced X-ray spectroscopy. Collectors have been exposed to a wide range of discharges in the DITE tokamak both with and without the bundle divertor and with and without neutral beam injection up to 1 MW.
2. Limiter analysis The DITE tokamak,was used with two pairs of fixed poloidal titanium limiters (r = 0.26 m) during this period. Each half limiter subtended an angle of 160”. In addition to the fixed limiters there were a pair of adjustable limiters made of ATJ graphite. These limiters could be moved in to a radius of 0.18 m or withdrawn behind the fixed limiters. A detailed analysis of the surface of both the carbon and titanium limiters was undertaken after exposure to - 100 discharges, in order to assess the impurities likely to be present. The impurity concentrations were remarkably consistent over the surface of both types of limiter probably indicating that
0022-3 115/82/0000-0000/$02.75
0 1982 North-Holland
an equilibrium had been reached. The concentration of titanium on the carbon limiter was 1.5 ? 0.5 X 102’ atoms/m2 and the relative concentration of other impurities in atoms/m2 Ti 100
Fe 20-30
Cu 5
MO 0.6
Pb 0.02
The concentration of the other impurities on the titanium limiter was the same within a factor 2. In all cases these levels were at least a factor of 10 greater than the background level on clean carbon. Other impurities measured using ion induced X-rays included S, Cl, K, Ca, Cr and Ni, all at about the 1% level. Compared with earlier measurements the titanium is now larger than iron; the molybdenum from the limiters used previously has dropped to an insignificant level. An important point however is the presence of iron on the limiters. This probably results from arcing and disruptions in unstable discharges producing impurities from the wall which are then ionised in the plasma and deposited on the limiters. Thus the presence of iron in stable non-disruptive plasmas does not necessarily imply impurities coming from the wall.
3. Time-resolved measurements Measurements of the time-resolved impurity fluxes have been made over a range of operating conditions with currents from 60 kA to 170 kA, densities from iie = 5 X lOI mm3 to 6 X lOI me3 with neutral beam injection up to 1 MW and at radii from 0.25 to 0.27m. The geometry of the collector discs was as used earlier [2] and the collectors were connected to torus potential.
160
G.M. McCracken et al. / Impurity inoestigatrons rn the DITE tokamak
The incident deuterium flux was measured on the same collectors for the same discharges in a number of cases. The measurements were made over a period of a years operation and over forty separate time-resolved measurements have been made. In most cases these measurements were made at one radius through a 2 mm square aperture. Corrections to obtain the incident flux have to be made for the transmission through such an aperture (3,4]. Measurements of the radial d~st~bution have been made in a few cases using a cylindrical collector with a slit, similar to that used in PLT [S]. A series of measurements have also been made of the ion Larmor radii using a fixed collector and subsequently analysing the spatial distribution behind the collimating hoIe [4]. These measurements indicated a deuterium ion energy of SO-80 eV and an average charge state of 4 for iron and titanium ions if it is assumed that they are in thermal equilibrium with the plasma. In the present discussion we will concentrate on the time-resolved measurements of the impurity flux. Most of these measurements have been made with the eollector probe at the same radius as the fixed titanium limiters (r = 0.26 m) and measurements are concentrated on the titanium impurities. However a significant amount of iron was also present typically at a level of 30% of the titanium flux. Measurable amounts of copper, molybdenum and lead were also occasionally present in similar relative abundance to that observed on the limiter. These quantities were normally small, near the limit of detection, but could be unequivocally identified using time integrated exposures. Apart from the iron their contribution to radiation and to z,rr was generally insignificant. Looking at the time-resolved titanium fIux a rather large variation has been observed. The collected flux varies typically from 10’8-1020 ions/m2. s-‘. If we consider that the ions travel at 0.3 C, where C, is the ion sound speed and that the electron temperature is typically 20 eV [6] then these fluxes correspond to local impurity concentration of 5 X lOI m-s to 5 X lOi mp3. Unfortunately there are few spectroscopic measurements which can be directly compared with these measurements but some spatially resolved measurements have shown titanium concentrations as high as IO” rnp3 in the centre of the plasma [7]. Within the large variations from discharge to discharge it is difficult to see any definite trend with increasing plasma current. The average value of the titanium flux is - 510 X 101s/m2+s-‘. The variation in flux with radial position is similarly difficult to determine within the reproducibility of the data. However there could be a variation of up to a factor of 4 between the value at a
radial position of 0.27 m compared with 0.25 m. One of the factors making comparisons difficult is that the discharges at high currents typically have larger densities. It is well known that gas puffing tends to reduce impurity levels. Direct evidence of a reduction in the heat flux and the edge temperature have been also observed [S]. However low rates of gas puffing are not effective in reducing impurities. An approximate criterion that dn,/dr must be greater than 2 X 102’/cm -‘, SC’, in order to cause a significant reduction in the edge impurity concentration, has been established. An example of impurity fluxes in a discharge with high gas puffing is shown in fig. 1. After a high flux at the start of the discharge the impurity level drops rapidly as has been observed previously f2]. However in this case the impurity level increases by an order of magnitude in an unexplained way during the discharge. A number of examples of this effect have been seen. A possible explanation is that it is due to a flaking event or “UFO” caused by a small solid piece of titanium entering the plasma [9]. The other major effect that has been investigated is
TIME
Fig. 1.Titanium
(ms)
impurity flux on a collector at 0.27 m radius in the DITE tokamak. Collector facing ion drift direction. Two discharges superimposed.
