- Email: [email protected]
Status of impurity studies in doublet III
STATUS OF IMPURITY STUDIES IN DOUBLET III* DOUBLET
III GROUPSt
General Atomic Company and Japan Atomic Energy Research Institute. P.O. Box 81608. San Diego. California 92138. USA
Recent impurity results obtained in Doublet III (R = 1.43 m, a = 0.45 m) are summarized. Topin discussed include wall compositon measurements, the effects of plasma edge temperature on impurity influx, and the role of MHD activity in determining whether or not impurities concentrate near the plasma axis. Macroscopically similar discharges with and without central impurity peaking are described and compared. Soft X-ray data implies that the peaked discharges have a Z,r profile that is peaked on axis, a hollow current density profile, and a double valued q-profile until the sudden onset of a large amplitude internal MHD mode reverses the impurity peaking.
1. Experimental parameters and surface analysis results Doublet III is a large tokamak device in which noncircular (doublet, droplet, and ellipse) configurations are studied [l]. A schematic of the machine is shown in fig. 1. The vacuum vessel and limiters are Inconel. The primary limiters have a double curvature and the secondary limiters are short segments encircling the plasma poloidally. The primary limiters are mounted with their front surface about 5-6cm from the vessel wall. The secondary limiters are attached directly to the vessel wall with front surfaces 3 cm from the wall. Extensive low power discharge cleaning combined with precise plasma position control and gas puffing has resulted in clean plasmas with Z,+, near unity. The phenomena described in the following sections were observed in all plasma configurations. For the sake of simplicity we discuss results for slightly elliptical hydrogen plasmas with elongation -1.2, B, = 20 kG, and I,--6ookA. Observations during spectrographic mode operation of the vacuum ultraviolet (VUV) indicate that oxygen is the spectrometer dominant low 2 impurity and nickel (the prin-
cipal constituent of Inconel) the dominant medium 2 impurity. Chromium lines are also identified. The carbon level is roughly one order of magnitude smaller than oxygen. No high 2 impurity radiation is observed, The VUV system has not yet been absolutely calibrated for intensity measurements. A sinlilar impurity situation is inferred from surface composition data obtained with Auger electron spectroscopy. Surface measurements were made in February 1979 with equipment
DIAG. PORTS /GAS PUFF)
FIELO SHAPING .. :.. ‘. ‘,.,‘.: MICROWAVE INTERFEROMETER Zwce RADIOMETER SXR ARRAY OETECTOR SXR SPECTROMETER VUV SPECTROSCOPY SURFACE STATION CONEL)
“’4,,$ -‘ .‘& _,of)f’ J... x.,#2.$2c ‘.I ;;. iii! DOUBLET *=‘-;
>.,r.. _ . ....’.. ..<_.: ..,:ii
w ELLIPSE
* Research carried out under the fusion research cooperation agreement on Doublet III between the United States Department of Energy and the Japan Atomic Energy Research Institute. Work supported by Department of Energy Contract no. DE-ATO3-76ET51011. t See for List of authors section 5.
THOMSON
SCATTERING
Fig. 1. Schematic of the Doublet III device and plasma diagnostics. Different plasma configurations, doublet, droplet, and ellipse are formed by control of the field shaping coil flux.
Journal of Nuclear Materials 93 & 94 (1980) 259-266 @ North-Holland Publishing Company
259
260
Doublet III Groups / Impwity studies in Doublet III
Table 1 Surface compositions Element
0
C N Ni Cr Fe MO Al Ti
After 15 h discharge cleaning
After plasma shots 8842-8892
Feb. 1979
Feb. 1980
20 Feb. 1980
6.9% 23.3 2.1 47.0 12.9 2.7 1.2 2.5 0.4
9.0% 23.0 <0.5”’ 42.0 13.0 4.0 11.0 0.0
9.0% 23.0
“Quantitative analysis for N and Ti is ambiguous due to overlapping of the respective Auger peaks. T’he numbers given are upper bounds assuming that the peak at -385 eV is due to N or only the Ti.
