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Particle balance studies in TEXTOR during experiments of pellet injection, helium injection, and ICR-heating
ELSEVIER
Journal of Nuclear Materials 220-222 (1995) 478-482
Particle balance studies in TEXTOR during experiments of pellet injection, helium injection, and ICR-heating T. Banno a, N. Noda b, K.H. Finken c, D.S. Gray TEXTOR
d, j. Winter c, A L T - I I
Team
c,d,
Team c
Department of Applied Physics, University of Tokyo, Japan b National Institute for Fusion Science, Japan c Institut fiir Plasmaphysik, Forschungszentrum Jiilich, Association EURATOM/KFA, Jiilich, Germany d University of California, Los Angeles, USA
Abstract Analysis based on the particle conservation law has been carried out to observe the global fuelling process in tokamak discharges. The response of the net recycling flux from the first wall is investigated in the tokamak TEXTOR, using calibrated signals of the gas feed rate, the neutral gas pressure in the vessel, the total amount of electrons, and the particle removal rates by the ALT-II belt-pump limiter and by a main pump unit. Net absorption (pumping) of hydrogen by the wall is observed for almost all tokamak discharges since a new wall conditioning technique called siliconisation is employed. The net absorption or fuelling depending on the discharge condition influenced by injection of pellets, by helium gas injection combined with neutral beam injection, and by rf heating can be interpreted in terms of the particle-induced desorption effect with depth profile taken into consideration.
1. Introduction The study of particle balance is an important issue for the operation of future long pulse fusion devices e.g., an increase of fuelling rate by pellets leads to an increase of fusion power, which induces the temperature increase of the first wall, then the release of dissolved hydrogen in the wall, and results in another increase of fuelling rate. There could be a wide range of time constants in this loop including the retention and depletion [1]. It is therefore necessary to obtain its data base from present tokamaks as well as from laboratory experiments [2,3,4] for the future devices. The tokamak TEXTOR is a suitable machine for this purpose, equipped with well-calibrated diagnostic tools, a belt pump limiter ALT-II, a pellet injector, and auxiliary means for plasma heating.
2. Modelling An employed method for the particle balance analysis is based on the equation describing the simple
conservation law of particles with rates of supplies and removals: dN
dNn
dt
dt
dt
= - S o n n - RALT -- ~w -t- a/tw + Qg + QNSX+ Qoell"
(1)
Here N is the total amount of ions in plasma, N, is the total amount of neutral particles in the vessel, and N~ is the total amount of neutrals in the plenum and in the duct of the pump limiter, which is not negligible when the gate valve to the pump unit is closed. Son n is the pumping rate of the main turbo molecular pump (nn: the neutral gas density at the entrance of the pumping port, Sp: its effective pump speed), RALT is the particle removal rate by the pump limiter, ~w is the particle flux to the Wall, ~w is the particle flux from the wall, Qg is the gas inlet rate, QNm is the intensity of neutral beam injectors, and Qoell is the fuelling rate by the pellet injector. It is often convenient to illustrate the behaviour of the total amount of
0022-3115/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-3115(94)00645-8
T. Banno et al. /Journal of Nuclear Materials 220-222 (1995) 478-482 ions N(t) instead of its derivative, so that we integrate Eq. (1) and obtain an expression: t
¢
N ( t ) + Nn(t ) + N ' ( t ) + S p f nn(t ) dt' t
¢
t
!
+ f RAL-r( t ) dt'-- f O,x( t ) dt' = ft(~P w - tOw) dt',
(2)
where Qex = Qg + QNBI + Qpeu.
479
of the main pump unit on the assumption of uniform distribution of neutrals outside the plasma. The amount of ions N, is assumed to be equal (Zeff = 1) to that of total number of electrons that is measured with an interferometer capable of resolving the electron density distribution in radial direction.
4. Results and discussion
4.1. General remarks and results of pellet injection (3)
In T E X T O R the values of the terms on the left hand side of Eq. (2) are measurable, whereas the term on the right-hand side, which indicates the net change of dissolved atoms in the wall, is a quantity to be evaluated in this study.
