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Impurity generation mechanism and remote radiative cooling in JT-60U divertor discharges
jnurnalnf nuclear materials
Journal of Nuclear Materials 196-198 (1992) 71-79 North-Holland
Impurity generation mechanism and remote radiative cooling in JT-60U divertor discharges H. Kubo, M. Shimada, T. Sugie, N. Hosogane, K. Itami, S. Tsuji, H. Nakamura, N. Asakura, A. Sakasai, Y. Kawano and the JT-60 Team Japan Atomic Energy Research Institute, Mukoyama, Naka-gun, lbaraki-ken, 311-01, Japan
The impurity behavior and the remote radiative cooling have been studied in the JT-60U divertor discharges with high power NB heating. NB at a power level of 20 MW was injected for 2 s, and no carbon bloom has been observed. For the first time, the measurement and the calculation for the carbon influx from the divertor plates are compared quantitatively. The carbon generation mechanism can be explained by the sputtering with deuterium (physical sputtering), oxygen and carbon. It is found that the contribution of the chemical sputtering with deuterium is small and that of the sputtering with oxygen is important. Remote radiative cooling can reduce the heat flux onto divertor plates. The dependence of the radiative cooling power on the electron density and the safety factor is presented. The favorable operation regime for discharges with high power auxiliary heating is discussed based on the experimental results.
1. Introduction Impurity control, alleviation of divertor tile erosion, and heat removal are crucial issues in present and next tokamaks. In order to reduce the radiation losses from heavy impurities, most of the major tokamak experiments have protected near vessel structures with carbon material. However, in discharges with high power auxiliary heating, the use of carbon materials often leads to a carbon bloom and reduces drastically the numbers of fusion products [1,2]. Impurity behavior is explained by three principle elements: generation, shielding, and transport. Carbon generation from limiters has been investigated in some tokamaks, and the mechanism of carbon generation has been discussed [3,4]. However, most of them werc qualitative discussion. Recently, the JET team reported a comparison between the observation and the calculation [5]. However, they assumed oxygen flux while the sputtering by oxygen was an important process [6]. Although the study of the impurity generation from the divertor plates is more important to design next tokamaks (for example, ITER), the measurement and the calculation for the carbon influx in the divertor region has not been compared quantitatively. Divertor tile erosion is a serious issue regarding the lifetime of the tiles [7]. Remote radiative cooling is the most straightforward way to reduce the heat load onto the divertor plates [8-10]. This radiative cooling is also effective in making dense and cold divertor plasmas. Cold divertor plasma is indispensable in reducing the erosion of divertor plates to an acceptable level [8,11,12].
The JT-60 [13] was upgraded in order to conduct the experiments in an intermediate region between the JT-60 and next-step tokamaks, and the upgrade device was named JT-60U [14,15]. Experiments with plasma currents up to 4 MA and NB heating powers up to 22 MW have been performed [16]. This paper presents the studies of impurities and radiation losses in the JT-60U tokamak. In section 2, the operation of the JT-60U tokamak and the overview of the impurity behavior in NB heated discharges are described. In section 3, the measurement and the calculation for the carbon influx in the divertor region are compared quantitatively for the first time. The mechanisms of carbon generation are discussed. In section 4, the dependence of radiative energy losses on the electron density and the safety factor is discussed. In section 5, the favorable operation regime for discharges with high power auxiliary heating is discussed based on the experimental results.
2. JT-60U operation and impurity contents In the JT-60U tokamak, the nominal plasma current is 6 MA for divertor discharges and the toroidal field is 4.2 T. The major radius is 3.4 m, and the minor radius is 1.1 m horizontally and 1.4 m vertically. A detailed description of the device is available elsewhere [14,15]. In 1991, experiments with plasma currents of up to 4 MA and NB heating powers of up to 22 MW have been performed. L-mode, H-mode and high poloidal beta regime have been investigated in the open divertor discharges with a lower null point. In the present
0022-3115/92/$05.00 9 1992 - Elsevier Science Publishers B.V. All rights reserved
72
H. Kubo et aL / Impurity generation mechanism in JT-60U
paper, the impurity behavior and radiation losses in L-mode discharges are discussed. A carbon-fiber composite with a thermal conductivity of 300 W / m ~ is used for the divertor plates. All plates are placed continuously and the edges of the divertor tiles with a difference in height between adjacent tiles larger than 0.5 mm are bevelled in the toroidal direction to avoid localized heat deposition. The inner surface of the vacuum vessel except for the divertor plates is covered with isotropic graphite tiles. Wall conditioning has been performed by a combination of the baking of the vessel ( ~ 300~ Taylor discharge cleaning (TDC) and He glow discharge cleaning (GDC) [17]. TDC was performed with a pressure of 0.2-1 • 10 -2 Pa, I o = 10-40 kA, B T = 0.7-1 T, a pulse duration of 30 ms and a repetition time of 1 s. GDC was performed with a pressure of ~ 0.1 Pa, a discharge voltage of 500-600 V and a discharge current of 0.5-1.5 A. The main impurity species were oxygen and carbon. GDC was effective for oxygen reduction, even though the discharge current was low. In ohmically heated discharges, the oxygen concentration after the GDC decreased by a factor of 2 down to 1%. On the other hand, the carbon concentration did not change. By the introduction of the GDC, Zeef decreased from 3.5 to 2.0 in ohmically heated discharges with an electron density of ~ 2 • 1019 m -3. A schematic diagram of the experimental setup of the important diagnostics in this work is shown in fig. 1. Electron density, radiation losses and impurity spectra were observed with FIR interferometers (two channels), bolometers (32 channels) and a VUV polychromator, respectively. The intensity of visible bremsstrahlung for Z~ff measurements was tangentially measured through a 10-channel optical fiber array with interference filters. For study of the divertor region, a variety of diagnostics were prepared (fig. lb). The intensities of line emissions of deuterium and impurities were measured with a 0.5 m visible spectrometer with fiber optics. The spatial distribution of the line intensities was observed through a 38-channel optical fiber array with interference filters. The electron temperature and the electron density were measured by the use of Langmuir probes [18]. The surface temperature of the divertor tiles was measured with an IRTV and the heat load onto the divertor tiles was derived. Time evolution of a discharge with NB heating of high power is shown in fig. 2. NB at a power level of 20 MW is injected for 2 s. The intensity of visible bremsstrahlung normalized by the square of the electron density indicates a value that is proportional to Zeef when the electron temperature is constant. During the NB heating, the normalized intensity of the visible bremsstrahlung does not increase, and this suggests that Zeff does not increase. The radiation losses do not suggest the occurrence of carbon bloom, either. Carbon blooms have not been observed even when the NB
(a) FIR Interterometer 2 Visible Spectrometer( 2 ch) Optical Fiber Array (38ch) I
_~_ Bolometer Array {16 ch) Optical Fiber Array for Zeff (10 ch) Bolometer Array (16 ch)
0
VUV Spectrometer 2
2.5
5.5 ~
4.5 Langmuir Probes (15ch. AR=2cm
(b) I
IRTV 38ch Optical FiberArray
/
Longmuir Probe I
R = 2.8
3.0
i
3.2
I
3.4 (m)
Fig. 1. (a) A schematic diagram of the experimental setup at the diagnostics. (b) The experimental setup of the diagnostics for the diverter study.
with a power of ~ 20 MW was injected for 2 s. The level of the injected NB power is close to the limit operation level without carbon bloom in TFTR (25 MW for 2 s) [19]. The surface temperature of the divertor plates was rather low because of the high thermal conductivity of the divertor tiles and the reduction of the heat load by remote radiative cooling. The maximum of the surface temperature measured with the IRTV was about 700~ for NB heated discharges. Fig. 3 shows Zef f as a function of absorbed heating power. In the low density region, Ze~f increases up to 4, as the absorbed power increases up to 18 MW. However, in the high density region, Zeff stays in the
H. Kubo et al. / Impurity generation mechanism in JT-60U 13780
[p=
2MA~
73
I
BT=4T
I
I
3 r ...........'.""--"', .... -'; v v ~,........................... ~ .