supplied by Oak Ridge National Laboratory [2] and more recently with a General Atomic surface station. The ORNL measurements were made during a period in which Doublet III was operated with tantalum-tungsten primary and molybdenum secondary limiters, which were later replaced with Inconel in August 1979. The surface composition of Inconel samples exposed to discharge cleaning and tokamak plasmas is shown in table 1. Metallic constituents are similar to Inconel in all cases. High-Z metals (Ia or W) were not observed. Low-Z impurities were 7-9% oxygen and 23% carbon, all in carbide form. 2. Effect of plasma edge temperature and density of impurity i&lx The role of plasma edge temperature in determining the influx of metallic impurities is well documented [3,4]. The effect is clearly visible in Doublet III in an experiment with a delayed current ramp. The experiment consists of establishing a current flat-top at a relatively low level, and then ramping the current to a higher flat-top level. The current levels and ramping rate are chosen to avoid disruptions and their ensuing impurity influx. Typical results of
ramps with and without a simultaneous increase in gas puffing rate are shown in fig. 2. The influx of nickel into the plasma is monitored by observing the Ni XI line at 148.4 A. The intensity of this line is proportional to the absolute magnitude of the influx. As shown in fig. 2, with constant gas puffing, the influx increases with the second current ramp. With higher puffing the in&ix during the ramp is suppressed. In either case the OV 192.9 8, signal is similar, indicating an unchanged oxygen influx. The cyclotron harmonic radiometer data indicates that the edge temperature (measured at r/a = 0.75) increases during the current ramp with constant gas, and is suppressed with additional puffing. The correlation of higher edge temperature and enhanced metallic influx is clearly illustrated. 3. Dynamic behavior of impurities (type 0 and types-rs4 The radial distribution of impurities in tokamak plasmas can significantly affect discharge characteristics, especially if axial peaking occurs. In a number of small devices a flat distribution of impurities over the plasma cross section has been measured, while in some others peaking toward the center has been observed [5-81. In Doublet III there is evidence that in a certain type of discharge both low 2 and metallic impurities concentrate strongly toward the plasma center, producing a hollow current density profile and a double-valued safety factor profile. Such discharges exhibit a large-scale MHD oscillation in the core of the plasma which is observed to flatten the impurity profiles and restore a centrally peaked current profile. The accumulation and profile changes are accompanied by a distinctive soft X-ray enhancement signature, but other directly observable aspects of the discharge such as the loop voltage and temperature profiles differ only slightly from those of a normal discharge in which the impurity accumulation does not occur. Examples of the two types of possible discharges are shown in fig. 3. The discharges are obtained with identical control parameters, but
Doublet III Groups / Impurity studies in Doublet 111 fa)WITH CONST.GASPUFF
(b)WlTHAOOlTlGNALGASPUFF SHOT8887
SHOT0888
~,_____,,_ ____ I-l ‘L
-e-e
I
261
I,
\._____ _ ____--_ 0
0.2
0.4
0.6
0.8
1
0.2
0
(SEC)
0.4
0.6
0.8
(SEC) 1
,BEFOAE$,RAMP 0.8
0.6
2 Y :
0.4
0.2 0
0.2
0.4 (SEC)
0.6
0.8
1 0 1.4
1.6
1.8
R (ml Fig. 2. Plasma current ramp with constant gas pufling (a) and with additional gas puffing (b). The edge temperature is measured at r/a = 3/4. Electron temperature profiles before (0.25 s) and after (0.44 s) the current ramp with and without additional gas puffing are show’n.