3. Experimental The experimental condition of the tokamak discharges is the following: line averaged electron density Re = 3.5 x 1019 m -3, plasma current 340 kA (at fiat top t ~ 1.5 s), toroidal magnetic field 2.25 T. The minor radius is determined to be 0.46 m by the ALT-II belt pump limiter. Ice pellets of hydrogen are injected for tens of discharges at t = 1.54 s. A helium gas pulse is supplied to several other discharges at t = 1.0 s with a duration of 10 ms ( ~ 1019 atoms) while a neutral beam injector is switched on between 0.4 and 2.2 s with the energy of 50 keV (its intensity 1.3 A) to heat the plasma edge. Ion cyclotron resonance heating (ICRH) experiments are also performed with various rf-power 0.6-2.8 MW. The wall elements were conditioned with a newly developed technique of siliconisation [5] one week before the experiments of helium gas injection and two weeks before the pellet injection. A technique of boronisation modified to reduce the hydrogen content in the coating was employed prior to the experiment on ICRH. The removal rate of particles from the plasma by the ALT-II belt pump limiter is measured using ion gauges (calibrated for the flow rate) in the duct connected to the pump units [6]. Piezoelectric valves in the gas inlet system are operated in the pulse width modulation (PWM) method so that the gas inlet rate Qg is almost proportional to a signal of the duty ratio of PWM. The time-integrated amount of the gas feed is evaluated independently from the pressure decrease Apg in the gas reservoir, and is used only for the calibration of the above-mentioned rate Qg because of the limited time response of Apg. The amount of neutral particles N., in the vessel of TEXTOR is estimated using a fast ion gauge located in a porthole
A diagram, which illustrates the results of each term of the left-hand side of Eq. (2), is drawn in Fig. 1 for a discharge (two weeks after the siliconisation) with a pellet: it shows the total external amount of fuel added until time t (thick curve) and also shows the particles that are found remaining as ions in plasma at t (height #1 with shading), in the portion removed by the pump iimiter ALT-II up to t (height # 2 with vertical hatching), in the portion removed by the main pump unit up to t (height #3 with diagonal hatching), or remaining as neutrals in the vessel but outside the plasma at t (height # 4 dotted hatching). The removal rate of the pump limiter ALT-II is evaluated to be 4 x 1019 atoms/s during the fiat top in accord with the results reported by Gray et al. [6]. As discussed by Ehrenberg [7], the fuel efficiency is defined as the ratio N(t)/ftQexdt ', the value of which is typically of the order of 30%. When the pellet is injected at 1.54 s, measurable increase can only be seen in N(t): the fuel efficiency of the pellet is quite close to 100%.
[xl020] Shot No.52699: on 20.Oct.92, (Siliconisafion: on 3.Oct.92) 8
'~
6
oE y~ 5
~
3
E a Z 1 0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
time [secl Fig. 1. Diagram indicating amounts of particles of a tokamak discharge additionally fuelled by a pellet injected at t = 1.54 s.
T. Banno et al. /Journal of Nuclear Materials 220-222 (1995) 478-482
480
The portion #5 in Fig. 1 corresponds to the net amount of dissolved particles in the wall. If we assume a homogeneous distribution of hydrogen in the wall of 35 m 2 (the liner surface is the dominant plasma density controlling material in T E X T O R [7]), 1.5 × 1015 atoms are retained in 1 cm 2 up to 2 s. Its time variation is the net fuel rate (q)w - qw) from the wall according to Eq. (1), and is plotted in Fig. 2. The value remains negative about - 1.5 × 10 21 a t o m s / s , indicating that the wall is kept capable of pumping even 2 weeks after the siliconisation. As soon as a pellet is injected, the net fuel rate becomes more negative i.e., further absorption of the wall takes place for a period of 100 ms during which the electron temperature decreases abruptly from 0.9 to 0.6 keV and then increases to 0.8 keV. This enhancement of absorption could be explained in terms of the ion-induced desorption: ions (or charge-exchanged neutrals) with lower kinetic energy E (roughly in proportion to the electron temperature) have a smaller value of cross section /3(E) for desorption and hence the concentration of dissolved hydrogen atoms at steady state is higher [8].
