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Energy distribution of deuterium atoms recycling on molybdenum limiter surface
ELSEVIER
Journal of Nuclear Materials 220-222 (1995) 1135-1139
Energy distribution of deuterium atoms recycling on molybdenum limiter surface Shigeyuki Sekine a, Yoichi Hirano a, Yasuyuki Yagi a, Toshio Shimada a, Yoshiki Maejima a, Hazime Shimizu a, Tetsuo Tanabe b a Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan b Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan
Abstract
The emission spectrum of deuterium atoms ( D , ) was measured at the surface of a movable molybdenum limiter in the reversed field pinch machine (RFP: TPE-1RM20). The intensity of the D~ line increased with the insertion depth of the limiter. The observed line profile was fitted by a combination of two Gaussian functions with different temperatures. The temperatures derived from the fitted functions were approximately 5000 K and 50 000 K. The temperatures were independent of the insertion depth. No Doppler shifted shoulder, which would correspond to neutralized reflected ions at the limiter surface, was observed in the present experiment.
1. Introduction
Low-Z materials (graphite, beryllium, etc.) are mainly used as plasma facing materials in currently operated large scale machines with the great progress in the plasma performance. However, there still remain several problems; e.g. high erosion, high tritium retention and poor thermal conductivity for graphite, and low melting point and high toxicity for beryllium, etc. Great effort is being devoted to solve these problems but it is not yet clear that the low-Z materials can be used as the plasma facing materials in a future nuclear fusion reactor. Therefore, it is worthwhile to consider again high-Z materials, i.e. molybdenum (Mo), tungsten (W), etc., as a candidate for the plasma facing materials [1]. They have several advantages, such as the high melting point, low erosion, low retention, high thermal conductivity, etc. Improvements of the plasma performance and the long pulse operation have been observed in several experiments on reversed field pinch (RFP) machines [2] and TRIAM-1M [3] with molybdenum limiters. In the RFP experiment on TPE-1RM15 ( R / a = 700/135 mm), two kinds of limiter materials, graphite (POCO DBP-3-2) and Mo were used [2]. TPE-1RM15 was operated in the range with the plasma current I o =
100-250 kA, electron density n e = (1-3)X 1019 m -3, central electron temperature Te = 300-1000 eV and discharge duration 10-13 ms. Large heating power density and high heat flux by hot electron flow along the magnetic field were characteristics of RFP (typically 5 M W / m 3 and 500 M W / m 2, respectively). In the case of the graphite limiter, the toroidal one turn loop voltage, Vioop, was relatively high and Te low. Moreover, as Ip increased over 175 kA, Vloop sharply increased, T~ decreased and the global confinement property was degraded. On the other hand, by using the Mo limiter, degradation did not occur up to Ip = 200 kA, the scaling factor of T~ on Ip was improved from 2.8 e V / k A for graphite to 4.7 e V / k A for Mo, and that Te went up to as high as almost 1000 eV. Compared with the graphite limiters, two advantages of the Mo limiter, smaller temperature rise of the limiter for the same heat load and smaller impurity release at a given surface temperature, were the cause for this improvement. These observations may support tlae usefulness of high-Z materials as the plasma facing materials in future machines; if a method can be found by which the inward flow and accumulation of high-Z impurities is largely suppressed. However, there are little systematic data bases of the interaction between plasmas and
0022-3115/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-3115(94)00489-7
1136
S. Sekine et aL /Journal of Nuclear Materials 220 222 (199.5) 1135-1139 (a) mt~h,~t4~nl , ~
(b)
E-side Fig. l. (a) Schematic of the experimental setup. D,, emission at the limiter surface was measured through an optical window at the opposite side of the chamber. OMA: optical multichannel analyzer. (b) Enlarged view around the limiter head. The hatched surface at the head had been melted by a flux of drifting electrons with X > 21 mm,
high-Z materials. In particular, study of the interaction u n d e r high h e a t load is necessary for such a data base. As for hydrogen recycling at the surface of low-Z materials, S a m m et al. m e a s u r e d the H~ profile on the surface of a carbonized stainless-steel limiter and observed a strong s h o u l d e r at the shorter wavelength of the H , line in T E X T O R [4]. R e i t e r et al. also observed the s h o u l d e r of the D~ line on the surface of graphite a n d stainless-steel limiters in T E X T O R [5]. They interp r e t e d that the s h o u l d e r was due to reflected atoms at the limiter surface. However, t h e r e has b e e n little c o r r e s p o n d i n g data for M o or W limiters, especially for high heat flux conditions the data base does not reach a sufficient level. In this paper, the profile of the D,~ line emission at the m o l y b d e n u m surface was measured u n d e r very high heat flux by hot electron flow in the R F P plasma in o r d e r to obtain the energy distribution of lhe recycling d e u t e r i u m atoms on m o l y b d e n u m surfaces.