NiXXl
CH. 9 (r = 18 cm)
SXR CH. 3(r = 0 em) 0
N
P
0,
ii e (x 10'3 chr31 PUFF 0
0 in
Ip (MA)
t
V(v)
-0
Doublet III Groups 1 Impurity studies in Doublet III
the soft X-ray (SXR) behavior is completely different. The first type (type 0) has an increasing central SXR emission until about 0.45 s followed by a rapid decrease thereafter with large scale MHD Oscillations decaying into normal sawtooth activity. The other (type S) is characterized by Sawtooth activity throughout most of the discharge. The loop voltage, plasma current and line average plasma density are otherwise nearly identical. A 16 channel SXR detector array views the plasma tangentially. The Ni XXI line, which comes from the central portion of plasma, shows a temporal evolution very similar to the central SXR signal for either type of discharge. The Ni XI line, which is proportional to the influx of nickel from the plasma edge, has a slightly larger signal in the first 0.15 s for type 0 discharges than for type S discharges. Little difference is seen in the 0 VI signal between the two types. Bolometric measurements of radiated power (fig. 4) show higher radiation from the center for type 0 discharges than for type S. The central radiated power is well correlated with the central SXR and Ni XXI signals. The power radiated from the intermediate and boundary regions is only slightly different. Total radiated power increases to about 70% of ohmic input toward the end of the discharge for both types. The other principal difference between the two types of discharges is in the electron temperature evolution. Type S discharges have a higher initial central temperature and a more peaked profile than type 0. After the MHD oscillation the profiles are essentially identical for both types. Estimates of the impurity and .& profile derived from the soft X-ray data provide an explanation of the phenomenon observed in type 0 discharges. A complete discussion of the analysis is too lengthy to be included here and will be published elsewhere [lo]. A summary of the analysis follows. We apply the method of Von Goeler et al. [6]. The VUV data shows that oxygen and nickel are the principal impurities. The maximum central concentrated power and coronal equilibrium [9] is 0.05%. The calculated contribution of this
263
amount of nickel to the SXR enhancement factor 4’ is less than 5, a value small compared to the observed central enhancement of 45. Accordingly, in using the SXR data to calculate Zea we assume oxygen is the only impurity. The electron density profile is assumed to be nominally parabolic. However, in cases with high axial 5, the required fractional oxygen density exceeds the limit of 0.125 imposed by charge neutrality. In such cases we add additional central electron density to maintain n&z, 5 0.125 everywhere. The perturbed profile is renormalized to match the measured line-average. Once the profiles are specified, it is straightforward to calculate Z,,+ The current density 0’) and safety factor (4) profiles are then determined, using Spitzer resistivity without trapped particle corrections. The electric field is assumed to be uniform throughout the plasma. Results of the analysis for a type 0 discharge before and after the oscillation (t = 0.44 and 0.75 s, respectively) are shown in fig. 5. Before the oscillation both the relative oxygen density and Z,, profiles are strongly peaked on axis. The peaking of Zeff results in a hollow i(r) profile and a double-valued q(r) profile with a minimum just above unity at t = 8-10cm. After the oscillation the relative oxygen density and Zefi profiles are flat, i(r) becomes centrally peaked and q(r) is single valued, with q = 1 at t = 8 cm. The analysis suggests the following model: depending on the initial conditions in the startup phase the discharge can evolve in either one of two ways. If the initial impurity content is low and the influx of impurities due to minor disruptions is low, then the central temperature peaks early, sawteeth develop, and impurities do not accumulate. However, if the initial impurity content is higher, or if the initial phase is more disruptive, the central temperature is lower, sawteeth fail to develop, impurities accumulate, and a hollow current profile develops. The condition persists until q = 1 is reached at some intermediate radius, resulting in an intense MHD oscillation, probably a double tearing mode, that reverses the accumulation of impurities and allows the discharge to re-establish normal profiles and sawtooth activity.
264
Doublet III Groups / Impurity studies in Doublet III
1.20
TYPE 0-
8858
TYPES ---
8846
r
a
,/
0.8 5 2 F 0.4
: 0 1.0
b)
(TOTAL)
0.8 z E
1
06 *
9
5
0.8
04 * 0.2 5: y"
0
0.6
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g z OF?
0
"r 2
0.8 0.1
< OF
F
0.6
s
0
r-" 0.4 s y s
0.1
z 0
0
0.2
0.4 (SEC)
0.6
0.8
1.4
1.8
1.8
R (m)
Fig. 4. Electron temperature and radiation power data for type 0 and type S discharges: (a) temporal evolution of peak electron temperature was measured by cyclotron harmonic radiometer and by the soft X-ray spectrometer, (b) total radiation power monitored by the 5 channel bolometer, (c)-(e) radiation power densities from three regions calculated by Abel inversion, (e(h) electron temperature profiles at three different times.
Doublet III Groups I Impurity studies in Doublet III SHOT8858
SHOT8858
8)
1
@ OMSEC CH.
265
5
@ 0.44SEC
2
80 80 40
8
20
4
0
0
5 60
0.8 -
8 i -ii
6 0.75SEC
a
8
P
P z oz H
0.8 -
60
6
12
0.4 -
40
4
8
0.2 -
4
20
-5
0 0
I
I
10
20
0 0
10
20
r(cm) r km) Fig. 5. Analysis of SXR enhancement factors at 0.4 s and at 0.75 s for a type 0 discharge: (a) SXR signal levels and enhancement factor, (b) calculated profiles of impurity densities, Zd, current density j(r) and safety factor q.