4.2. Helium gas injection experiments with neutral beams Fig. 3 shows the variation of the net f u e l / a b s o r p tion rate from the wall with ALT-II gate valves open (thick curve) and closed (thinner curve) i.e., not pumping. Helium gas is injected at t = 1 s with NBI from t = 0.4 to 2.2 s. The total numbers of ions for both discharges are almost identical. W h e n the plasma edge is heated by the neutral beam, the kinetic energy of ions striking the wall is increased, the ion-induced desorption rate is once enhanced to a maximum of ~ 10 21 a t o m s / s , i.e. ~ 3 × 1015 atoms c m - 2 s 1 at t = 0.43 s. Thus the near surface layers tend to be depleted while the higher energy ions are implanted to
[xl02°atoms/s] . . . . . . . .
I
. . . .
I
4
. . . .
I
. . . .
I
. . . .
I"
'
Shot No.52699: on 20.Oct.92
-2 e,¢
-4
lx 102°]
i .52559 (ALT-II pumping)
& 5
.52560 (ALT-[Inot pumping)
-
E
.5
~ -10 ~
-15
r-~ g -2C
f.v~,~
z 0.0
0.5
1.0
lnjecuo. !~O'~a,om~. . . . 1.5
2.0
2.5
3.0
time [sec] Fig. 3. Comparison between the net fuel rates from the wall during experiments with helium injection and weak neutral beam injection: a discharge with the gate valve of ALT-II open (#52559: thicker curve) and another with the valve closed (#52560: thinner curve).
deeper layers so that the net absorption is induced [3] from t ~ 0.5 s. As soon as helium gas is injected, hydrogen is released (stronger depletion of near surface layers) due to a larger cross section of helium than that of hydrogen in accord with the laboratory results, f l H e ( E ) > f l ( E ) [3]. McCracken et al. have reported that the effective confinement time of helium exceeds significantly 1 s i.e., a recycling impurity [9]. The concentration of helium in the plasma is about 4% for the discharge with gate valves closed [10]. The desorption rate remains high and the net absorption rate is small at 3 - 5 × 10 20 a t o m s / s . But as the removal of helium by the A L T - I I proceeds with gate valves open, the desorption rate is decreased and the net absorption rate reaches a value of ~ 1.5 x 10 21 a t o m s / s i.e., the level before the helium injection. Since the plasma parameters are almost identical between these two discharges, the hydrogen flux out of the plasma to the wall q~w should also be identical except for the difference in the helium flux intensity. Thus the difference between the two quasi-constant values of the net absorption rate can be attributed to the effect of heliuminduced desorption: its rate ~ 1 × 10 21 a t o m s / s is large, compared with the removal rate by ALT-II.
4.3. Ion cyclotron resonance heating experiment
~-6
-10 0.0
~
.
0.5
1.0
.
I
1.5
. . . .
I
2.0
. . . .
I
2.5
. . . .
I
.
,
3.0
time [sec] Fig. 2. Net fuel rate from the wall during the same discharge as in Fig. 1. Negative values correspond to the net absorption (pumping) rate of the wall.
A diagram similar to Fig. 1 is plotted in Fig. 4 for a discharge heated by the rf power of 2.4 MW. This figure indicates that the plasma density increases from t = 1 s even though the external fuel is stopped, and that the sum of the portions 1-4 exceeds the value of the external fuel at ~ 1.2 s. If this excess continues as seen in the case of carbonised walls [2], the system will
T. Banno et at/Journal of Nuclear Materials 220-222 (1995) 478-482
[xtO 2°] I0I
I [] [] []
of absorption may be attributed to the ion implantation to the near surface layers which have been depleted of hydrogen during ICRH. The strong depletion may furthermore lead to the remarkable net absorption during the soft landing phase (t > 2.5 s), as is also shown in Fig. 4: the external gas feed is required again from t = 3.2 s. Both absorption and desorption rates become smaller for discharges with the lower rf power.