20
4
'[: 2
n'e =3-3.4x1019m-3
0
%, ~,/
~0
I
I
I
i=
3
0
./Bremss d~t/(.l'ned~) 2
S5
O
[
I
I
I
4
1
~2
~e= 3 . 8 . 4 . 5 x 1 019m.3
0 0
I j
6
I
I
7
8
9
power, fBremss d l / ( f n ~ dl) 2 the intensity of visible bremsstrahlung normalized by the square of the electron density, p~m~in the radiation loss in the main plasma, and 9 the radiation loss in the divertor region.
region of 2-3, while the absorbed power is increased up to 16 MW. The relation between impurity concentrations and the line averaged electron density for NB heated discharges is shown in fig. 4a. The concentrations were derived from Zef f and the intensity ratio of C V I (33.7 ,~) and O V I I I (19 ,~) [20]. As the electron density increases to 4.5 x 1019 m 3, the carbon concentration decreases to 2% and the oxygen concentration decreases to 1%. The dependence of carbon con-
, a , ~ o~ 4
Carbon
i
o
I
dD
3
.
r
\
j
o
4
3. C a r b o n
generation
mechanism
We discuss the impurity flux at the outer strike point of the separatrix on the divertor plates, because the electron temperature, pressure and heat load at
6 9
v
4
ne = 3 - 3 . 4 x 1 0 1 9 m 3
o
9
/ ne=3.8-4.5X1019m'3
~
1
0
o ~==/
2
Oxygen 0
20
r0
E 2
0
i
16
centration on absorbed power for NB heated discharges is shown in fig. 4b. In the low density region, the concentration increases up to 5.5%, as the absorbed power increases. On the other hand, in the high density region, the concentration stays in the region of 2-3.5%, while the absorbed power is increased up to 16 MW. To summarize, high density operation is effective in maintaining lower carbon concentration during the high power NB injection.
,m
1
i
12
Fig. 3. Zorf as a function of absorbed heating power for NB heated discharges.
r
,<~ ".o ~ o 9 o'-~.o
i
8
Pabs ( M W )
(b) 5 -
- ~ -
0
""
TIME (sec) Fig. 2. Time evolution of a discharge with NB heating at a power level of 20 MW. The plasma current was 2 MA, and the toroidal field was 4 T. ~ indicates the line averaged electron density in the main plasma, P ~ the NB heating
r
9
O I
0
r
9
9
J
2
3
fie (1 01 9m- 3)
4
5
0
5
10
15
20
Pabs ( M W )
Fig. 4. (a) Relation between impurity concentrations and line averaged electron density for NB heated discharges with beating powers of 10-18 MW. The open circles indicate the carbon concentration, the closed circles the oxygen concentration. (b) Carbon concentration as a function of absorbed power for NB heated discharges.
H. Kubo et aL / Impurity generation mechanism in JT-60U
74
(a)
60
(b) ~ \
1.5
F
,
NBI
0, k \
R"
\
.....
>
40
\
',0 \
-
i= 1.o
\ k
\
9
\
v
o.-"
9 -
/
O~
\
,. /
~:)
/
/
NBI
> nO
I-
20
,
:~
0.5 -
OH
4 -o
,"
0
0
I 1
I 2
I 3
0.0 0
4
t
i
1
n--e(1 01 9 m- 3)
2
i
3
4
n--e ( 1 01 9 m- 3)
Fig. 5. (a) Electron temperature and (b) electron density in the divertor region as a function of the line averaged electron density in the main plasma.
ing powers of 4.6-9.2 MW. In the ohmically heated discharges, the electron temperature in the divertor region decreases from 60 to 40 eV, as the electron density in the main plasma increases from 0.9 x 1019 to 1.8 x 1019 m 3. The electron density in the divertor region increases from 0.2 x 1019 to 0.4 x 1019 m -3. In the NB heated discharges, the electron temperature and density stay in the regions of 20-55 eV and (0.71.2) • 1019 m -3, respectively. For the discharges, the baking temperature of the vacuum vessel was 260~
the outer strike point were larger than those at the inner strike point [18]. The electron temperature and density at the outer strike point are shown as functions of the line averaged density in the main plasma in fig. 5. The data in the low density region were obtained in the ohmically heated discharges with plasma currents of 1.5-2.5 MA and toroidal fields of 2.5-3.8 T. The data in the high density region were obtained in the NB heated discharges with plasma currents of 2-2.5 MA, toroidal fields of 2.5-4 T and absorbed NB heat-
0.10
0.25
I
o
C
P
r ..Q
0v
(t)
t.-
0.20
OH NB
0.15
o t~ -"t _9.o 0
0.05
(:3
D
0.00 0
0.10
"'....
0
I
I
I
1
2
3
n e (1019
o
0.05 0.00 4
m "3)
Fig. 6. Observed (closed circles) and calculated (open symbols) relative carbon influxes as a function of the electron density in the main plasma. The left vertical axis is for the observation, and the right vertical axis is for the calculation. The open triangles indicate the calculated flux due to the sputtering by deuterium, the open squares the flux due to the sputtering by deuterium and oxygen, and the open circles the total flux. The data in the electron density region lower than 2x1019 m -3 were obtained in ohmically heated discharges, and the data in the higher density region were obtained in the NB heated discharges with heating powers of 4.6-9.2 MW.
75
H. Kubo et al. / Impurity generation mechanism in JT-60U
Under the condition of negligible recombination, the neutral influx equals the ionization rate. The ionization process is accompanied by the emission of spectral lines and the numbers of the ionization events and those of emitted photons are closely correlated. Therefore, the neutral particle influx can be derived from the measured intensity of the neutral line emission. In the region of low electron density, the relation between the neutral particle influx F and the line intensity I is basically given by the equation S (1)
r = 4 ~r--x~ l ,
where S and X are the ionization and excitation rate coefficients, respectively. B is the branching ratio for the observed line. This equation is commonly used to derive hydrogen influx from H a measurements [21], and it can also be used for influx measurements of low-charge-state impurities [22]. The influxes of deuterium, carbon and oxygen were derived respectively from the measured line intensities of D~, C II 6578, and O I I 4415, 4417 by using eq. (1). The ionization events per photon ( S / X B ) [22] at the electron temperature measured with the Langmuir probe was used in the analysis. In fig. 6, the observed carbon influx relative to the deuterium influx ( F c / F D) in the divertor region is shown as closed circles against the electron density. The relative carbon influx in the divertor region decreases from 10 to 1.5% as the electron density increases from 0.9 • 1019 to 3.4 • 1019 m 3. The carbon flux in the NB heated discharges seems to be a bit larger than that in the ohmically heated discharges, taking account of the electron density dependence. In a simple sputtering model, the carbon influx F c can be expressed as F c = Y D F ~ u' + Y o F ~ ~' + Ec s c
,
where i indicates the ion, C s the velocity of sound, 4~s the sheath potential, and qi the charge number of the ion. The ion temperature was assumed to be equal to the electron temperature. The sheath potential was assumed to be 3Tf iv. By considering the recycling time and the ionization time, we assumed that all of the produced carbon returned to the divertor plates with a charge number of 4. The charge number of incident oxygen ions is not important, because the energy dependence of the sputtering yield for oxygen is weak. In experiments with ion beams, the chemical sputtering yield for deuterium is much larger than the physical sputtering yield for deuterium. The chemical sputtering yield depends on the surface temperature and has a maximum value at the temperature of 550~ For oxygen, the chemical sputtering yield is as large as the physical sputtering yield. The chemical sputtering yield depends neither on the surface temperature nor on the incident energy of the ion. The total sputtering yield for oxygen is always nearly unity. In the following calculation, the sputtering yields for normal incidence are used for simplicity, although the yields increase with the incidence angle. For ohmically heated discharges with T~ iv = 39-45 eV and the maximum surface temperature of the divertot tiles of 300-310~ the relation between the relative carbon influx and the relative oxygen influx is shown in fig. 7. The relative carbon influx increases with the relative oxygen influx. Because the electron temperature and the surface temperature of the divertor plates are nearly constant, all of the sputtering yields in eq. (3) are considered to be constant. The observed linear correlation can be explained by eq. (3). It is clearly shown that the sputtering with oxygen is an important process for the carbon production.