The mode of the MHD oscillation has not yet been identified, but it is likely to be ~tl = 1, II = 1 at the q = 1 surface. The mechanism by which the MHD activity destroys the impurity confinement is also not yet understood. The 5 analysis obviously incorporates a number of modeling and physics simplifications, so some of the quantitative results (e.g. the exact impurity concentrations and profiles) should not be taken too literally, particularly for I > 15 cm. However, the centrally peaked nature of the impurity and 2.~ profiles before the MHD oscillation and their flattening after the oscillation is quite insensitive to modeling assumptions. In addition, the current density and q profiles inferred from the soft X-ray analysis are consistent with the observed location of the type 0 MHD oscillation. Type S discharges are obviously more desirable for normal plasma operation. Type S plasmas are more frequently obtained with slower current
rise, strong gas puffing and precise plasma position control. Type 0 plasmas are more common with low density, faster current rise and poor initial phase control. 4. Summary The principal impurities in Doublet III plasmas are oxygen and nickel. The influx of nickel is increased by higher plasma edge temperature and suppressed by gas puffing and higher plasma density. Discharges with either of two types of impurity behavior are observed. In the normal type discharge (type S) the initial central temperature is higher, sawtooth oscillations develop early and no accumulation of impurities is observed. In the second type of discharge (type 0), the initial temperature is lower, the initial influx of impurities is higher and sawteeth do not develop. Self-consistent calculations suggest that low-2 and metallic impurities accumulate on
266
Doublet III Groups 1 Impurity studies in Doublet II1
axis, leading to a peaked Z,* profile, a hollow current density profile. The impurity peaking is eliminated by the onset of an internal MHD oscillation that restores the discharge to type S behavior. 5. List of authors Doublet III Group, General Atomic Company T.R. Angel, C.J. Armentrout, F.P. Blau, G. Bramson, R.P. Chase, R.W. Callis, J.C. DeBoo, J.S. deGrassie, E. Fairbanks, S. Ejima, R.L. Freeman, R.J. Groebner, C.L. Hsieh, G.L. Jackson, T.H. Jensen, J. Lohr, M.A. Mahdavi, F.B. Marcus, N. Ozakit, P.I. Petersen, R. Seraydarian, A.M. Sleeper,
D.R. Baker, N.H. Brooks, R.E. Clausing*, L.C. Emerson*, R.K. Fisher, L. Heatherly*, G.L. Jahns, J.L. Luxon, C.H. Meyer, T.W. Petrie, J.N. Smith, Jr.,
* Permanent address: Oak Ridge National Laboratories, Oak Ridge, TN, USA. t Permanent address: Energy Research Laboratory, Hitachi Ltd., Hitachi, Japan.
R. Snider, R.D. Stambaugh, T. Tamano, T.S. Taylor, J.C. Wesley, S.S. Wojtowicz. Doublet III Group, Research Institute
Japan
Atomic
Energy
N. Fujisawa, A. Kitsunezaki, S. Konoshima, M. Nagami, S. Seki, M. Shimada, H. Toyama, H. Yokomizo. References [l] T. Ohkawa, 9th European Conf. on Controlled Fusion and Plasma Physics, Oxford, UK, 1979. [2] Doublet III group, to be published. [3] K. Bol et al., in: Proc. 7th Intern. ConI. on Plasma Physics and Controlled Nuclear Fusion Research, Innsbruck, 1978, IAEA-CN-37-Al. [4] R.S. Granetz et al., Nucl. Fusion 19 (1979) 1587. [5] V.A. Vershkov and S.V. Mimov, Nucl. Fusion 14 (1974) 383. [6] S. von Goeler et al., Nucl. Fusion 15 (1975) 301. [7] W. Englehardt et al., in: Proc. 7th Intern. ConI. on Plasma Physics and Controlled Nuclear Fusion Research, Innsbruck, 1978, IAEA-CN-37-A-5. [8] W. Engelhardt et al., 9th European Cord. on Controlled Fusion and Plasma Physics, OxIord, UK, 1979. [9] D.E. Post et al., Princeton Plasma Phys. Lab. Report PPPL-1352 (1977). [lo] Doublet III group, to be published.