using the feedthrough. T h e scale of the f e e d t h r o u g h ( X ) was used as a p a r a m e t e r of the insertion d e p t h hereafter. Fixed M o limiters were also a t t a c h e d inside the c h a m b e r as shown in Fig. la. They limited the plasma m i n o r radius to a = 192 mm. W h e n the scale of the f e e d t h r o u g h X = 10 ram, the surface of the movable limiter h e a d was at a = 192 mm. D~ emission on the limiter surface was m e a s u r e d t h r o u g h an optical window at the opposite side of the c h a m b e r . T h e spatial resolution at the limiter surface was e s t i m a t e d to b e approximately 3 m m Q. T h e monitoring area was d e t e r m i n e d by using a telescope and was fixed to the electron drift side (E-side) as shown in Fig. lb. T h e m o n i t o r i n g area was off-center of the r o u n d h e a d a n d the main part of the reflected light at the h e a d would not come into the detector. T h e D , l i n e - b r o a d e n i n g was analyzed by a 1 m double monochrometer (JASCO CT-1000D) with a gated image intensifier a n d a m u l t i c h a n n e l d e t e c t o r (OMA). T h e gate timing was set at the elapsed time of 3 - 4 ms from the start of the discharge pulse. Overall wavelength resolution ( F W H M ) was approximately 0.39 cnl
1.
T h e surface t e m p e r a t u r e of the E-side of the limiter during discharge was m e a s u r e d by a far-infrared temp e r a t u r e m o n i t o r whose time resolution was a b o u t 2 ms. It could m e a s u r e t e m p e r a t u r e s b e t w e e n 1000 a n d 3000°C.
3. Results and discussions Typical time evolutions of D,~ emission intensity, plasma c u r r e n t a n d M o atomic emission (379 n m ) are shown in Fig. 2. In this m e a s u r e m e n t , the limiter was not inserted into the plasma ( X = 0 mm). I n t e n s e peaks of the D~ trace at the b e g i n n i n g and e n d of the discharge are due to a strong p l a s m a - w a l l interaction
m
,i
"'0[ >, 2. Experimental arrangement Fig. la shows the experimental setup. In this measurement the plasma current was kept at approximately 130 kA. A movable 20 mm G columnar limiter with a round head was made of polycrystal Mo. The limiter was mounted on a linear-motion feedthrough and its head was inserted into the boundary region of the RFP plasma. The insertion depth was changed by
E
b)
~3 =2[
;L ....... o
5 Time /
lo
:i:,
ms
Fig. 2. Typical time evolutions of (a) D,~ emission intensity, (b) plasma current and Mo atomic emission (MoI 379 nm). In these measurements, the movable limiter was not inserted into the plasma.
S. Sekine et aL /Journal of Nuclear Materials 220-222 (1995) 1135-1139
3000
150
1300 position / mm
limiter
-~. 2 0 0 0
|
r- 100
1oo ~.