(2)
where F oout , r ~ u t and F,~ut are the fluxes of deuterium, oxygen, and carbon ions onto the divertor plates, respectively. YD, Yo and u are sputtering yields of carbon by deuterium, oxygen and carbon ions, respectively [6,23]. Considering a steady state, we can assume that the outflux is equal to the influx measured with the spectrometers. Then eq. (2) can be transformed into
rc
Yo
ro
YD
0.10
r..-,
0.00
1
2
E i = ~ k T i + ~ m i C s + eqi&s ,
(4)
o/"
/
/
0.04
Physical and chemical processes are considered for the sputtering by deuterium and oxygen. The sputtering by carbon (self-sputtering) is a physical process. The physical sputtering yield depends on the incident energy of the ions. The energy of the incident ions E was assumed to be given by ref. [24]
-
1-Yc'
/" oj
0.02
-
at"
0.06
(3)
+
F D - 1 - Y c FD
/
0.08
,/o
,
l
I
I
0.00 0.02 0.04
i
I
I
I
n
0 . 0 6 0 . 0 8 0.10
Fo/FD Fig. 7. Relation between the relative carbon influx and the relative oxygen influx for ohmically heated discharges with the electron temperature in the divertor region of 39-45 eV and the maximum surface temperature of the diverter plates of 300-310~
76
H. Kubo et al. / Impurity generation mechanism in JT-60U
r
o
"~ 0
a
~
0
"-
10.0
8.0 6.0
0 0
OH
i 0
i
i
NBI 0
0
0 O
4.0
C 0 m
4
2.0,
t.
--
-SS--
--0--
--
I I I I 0.0 3 0 0 350 400 450 5 0 0 5 5 0 6 0 0
Temperature of Divertor Plates (~
Fig. 8. Ratio of the calculated relative carbon influx to the observed influx as a function of the surface temperature of the divertor plates. The chemical sputtering by deuterium is included in the calculation for open circles and excluded for closed circles. The data in the temperature region of ~ 310~ were obtained in ohmically heated discharges, and the data in the temperature region higher than 400~ were obtained in NB heated discharges with heating powers of 4-13 MW.
The ratio of the calculated carbon influx to the observed influx is shown as a function of the surface temperature of the divertor plates in fig. 8. The open circles indicate the ratio for the carbon flux calculated including the chemical sputtering by deuterium and the closed circles indicate the ratio for that calculated neglecting the chemical sputtering. For the chemical sputtering yield by deuterium, the dependence on the ion flux density was considered as the sputtering yield was proportional t o FD 0"1. The surface temperature of the divertor plates in ohmically heated discharges is about 310~ and the temperature increases up to 540~ in the NB heated discharges. The chemical sputtering yield by deuterium increases with the surface temperature of the carbon plates and has a maximum at 550~ For the calculation including the chemical sputtering, the ratio of the calculated carbon flux to the observed flux in the ohmically heated discharges is 3.3. The ratio increases in NB heated discharges from 5 to 10, as the surface temperature increases. On the contrary, the ratio for the calculation neglecting the chemical sputtering is always nearly constant. Therefore, the observed carbon flux does not depend on the surface temperature. It suggests that the contribution of the chemical sputtering is small. Such a phenomenon has been observed in other tokamaks [4]. Some mechanisms of the reduction of the chemical sputtering by the deuterium were suggested. It has been reported that the chemical sputtering yield decreases as the ion flux density increases [25]. The ion
flux density onto the divertor plates increased from 1 x 1021 to 1.3 X 1022 m -2 s -1, as the line averaged electron density in the main plasma increased from 0.9 x 1019 to 3.4 x 1019 m 3. The flux density was higher by 1-2 orders of magnitude than that in ion beam experiments. In the calculation, the dependence has already been included as mentioned previously and it cannot explain the experimental result. The presence of metal contamination on the surface suppresses the chemical sputtering [26]. However, this effect would be unlikely in the JT-60U tokamak, because most of the inner surface of the vacuum vessel is covered with graphite tiles and metal contamination would be little. The sputtered carbon might recycle rapidly near the divertor plates before emitting the observed line [27]. It is probable that the carbon produced in CO 4 form hardly affects the intensity of the C II line. The relative carbon influx calculated by neglecting the chemical sputtering with deuterium is shown as open symbols in fig. 6. It is shown that the calculated carbon flux agrees with the measured flux not only in the ohmically heated discharges but also in the NB heated discharges, except for a discrepancy of a factor of 2.5. In the low density region (ne = 0.9 X 1019 m-3), the self-sputtering and the sputtering by oxygen are important. The relative carbon influx decreases for a higher electron density because of the smaller selfsputtering yield and the decrease in the relative oxygen density. In the medium density regime (~e = 1.8 x 1019 m-3), the contributions of the sputterings by deuterium, oxygen and carbon are about 17%, 44% and 39% of the total carbon influx, respectively. The carbon flux in the NB heated discharges is larger than the flux in the ohmically heated discharges, taking account of the electron density dependence. The increase is attributed to the increase of the self-sputtering with the electron temperature in the divertor region. In the high electron density region (~e = 3.4 X 1019 m -3) , the contributions of the sputterings by deuterium, oxygen and carbon are about 32%, 36%, and 32% of the total carbon influx, respectively. There are uncertainties in the estimation of the incident energy of ions. However, the error in the estimation of the incident energy would hardly affect the calculated result in the high electron temperature region, because the dependence of the sputtering yield on the incident energy is weak in the high energy region. One possible reason for the discrepancy between the observed carbon flux and the calculated one might be due to the error in the estimation of the influx from the line intensity. The error in the absolute influx measurement is usually estimated to be a factor of 2, while the deduction of relative influx is expected to be more reliable [28]. In addition, a more sophisticated calculation would be required. The effect of the spatial distribution and the recycling of sputtered ions should be considered.
77
H. Kubo et al. / Impurity generation mechanism in JT-60U
As discussed in section 2, the carbon bloom has not been observed in the discharges with high power heating. The surface temperature of the divertor plates measured with the IRTV did not exceed 700~ and the concentration of heat deposition at edges of carbon tiles would be reduced by good alignment and the edge bevelling of the carbon tiles. Therefore, up to now, the radiation enhanced sublimation might not play an important role in JT-60U. In fig. 9, the ratio of the carbon density normalized by the deuterium density in main plasmas to the carbon influx normalized by the deuterium influx is shown as a function of the electron density in the main plasma. The data were obtained for ohmically heated discharges. The ratio decreases as the electron density increases. Therefore, this suggests that the shielding efficiency for carbon is higher as the electron density is higher. The tendency can be explained by the penetration probabilities of carbon and deuterium. Here we assumed that carbon in the core plasma came from the strike point. In ohmically heated discharges, the carbon influx from the vessel wall was smaller by an order of magnitude than the influx from the divertor plates. However, the carbon influx from the wall may affect the carbon concentration in the core plasma, because the shielding efficiency for carbon generated at the wall would be lower than that for carbon generated at the strike point. At the present stage, it is difficult to clarify the contribution of the carbon generated at the wall experimentally. Considering these results, the reduction in the oxygen concentration is effective to reduce the carbon flux. It would be realized by wall conditionings such as boronization [29]. The oxygen concentration in He discharges was lower than that in D 2 discharges, and the carbon concentration in He discharges was also lower than that in D 2 discharges. As the electron
'E C~ ~<
0 div . . . . . . . . . . . .
2
0 - 1.5 - ~div.
% \
9
r.-9
\
r--.,
9
9
\ X o IN N
0.5 c"(...)
I
1.0
i
I
i
1.5
i
2.0
r
2.5
ne (lOI9m-3) Fig. 9. Ratio of normalized carbon concentration in the main plasma (n c / n D) to normalized carbon influx in the divertor region ( F c / F D) against line averaged electron density for ohmically heated discharges.
.
% 1.0
0.5 0 55O
~
3OO 250
"
moin
~
J
•
L
T div.
22__/
" J
5
L
L
7 8 9 10 TIME (sec) Fig. 10. Time evolution of a typical remote radiative cooling. The plasma current was 1.7 MA, and the toroidal field was 3 W. qmax div indicates the maximum heat flux density onto the divertor plates, and Tma~x indicates the maximum surface temperature of the divertor plates.
6
temperature in the divertor region decreases, the physical sputtering yield decreases and the carbon flux is reduced. Operation in the high electron density region reduces the relative oxygen concentration and the electron temperature in the divertor region. It is more effective to lower the electron temperature with the aid of the dense divertor. In addition to that, the shielding efficiency is higher in the high electron density region.
4. R e m o t e
1.0
O.Od.