III GROUPSt
General Atomic Company and Japan Atomic Energy Research Institute. P.O. Box 81608. San Diego. California 92138. USA
Recent impurity results obtained in Doublet III (R = 1.43 m, a = 0.45 m) are summarized. Topin discussed include wall compositon measurements, the effects of plasma edge temperature on impurity influx, and the role of MHD activity in determining whether or not impurities concentrate near the plasma axis. Macroscopically similar discharges with and without central impurity peaking are described and compared. Soft X-ray data implies that the peaked discharges have a Z,r profile that is peaked on axis, a hollow current density profile, and a double valued q-profile until the sudden onset of a large amplitude internal MHD mode reverses the impurity peaking.
1. Experimental parameters and surface analysis results Doublet III is a large tokamak device in which noncircular (doublet, droplet, and ellipse) configurations are studied [l]. A schematic of the machine is shown in fig. 1. The vacuum vessel and limiters are Inconel. The primary limiters have a double curvature and the secondary limiters are short segments encircling the plasma poloidally. The primary limiters are mounted with their front surface about 5-6cm from the vessel wall. The secondary limiters are attached directly to the vessel wall with front surfaces 3 cm from the wall. Extensive low power discharge cleaning combined with precise plasma position control and gas puffing has resulted in clean plasmas with Z,+, near unity. The phenomena described in the following sections were observed in all plasma configurations. For the sake of simplicity we discuss results for slightly elliptical hydrogen plasmas with elongation -1.2, B, = 20 kG, and I,--6ookA. Observations during spectrographic mode operation of the vacuum ultraviolet (VUV) indicate that oxygen is the spectrometer dominant low 2 impurity and nickel (the prin-
cipal constituent of Inconel) the dominant medium 2 impurity. Chromium lines are also identified. The carbon level is roughly one order of magnitude smaller than oxygen. No high 2 impurity radiation is observed, The VUV system has not yet been absolutely calibrated for intensity measurements. A sinlilar impurity situation is inferred from surface composition data obtained with Auger electron spectroscopy. Surface measurements were made in February 1979 with equipment
DIAG. PORTS /GAS PUFF)
FIELO SHAPING .. :.. ‘. ‘,.,‘.: MICROWAVE INTERFEROMETER Zwce RADIOMETER SXR ARRAY OETECTOR SXR SPECTROMETER VUV SPECTROSCOPY SURFACE STATION CONEL)
“’4,,$ -‘ .‘& _,of)f’ J... x.,#2.$2c ‘.I ;;. iii! DOUBLET *=‘-;
>.,r.. _ . ....’.. ..<_.: ..,:ii
w ELLIPSE
* Research carried out under the fusion research cooperation agreement on Doublet III between the United States Department of Energy and the Japan Atomic Energy Research Institute. Work supported by Department of Energy Contract no. DE-ATO3-76ET51011. t See for List of authors section 5.
THOMSON
SCATTERING
Fig. 1. Schematic of the Doublet III device and plasma diagnostics. Different plasma configurations, doublet, droplet, and ellipse are formed by control of the field shaping coil flux.
Journal of Nuclear Materials 93 & 94 (1980) 259-266 @ North-Holland Publishing Company
259
260
Doublet III Groups / Impwity studies in Doublet III
Table 1 Surface compositions Element
0
C N Ni Cr Fe MO Al Ti
After 15 h discharge cleaning
After plasma shots 8842-8892
Feb. 1979
Feb. 1980
20 Feb. 1980
6.9% 23.3 2.1 47.0 12.9 2.7 1.2 2.5 0.4
9.0% 23.0 <0.5”’ 42.0 13.0 4.0 11.0 0.0
9.0% 23.0
“Quantitative analysis for N and Ti is ambiguous due to overlapping of the respective Auger peaks. T’he numbers given are upper bounds assuming that the peak at -385 eV is due to N or only the Ti.
