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Limiter damage due to over-heat load in doublet III tokamak
Nuclear Engineering and Design/Fusion 1 (1984) 279-285 North-Holland, Amsterdam
LIMITER D A M A G E D U E TO O V E R - H E A T
279
LOAD IN DOUBLET
! I i TOKAMAK
H. Y O K O M I Z O , M. K A S A I *, T. T A Y L O R **, R. C A L L I S , **, D. D O L L **, H. A I K A W A , A. K I T S U N E Z A K I , S. K O N O S H I M A , T. M A T S U D A , M. N A G A M I a n d M. S H I M A D A Japan Atomic Energy Research Institute, Tokai, Ibaraki, Japan
Received 1 September 1983
Two tiles of the primary limiter, which are composed of TiC coated Poco graphite, are heavily cracked during neutral beam heating experiment (beam power of 2 MW). These damages are considered to be caused by the thermal stress due to the over-heat load (2.3 kW/cm2). The major disruption occurs after a rapid increase of the impurity influx following to the tile cracks. A base temperature of the limiter tile is over 300 o C at the time of this accident. These kind of accidents are avoidable with a careful monitoring of the limiter temperature.
1. Introduction The tokamak device seems to be one of the promising candidates for the fusion reactors [1]. The tokamak, however, has a dangerous phenomena regarding the machine safety during operation. The heat deposition onto the structure of the first wall and the limiter are of concern for the machine's safe operation. Especially, if during a major disruption, the major part of the stored energy is released within a short period and the heat flux onto the first wall exceeds the allowable value of the materials. Plasma operation which avoids major disruptions will be of the upmost importance in future devices. The main features of disruptions are well studied, however, causes of disruptions are less understood because of the variety of observations [2]. In general, the major disruption is triggered by the current related or the density-related instability. In the initial phase of the disruption, some M H D instabilities are usually observed to grow. In this paper, one type of disruption is discussed, which is correlated with the high heat flux to the primary limiter. Major disruptions were observed during neutral beam heating phase (beam power of 2 MW) in Doublet Ill. Two limiter tiles were broken at these disruptions. Plasmas had a small minor radius which was limited by the primary limiter. Since the heat deposition con* On leave from Mitsubishi Atomic Power Industry, Japan. ** GA Technologies, Inc. San Diego, California, USA.
centrated in a small area on the primary limiter, the base temperature of the limiter tile became high although the backing plate of the limiter was water-cooled. Impurity influx suddenly increased and the perturbations of soft X-ray signals propagated from the outer region to the central region. Thereafter a current disruption occured. Mirnov coil did not show any enhanced fluctuation until the plasma current started to decrease. Plasma TV showed that a large eruption occurred on the limiter surface after the disruption. Section 2 describes the experimental conditions of the plasma configuration and the plasma diagnostics. Section 3 shows time behaviours obtained from several diagnostics during a disruption and section 4 presents the results of thermocouple measurement of the limiter tiles and their analysis. Section 5 discusses the mechanisms of the tile crack and the disruption. Section 6 presents the conclusion.
2. Experimental apparatus D O U B L E T III [3] has a capability of making several kinds of plasma shapes. The present experiment was carried out in the following conditions: a major radius of 1.45 m, a minor radius of 0.36 m (so called a small minor radius plasma), an elongation of 1.2, a toroidal field of 24 kG, and neutral beam heating power of 2 MW. One poloidal limiter, which is called as the primary limiter, is located on the outer-side vessel wall at one toroidal angle. It is composed of six graphite tiles in a
0 1 6 7 - 8 9 9 x / 8 4 / $ 0 3 . 0 0 © Elsevier S c i e n c e P u b l i s h e r s B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g D i v i s i o n )
280
H. Yokomizo et aL / Limiter damage due to over-heat load
75 cm x 18 cm array m o u n t e d to a water-cooled metal structure with barrel nuts [4,5]. The graphite tiles are Poco A X F - 5 Q coated with 20 /~m of TiC. The heat deposition due to the conduction and convection losses of the plasma energy was concentrated in a small area of the primary limiter because the plasma surface was in contact only with the primary limiter as shown in fig. 1. Plasma diagnostics used in the present study are summarized as follows; TiXX(254 ,~) line radiation was measured by V U V spectrometer a n d CIII(4649 ,~,) and H , ( 6 5 6 3 A) line radiations were measured by photodiodes with b a n d - p a s s filters. Their time resolutions are 0.4 ms which are limited by the sampling time of digitizers. A n array of 21 silicon barrier diodes was used to measure the soft X-ray flux emitted along horizontal chords with a sampling time of 0.2 ms. The spacial resolution is 3 cm a r o u n d the plasma center. T h e vertical distribution of the radiation power was measured by a 21-channel bolometer array. One M i r n o v coil was located inside the v a c u u m vessel and measured M H D fluctuations up to several 10 kHz recorded with a sampling time of 25 p,s. A tangential TV observed the interaction between the plasma surface a n d the primary limiter with a s t a n d a r d speed of 16 ms. M a n y thermocouples were installed in the tiles of the primary limiter, back-up limiters a n d a r m o r tiles. Their temperatures were automatically recorded at the time of 10 s before a n d after each discharge. The increment of the temperature before a n d after a discharge provides the information of the integrated heat deposition during one discharge. Photographs of the inside v a c u u m vessel were taken from outside through the same window as the tangential T V m e a s u r e m e n t on a m a i n t e n a n c e day.