E14238
radiative
cooling
Time evolution of a typical remote radiative cooling is shown in fig. 10. During the NB heating with a power of 7.5 MW, the electron density was raised due to gas puffing. As the electron density increases, the radiation loss in the divertor region increases up to 45% of the NB heating power. The increase in the radiation loss in the main plasma is gradual and the fraction is less than 20%. The maximum value of heat flux density onto the divertor plates decreases simultaneously. The maximum value of the surface temperature of the divertor plates decreases gradually. Remote radiative cooling reduces the heat load onto the divertor plates. Fig. 11 shows radiation power as a function of absorbed power. The radiation power in the main plasma and that in the divertor region increase, as the
78
H. Kubo et al. / Impurity generation mechanism in JT-60U
510 "-"
i
i
,
I
9 9 9
8 6
J
i
I
,
F
I
~
2-2.3 MA 2.4-2.7MA 3.3-3.5 MA =D t' 9
+
In
9 A9
o -
4
.o
2
=
i
,
i
I
~
i
F
1.3 MA 1.7 MA
9
~.~
i
length plays a role. The dependence might be explained by the decrease in the electron temperature in the peripheral plasma region with the increase in connection length. To summarize, the remote radiative cooling power is higher for discharges with high density and high safety factor.
0~-
,oqlD= 9 1 4 9
e
t=
r ""
5. Summary
-%
9 I 9 ,-c~,~.s.~ o n-
0
4
8
Absorbed
12
16
2G
Power(MW)
Fig. 11. Radiation power against absorbed power for NB heated discharges. Open symbols indicate radiation loss in the main plasma, and closed symbols indicate total radiation loss.
absorbed power increases. In the discharges with absorbed powers of 16 MW, the radiation power in the main plasma and that in the divertor region is 2 and 4 MW, respectively. The correlation between the fraction of radiation loss and the line averaged electron density is shown in fig. 12a. The fraction of the radiation loss in the divertor region increases as the electron density increases. In the high density region, the fraction of the radiation loss in the divertor region and that in the main plasma are about 35% and 15%, respectively. These fractions are close to those observed in discharges with lower null points in JT-60 [30,10]. In fig. 12b, the fraction of the radiation loss is plotted against neqeff. Comparing fig. 12a with fig. 12b, the data are relatively scattered in fig. 12a and the correlation is clearer in fig. 12b. The radiation loss increases as neqeff increases. This suggests that the connection
In In
0.6
"r
0.5
I
0.6
I
(a)
o
"o
" c O
0= n"
0.4 0.3
c-
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~
tr
divertorr
N,,=
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:= .~_ -o
+
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N,=,
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main I ~176 0
9 9 9 + x
0.5 0.4
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I
t
i
2
3
-fie (1019 m-3)
P 4
u.
9
9
9 +
divertor
z~
~
0.1
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main
o
1
(b)
2 MA/3T 2 MN4T 2.2 MA/4T 2.7 MA/4T 3.3 MA/4T
0.3
c
q c ~ c ~A o ~ O
0.1
9 2 MA/ZST
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O
'~,
In JT-60U, experiments with plasma currents up to 4 MA and NB heating powers up to 22 MW have been performed. The impurity content, the impurity generation mechanism and the remote radiative cooling have been studied. NB with a power of ~ 20 MW was injected for 2 s, and the carbon bloom has not been observed. Carbon is the dominant impurity and the concentration stays in the region of 2-3.5% in the high electron density region even if the NB power increases up to 16 MW. The relative carbon influx in the divertor region decreases from 10 to 1.5% as the electron density in the main plasma increases from 0.9 • 1019 to 3.4 • 1019 m -3. The dependence of the carbon influx on the electron density can be explained by the sputtering with deuterium (physical sputtering), oxygen and carbon. The contribution of the chemical sputtering by deuterium is small. The importance of the sputtering by oxygen is clear. The decrease in relative carbon flux in the high density region is explained by the reduction in the relative oxygen concentration and the decrease in the electron temperature in the divertor region. In ohmically heated discharges, the shielding efficiency for carbon becomes higher as the electron density increases. The fraction of the radiation loss in the divertor region and the fraction of that in the main plasma is 35% and 15% in the NB heated discharges with high
o
t
0
I
t
I
I
5
t
I
t
I
t
10
t
i
r
15
fieqeff (1019 m-3)
Fig. 12. (a) Dependence of the fraction of the radiation loss power on line average electron density. (b) Dependence of the fraction of the radiation loss power on the electron density multiplied by the safety factor.
H. Kubo et al. / Impurity generation mechanism in JT-60U
electron density. T h e fraction of the r a d i a t i o n power increases with n~qef f. This suggests that the c o n n e c t i o n length plays a role. In the p r e s e n t work, the a d v a n t a g e of high density o p e r a t i o n has b e e n shown for impurity dilution, impurity g e n e r a t i o n a n d impurity shielding. A n d it has b e e n shown that o p e r a t i o n with the high density and the high safety factor increases the r e m o t e radiative cooling. In ref. [18] it was p r e s e n t e d that the peaking factor of the heat load in the scrape-off layer was propor0.45 (I.67 tional to n~ qeff , also suggesting the a d v a n t a g e of high density and high safety factor operation. To summarize, the o p e r a t i o n with the high density a n d the high safety factor is favorable for impurity control, alleviation of divertor tile erosion a n d h e a t removal.
Acknowledgements T h e authors would like to express sincere gratitude to Dr, T. Takizuka a n d Dr. S. T a n a k a of J a p a n Atomic Energy R e s e a r c h Institute for useful discussion. T h e authors appreciate the fruitful discussion on the sputtering m e c h a n i s m with Dr. J. R o t h of Max-Planck-Institut fflr Plasmaphysik. T h e authors wish to t h a n k the m e m b e r s who have c o n t r i b u t e d to the J T - 6 0 U project.
References [1] M. Ulrickson, JET Team and TFTR Team, J. Nucl. Mater. 176 & 177 (1990) 44. [2] R. Reichle, D.D.R. Summers and M.F. Stamp, J. Nucl. Mater. 176 & 177 (1990) 375. [3] C.S. Pitcher et al., Nucl. Fusion 26 (1986) 1641. [4] A. Pospieszczyk et al., J. Nucl. Mater. 145-147 (1987) 574.
79
[5] JET Team, J. Nucl. Mater. 176 & 177 (1990) 3. [6] J. Roth, J. Nucl. Mater. 145-147 (1987) 87. [7] R.K. Janev and A. Miyahara, Suppl. Nucl. Fusion (1991) 123. [8] M. Shimada et al., Nucl. Fusion 22 (1982) 643. [9] M.G. Bell et al., J. Nucl. Mater. 121 (1984) 132. [10] M. Shimada et al., J. Nucl. Mater. 176 & 177 (1990) 122. [11] Y. Shimomura et al., Nucl. Fusion 23 (1983) 869. [12] S. Sengoku et al., Nucl. Fusion 24 (1984) 415. [13] JT-60 team, Proc. 12th Int. Conf. on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 1988, vol. 1 (IAEA, Vienna, 1989) p. 67. [14] H. Ninomiya et al., Plasma Devices and Operation 1 (1990) 43. [15] H. Horiike et al., Fusion Eng. Des. 16 (1991) 285. [16] H. Ninomiya and JT-60 Team, Phys. Fluids B4 (1992) 2070. [17] M. Shimada et al., these Proceedings (PSI-10), J. Nucl. Mater. 196 198 (1992) 164. [18] K. ltami et al., these Proceedings (PSI-10), J. Nucl. Mater. 196-198 (1992) 755. [19] D.M. Meade et al., Proc. 13th Int. Conf. on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 19911, vol. 1 (IAEA, Vienna, 1991) p. 9. [20] H. Kubo el al., Nucl. Fusion 29 (1989) 571. [21] L.C. Johnson and E. Hinnov, J. Quant. Spectrosc. Radiat. Transf. 13 (1973) 333. [22] K.H. Behringer, J. Nucl. Mater 145-147 (1987) 145. [23] J. Roth, E. Vietzke and A.A. Haasz, Nucl. Fusion, suppl. (1991) 63. [24] R. Chodura, J. Nucl. Mater. 111 & 112 (1982) 420. [25] D.M. Goebel et al., Nucl. Fusion 28 (1988) 1041. [26] J. Roth, J. Bohdansky and J.B. Roberto, J. Nucl Mater. 128 & 129 (1984) 534. [27] W.D. Langer, Nucl. Fusion 22 (1982) 751. [28] K. Behringer et al., Plasma Phys, Control. Fusion 31 (1989) 2059. [29] J, Winter et al., J. Nucl. Mater. 162-164 (1989) 713. [30] S. Tsuji et al., Proc. 12th Int. Conf. on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 1988, vot. 1 (IAEA, Vienna, 1989) p. 265.