supplied by Oak Ridge National Laboratory [2] and more recently with a General Atomic surface station. The ORNL measurements were made during a period in which Doublet III was operated with tantalum-tungsten primary and molybdenum secondary limiters, which were later replaced with Inconel in August 1979. The surface composition of Inconel samples exposed to discharge cleaning and tokamak plasmas is shown in table 1. Metallic constituents are similar to Inconel in all cases. High-Z metals (Ia or W) were not observed. Low-Z impurities were 7-9% oxygen and 23% carbon, all in carbide form. 2. Effect of plasma edge temperature and density of impurity i&lx The role of plasma edge temperature in determining the influx of metallic impurities is well documented [3,4]. The effect is clearly visible in Doublet III in an experiment with a delayed current ramp. The experiment consists of establishing a current flat-top at a relatively low level, and then ramping the current to a higher flat-top level. The current levels and ramping rate are chosen to avoid disruptions and their ensuing impurity influx. Typical results of
ramps with and without a simultaneous increase in gas puffing rate are shown in fig. 2. The influx of nickel into the plasma is monitored by observing the Ni XI line at 148.4 A. The intensity of this line is proportional to the absolute magnitude of the influx. As shown in fig. 2, with constant gas puffing, the influx increases with the second current ramp. With higher puffing the in&ix during the ramp is suppressed. In either case the OV 192.9 8, signal is similar, indicating an unchanged oxygen influx. The cyclotron harmonic radiometer data indicates that the edge temperature (measured at r/a = 0.75) increases during the current ramp with constant gas, and is suppressed with additional puffing. The correlation of higher edge temperature and enhanced metallic influx is clearly illustrated. 3. Dynamic behavior of impurities (type 0 and types-rs4 The radial distribution of impurities in tokamak plasmas can significantly affect discharge characteristics, especially if axial peaking occurs. In a number of small devices a flat distribution of impurities over the plasma cross section has been measured, while in some others peaking toward the center has been observed [5-81. In Doublet III there is evidence that in a certain type of discharge both low 2 and metallic impurities concentrate strongly toward the plasma center, producing a hollow current density profile and a double-valued safety factor profile. Such discharges exhibit a large-scale MHD oscillation in the core of the plasma which is observed to flatten the impurity profiles and restore a centrally peaked current profile. The accumulation and profile changes are accompanied by a distinctive soft X-ray enhancement signature, but other directly observable aspects of the discharge such as the loop voltage and temperature profiles differ only slightly from those of a normal discharge in which the impurity accumulation does not occur. Examples of the two types of possible discharges are shown in fig. 3. The discharges are obtained with identical control parameters, but
Doublet III Groups / Impurity studies in Doublet 111 fa)WITH CONST.GASPUFF
(b)WlTHAOOlTlGNALGASPUFF SHOT8887
SHOT0888
~,_____,,_ ____ I-l ‘L
-e-e
I
261
I,
\._____ _ ____--_ 0
0.2
0.4
0.6
0.8
1
0.2
0
(SEC)
0.4
0.6
0.8
(SEC) 1
,BEFOAE$,RAMP 0.8
0.6
2 Y :
0.4
0.2 0
0.2
0.4 (SEC)
0.6
0.8
1 0 1.4
1.6
1.8
R (ml Fig. 2. Plasma current ramp with constant gas pufling (a) and with additional gas puffing (b). The edge temperature is measured at r/a = 3/4. Electron temperature profiles before (0.25 s) and after (0.44 s) the current ramp with and without additional gas puffing are show’n.