Shot NO.54780 on 9.Mar.93 1.1ons 2.ALT- Removal 3.Removal of Neutrals 4.Neutrals External Fuel
481
1¢
8
6
5. Concluding remarks 4 Z 2
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
time [sec] Fig. 4. Diagram indicating amounts of particles of a tokamak discharge heated by ICR at 2.4 MW. Meanings of the simbols are the same as in Fig. 1. not be able to be controlled. But this modified technique of boronisation enables us again to control the plasma density in a short time. The net fuel/absorption rate of the wall in Fig. 5 shows that an abrupt desorption occurs upon initiation of the rf heating due to the similar effect of the edge plasma as in the above-mentioned NBI heating [11]. Then absorption starts somewhat more slowly than the one that occurred in the case of NBI (Fig. 3) and the net absorption rate reaches a maximum of 2 x 1021 atoms/s. From t = 1.5 s this rate decreases probably due to an increase of the desorption rate resulting from the temperature increase of the wall during ICRH, as is pointed out by Noda [1]. When the rf power is switched off at t = 2 s, a small abrupt increase of the absorption rate can be seen in Fig. 5. This occurrence
[xl02°] "~
IS.''''i i
....
I .... I=
I ....
I .... =I
J .... i .... Shot N o , 5 4 7 8 0
I
5
0
t.~ -10 ~.~ -15
z -20 0.0
0.5
1.0
1.5 2.0 2.5 3.0 3.5 time [sec] Fig. 5. Net fuel rate from the wall during the same discharge as in Fig. 4.
The analysis on the global particle balance has been performed for tokamak discharges at TEXTOR. It allows us to observe quantitatively the behavior of the wall with respect to the absorption (wall pumping) or desorption (wall fuelling) of hydrogen. The results obtained from the experiments of pellet injection, helium gas injection combined with NBI, and ICRH are able to be explained qualitatively in terms of the ion- (or high energy neutral particle-) induced desorption of hydrogen from the wall with the consideration of implantation depth profile. The net absorption is observed for almost all the discharges since the wall conditioning techniques of siliconisation and boronisation are employed. Further study will be continued to distinguish some local effect from the global behaviour e.g., the interaction of limiter surfaces and of rf antennas with plasma. Quantitative comparison of the data of the net wall fuel rate with the laboratory results on the cross section/3(E) is also required in order to have the data base for the operation of future larger devices.
Acknowledgements We thank K.N. Sato and A. Hiller for operating the pellet injector, and appreciate J.A. Boedo and V. Philipps for their fruitful discussion.
References [1] N. Noda, in: Contributions to High Temperature Plasma Physics, eds. J. Uhlenbusch and K.H. Spatschek (Akademie Verlag, Berlin, 1994). [2] R.E. Clausing and L. Heatherly, J. Nucl. Mater. 145-147 (1987) 317. [3] S. Michizono, T. Banno and A. Kinbara, Vacuum 41 (1990) 1493. [4] D.S. Walsh, B.L. Doyle, W.R. Wampler and A.K. Hays, J. Vac. Sci. Technol. A 9 (1991) 727. [5] J. Winter, H.G. Esser, G. Jackson, L. K6nen, V. Philipps, A. Pospieszczyk, U. Saturn, B. Schweer, B. Unterberg and P. Wienhold, Proc. 20th Europ. Phys. Soc. Conf. on Controlled Fusion and Plasma Physics, vol. 1, Lisbon, Portugal, 1993, p. 279.