Journal of Nuclear Materials 196-198 (1992) 71-79 North-Holland
Impurity generation mechanism and remote radiative cooling in JT-60U divertor discharges H. Kubo, M. Shimada, T. Sugie, N. Hosogane, K. Itami, S. Tsuji, H. Nakamura, N. Asakura, A. Sakasai, Y. Kawano and the JT-60 Team Japan Atomic Energy Research Institute, Mukoyama, Naka-gun, lbaraki-ken, 311-01, Japan
The impurity behavior and the remote radiative cooling have been studied in the JT-60U divertor discharges with high power NB heating. NB at a power level of 20 MW was injected for 2 s, and no carbon bloom has been observed. For the first time, the measurement and the calculation for the carbon influx from the divertor plates are compared quantitatively. The carbon generation mechanism can be explained by the sputtering with deuterium (physical sputtering), oxygen and carbon. It is found that the contribution of the chemical sputtering with deuterium is small and that of the sputtering with oxygen is important. Remote radiative cooling can reduce the heat flux onto divertor plates. The dependence of the radiative cooling power on the electron density and the safety factor is presented. The favorable operation regime for discharges with high power auxiliary heating is discussed based on the experimental results.
1. Introduction Impurity control, alleviation of divertor tile erosion, and heat removal are crucial issues in present and next tokamaks. In order to reduce the radiation losses from heavy impurities, most of the major tokamak experiments have protected near vessel structures with carbon material. However, in discharges with high power auxiliary heating, the use of carbon materials often leads to a carbon bloom and reduces drastically the numbers of fusion products [1,2]. Impurity behavior is explained by three principle elements: generation, shielding, and transport. Carbon generation from limiters has been investigated in some tokamaks, and the mechanism of carbon generation has been discussed [3,4]. However, most of them werc qualitative discussion. Recently, the JET team reported a comparison between the observation and the calculation [5]. However, they assumed oxygen flux while the sputtering by oxygen was an important process [6]. Although the study of the impurity generation from the divertor plates is more important to design next tokamaks (for example, ITER), the measurement and the calculation for the carbon influx in the divertor region has not been compared quantitatively. Divertor tile erosion is a serious issue regarding the lifetime of the tiles [7]. Remote radiative cooling is the most straightforward way to reduce the heat load onto the divertor plates [8-10]. This radiative cooling is also effective in making dense and cold divertor plasmas. Cold divertor plasma is indispensable in reducing the erosion of divertor plates to an acceptable level [8,11,12].
The JT-60 [13] was upgraded in order to conduct the experiments in an intermediate region between the JT-60 and next-step tokamaks, and the upgrade device was named JT-60U [14,15]. Experiments with plasma currents up to 4 MA and NB heating powers up to 22 MW have been performed [16]. This paper presents the studies of impurities and radiation losses in the JT-60U tokamak. In section 2, the operation of the JT-60U tokamak and the overview of the impurity behavior in NB heated discharges are described. In section 3, the measurement and the calculation for the carbon influx in the divertor region are compared quantitatively for the first time. The mechanisms of carbon generation are discussed. In section 4, the dependence of radiative energy losses on the electron density and the safety factor is discussed. In section 5, the favorable operation regime for discharges with high power auxiliary heating is discussed based on the experimental results.
2. JT-60U operation and impurity contents In the JT-60U tokamak, the nominal plasma current is 6 MA for divertor discharges and the toroidal field is 4.2 T. The major radius is 3.4 m, and the minor radius is 1.1 m horizontally and 1.4 m vertically. A detailed description of the device is available elsewhere [14,15]. In 1991, experiments with plasma currents of up to 4 MA and NB heating powers of up to 22 MW have been performed. L-mode, H-mode and high poloidal beta regime have been investigated in the open divertor discharges with a lower null point. In the present
0022-3115/92/$05.00 9 1992 - Elsevier Science Publishers B.V. All rights reserved
72
H. Kubo et aL / Impurity generation mechanism in JT-60U
paper, the impurity behavior and radiation losses in L-mode discharges are discussed. A carbon-fiber composite with a thermal conductivity of 300 W / m ~ is used for the divertor plates. All plates are placed continuously and the edges of the divertor tiles with a difference in height between adjacent tiles larger than 0.5 mm are bevelled in the toroidal direction to avoid localized heat deposition. The inner surface of the vacuum vessel except for the divertor plates is covered with isotropic graphite tiles. Wall conditioning has been performed by a combination of the baking of the vessel ( ~ 300~ Taylor discharge cleaning (TDC) and He glow discharge cleaning (GDC) [17]. TDC was performed with a pressure of 0.2-1 • 10 -2 Pa, I o = 10-40 kA, B T = 0.7-1 T, a pulse duration of 30 ms and a repetition time of 1 s. GDC was performed with a pressure of ~ 0.1 Pa, a discharge voltage of 500-600 V and a discharge current of 0.5-1.5 A. The main impurity species were oxygen and carbon. GDC was effective for oxygen reduction, even though the discharge current was low. In ohmically heated discharges, the oxygen concentration after the GDC decreased by a factor of 2 down to 1%. On the other hand, the carbon concentration did not change. By the introduction of the GDC, Zeef decreased from 3.5 to 2.0 in ohmically heated discharges with an electron density of ~ 2 • 1019 m -3. A schematic diagram of the experimental setup of the important diagnostics in this work is shown in fig. 1. Electron density, radiation losses and impurity spectra were observed with FIR interferometers (two channels), bolometers (32 channels) and a VUV polychromator, respectively. The intensity of visible bremsstrahlung for Z~ff measurements was tangentially measured through a 10-channel optical fiber array with interference filters. For study of the divertor region, a variety of diagnostics were prepared (fig. lb). The intensities of line emissions of deuterium and impurities were measured with a 0.5 m visible spectrometer with fiber optics. The spatial distribution of the line intensities was observed through a 38-channel optical fiber array with interference filters. The electron temperature and the electron density were measured by the use of Langmuir probes [18]. The surface temperature of the divertor tiles was measured with an IRTV and the heat load onto the divertor tiles was derived. Time evolution of a discharge with NB heating of high power is shown in fig. 2. NB at a power level of 20 MW is injected for 2 s. The intensity of visible bremsstrahlung normalized by the square of the electron density indicates a value that is proportional to Zeef when the electron temperature is constant. During the NB heating, the normalized intensity of the visible bremsstrahlung does not increase, and this suggests that Zeff does not increase. The radiation losses do not suggest the occurrence of carbon bloom, either. Carbon blooms have not been observed even when the NB
(a) FIR Interterometer 2 Visible Spectrometer( 2 ch) Optical Fiber Array (38ch) I
_~_ Bolometer Array {16 ch) Optical Fiber Array for Zeff (10 ch) Bolometer Array (16 ch)
0
VUV Spectrometer 2
2.5
5.5 ~
4.5 Langmuir Probes (15ch. AR=2cm
(b) I
IRTV 38ch Optical FiberArray
/
Longmuir Probe I
R = 2.8
3.0
i
3.2
I
3.4 (m)
Fig. 1. (a) A schematic diagram of the experimental setup at the diagnostics. (b) The experimental setup of the diagnostics for the diverter study.
with a power of ~ 20 MW was injected for 2 s. The level of the injected NB power is close to the limit operation level without carbon bloom in TFTR (25 MW for 2 s) [19]. The surface temperature of the divertor plates was rather low because of the high thermal conductivity of the divertor tiles and the reduction of the heat load by remote radiative cooling. The maximum of the surface temperature measured with the IRTV was about 700~ for NB heated discharges. Fig. 3 shows Zef f as a function of absorbed heating power. In the low density region, Ze~f increases up to 4, as the absorbed power increases up to 18 MW. However, in the high density region, Zeff stays in the
H. Kubo et al. / Impurity generation mechanism in JT-60U 13780
[p=
2MA~
73
I
BT=4T
I
I
3 r ...........'.""--"', .... -'; v v ~,........................... ~ .