NiXXl
CH. 9 (r = 18 cm)
SXR CH. 3(r = 0 em) 0
N
P
0,
ii e (x 10'3 chr31 PUFF 0
0 in
Ip (MA)
t
V(v)
-0
Doublet III Groups 1 Impurity studies in Doublet III
the soft X-ray (SXR) behavior is completely different. The first type (type 0) has an increasing central SXR emission until about 0.45 s followed by a rapid decrease thereafter with large scale MHD Oscillations decaying into normal sawtooth activity. The other (type S) is characterized by Sawtooth activity throughout most of the discharge. The loop voltage, plasma current and line average plasma density are otherwise nearly identical. A 16 channel SXR detector array views the plasma tangentially. The Ni XXI line, which comes from the central portion of plasma, shows a temporal evolution very similar to the central SXR signal for either type of discharge. The Ni XI line, which is proportional to the influx of nickel from the plasma edge, has a slightly larger signal in the first 0.15 s for type 0 discharges than for type S discharges. Little difference is seen in the 0 VI signal between the two types. Bolometric measurements of radiated power (fig. 4) show higher radiation from the center for type 0 discharges than for type S. The central radiated power is well correlated with the central SXR and Ni XXI signals. The power radiated from the intermediate and boundary regions is only slightly different. Total radiated power increases to about 70% of ohmic input toward the end of the discharge for both types. The other principal difference between the two types of discharges is in the electron temperature evolution. Type S discharges have a higher initial central temperature and a more peaked profile than type 0. After the MHD oscillation the profiles are essentially identical for both types. Estimates of the impurity and .& profile derived from the soft X-ray data provide an explanation of the phenomenon observed in type 0 discharges. A complete discussion of the analysis is too lengthy to be included here and will be published elsewhere [lo]. A summary of the analysis follows. We apply the method of Von Goeler et al. [6]. The VUV data shows that oxygen and nickel are the principal impurities. The maximum central concentrated power and coronal equilibrium [9] is 0.05%. The calculated contribution of this
263
amount of nickel to the SXR enhancement factor 4’ is less than 5, a value small compared to the observed central enhancement of 45. Accordingly, in using the SXR data to calculate Zea we assume oxygen is the only impurity. The electron density profile is assumed to be nominally parabolic. However, in cases with high axial 5, the required fractional oxygen density exceeds the limit of 0.125 imposed by charge neutrality. In such cases we add additional central electron density to maintain n&z, 5 0.125 everywhere. The perturbed profile is renormalized to match the measured line-average. Once the profiles are specified, it is straightforward to calculate Z,,+ The current density 0’) and safety factor (4) profiles are then determined, using Spitzer resistivity without trapped particle corrections. The electric field is assumed to be uniform throughout the plasma. Results of the analysis for a type 0 discharge before and after the oscillation (t = 0.44 and 0.75 s, respectively) are shown in fig. 5. Before the oscillation both the relative oxygen density and Z,, profiles are strongly peaked on axis. The peaking of Zeff results in a hollow i(r) profile and a double-valued q(r) profile with a minimum just above unity at t = 8-10cm. After the oscillation the relative oxygen density and Zefi profiles are flat, i(r) becomes centrally peaked and q(r) is single valued, with q = 1 at t = 8 cm. The analysis suggests the following model: depending on the initial conditions in the startup phase the discharge can evolve in either one of two ways. If the initial impurity content is low and the influx of impurities due to minor disruptions is low, then the central temperature peaks early, sawteeth develop, and impurities do not accumulate. However, if the initial impurity content is higher, or if the initial phase is more disruptive, the central temperature is lower, sawteeth fail to develop, impurities accumulate, and a hollow current profile develops. The condition persists until q = 1 is reached at some intermediate radius, resulting in an intense MHD oscillation, probably a double tearing mode, that reverses the accumulation of impurities and allows the discharge to re-establish normal profiles and sawtooth activity.
264
Doublet III Groups / Impurity studies in Doublet III
1.20
TYPE 0-
8858
TYPES ---
8846
r
a
,/
0.8 5 2 F 0.4
: 0 1.0
b)
(TOTAL)
0.8 z E
1
06 *
9
5
0.8
04 * 0.2 5: y"
0
0.6
gt 0.4 (O
g z OF?
0
"r 2
0.8 0.1
< OF
F
0.6
s
0
r-" 0.4 s y s
0.1
z 0
0
0.2
0.4 (SEC)
0.6
0.8
1.4
1.8
1.8
R (m)
Fig. 4. Electron temperature and radiation power data for type 0 and type S discharges: (a) temporal evolution of peak electron temperature was measured by cyclotron harmonic radiometer and by the soft X-ray spectrometer, (b) total radiation power monitored by the 5 channel bolometer, (c)-(e) radiation power densities from three regions calculated by Abel inversion, (e(h) electron temperature profiles at three different times.
Doublet III Groups I Impurity studies in Doublet III SHOT8858
SHOT8858
8)
1
@ OMSEC CH.
265
5
@ 0.44SEC
2
80 80 40
8
20
4
0
0
5 60
0.8 -
8 i -ii
6 0.75SEC
a
8
P
P z oz H
0.8 -
60
6
12
0.4 -
40
4
8
0.2 -
4
20
-5
0 0
I
I
10
20
0 0
10
20
r(cm) r km) Fig. 5. Analysis of SXR enhancement factors at 0.4 s and at 0.75 s for a type 0 discharge: (a) SXR signal levels and enhancement factor, (b) calculated profiles of impurity densities, Zd, current density j(r) and safety factor q.