0 10 / ms Fig. 3. Time evolutions of the surface temperature at the limiter head for different movable limiter positions X and typical plasma current. The used pyrometer can measure temperatures only above 1000°C. 0
~
I
C
I
limiter position X= 20 mm
A~. J
r~
= 5600 K "
50
5
Time
at those times. During the fiat top phase, t = 2-10 ms, the D~ intensity gradually decreased. On the other hand, the emission of the Mo atomic line exhibited an almost constant intensity. Moreover, no significant increase of the Mo emission intensity was observed by insertion of the limiter up to 19 mm in the flat top phase. However, at the end of discharge (t > 10 ms), a fairly intense emission was detected, which increased with the insertion depth. When the limiter was inserted into the plasma, the E-side of the limiter head was irradiated by an intense electron beam [4] and the surface temperature increased with time and insertion depth as shown in Fig. 3. (Used temperature monitor can not measure temperatures below 1000°C.) A dent observed in the temperature trace at the end of the discharge for all X may be due to an electric noise at the plasma termination. We observed black-body radiation in the visible range (about 650 nm) from the limiter surface at 8-10 ms giving a background noise to the D~ line if the gate of the image intensifier was set at 8-9 ms, which caused a poor S/N ratio of the observed spectra. Another measurement of MoI line emission revealed that the emission intensity started to increase sharply (more than one order of magnitude) when the surface temperature of Mo approx, reached the melting point of Mo (2620°C). According to these observations the gate timing was set at 3 - 4 ms to observe the emission from recycling (reflected) D atoms at the limiter surface. The observed emission spectrum of the DQ line and the fitted curve at X = 20 mm are shown in Fig. 4 (the movable limiter was inserted into the plasma for 10 mm). This spectrum was obtained by an accumulation for five discharges. The spectrum is fitted by a combination of two Gaussian functions with the same center
I
~l t'~
~
0~
I
"o
200 3=
~ooo I
1137
I
0
15235
15237
I
15239
.~.Tm,~ 15241
Wavenumber / cm -1
Fig. 4. Observed emission spectrum of the D,, line at the movable limiter position, X = 20 ram. The spectrum was obtained by an accumulation for five discharge pulses. The spectrum is fitted by a combination of two Gaussian functions which are also shown.
and different temperature. These Gaussian components are also shown in Fig. 4. Temperatures of two Gaussian components of fitted curves are plotted in Fig. 5 as a function of 3(. Any dependence of the temperatures on X was not observed in the present S/N ratio of the spectrum. Moreover, no significant Doppler shift nor the shoulder was observed for all X values in the present experiment on TPE-1RM20. This result obtained on the Mo limiter is considerably different from that for a carbonized stainless-steel [4],
100000 plasma surface -
80000
60000 ~. 40000
° I~
Ti I
Thigh
)
p20000
outside plasma
inside plasma
-
T,ow
0
-5
~[
'
0
5
10
15
20
Limiterposition X /
mm
25
Fig. 5. Derived temperature of two Gaussian components of the fitted curve in various movable limiter positions X. Each plot was derived from the accumulated spectrum.