20
4
'[: 2
n'e =3-3.4x1019m-3
0
%, ~,/
~0
I
I
I
i=
3
0
./Bremss d~t/(.l'ned~) 2
S5
O
[
I
I
I
4
1
~2
~e= 3 . 8 . 4 . 5 x 1 019m.3
0 0
I j
6
I
I
7
8
9
power, fBremss d l / ( f n ~ dl) 2 the intensity of visible bremsstrahlung normalized by the square of the electron density, p~m~in the radiation loss in the main plasma, and 9 the radiation loss in the divertor region.
region of 2-3, while the absorbed power is increased up to 16 MW. The relation between impurity concentrations and the line averaged electron density for NB heated discharges is shown in fig. 4a. The concentrations were derived from Zef f and the intensity ratio of C V I (33.7 ,~) and O V I I I (19 ,~) [20]. As the electron density increases to 4.5 x 1019 m 3, the carbon concentration decreases to 2% and the oxygen concentration decreases to 1%. The dependence of carbon con-
, a , ~ o~ 4
Carbon
i
o
I
dD
3
.
r
\
j
o
4
3. C a r b o n
generation
mechanism
We discuss the impurity flux at the outer strike point of the separatrix on the divertor plates, because the electron temperature, pressure and heat load at
6 9
v
4
ne = 3 - 3 . 4 x 1 0 1 9 m 3
o
9
/ ne=3.8-4.5X1019m'3
~
1
0
o ~==/
2
Oxygen 0
20
r0
E 2
0
i
16
centration on absorbed power for NB heated discharges is shown in fig. 4b. In the low density region, the concentration increases up to 5.5%, as the absorbed power increases. On the other hand, in the high density region, the concentration stays in the region of 2-3.5%, while the absorbed power is increased up to 16 MW. To summarize, high density operation is effective in maintaining lower carbon concentration during the high power NB injection.
,m
1
i
12
Fig. 3. Zorf as a function of absorbed heating power for NB heated discharges.
r
,<~ ".o ~ o 9 o'-~.o
i
8
Pabs ( M W )
(b) 5 -
- ~ -
0
""
TIME (sec) Fig. 2. Time evolution of a discharge with NB heating at a power level of 20 MW. The plasma current was 2 MA, and the toroidal field was 4 T. ~ indicates the line averaged electron density in the main plasma, P ~ the NB heating
r
9
O I
0
r
9
9
J
2
3
fie (1 01 9m- 3)
4
5
0
5
10
15
20
Pabs ( M W )
Fig. 4. (a) Relation between impurity concentrations and line averaged electron density for NB heated discharges with beating powers of 10-18 MW. The open circles indicate the carbon concentration, the closed circles the oxygen concentration. (b) Carbon concentration as a function of absorbed power for NB heated discharges.
H. Kubo et aL / Impurity generation mechanism in JT-60U
74
(a)
60
(b) ~ \
1.5
F
,
NBI
0, k \
R"
\
.....
>
40
\
',0 \
-
i= 1.o
\ k
\
9
\
v
o.-"
9 -
/
O~
\
,. /
~:)
/
/
NBI
> nO
I-
20
,
:~
0.5 -
OH
4 -o
,"
0
0
I 1
I 2
I 3
0.0 0
4
t
i
1
n--e(1 01 9 m- 3)
2
i
3
4
n--e ( 1 01 9 m- 3)
Fig. 5. (a) Electron temperature and (b) electron density in the divertor region as a function of the line averaged electron density in the main plasma.
ing powers of 4.6-9.2 MW. In the ohmically heated discharges, the electron temperature in the divertor region decreases from 60 to 40 eV, as the electron density in the main plasma increases from 0.9 x 1019 to 1.8 x 1019 m 3. The electron density in the divertor region increases from 0.2 x 1019 to 0.4 x 1019 m -3. In the NB heated discharges, the electron temperature and density stay in the regions of 20-55 eV and (0.71.2) • 1019 m -3, respectively. For the discharges, the baking temperature of the vacuum vessel was 260~
the outer strike point were larger than those at the inner strike point [18]. The electron temperature and density at the outer strike point are shown as functions of the line averaged density in the main plasma in fig. 5. The data in the low density region were obtained in the ohmically heated discharges with plasma currents of 1.5-2.5 MA and toroidal fields of 2.5-3.8 T. The data in the high density region were obtained in the NB heated discharges with plasma currents of 2-2.5 MA, toroidal fields of 2.5-4 T and absorbed NB heat-
0.10
0.25
I
o
C
P
r ..Q
0v
(t)
t.-
0.20
OH NB
0.15
o t~ -"t _9.o 0
0.05
(:3
D
0.00 0
0.10
"'....
0
I
I
I
1
2
3
n e (1019
o
0.05 0.00 4
m "3)
Fig. 6. Observed (closed circles) and calculated (open symbols) relative carbon influxes as a function of the electron density in the main plasma. The left vertical axis is for the observation, and the right vertical axis is for the calculation. The open triangles indicate the calculated flux due to the sputtering by deuterium, the open squares the flux due to the sputtering by deuterium and oxygen, and the open circles the total flux. The data in the electron density region lower than 2x1019 m -3 were obtained in ohmically heated discharges, and the data in the higher density region were obtained in the NB heated discharges with heating powers of 4.6-9.2 MW.
75
H. Kubo et al. / Impurity generation mechanism in JT-60U
Under the condition of negligible recombination, the neutral influx equals the ionization rate. The ionization process is accompanied by the emission of spectral lines and the numbers of the ionization events and those of emitted photons are closely correlated. Therefore, the neutral particle influx can be derived from the measured intensity of the neutral line emission. In the region of low electron density, the relation between the neutral particle influx F and the line intensity I is basically given by the equation S (1)
r = 4 ~r--x~ l ,
where S and X are the ionization and excitation rate coefficients, respectively. B is the branching ratio for the observed line. This equation is commonly used to derive hydrogen influx from H a measurements [21], and it can also be used for influx measurements of low-charge-state impurities [22]. The influxes of deuterium, carbon and oxygen were derived respectively from the measured line intensities of D~, C II 6578, and O I I 4415, 4417 by using eq. (1). The ionization events per photon ( S / X B ) [22] at the electron temperature measured with the Langmuir probe was used in the analysis. In fig. 6, the observed carbon influx relative to the deuterium influx ( F c / F D) in the divertor region is shown as closed circles against the electron density. The relative carbon influx in the divertor region decreases from 10 to 1.5% as the electron density increases from 0.9 • 1019 to 3.4 • 1019 m 3. The carbon flux in the NB heated discharges seems to be a bit larger than that in the ohmically heated discharges, taking account of the electron density dependence. In a simple sputtering model, the carbon influx F c can be expressed as F c = Y D F ~ u' + Y o F ~ ~' + Ec s c
,
where i indicates the ion, C s the velocity of sound, 4~s the sheath potential, and qi the charge number of the ion. The ion temperature was assumed to be equal to the electron temperature. The sheath potential was assumed to be 3Tf iv. By considering the recycling time and the ionization time, we assumed that all of the produced carbon returned to the divertor plates with a charge number of 4. The charge number of incident oxygen ions is not important, because the energy dependence of the sputtering yield for oxygen is weak. In experiments with ion beams, the chemical sputtering yield for deuterium is much larger than the physical sputtering yield for deuterium. The chemical sputtering yield depends on the surface temperature and has a maximum value at the temperature of 550~ For oxygen, the chemical sputtering yield is as large as the physical sputtering yield. The chemical sputtering yield depends neither on the surface temperature nor on the incident energy of the ion. The total sputtering yield for oxygen is always nearly unity. In the following calculation, the sputtering yields for normal incidence are used for simplicity, although the yields increase with the incidence angle. For ohmically heated discharges with T~ iv = 39-45 eV and the maximum surface temperature of the divertot tiles of 300-310~ the relation between the relative carbon influx and the relative oxygen influx is shown in fig. 7. The relative carbon influx increases with the relative oxygen influx. Because the electron temperature and the surface temperature of the divertor plates are nearly constant, all of the sputtering yields in eq. (3) are considered to be constant. The observed linear correlation can be explained by eq. (3). It is clearly shown that the sputtering with oxygen is an important process for the carbon production.