G.M. McCracken
Impuritv
et al.
investigations
in the DITE
tokamak
161
surements of the deuterium flux to the probe shows this to rise during injection by a factor of 2 to 5, fig. 2. Since the neutral injection is hydrogen the increased deuterium flux appears to result from a decreased confinement time. At the end of this discharge the plasma disrupts due to reaching the density limit. It is interesting to note that the decay in the deuteron flux to the probe is relatively slow, taking almost 50 ms. The effective time resolution of the collector due to the finite window size is 15 ms.
DISRUPTION
5
Conclusions 2 7 NW ‘E -
10
Much remains to be done to understand the observed impurity levels. The results obtained in the present investigation show large fluctuations both during discharges and from one discharge to the next. The radial density gradient of impurities in the boundary is not large. It is clear that the density and in particular dn/dt can have a strong effect on the impurity level. However there are apparently a number of other factors causing changes in impurity level with have not been well controlled in the present experiments. Possibilities include flaking from the walls, and changes in the level of the light impurities, oxygen and carbon, in the discharges.
20
5
x 3 c
2
t g
10 19
c 5
2 10 18
5
0
100 TIME
200
300
References
(ms)
Fig. 2. Titanium and deuterium flux on a collector at 0.255 m radius. Collector facing ion drift direction. Single discharge.
[I] G.M. McCracken
810 kW neutral at - 150 ms.
[2]
beam injection
at 60 ms. The plasma
disrupts
[3]
the effect of neutral beam injection. Often the impurity level increases when injection is turned on. However this is by no means always true and there appears to be a tendency for the increases in impurity level to be small, often negligible, for discharges with higher plasma current i.e. > 1.50 kA. This is consistent with the expected better confinement of the fast ions at larger plasma currents. For discharges with Ir, = 80- 120 kA an increase in titanium flux of 5-6 can be observed, fig. 2. The situation is again complicated by the normal practice of strong gas puffing with neutral injection. Mea-
[4]
[5]
[6] [7] [8] [9]
and P.E. Stott, Nucl. Fusion 19 (1979) 889. G.M. McCracken, G. Dearnaley, R.D. Gill, J. Hugill, J.W.M. Paul, B.A. Powell, P.E. Scott, J.F. Turner and J. Vince, J. Nucl. Mater. 76/77 (1979) 431. G. Staudenmaier, P. Staib and W. Poschenrieder, J. Nucl. Mater. 93/94 (1980) 121. C.J. Sofield, G.M. McCracken, L.B. Bridwell, J. Shea, E.S. Hotston and S.K. Erents, Nucl. Instrum. Methods 191 (1981) 383. W.R. Wampler, S.T. Picraux, S.A. Cohen, H.F. Dylla, G.M. McCracken, S.M. Rossnagel, and C.W. Magee, J. Nucl. Mater. 85/86 (1979) 983. G. Proudfoot and P.J. Harbour, these proceedings. W.H.M. Clark, J.G. Cordey, S.J. Fielding, R.D. Gill et al., Nucl. Fusion 22 (1982) 333. P.C. Srangeby, G.M. McCracken and J.E. Vince, these proceedings. D.H.J. Goodall, these proceedings.