482
T. Banno et al. /Journal of Nuclear Materials 220-222 (1995) 478-482
[6] D.S. Gray, J.A. Boedo, R.W. Conn, K.H. Dippel, K.H. Finken, R.A. Moyer, A. Pospieszczyk, D. Reiter, U. Samm and G.H. Wolf, J. Nucl. Mater. 196-198 (1992) 1096. [7] J. Ehrenberg, V. Philipps, L. De Kock, R.A. Causey and W.L. Hsu, J. Nucl. Mater. 176&177 (1990) 226; J. Ehrenberg, J. Nucl. Mater. 162-164 (1989) 63. [8] F.G. Waelbroeck, Vacuum 39 (1989) 821. [9] G.M. McCracken, U. Samm, P.C. Stangeby, G. Bert-
schinger, J.A. Boedo, S.J. Davies, D.S. Gray, V. Philipps, R.A. Pitts, D.H.J. Goodall, A. Pospieszczyk, R.P. Schorn, B. Schweer, G. Telesca, B. Unterberg and G. Waidmann, Nucl. Fusion 33 (1993) 1409. [10] K.H. Finken, D.L. Hillis et al., J. Nucl. Mater. 176&177 (1990) 816. [11] R.A. Moyer, R. van Nieuwenhove, et a|. J. Nucl. Mater. 176&177 (1990) 293.
Journal of Nuclear Materials 220-222 (1995) 478-482
Particle balance studies in TEXTOR during experiments of pellet injection, helium injection, and ICR-heating T. Banno a, N. Noda b, K.H. Finken c, D.S. Gray TEXTOR
d, j. Winter c, A L T - I I
Team
c,d,
Team c
Department of Applied Physics, University of Tokyo, Japan b National Institute for Fusion Science, Japan c Institut fiir Plasmaphysik, Forschungszentrum Jiilich, Association EURATOM/KFA, Jiilich, Germany d University of California, Los Angeles, USA
Abstract Analysis based on the particle conservation law has been carried out to observe the global fuelling process in tokamak discharges. The response of the net recycling flux from the first wall is investigated in the tokamak TEXTOR, using calibrated signals of the gas feed rate, the neutral gas pressure in the vessel, the total amount of electrons, and the particle removal rates by the ALT-II belt-pump limiter and by a main pump unit. Net absorption (pumping) of hydrogen by the wall is observed for almost all tokamak discharges since a new wall conditioning technique called siliconisation is employed. The net absorption or fuelling depending on the discharge condition influenced by injection of pellets, by helium gas injection combined with neutral beam injection, and by rf heating can be interpreted in terms of the particle-induced desorption effect with depth profile taken into consideration.
1. Introduction The study of particle balance is an important issue for the operation of future long pulse fusion devices e.g., an increase of fuelling rate by pellets leads to an increase of fusion power, which induces the temperature increase of the first wall, then the release of dissolved hydrogen in the wall, and results in another increase of fuelling rate. There could be a wide range of time constants in this loop including the retention and depletion [1]. It is therefore necessary to obtain its data base from present tokamaks as well as from laboratory experiments [2,3,4] for the future devices. The tokamak TEXTOR is a suitable machine for this purpose, equipped with well-calibrated diagnostic tools, a belt pump limiter ALT-II, a pellet injector, and auxiliary means for plasma heating.
2. Modelling An employed method for the particle balance analysis is based on the equation describing the simple
conservation law of particles with rates of supplies and removals: dN
dNn
dt
dt
dt
= - S o n n - RALT -- ~w -t- a/tw + Qg + QNSX+ Qoell"
(1)
Here N is the total amount of ions in plasma, N, is the total amount of neutral particles in the vessel, and N~ is the total amount of neutrals in the plenum and in the duct of the pump limiter, which is not negligible when the gate valve to the pump unit is closed. Son n is the pumping rate of the main turbo molecular pump (nn: the neutral gas density at the entrance of the pumping port, Sp: its effective pump speed), RALT is the particle removal rate by the pump limiter, ~w is the particle flux to the Wall, ~w is the particle flux from the wall, Qg is the gas inlet rate, QNm is the intensity of neutral beam injectors, and Qoell is the fuelling rate by the pellet injector. It is often convenient to illustrate the behaviour of the total amount of
0022-3115/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-3115(94)00645-8
T. Banno et al. /Journal of Nuclear Materials 220-222 (1995) 478-482 ions N(t) instead of its derivative, so that we integrate Eq. (1) and obtain an expression: t
¢
N ( t ) + Nn(t ) + N ' ( t ) + S p f nn(t ) dt' t
¢
t
!