1138
S. Sekine et al. /Journal of Nuclear Materials 220-222 (1995) 1135-1139
graphite and stainless-steel limiters [5], where the strong shoulder due to reflected atoms at the limiter surface was observed in the spectrum. These facts might indicate that the reflection of D + ions at the Mo surface may not be significant or that the D , emission from the plasma may be too strong comparing to that from reflected atoms. Bogen ct al. measured Doppler broadening of the hydrogen L,~ line at the A S D E X - t o k a m a k with the laser-induced fluorescence method [7]. They derived a temperature of 6000 K in the 600 A, 50 Hz cleaning discharge. Muraoka and Maeda also measured the L,, profile in T E X T O R with a plasma current of 250 kA [8]. Thc derived temperature was approximately 8000 K. These temperatures were almost the same as the low-temperature component (3000-8000 K) of the present measurement. Muraoka and Maeda concluded that the velocity of the D atoms was determined from the dissociation process of D 2 or D + and not from the ion temperature of the edge plasma. The S / N ratio of the line profile observed in T E X T O R [8] might not be high enough for showing the high temperature component in the curve fitting. The observed high-temperature component (40000-70000 K) in lhc present experiment possibly corresponds to
atoms charge-exchanged with ions in the boundary region. This assignment was estimated with reference to the plasma parameters around the edge region, T~,V= 10 eV and new = 3 × 1018 m ~. The dependence of the D~ emission intensity on X is shown in Fig. 6a. Peak heights of two Gaussian components of the fitted curves arc plotted. Each data point was derived from an accumulated spectrum for the same X. Intensity increased slightly with insertion depth beyond the plasma surface. The intensity ratio of the two Gaussian components did not exhibit any variation in the present S / N ratio of the spectrum as shown in Fig. 6b. When the monitoring area was changed in the ion drift side, no increase of the emission was observed. If the reflection of ions on the Mo surface plays a significant role in D , emission, the D intensity of hot D atoms should increase with the insertion depth. O n e possible explanation to the observed behavior of the intensity ratio is that the reflection of a D atom on a Mo limiter surface may not be significant in the present conditions; the Mo limiter was irradiated by a high heat flux of hot electrons. The increase of the emission intensity may be due to an increase of thermally desorbing D 2 molecules. In addition, it should be noted that if most of the ions arc reflected on the Mo surface without neutralization the estimation of the recycling effect is not possible by the present method.
140
(a) 120 ~
4. Conclusions
plasma surface outside plasma
inside plasma
1oo
E
Doppler broadening of the D , emission line was measured on the surface of a movable molybdenum limiter in a R F P plasma. The line profile was fitted by a combination of two Gaussian components with low (3000-8000 K) and high (40000-70000 K) temperatures. The low temperature component probably corresponds to the neutral D atom temperature in the vicinity of a limiter due to dissociation of D 2 and the high-temperature component may be due to atoms charge-exchanged with the D + ions. Both temperatures were not influenced by insertion depth, and the intensity of the emission increased with insertion depth. However, the intensity ratio of the high temperature component to the low temperature component did not show any dependence on the insertion depth. Neutralized reflected D atoms on the Mo surface were not observed in the present experiment.
8o low temp.
•
>, 60 E
c
40 20 0
t
.4
i& 90 8O ---= 7O +
(b)
!
•
..
.
±i
!
•
~o 6o -5
i
i
0
5
10
Limiter position
I
1
15
20
25
X /mm
Fig. 6. (a) Variation of the intensity of the line emission in various movable limiter positions X. Peak values of two Gaussian components were plotted. Each plot was derived from the accumulated spectrum. (b) Intensity ratio of two Gaussian components•
References [1] T. Tanabe, N. Noda and H. Nakamura, J. Nucl. Mater• 196-198 (1992) 11. [2] Y. Hirano, Y. Yagi, T. Shimada et al., Plasma Physics and
S. Sekine et al. /Journal of Nuclear Materials 220-222 (1995) 1135-1139 Controlled Nuclear Fusion Research, 1990 (Proc. 13th Int. Conf. on Plasma Phys. and Controlled Nuclear Fusion, IAEA, Washington, 1990) vol. 2, p. 717, and references cited therein. [3] N. Yoshida, K. Tokunaga, T. Fujisawa, K. Tawara, T. Muroga, S. Itoh and the TRIAM-group, J. Nucl. Mater. 196-198 (1992) 415. [4] U. Samm, P. Bogen, H, Hartwig E. Hintz, K. Hfthker, Y.T. Lie, A. Pospieszczyk, D. Rusbiildt, B. Schweer and Y.J. Yu, J. Nucl. Mater. 162-164 (1989) 24.
1139
[5] D. Reiter, P. Bogen and U. Samm, J. Nucl. Mater. 196-198 (1992) 1059. [6] Y. Yagi, T. Shimada et al., J. Nucl. Mater. 162-164 (1989) 702. [7] P. Bogen, R.W. Dreyfus, Y.T. Lie and H. Langer, J. Nucl. Mater. 111/112(1982) 75. [8] K. Muraoka and M. Maeda, Plasma Phys. Control. Fusion 35 (1993) 633.