(2)
where F oout , r ~ u t and F,~ut are the fluxes of deuterium, oxygen, and carbon ions onto the divertor plates, respectively. YD, Yo and u are sputtering yields of carbon by deuterium, oxygen and carbon ions, respectively [6,23]. Considering a steady state, we can assume that the outflux is equal to the influx measured with the spectrometers. Then eq. (2) can be transformed into
rc
Yo
ro
YD
0.10
r..-,
0.00
1
2
E i = ~ k T i + ~ m i C s + eqi&s ,
(4)
o/"
/
/
0.04
Physical and chemical processes are considered for the sputtering by deuterium and oxygen. The sputtering by carbon (self-sputtering) is a physical process. The physical sputtering yield depends on the incident energy of the ions. The energy of the incident ions E was assumed to be given by ref. [24]
-
1-Yc'
/" oj
0.02
-
at"
0.06
(3)
+
F D - 1 - Y c FD
/
0.08
,/o
,
l
I
I
0.00 0.02 0.04
i
I
I
I
n
0 . 0 6 0 . 0 8 0.10
Fo/FD Fig. 7. Relation between the relative carbon influx and the relative oxygen influx for ohmically heated discharges with the electron temperature in the divertor region of 39-45 eV and the maximum surface temperature of the diverter plates of 300-310~
76
H. Kubo et al. / Impurity generation mechanism in JT-60U
r
o
"~ 0
a
~
0
"-
10.0
8.0 6.0
0 0
OH
i 0
i
i
NBI 0
0
0 O
4.0
C 0 m
4
2.0,
t.
--
-SS--
--0--
--
I I I I 0.0 3 0 0 350 400 450 5 0 0 5 5 0 6 0 0
Temperature of Divertor Plates (~
Fig. 8. Ratio of the calculated relative carbon influx to the observed influx as a function of the surface temperature of the divertor plates. The chemical sputtering by deuterium is included in the calculation for open circles and excluded for closed circles. The data in the temperature region of ~ 310~ were obtained in ohmically heated discharges, and the data in the temperature region higher than 400~ were obtained in NB heated discharges with heating powers of 4-13 MW.
The ratio of the calculated carbon influx to the observed influx is shown as a function of the surface temperature of the divertor plates in fig. 8. The open circles indicate the ratio for the carbon flux calculated including the chemical sputtering by deuterium and the closed circles indicate the ratio for that calculated neglecting the chemical sputtering. For the chemical sputtering yield by deuterium, the dependence on the ion flux density was considered as the sputtering yield was proportional t o FD 0"1. The surface temperature of the divertor plates in ohmically heated discharges is about 310~ and the temperature increases up to 540~ in the NB heated discharges. The chemical sputtering yield by deuterium increases with the surface temperature of the carbon plates and has a maximum at 550~ For the calculation including the chemical sputtering, the ratio of the calculated carbon flux to the observed flux in the ohmically heated discharges is 3.3. The ratio increases in NB heated discharges from 5 to 10, as the surface temperature increases. On the contrary, the ratio for the calculation neglecting the chemical sputtering is always nearly constant. Therefore, the observed carbon flux does not depend on the surface temperature. It suggests that the contribution of the chemical sputtering is small. Such a phenomenon has been observed in other tokamaks [4]. Some mechanisms of the reduction of the chemical sputtering by the deuterium were suggested. It has been reported that the chemical sputtering yield decreases as the ion flux density increases [25]. The ion
flux density onto the divertor plates increased from 1 x 1021 to 1.3 X 1022 m -2 s -1, as the line averaged electron density in the main plasma increased from 0.9 x 1019 to 3.4 x 1019 m 3. The flux density was higher by 1-2 orders of magnitude than that in ion beam experiments. In the calculation, the dependence has already been included as mentioned previously and it cannot explain the experimental result. The presence of metal contamination on the surface suppresses the chemical sputtering [26]. However, this effect would be unlikely in the JT-60U tokamak, because most of the inner surface of the vacuum vessel is covered with graphite tiles and metal contamination would be little. The sputtered carbon might recycle rapidly near the divertor plates before emitting the observed line [27]. It is probable that the carbon produced in CO 4 form hardly affects the intensity of the C II line. The relative carbon influx calculated by neglecting the chemical sputtering with deuterium is shown as open symbols in fig. 6. It is shown that the calculated carbon flux agrees with the measured flux not only in the ohmically heated discharges but also in the NB heated discharges, except for a discrepancy of a factor of 2.5. In the low density region (ne = 0.9 X 1019 m-3), the self-sputtering and the sputtering by oxygen are important. The relative carbon influx decreases for a higher electron density because of the smaller selfsputtering yield and the decrease in the relative oxygen density. In the medium density regime (~e = 1.8 x 1019 m-3), the contributions of the sputterings by deuterium, oxygen and carbon are about 17%, 44% and 39% of the total carbon influx, respectively. The carbon flux in the NB heated discharges is larger than the flux in the ohmically heated discharges, taking account of the electron density dependence. The increase is attributed to the increase of the self-sputtering with the electron temperature in the divertor region. In the high electron density region (~e = 3.4 X 1019 m -3) , the contributions of the sputterings by deuterium, oxygen and carbon are about 32%, 36%, and 32% of the total carbon influx, respectively. There are uncertainties in the estimation of the incident energy of ions. However, the error in the estimation of the incident energy would hardly affect the calculated result in the high electron temperature region, because the dependence of the sputtering yield on the incident energy is weak in the high energy region. One possible reason for the discrepancy between the observed carbon flux and the calculated one might be due to the error in the estimation of the influx from the line intensity. The error in the absolute influx measurement is usually estimated to be a factor of 2, while the deduction of relative influx is expected to be more reliable [28]. In addition, a more sophisticated calculation would be required. The effect of the spatial distribution and the recycling of sputtered ions should be considered.
77
H. Kubo et al. / Impurity generation mechanism in JT-60U
As discussed in section 2, the carbon bloom has not been observed in the discharges with high power heating. The surface temperature of the divertor plates measured with the IRTV did not exceed 700~ and the concentration of heat deposition at edges of carbon tiles would be reduced by good alignment and the edge bevelling of the carbon tiles. Therefore, up to now, the radiation enhanced sublimation might not play an important role in JT-60U. In fig. 9, the ratio of the carbon density normalized by the deuterium density in main plasmas to the carbon influx normalized by the deuterium influx is shown as a function of the electron density in the main plasma. The data were obtained for ohmically heated discharges. The ratio decreases as the electron density increases. Therefore, this suggests that the shielding efficiency for carbon is higher as the electron density is higher. The tendency can be explained by the penetration probabilities of carbon and deuterium. Here we assumed that carbon in the core plasma came from the strike point. In ohmically heated discharges, the carbon influx from the vessel wall was smaller by an order of magnitude than the influx from the divertor plates. However, the carbon influx from the wall may affect the carbon concentration in the core plasma, because the shielding efficiency for carbon generated at the wall would be lower than that for carbon generated at the strike point. At the present stage, it is difficult to clarify the contribution of the carbon generated at the wall experimentally. Considering these results, the reduction in the oxygen concentration is effective to reduce the carbon flux. It would be realized by wall conditionings such as boronization [29]. The oxygen concentration in He discharges was lower than that in D 2 discharges, and the carbon concentration in He discharges was also lower than that in D 2 discharges. As the electron
'E C~ ~<
0 div . . . . . . . . . . . .
2
0 - 1.5 - ~div.
% \
9
r.-9
\
r--.,
9
9
\ X o IN N
0.5 c"(...)
I
1.0
i
I
i
1.5
i
2.0
r
2.5
ne (lOI9m-3) Fig. 9. Ratio of normalized carbon concentration in the main plasma (n c / n D) to normalized carbon influx in the divertor region ( F c / F D) against line averaged electron density for ohmically heated discharges.
.
% 1.0
0.5 0 55O
~
3OO 250
"
moin
~
J
•
L
T div.
22__/
" J
5
L
L
7 8 9 10 TIME (sec) Fig. 10. Time evolution of a typical remote radiative cooling. The plasma current was 1.7 MA, and the toroidal field was 3 W. qmax div indicates the maximum heat flux density onto the divertor plates, and Tma~x indicates the maximum surface temperature of the divertor plates.
6
temperature in the divertor region decreases, the physical sputtering yield decreases and the carbon flux is reduced. Operation in the high electron density region reduces the relative oxygen concentration and the electron temperature in the divertor region. It is more effective to lower the electron temperature with the aid of the dense divertor. In addition to that, the shielding efficiency is higher in the high electron density region.
4. R e m o t e
1.0
O.Od.