+ f RAL-r( t ) dt'-- f O,x( t ) dt' = ft(~P w - tOw) dt',
(2)
where Qex = Qg + QNBI + Qpeu.
479
of the main pump unit on the assumption of uniform distribution of neutrals outside the plasma. The amount of ions N, is assumed to be equal (Zeff = 1) to that of total number of electrons that is measured with an interferometer capable of resolving the electron density distribution in radial direction.
4. Results and discussion
4.1. General remarks and results of pellet injection (3)
In T E X T O R the values of the terms on the left hand side of Eq. (2) are measurable, whereas the term on the right-hand side, which indicates the net change of dissolved atoms in the wall, is a quantity to be evaluated in this study.
3. Experimental The experimental condition of the tokamak discharges is the following: line averaged electron density Re = 3.5 x 1019 m -3, plasma current 340 kA (at fiat top t ~ 1.5 s), toroidal magnetic field 2.25 T. The minor radius is determined to be 0.46 m by the ALT-II belt pump limiter. Ice pellets of hydrogen are injected for tens of discharges at t = 1.54 s. A helium gas pulse is supplied to several other discharges at t = 1.0 s with a duration of 10 ms ( ~ 1019 atoms) while a neutral beam injector is switched on between 0.4 and 2.2 s with the energy of 50 keV (its intensity 1.3 A) to heat the plasma edge. Ion cyclotron resonance heating (ICRH) experiments are also performed with various rf-power 0.6-2.8 MW. The wall elements were conditioned with a newly developed technique of siliconisation [5] one week before the experiments of helium gas injection and two weeks before the pellet injection. A technique of boronisation modified to reduce the hydrogen content in the coating was employed prior to the experiment on ICRH. The removal rate of particles from the plasma by the ALT-II belt pump limiter is measured using ion gauges (calibrated for the flow rate) in the duct connected to the pump units [6]. Piezoelectric valves in the gas inlet system are operated in the pulse width modulation (PWM) method so that the gas inlet rate Qg is almost proportional to a signal of the duty ratio of PWM. The time-integrated amount of the gas feed is evaluated independently from the pressure decrease Apg in the gas reservoir, and is used only for the calibration of the above-mentioned rate Qg because of the limited time response of Apg. The amount of neutral particles N., in the vessel of TEXTOR is estimated using a fast ion gauge located in a porthole
A diagram, which illustrates the results of each term of the left-hand side of Eq. (2), is drawn in Fig. 1 for a discharge (two weeks after the siliconisation) with a pellet: it shows the total external amount of fuel added until time t (thick curve) and also shows the particles that are found remaining as ions in plasma at t (height #1 with shading), in the portion removed by the pump iimiter ALT-II up to t (height # 2 with vertical hatching), in the portion removed by the main pump unit up to t (height #3 with diagonal hatching), or remaining as neutrals in the vessel but outside the plasma at t (height # 4 dotted hatching). The removal rate of the pump limiter ALT-II is evaluated to be 4 x 1019 atoms/s during the fiat top in accord with the results reported by Gray et al. [6]. As discussed by Ehrenberg [7], the fuel efficiency is defined as the ratio N(t)/ftQexdt ', the value of which is typically of the order of 30%. When the pellet is injected at 1.54 s, measurable increase can only be seen in N(t): the fuel efficiency of the pellet is quite close to 100%.
[xl020] Shot No.52699: on 20.Oct.92, (Siliconisafion: on 3.Oct.92) 8
'~
6
oE y~ 5
~
3
E a Z 1 0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
time [secl Fig. 1. Diagram indicating amounts of particles of a tokamak discharge additionally fuelled by a pellet injected at t = 1.54 s.