Journal of Nuclear Materials 220-222 (1995) 1135-1139
Energy distribution of deuterium atoms recycling on molybdenum limiter surface Shigeyuki Sekine a, Yoichi Hirano a, Yasuyuki Yagi a, Toshio Shimada a, Yoshiki Maejima a, Hazime Shimizu a, Tetsuo Tanabe b a Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan b Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan
Abstract
The emission spectrum of deuterium atoms ( D , ) was measured at the surface of a movable molybdenum limiter in the reversed field pinch machine (RFP: TPE-1RM20). The intensity of the D~ line increased with the insertion depth of the limiter. The observed line profile was fitted by a combination of two Gaussian functions with different temperatures. The temperatures derived from the fitted functions were approximately 5000 K and 50 000 K. The temperatures were independent of the insertion depth. No Doppler shifted shoulder, which would correspond to neutralized reflected ions at the limiter surface, was observed in the present experiment.
1. Introduction
Low-Z materials (graphite, beryllium, etc.) are mainly used as plasma facing materials in currently operated large scale machines with the great progress in the plasma performance. However, there still remain several problems; e.g. high erosion, high tritium retention and poor thermal conductivity for graphite, and low melting point and high toxicity for beryllium, etc. Great effort is being devoted to solve these problems but it is not yet clear that the low-Z materials can be used as the plasma facing materials in a future nuclear fusion reactor. Therefore, it is worthwhile to consider again high-Z materials, i.e. molybdenum (Mo), tungsten (W), etc., as a candidate for the plasma facing materials [1]. They have several advantages, such as the high melting point, low erosion, low retention, high thermal conductivity, etc. Improvements of the plasma performance and the long pulse operation have been observed in several experiments on reversed field pinch (RFP) machines [2] and TRIAM-1M [3] with molybdenum limiters. In the RFP experiment on TPE-1RM15 ( R / a = 700/135 mm), two kinds of limiter materials, graphite (POCO DBP-3-2) and Mo were used [2]. TPE-1RM15 was operated in the range with the plasma current I o =
100-250 kA, electron density n e = (1-3)X 1019 m -3, central electron temperature Te = 300-1000 eV and discharge duration 10-13 ms. Large heating power density and high heat flux by hot electron flow along the magnetic field were characteristics of RFP (typically 5 M W / m 3 and 500 M W / m 2, respectively). In the case of the graphite limiter, the toroidal one turn loop voltage, Vioop, was relatively high and Te low. Moreover, as Ip increased over 175 kA, Vloop sharply increased, T~ decreased and the global confinement property was degraded. On the other hand, by using the Mo limiter, degradation did not occur up to Ip = 200 kA, the scaling factor of T~ on Ip was improved from 2.8 e V / k A for graphite to 4.7 e V / k A for Mo, and that Te went up to as high as almost 1000 eV. Compared with the graphite limiters, two advantages of the Mo limiter, smaller temperature rise of the limiter for the same heat load and smaller impurity release at a given surface temperature, were the cause for this improvement. These observations may support tlae usefulness of high-Z materials as the plasma facing materials in future machines; if a method can be found by which the inward flow and accumulation of high-Z impurities is largely suppressed. However, there are little systematic data bases of the interaction between plasmas and
0022-3115/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-3115(94)00489-7
1136
S. Sekine et aL /Journal of Nuclear Materials 220 222 (199.5) 1135-1139 (a) mt~h,~t4~nl , ~
(b)
E-side Fig. l. (a) Schematic of the experimental setup. D,, emission at the limiter surface was measured through an optical window at the opposite side of the chamber. OMA: optical multichannel analyzer. (b) Enlarged view around the limiter head. The hatched surface at the head had been melted by a flux of drifting electrons with X > 21 mm,
high-Z materials. In particular, study of the interaction u n d e r high h e a t load is necessary for such a data base. As for hydrogen recycling at the surface of low-Z materials, S a m m et al. m e a s u r e d the H~ profile on the surface of a carbonized stainless-steel limiter and observed a strong s h o u l d e r at the shorter wavelength of the H , line in T E X T O R [4]. R e i t e r et al. also observed the s h o u l d e r of the D~ line on the surface of graphite a n d stainless-steel limiters in T E X T O R [5]. They interp r e t e d that the s h o u l d e r was due to reflected atoms at the limiter surface. However, t h e r e has b e e n little c o r r e s p o n d i n g data for M o or W limiters, especially for high heat flux conditions the data base does not reach a sufficient level. In this paper, the profile of the D,~ line emission at the m o l y b d e n u m surface was measured u n d e r very high heat flux by hot electron flow in the R F P plasma in o r d e r to obtain the energy distribution of lhe recycling d e u t e r i u m atoms on m o l y b d e n u m surfaces.