Journal of Nuclear Materials 111& 112 (1982) 159-161 North-Holland Publishing Company
IMPURITY INVESTIGATIONS TOKAMAK G.M. MCCRACKEN, Culham Laboratory,
IN THE BOUNDARY
J.W. PARTRIDGE,
Abingdon,
LAYER OF THE DITE
S.K. ERENTS
OX14 3DB, UK (Euraiom /UKAEA
Associatroni
and C.J. SOFIELD, AERE
SM. FERGUSON
Harwell, Oxon, UK
1. Introduction The mechanisms responsible for the production of impurities and the behaviour of the impurities in the boundary layers of tokamaks are still not well understood. Previous investigations have shown that a valuable technique for studying impurity fluxes is the use of collecting surfaces followed by surface analysis [l]. In the present study rotating carbon discs and cylinders have been used as collectors. After exposure the samples have been transferred in atmosphere to a chamber for surface analysis. Analytical techniques used have included Auger spectroscopy, Rutherford Backscattering, 2H( 3He, p) 4He nuclear reaction analysis and ion induced X-ray spectroscopy. Collectors have been exposed to a wide range of discharges in the DITE tokamak both with and without the bundle divertor and with and without neutral beam injection up to 1 MW.
2. Limiter analysis The DITE tokamak,was used with two pairs of fixed poloidal titanium limiters (r = 0.26 m) during this period. Each half limiter subtended an angle of 160”. In addition to the fixed limiters there were a pair of adjustable limiters made of ATJ graphite. These limiters could be moved in to a radius of 0.18 m or withdrawn behind the fixed limiters. A detailed analysis of the surface of both the carbon and titanium limiters was undertaken after exposure to - 100 discharges, in order to assess the impurities likely to be present. The impurity concentrations were remarkably consistent over the surface of both types of limiter probably indicating that
0022-3 115/82/0000-0000/$02.75
0 1982 North-Holland
an equilibrium had been reached. The concentration of titanium on the carbon limiter was 1.5 ? 0.5 X 102’ atoms/m2 and the relative concentration of other impurities in atoms/m2 Ti 100
Fe 20-30
Cu 5
MO 0.6
Pb 0.02
The concentration of the other impurities on the titanium limiter was the same within a factor 2. In all cases these levels were at least a factor of 10 greater than the background level on clean carbon. Other impurities measured using ion induced X-rays included S, Cl, K, Ca, Cr and Ni, all at about the 1% level. Compared with earlier measurements the titanium is now larger than iron; the molybdenum from the limiters used previously has dropped to an insignificant level. An important point however is the presence of iron on the limiters. This probably results from arcing and disruptions in unstable discharges producing impurities from the wall which are then ionised in the plasma and deposited on the limiters. Thus the presence of iron in stable non-disruptive plasmas does not necessarily imply impurities coming from the wall.
3. Time-resolved measurements Measurements of the time-resolved impurity fluxes have been made over a range of operating conditions with currents from 60 kA to 170 kA, densities from iie = 5 X lOI mm3 to 6 X lOI me3 with neutral beam injection up to 1 MW and at radii from 0.25 to 0.27m. The geometry of the collector discs was as used earlier [2] and the collectors were connected to torus potential.
160
G.M. McCracken et al. / Impurity inoestigatrons rn the DITE tokamak
The incident deuterium flux was measured on the same collectors for the same discharges in a number of cases. The measurements were made over a period of a years operation and over forty separate time-resolved measurements have been made. In most cases these measurements were made at one radius through a 2 mm square aperture. Corrections to obtain the incident flux have to be made for the transmission through such an aperture (3,4]. Measurements of the radial d~st~bution have been made in a few cases using a cylindrical collector with a slit, similar to that used in PLT [S]. A series of measurements have also been made of the ion Larmor radii using a fixed collector and subsequently analysing the spatial distribution behind the collimating hoIe [4]. These measurements indicated a deuterium ion energy of SO-80 eV and an average charge state of 4 for iron and titanium ions if it is assumed that they are in thermal equilibrium with the plasma. In the present discussion we will concentrate on the time-resolved measurements of the impurity flux. Most of these measurements have been made with the eollector probe at the same radius as the fixed titanium limiters (r = 0.26 m) and measurements are concentrated on the titanium impurities. However a significant amount of iron was also present typically at a level of 30% of the titanium flux. Measurable amounts of copper, molybdenum and lead were also occasionally present in similar relative abundance to that observed on the limiter. These quantities were normally small, near the limit of detection, but could be unequivocally identified using time integrated exposures. Apart from the iron their contribution to radiation and to z,rr was generally insignificant. Looking at the time-resolved titanium fIux a rather large variation has been