T. Banno et al. /Journal of Nuclear Materials 220-222 (1995) 478-482
480
The portion #5 in Fig. 1 corresponds to the net amount of dissolved particles in the wall. If we assume a homogeneous distribution of hydrogen in the wall of 35 m 2 (the liner surface is the dominant plasma density controlling material in T E X T O R [7]), 1.5 × 1015 atoms are retained in 1 cm 2 up to 2 s. Its time variation is the net fuel rate (q)w - qw) from the wall according to Eq. (1), and is plotted in Fig. 2. The value remains negative about - 1.5 × 10 21 a t o m s / s , indicating that the wall is kept capable of pumping even 2 weeks after the siliconisation. As soon as a pellet is injected, the net fuel rate becomes more negative i.e., further absorption of the wall takes place for a period of 100 ms during which the electron temperature decreases abruptly from 0.9 to 0.6 keV and then increases to 0.8 keV. This enhancement of absorption could be explained in terms of the ion-induced desorption: ions (or charge-exchanged neutrals) with lower kinetic energy E (roughly in proportion to the electron temperature) have a smaller value of cross section /3(E) for desorption and hence the concentration of dissolved hydrogen atoms at steady state is higher [8].
4.2. Helium gas injection experiments with neutral beams Fig. 3 shows the variation of the net f u e l / a b s o r p tion rate from the wall with ALT-II gate valves open (thick curve) and closed (thinner curve) i.e., not pumping. Helium gas is injected at t = 1 s with NBI from t = 0.4 to 2.2 s. The total numbers of ions for both discharges are almost identical. W h e n the plasma edge is heated by the neutral beam, the kinetic energy of ions striking the wall is increased, the ion-induced desorption rate is once enhanced to a maximum of ~ 10 21 a t o m s / s , i.e. ~ 3 × 1015 atoms c m - 2 s 1 at t = 0.43 s. Thus the near surface layers tend to be depleted while the higher energy ions are implanted to
[xl02°atoms/s] . . . . . . . .
I
. . . .
I
4
. . . .
I
. . . .
I
. . . .
I"
'
Shot No.52699: on 20.Oct.92
-2 e,¢
-4
lx 102°]
i .52559 (ALT-II pumping)
& 5
.52560 (ALT-[Inot pumping)
-
E
.5
~ -10 ~
-15
r-~ g -2C
f.v~,~
z 0.0
0.5
1.0
lnjecuo. !~O'~a,om~. . . . 1.5
2.0
2.5
3.0
time [sec] Fig. 3. Comparison between the net fuel rates from the wall during experiments with helium injection and weak neutral beam injection: a discharge with the gate valve of ALT-II open (#52559: thicker curve) and another with the valve closed (#52560: thinner curve).
deeper layers so that the net absorption is induced [3] from t ~ 0.5 s. As soon as helium gas is injected, hydrogen is released (stronger depletion of near surface layers) due to a larger cross section of helium than that of hydrogen in accord with the laboratory results, f l H e ( E ) > f l ( E ) [3]. McCracken et al. have reported that the effective confinement time of helium exceeds significantly 1 s i.e., a recycling impurity [9]. The concentration of helium in the plasma is about 4% for the discharge with gate valves closed [10]. The desorption rate remains high and the net absorption rate is small at 3 - 5 × 10 20 a t o m s / s . But as the removal of helium by the A L T - I I proceeds with gate valves open, the desorption rate is decreased and the net absorption rate reaches a value of ~ 1.5 x 10 21 a t o m s / s i.e., the level before the helium injection. Since the plasma parameters are almost identical between these two discharges, the hydrogen flux out of the plasma to the wall q~w should also be identical except for the difference in the helium flux intensity. Thus the difference between the two quasi-constant values of the net absorption rate can be attributed to the effect of heliuminduced desorption: its rate ~ 1 × 10 21 a t o m s / s is large, compared with the removal rate by ALT-II.