E14238
radiative
cooling
Time evolution of a typical remote radiative cooling is shown in fig. 10. During the NB heating with a power of 7.5 MW, the electron density was raised due to gas puffing. As the electron density increases, the radiation loss in the divertor region increases up to 45% of the NB heating power. The increase in the radiation loss in the main plasma is gradual and the fraction is less than 20%. The maximum value of heat flux density onto the divertor plates decreases simultaneously. The maximum value of the surface temperature of the divertor plates decreases gradually. Remote radiative cooling reduces the heat load onto the divertor plates. Fig. 11 shows radiation power as a function of absorbed power. The radiation power in the main plasma and that in the divertor region increase, as the
78
H. Kubo et al. / Impurity generation mechanism in JT-60U
510 "-"
i
i
,
I
9 9 9
8 6
J
i
I
,
F
I
~
2-2.3 MA 2.4-2.7MA 3.3-3.5 MA =D t' 9
+
In
9 A9
o -
4
.o
2
=
i
,
i
I
~
i
F
1.3 MA 1.7 MA
9
~.~
i
length plays a role. The dependence might be explained by the decrease in the electron temperature in the peripheral plasma region with the increase in connection length. To summarize, the remote radiative cooling power is higher for discharges with high density and high safety factor.
0~-
,oqlD= 9 1 4 9
e
t=
r ""
5. Summary
-%
9 I 9 ,-c~,~.s.~ o n-
0
4
8
Absorbed
12
16
2G
Power(MW)
Fig. 11. Radiation power against absorbed power for NB heated discharges. Open symbols indicate radiation loss in the main plasma, and closed symbols indicate total radiation loss.
absorbed power increases. In the discharges with absorbed powers of 16 MW, the radiation power in the main plasma and that in the divertor region is 2 and 4 MW, respectively. The correlation between the fraction of radiation loss and the line averaged electron density is shown in fig. 12a. The fraction of the radiation loss in the divertor region increases as the electron density increases. In the high density region, the fraction of the radiation loss in the divertor region and that in the main plasma are about 35% and 15%, respectively. These fractions are close to those observed in discharges with lower null points in JT-60 [30,10]. In fig. 12b, the fraction of the radiation loss is plotted against neqeff. Comparing fig. 12a with fig. 12b, the data are relatively scattered in fig. 12a and the correlation is clearer in fig. 12b. The radiation loss increases as neqeff increases. This suggests that the connection
In In
0.6
"r
0.5
I
0.6
I
(a)
o
"o
" c O
0= n"
0.4 0.3
c-
.o u.
~
tr
divertorr
N,,=
o
:= .~_ -o
+
~x
0.2
N,=,
o
.o
main I ~176 0
9 9 9 + x
0.5 0.4
0.2
I
t
i
2
3
-fie (1019 m-3)
P 4
u.
9
9
9 +
divertor
z~
~
0.1
Gc~ []
main
o
1
(b)
2 MA/3T 2 MN4T 2.2 MA/4T 2.7 MA/4T 3.3 MA/4T
0.3
c
q c ~ c ~A o ~ O
0.1
9 2 MA/ZST
o
O
'~,
In JT-60U, experiments with plasma currents up to 4 MA and NB heating powers up to 22 MW have been performed. The impurity content, the impurity generation mechanism and the remote radiative cooling have been studied. NB with a power of ~ 20 MW was injected for 2 s, and the carbon bloom has not been observed. Carbon is the dominant impurity and the concentration stays in the region of 2-3.5% in the high electron density region even if the NB power increases up to 16 MW. The relative carbon influx in the divertor region decreases from 10 to 1.5% as the electron density in the main plasma increases from 0.9 • 1019 to 3.4 • 1019 m -3. The dependence of the carbon influx on the electron density can be explained by the sputtering with deuterium (physical sputtering), oxygen and carbon. The contribution of the chemical sputtering by deuterium is small. The importance of the sputtering by oxygen is clear. The decrease in relative carbon flux in the high density region is explained by the reduction in the relative oxygen concentration and the decrease in the electron temperature in the divertor region. In ohmically heated discharges, the shielding efficiency for carbon becomes higher as the electron density increases. The fraction of the radiation loss in the divertor region and the fraction of that in the main plasma is 35% and 15% in the NB heated discharges with high
o
t
0
I
t
I
I
5
t
I
t
I
t
10
t
i
r
15
fieqeff (1019 m-3)
Fig. 12. (a) Dependence of the fraction of the radiation loss power on line average electron density. (b) Dependence of the fraction of the radiation loss power on the electron density multiplied by the safety factor.
H. Kubo et al. / Impurity generation mechanism in JT-60U
electron density. T h e fraction of the r a d i a t i o n power increases with n~qef f. This suggests that the c o n n e c t i o n length plays a role. In the p r e s e n t work, the a d v a n t a g e of high density o p e r a t i o n has b e e n shown for impurity dilution, impurity g e n e r a t i o n a n d impurity shielding. A n d it has b e e n shown that o p e r a t i o n with the high density and the high safety factor increases the r e m o t e radiative cooling. In ref. [18] it was p r e s e n t e d that the peaking factor of the heat load in the scrape-off layer was propor0.45 (I.67 tional to n~ qeff , also suggesting the a d v a n t a g e of high density and high safety factor operation. To summarize, the o p e r a t i o n with the high density a n d the high safety factor is favorable for impurity control, alleviation of divertor tile erosion a n d h e a t removal.
Acknowledgements T h e authors would like to express sincere gratitude to Dr, T. Takizuka a n d Dr. S. T a n a k a of J a p a n Atomic Energy R e s e a r c h Institute for useful discussion. T h e authors appreciate the fruitful discussion on the sputtering m e c h a n i s m with Dr. J. R o t h of Max-Planck-Institut fflr Plasmaphysik. T h e authors wish to t h a n k the m e m b e r s who have c o n t r i b u t e d to the J T - 6 0 U project.
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79
[5] JET Team, J. Nucl. Mater. 176 & 177 (1990) 3. [6] J. Roth, J. Nucl. Mater. 145-147 (1987) 87. [7] R.K. Janev and A. Miyahara, Suppl. Nucl. Fusion (1991) 123. [8] M. Shimada et al., Nucl. Fusion 22 (1982) 643. [9] M.G. Bell et al., J. Nucl. Mater. 121 (1984) 132. [10] M. Shimada et al., J. Nucl. Mater. 176 & 177 (1990) 122. [11] Y. Shimomura et al., Nucl. Fusion 23 (1983) 869. [12] S. Sengoku et al., Nucl. Fusion 24 (1984) 415. [13] JT-60 team, Proc. 12th Int. Conf. on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 1988, vol. 1 (IAEA, Vienna, 1989) p. 67. [14] H. Ninomiya et al., Plasma Devices and Operation 1 (1990) 43. [15] H. Horiike et al., Fusion Eng. Des. 16 (1991) 285. [16] H. Ninomiya and JT-60 Team, Phys. Fluids B4 (1992) 2070. [17] M. Shimada et al., these Proceedings (PSI-10), J. Nucl. Mater. 196 198 (1992) 164. [18] K. ltami et al., these Proceedings (PSI-10), J. Nucl. Mater. 196-198 (1992) 755. [19] D.M. Meade et al., Proc. 13th Int. Conf. on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 19911, vol. 1 (IAEA, Vienna, 1991) p. 9. [20] H. Kubo el al., Nucl. Fusion 29 (1989) 571. [21] L.C. Johnson and E. Hinnov, J. Quant. Spectrosc. Radiat. Transf. 13 (1973) 333. [22] K.H. Behringer, J. Nucl. Mater 145-147 (1987) 145. [23] J. Roth, E. Vietzke and A.A. Haasz, Nucl. Fusion, suppl. (1991) 63. [24] R. Chodura, J. Nucl. Mater. 111 & 112 (1982) 420. [25] D.M. Goebel et al., Nucl. Fusion 28 (1988) 1041. [26] J. Roth, J. Bohdansky and J.B. Roberto, J. Nucl Mater. 128 & 129 (1984) 534. [27] W.D. Langer, Nucl. Fusion 22 (1982) 751. [28] K. Behringer et al., Plasma Phys, Control. Fusion 31 (1989) 2059. [29] J, Winter et al., J. Nucl. Mater. 162-164 (1989) 713. [30] S. Tsuji et al., Proc. 12th Int. Conf. on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 1988, vot. 1 (IAEA, Vienna, 1989) p. 265.