using the feedthrough. T h e scale of the f e e d t h r o u g h ( X ) was used as a p a r a m e t e r of the insertion d e p t h hereafter. Fixed M o limiters were also a t t a c h e d inside the c h a m b e r as shown in Fig. la. They limited the plasma m i n o r radius to a = 192 mm. W h e n the scale of the f e e d t h r o u g h X = 10 ram, the surface of the movable limiter h e a d was at a = 192 mm. D~ emission on the limiter surface was m e a s u r e d t h r o u g h an optical window at the opposite side of the c h a m b e r . T h e spatial resolution at the limiter surface was e s t i m a t e d to b e approximately 3 m m Q. T h e monitoring area was d e t e r m i n e d by using a telescope and was fixed to the electron drift side (E-side) as shown in Fig. lb. T h e m o n i t o r i n g area was off-center of the r o u n d h e a d a n d the main part of the reflected light at the h e a d would not come into the detector. T h e D , l i n e - b r o a d e n i n g was analyzed by a 1 m double monochrometer (JASCO CT-1000D) with a gated image intensifier a n d a m u l t i c h a n n e l d e t e c t o r (OMA). T h e gate timing was set at the elapsed time of 3 - 4 ms from the start of the discharge pulse. Overall wavelength resolution ( F W H M ) was approximately 0.39 cnl
1.
T h e surface t e m p e r a t u r e of the E-side of the limiter during discharge was m e a s u r e d by a far-infrared temp e r a t u r e m o n i t o r whose time resolution was a b o u t 2 ms. It could m e a s u r e t e m p e r a t u r e s b e t w e e n 1000 a n d 3000°C.
3. Results and discussions Typical time evolutions of D,~ emission intensity, plasma c u r r e n t a n d M o atomic emission (379 n m ) are shown in Fig. 2. In this m e a s u r e m e n t , the limiter was not inserted into the plasma ( X = 0 mm). I n t e n s e peaks of the D~ trace at the b e g i n n i n g and e n d of the discharge are due to a strong p l a s m a - w a l l interaction
m
,i
"'0[ >, 2. Experimental arrangement Fig. la shows the experimental setup. In this measurement the plasma current was kept at approximately 130 kA. A movable 20 mm G columnar limiter with a round head was made of polycrystal Mo. The limiter was mounted on a linear-motion feedthrough and its head was inserted into the boundary region of the RFP plasma. The insertion depth was changed by
E
b)
~3 =2[
;L ....... o
5 Time /
lo
:i:,
ms
Fig. 2. Typical time evolutions of (a) D,~ emission intensity, (b) plasma current and Mo atomic emission (MoI 379 nm). In these measurements, the movable limiter was not inserted into the plasma.
S. Sekine et aL /Journal of Nuclear Materials 220-222 (1995) 1135-1139
3000
150
1300 position / mm
limiter
-~. 2 0 0 0
|
r- 100
1oo ~.