observed. The collected flux varies typically from 10’8-1020 ions/m2. s-‘. If we consider that the ions travel at 0.3 C, where C, is the ion sound speed and that the electron temperature is typically 20 eV [6] then these fluxes correspond to local impurity concentration of 5 X lOI m-s to 5 X lOi mp3. Unfortunately there are few spectroscopic measurements which can be directly compared with these measurements but some spatially resolved measurements have shown titanium concentrations as high as IO” rnp3 in the centre of the plasma [7]. Within the large variations from discharge to discharge it is difficult to see any definite trend with increasing plasma current. The average value of the titanium flux is - 510 X 101s/m2+s-‘. The variation in flux with radial position is similarly difficult to determine within the reproducibility of the data. However there could be a variation of up to a factor of 4 between the value at a
radial position of 0.27 m compared with 0.25 m. One of the factors making comparisons difficult is that the discharges at high currents typically have larger densities. It is well known that gas puffing tends to reduce impurity levels. Direct evidence of a reduction in the heat flux and the edge temperature have been also observed [S]. However low rates of gas puffing are not effective in reducing impurities. An approximate criterion that dn,/dr must be greater than 2 X 102’/cm -‘, SC’, in order to cause a significant reduction in the edge impurity concentration, has been established. An example of impurity fluxes in a discharge with high gas puffing is shown in fig. 1. After a high flux at the start of the discharge the impurity level drops rapidly as has been observed previously f2]. However in this case the impurity level increases by an order of magnitude in an unexplained way during the discharge. A number of examples of this effect have been seen. A possible explanation is that it is due to a flaking event or “UFO” caused by a small solid piece of titanium entering the plasma [9]. The other major effect that has been investigated is
TIME
Fig. 1.Titanium
(ms)
impurity flux on a collector at 0.27 m radius in the DITE tokamak. Collector facing ion drift direction. Two discharges superimposed.
G.M. McCracken
Impuritv
et al.
investigations
in the DITE
tokamak
161
surements of the deuterium flux to the probe shows this to rise during injection by a factor of 2 to 5, fig. 2. Since the neutral injection is hydrogen the increased deuterium flux appears to result from a decreased confinement time. At the end of this discharge the plasma disrupts due to reaching the density limit. It is interesting to note that the decay in the deuteron flux to the probe is relatively slow, taking almost 50 ms. The effective time resolution of the collector due to the finite window size is 15 ms.
DISRUPTION
5
Conclusions 2 7 NW ‘E -
10
Much remains to be done to understand the observed impurity levels. The results obtained in the present investigation show large fluctuations both during discharges and from one discharge to the next. The radial density gradient of impurities in the boundary is not large. It is clear that the density and in particular dn/dt can have a strong effect on the impurity level. However there are apparently a number of other factors causing changes in impurity level with have not been well controlled in the present experiments. Possibilities include flaking from the walls, and changes in the level of the light impurities, oxygen and carbon, in the discharges.
20
5
x 3 c
2
t g
10 19
c 5
2 10 18
5
0
100 TIME
200
300
References
(ms)
Fig. 2. Titanium and deuterium flux on a collector at 0.255 m radius. Collector facing ion drift direction. Single discharge.
[I] G.M. McCracken
810 kW neutral at - 150 ms.
[2]
beam injection
at 60 ms. The plasma
disrupts
[3]
the effect of neutral beam injection. Often the impurity level increases when injection is turned on. However this is by no means always true and there appears to be a tendency for the increases in impurity level to be small, often negligible, for discharges with higher plasma current i.e. > 1.50 kA. This is consistent with the expected better confinement of the fast ions at larger plasma currents. For discharges with Ir, = 80- 120 kA an increase in titanium flux of 5-6 can be observed, fig. 2. The situation is again complicated by the normal practice of strong gas puffing with neutral injection. Mea-
[4]
[5]
[6] [7] [8] [9]
and P.E. Stott, Nucl. Fusion 19 (1979) 889. G.M. McCracken, G. Dearnaley, R.D. Gill, J. Hugill, J.W.M. Paul, B.A. Powell, P.E. Scott, J.F. Turner and J. Vince, J. Nucl. Mater. 76/77 (1979) 431. G. Staudenmaier, P. Staib and W. Poschenrieder, J. Nucl. Mater. 93/94 (1980) 121. C.J. Sofield, G.M. McCracken, L.B. Bridwell, J. Shea, E.S. Hotston and S.K. Erents, Nucl. Instrum. Methods 191 (1981) 383. W.R. Wampler, S.T. Picraux, S.A. Cohen, H.F. Dylla, G.M. McCracken, S.M. Rossnagel, and C.W. Magee, J. Nucl. Mater. 85/86 (1979) 983. G. Proudfoot and P.J. Harbour, these proceedings. W.H.M. Clark, J.G. Cordey, S.J. Fielding, R.D. Gill et al., Nucl. Fusion 22 (1982) 333. P.C. Srangeby, G.M. McCracken and J.E. Vince, these proceedings. D.H.J. Goodall, these proceedings.