4.3. Ion cyclotron resonance heating experiment
~-6
-10 0.0
~
.
0.5
1.0
.
I
1.5
. . . .
I
2.0
. . . .
I
2.5
. . . .
I
.
,
3.0
time [sec] Fig. 2. Net fuel rate from the wall during the same discharge as in Fig. 1. Negative values correspond to the net absorption (pumping) rate of the wall.
A diagram similar to Fig. 1 is plotted in Fig. 4 for a discharge heated by the rf power of 2.4 MW. This figure indicates that the plasma density increases from t = 1 s even though the external fuel is stopped, and that the sum of the portions 1-4 exceeds the value of the external fuel at ~ 1.2 s. If this excess continues as seen in the case of carbonised walls [2], the system will
T. Banno et at/Journal of Nuclear Materials 220-222 (1995) 478-482
[xtO 2°] I0I
I [] [] []
of absorption may be attributed to the ion implantation to the near surface layers which have been depleted of hydrogen during ICRH. The strong depletion may furthermore lead to the remarkable net absorption during the soft landing phase (t > 2.5 s), as is also shown in Fig. 4: the external gas feed is required again from t = 3.2 s. Both absorption and desorption rates become smaller for discharges with the lower rf power.
Shot NO.54780 on 9.Mar.93 1.1ons 2.ALT- Removal 3.Removal of Neutrals 4.Neutrals External Fuel
481
1¢
8
6
5. Concluding remarks 4 Z 2
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
time [sec] Fig. 4. Diagram indicating amounts of particles of a tokamak discharge heated by ICR at 2.4 MW. Meanings of the simbols are the same as in Fig. 1. not be able to be controlled. But this modified technique of boronisation enables us again to control the plasma density in a short time. The net fuel/absorption rate of the wall in Fig. 5 shows that an abrupt desorption occurs upon initiation of the rf heating due to the similar effect of the edge plasma as in the above-mentioned NBI heating [11]. Then absorption starts somewhat more slowly than the one that occurred in the case of NBI (Fig. 3) and the net absorption rate reaches a maximum of 2 x 1021 atoms/s. From t = 1.5 s this rate decreases probably due to an increase of the desorption rate resulting from the temperature increase of the wall during ICRH, as is pointed out by Noda [1]. When the rf power is switched off at t = 2 s, a small abrupt increase of the absorption rate can be seen in Fig. 5. This occurrence
[xl02°] "~
IS.''''i i
....
I .... I=
I ....
I .... =I
J .... i .... Shot N o , 5 4 7 8 0
I
5
0
t.~ -10 ~.~ -15
z -20 0.0
0.5
1.0
1.5 2.0 2.5 3.0 3.5 time [sec] Fig. 5. Net fuel rate from the wall during the same discharge as in Fig. 4.
The analysis on the global particle balance has been performed for tokamak discharges at TEXTOR. It allows us to observe quantitatively the behavior of the wall with respect to the absorption (wall pumping) or desorption (wall fuelling) of hydrogen. The results obtained from the experiments of pellet injection, helium gas injection combined with NBI, and ICRH are able to be explained qualitatively in terms of the ion- (or high energy neutral particle-) induced desorption of hydrogen from the wall with the consideration of implantation depth profile. The net absorption is observed for almost all the discharges since the wall conditioning techniques of siliconisation and boronisation are employed. Further study will be continued to distinguish some local effect from the global behaviour e.g., the interaction of limiter surfaces and of rf antennas with plasma. Quantitative comparison of the data of the net wall fuel rate with the laboratory results on the cross section/3(E) is also required in order to have the data base for the operation of future larger devices.
Acknowledgements We thank K.N. Sato and A. Hiller for operating the pellet injector, and appreciate J.A. Boedo and V. Philipps for their fruitful discussion.
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482
T. Banno et al. /Journal of Nuclear Materials 220-222 (1995) 478-482
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