0 10 / ms Fig. 3. Time evolutions of the surface temperature at the limiter head for different movable limiter positions X and typical plasma current. The used pyrometer can measure temperatures only above 1000°C. 0
~
I
C
I
limiter position X= 20 mm
A~. J
r~
= 5600 K "
50
5
Time
at those times. During the fiat top phase, t = 2-10 ms, the D~ intensity gradually decreased. On the other hand, the emission of the Mo atomic line exhibited an almost constant intensity. Moreover, no significant increase of the Mo emission intensity was observed by insertion of the limiter up to 19 mm in the flat top phase. However, at the end of discharge (t > 10 ms), a fairly intense emission was detected, which increased with the insertion depth. When the limiter was inserted into the plasma, the E-side of the limiter head was irradiated by an intense electron beam [4] and the surface temperature increased with time and insertion depth as shown in Fig. 3. (Used temperature monitor can not measure temperatures below 1000°C.) A dent observed in the temperature trace at the end of the discharge for all X may be due to an electric noise at the plasma termination. We observed black-body radiation in the visible range (about 650 nm) from the limiter surface at 8-10 ms giving a background noise to the D~ line if the gate of the image intensifier was set at 8-9 ms, which caused a poor S/N ratio of the observed spectra. Another measurement of MoI line emission revealed that the emission intensity started to increase sharply (more than one order of magnitude) when the surface temperature of Mo approx, reached the melting point of Mo (2620°C). According to these observations the gate timing was set at 3 - 4 ms to observe the emission from recycling (reflected) D atoms at the limiter surface. The observed emission spectrum of the DQ line and the fitted curve at X = 20 mm are shown in Fig. 4 (the movable limiter was inserted into the plasma for 10 mm). This spectrum was obtained by an accumulation for five discharges. The spectrum is fitted by a combination of two Gaussian functions with the same center
I
~l t'~
~
0~
I
"o
200 3=
~ooo I
1137
I
0
15235
15237
I
15239
.~.Tm,~ 15241
Wavenumber / cm -1
Fig. 4. Observed emission spectrum of the D,, line at the movable limiter position, X = 20 ram. The spectrum was obtained by an accumulation for five discharge pulses. The spectrum is fitted by a combination of two Gaussian functions which are also shown.
and different temperature. These Gaussian components are also shown in Fig. 4. Temperatures of two Gaussian components of fitted curves are plotted in Fig. 5 as a function of 3(. Any dependence of the temperatures on X was not observed in the present S/N ratio of the spectrum. Moreover, no significant Doppler shift nor the shoulder was observed for all X values in the present experiment on TPE-1RM20. This result obtained on the Mo limiter is considerably different from that for a carbonized stainless-steel [4],
100000 plasma surface -
80000
60000 ~. 40000
° I~
Ti I
Thigh
)
p20000
outside plasma
inside plasma
-
T,ow
0
-5
~[
'
0
5
10
15
20
Limiterposition X /
mm
25
Fig. 5. Derived temperature of two Gaussian components of the fitted curve in various movable limiter positions X. Each plot was derived from the accumulated spectrum.