The mode of the MHD oscillation has not yet been identified, but it is likely to be ~tl = 1, II = 1 at the q = 1 surface. The mechanism by which the MHD activity destroys the impurity confinement is also not yet understood. The 5 analysis obviously incorporates a number of modeling and physics simplifications, so some of the quantitative results (e.g. the exact impurity concentrations and profiles) should not be taken too literally, particularly for I > 15 cm. However, the centrally peaked nature of the impurity and 2.~ profiles before the MHD oscillation and their flattening after the oscillation is quite insensitive to modeling assumptions. In addition, the current density and q profiles inferred from the soft X-ray analysis are consistent with the observed location of the type 0 MHD oscillation. Type S discharges are obviously more desirable for normal plasma operation. Type S plasmas are more frequently obtained with slower current
rise, strong gas puffing and precise plasma position control. Type 0 plasmas are more common with low density, faster current rise and poor initial phase control. 4. Summary The principal impurities in Doublet III plasmas are oxygen and nickel. The influx of nickel is increased by higher plasma edge temperature and suppressed by gas puffing and higher plasma density. Discharges with either of two types of impurity behavior are observed. In the normal type discharge (type S) the initial central temperature is higher, sawtooth oscillations develop early and no accumulation of impurities is observed. In the second type of discharge (type 0), the initial temperature is lower, the initial influx of impurities is higher and sawteeth do not develop. Self-consistent calculations suggest that low-2 and metallic impurities accumulate on
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Doublet III Groups 1 Impurity studies in Doublet II1
axis, leading to a peaked Z,* profile, a hollow current density profile. The impurity peaking is eliminated by the onset of an internal MHD oscillation that restores the discharge to type S behavior. 5. List of authors Doublet III Group, General Atomic Company T.R. Angel, C.J. Armentrout, F.P. Blau, G. Bramson, R.P. Chase, R.W. Callis, J.C. DeBoo, J.S. deGrassie, E. Fairbanks, S. Ejima, R.L. Freeman, R.J. Groebner, C.L. Hsieh, G.L. Jackson, T.H. Jensen, J. Lohr, M.A. Mahdavi, F.B. Marcus, N. Ozakit, P.I. Petersen, R. Seraydarian, A.M. Sleeper,
D.R. Baker, N.H. Brooks, R.E. Clausing*, L.C. Emerson*, R.K. Fisher, L. Heatherly*, G.L. Jahns, J.L. Luxon, C.H. Meyer, T.W. Petrie, J.N. Smith, Jr.,
* Permanent address: Oak Ridge National Laboratories, Oak Ridge, TN, USA. t Permanent address: Energy Research Laboratory, Hitachi Ltd., Hitachi, Japan.
R. Snider, R.D. Stambaugh, T. Tamano, T.S. Taylor, J.C. Wesley, S.S. Wojtowicz. Doublet III Group, Research Institute
Japan
Atomic
Energy
N. Fujisawa, A. Kitsunezaki, S. Konoshima, M. Nagami, S. Seki, M. Shimada, H. Toyama, H. Yokomizo. References [l] T. Ohkawa, 9th European Conf. on Controlled Fusion and Plasma Physics, Oxford, UK, 1979. [2] Doublet III group, to be published. [3] K. Bol et al., in: Proc. 7th Intern. ConI. on Plasma Physics and Controlled Nuclear Fusion Research, Innsbruck, 1978, IAEA-CN-37-Al. [4] R.S. Granetz et al., Nucl. Fusion 19 (1979) 1587. [5] V.A. Vershkov and S.V. Mimov, Nucl. Fusion 14 (1974) 383. [6] S. von Goeler et al., Nucl. Fusion 15 (1975) 301. [7] W. Englehardt et al., in: Proc. 7th Intern. ConI. on Plasma Physics and Controlled Nuclear Fusion Research, Innsbruck, 1978, IAEA-CN-37-A-5. [8] W. Engelhardt et al., 9th European Cord. on Controlled Fusion and Plasma Physics, OxIord, UK, 1979. [9] D.E. Post et al., Princeton Plasma Phys. Lab. Report PPPL-1352 (1977). [lo] Doublet III group, to be published.