1138
S. Sekine et al. /Journal of Nuclear Materials 220-222 (1995) 1135-1139
graphite and stainless-steel limiters [5], where the strong shoulder due to reflected atoms at the limiter surface was observed in the spectrum. These facts might indicate that the reflection of D + ions at the Mo surface may not be significant or that the D , emission from the plasma may be too strong comparing to that from reflected atoms. Bogen ct al. measured Doppler broadening of the hydrogen L,~ line at the A S D E X - t o k a m a k with the laser-induced fluorescence method [7]. They derived a temperature of 6000 K in the 600 A, 50 Hz cleaning discharge. Muraoka and Maeda also measured the L,, profile in T E X T O R with a plasma current of 250 kA [8]. Thc derived temperature was approximately 8000 K. These temperatures were almost the same as the low-temperature component (3000-8000 K) of the present measurement. Muraoka and Maeda concluded that the velocity of the D atoms was determined from the dissociation process of D 2 or D + and not from the ion temperature of the edge plasma. The S / N ratio of the line profile observed in T E X T O R [8] might not be high enough for showing the high temperature component in the curve fitting. The observed high-temperature component (40000-70000 K) in lhc present experiment possibly corresponds to
atoms charge-exchanged with ions in the boundary region. This assignment was estimated with reference to the plasma parameters around the edge region, T~,V= 10 eV and new = 3 × 1018 m ~. The dependence of the D~ emission intensity on X is shown in Fig. 6a. Peak heights of two Gaussian components of the fitted curves arc plotted. Each data point was derived from an accumulated spectrum for the same X. Intensity increased slightly with insertion depth beyond the plasma surface. The intensity ratio of the two Gaussian components did not exhibit any variation in the present S / N ratio of the spectrum as shown in Fig. 6b. When the monitoring area was changed in the ion drift side, no increase of the emission was observed. If the reflection of ions on the Mo surface plays a significant role in D , emission, the D intensity of hot D atoms should increase with the insertion depth. O n e possible explanation to the observed behavior of the intensity ratio is that the reflection of a D atom on a Mo limiter surface may not be significant in the present conditions; the Mo limiter was irradiated by a high heat flux of hot electrons. The increase of the emission intensity may be due to an increase of thermally desorbing D 2 molecules. In addition, it should be noted that if most of the ions arc reflected on the Mo surface without neutralization the estimation of the recycling effect is not possible by the present method.
140
(a) 120 ~
4. Conclusions
plasma surface outside plasma
inside plasma
1oo
E
Doppler broadening of the D , emission line was measured on the surface of a movable molybdenum limiter in a R F P plasma. The line profile was fitted by a combination of two Gaussian components with low (3000-8000 K) and high (40000-70000 K) temperatures. The low temperature component probably corresponds to the neutral D atom temperature in the vicinity of a limiter due to dissociation of D 2 and the high-temperature component may be due to atoms charge-exchanged with the D + ions. Both temperatures were not influenced by insertion depth, and the intensity of the emission increased with insertion depth. However, the intensity ratio of the high temperature component to the low temperature component did not show any dependence on the insertion depth. Neutralized reflected D atoms on the Mo surface were not observed in the present experiment.
8o low temp.
•
>, 60 E
c
40 20 0
t
.4
i& 90 8O ---= 7O +
(b)
!
•
..
.
±i
!
•
~o 6o -5
i
i
0
5
10
Limiter position
I
1
15
20
25
X /mm
Fig. 6. (a) Variation of the intensity of the line emission in various movable limiter positions X. Peak values of two Gaussian components were plotted. Each plot was derived from the accumulated spectrum. (b) Intensity ratio of two Gaussian components•
References [1] T. Tanabe, N. Noda and H. Nakamura, J. Nucl. Mater• 196-198 (1992) 11. [2] Y. Hirano, Y. Yagi, T. Shimada et al., Plasma Physics and
S. Sekine et al. /Journal of Nuclear Materials 220-222 (1995) 1135-1139 Controlled Nuclear Fusion Research, 1990 (Proc. 13th Int. Conf. on Plasma Phys. and Controlled Nuclear Fusion, IAEA, Washington, 1990) vol. 2, p. 717, and references cited therein. [3] N. Yoshida, K. Tokunaga, T. Fujisawa, K. Tawara, T. Muroga, S. Itoh and the TRIAM-group, J. Nucl. Mater. 196-198 (1992) 415. [4] U. Samm, P. Bogen, H, Hartwig E. Hintz, K. Hfthker, Y.T. Lie, A. Pospieszczyk, D. Rusbiildt, B. Schweer and Y.J. Yu, J. Nucl. Mater. 162-164 (1989) 24.
1139
[5] D. Reiter, P. Bogen and U. Samm, J. Nucl. Mater. 196-198 (1992) 1059. [6] Y. Yagi, T. Shimada et al., J. Nucl. Mater. 162-164 (1989) 702. [7] P. Bogen, R.W. Dreyfus, Y.T. Lie and H. Langer, J. Nucl. Mater. 111/112(1982) 75. [8] K. Muraoka and M. Maeda, Plasma Phys. Control. Fusion 35 (1993) 633.