3. Time evolutions of diagnostics during major disruptions M a j o r disruptions occur after several successive discharges with a current of more than 700 kA. Waveforms of several plasma diagnostics during a disruption are s h o w n in fig. 2. T h e neutral beam is injected from 670 ms to 840 ms. T h e plasma is stable as with normal discharges before 820 ms. The sawtooth oscillation is normally observed d u r i n g the steady state with a repetition time of 17 ms and a last relaxation occurs at 807 ms. T h e impurity line radiations seem also the same as the normal discharges and" the total radiation power before the disruption is a b o u t 800 kW which is a b o u t 30% of the total input power.
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Fig. 1. Crosssectionof a smallradiusplasma.A plasmasurface contacts only to the primary limiter. The primary limiter and armor tiles are composedof PocographiteAXF-5Q and the back-up limitersare composedof inconelX-750.
~
0 4
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:~
12
10
•~¢ , ~
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1-___-_
Armor -Tiles
(a)
810
820 830 840 Time (msec)
850
Fig. 2. Wave-forms of several measurements during a major disruption. (a) plasma current and one turn voltage, (b) average electron density, (c) line radiations of H,, CIII and TiXX, (d) soft X-ray diodes looking through chords of Z = 0 cm and Z = 24 cm, (e) Mirnov oscillation.
H. Yokomizo et al. / Limiter damage due to ooer-heat load
281
J
Q
1 .,0
f Fig. 3. TV pictures during a major disruption and a photograph of the primary limiter. Black points in TV pictures have no meaning. (a) at 64 ms before the disruption, (b) at the disruption. (c) at 32 ms after the disruption. (d) at 64 ms after the disruption. (e) an illustration of the tangential view of TV, (f) a photograph of the damaged third and forth tiles from the top.
At the time of 822 ms, a sudden and rapid increment is observed in CIII line radiation at first, and then T i X X signal shows the sudden increase. C a r b o n and titanium are both the intrinsic impurities from the primary limiter. The perturbations of the soft X-ray diode signals are observed to propagate from outer location into inner location with a propagation time of 1
ms, which indicates a high propagation speed of 240 m / s . The start of the outer diode perturbation is coincident with the start time of CIII perturbation. The H,, signal starts to increase at the later time (about 2 ms) than TiXX signal. The H , perturbation is followed by the electron density perturbation. Mirnov coil signal shows no enhanced oscillation until the plasma current
282
H. Yokomizo et a L /
Limiter damage due to over-heat load
starts to decrease. The internal instability does not begin at the initial phase during which the impurity influx increases. These observations suggest that the cause of the perturbation is located at the outer region or outside of the plasma. At 1 ms after the sudden increase, each soft X-ray diode signal rapidly decreases with a decay time of 3 ms. The plasma temperature is considered to be cooled at this time from the outer region to the inner region. At 5 ms after the initial perturbation, the plasma current started to decrease with a speed of - 7 5 MA/s. The decay time is about 11 ms. This decaying speed is a typical value for a current disruption in Doublet III. It seems that the strong influx of cold impurities from the primary limiter cools the plasma temperature and extinguishes the plasma current. A similar type of the disruption which is correlated with the impurity accumulation was discussed in another report [6]. But their observations were different from the present experiment because the impurity accumulation was slow and the current density profile was changed and m / n = 2/1 mode was observed to grow exponentially in the initial phase of their disruptions. The plasma TV shows that a large eruption happens on the primary limiter, which is different from usual disruptions. At the time of the disruption the TV picture becomes bright filling the entire frame. A large eruption is observed on the limiter surface in the next frame and many fragments flew about in the frames that followed as shown in fig. 3. The photographs taken on a maintenance day following the experiment reveals that damages occured on the graphite tiles of the primary limiter at the exact location where TV showed the eruption. Two tiles were broken into several pieces and some of these pieces were missing as shown in fig. 3. Although it is unclear whether this disruption created the initial damage leading to the tile cracks or unobservable damage had already existed before this major disruption, the tiles of the primary limiter were obviously damaged by this disruption.
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Time ( Hour ) Fig. 4. Temperature history of the third tile of the primary limiter. (o) and (O) indicate the temperatures before and after discharges, respectively. Arrows shgw the temperature increments during discharges. ( x ) presents two disruptive discharges discussed in this report. and the repetition time between shots is long in order to cool down the tile temperature. Note that two disruptions occur when the base temperature of the primary limiter is higher than 300 o C. Fig. 5 shows the temperature increments during discharges with the various plasma current. The temperature increment presents the integrated heat deposition onto the tile during each discharge. As the plasma current becomes high, the heat deposition becomes large for normal discharges shown by circles. The disruptions shown by crosses occur at the high plasma current of 700-800 kA. The temperature increments of disruptive discharges are lower (407o) than normal discharges. The reason of the smaller increment is explained by a short ~80
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4. Thermal measurement and analysis
Fig. 4 shows a history of the temperature of the third tile from the top, which presents the highest temperature among six tiles of the primary limiter. Arrows indicate the increments of the limiter temperature during discharges. Cross marks show two disruptive discharges which are discussed in this paper. Discharges with a lower base temperature between two disruptions shown in fig. 4 have a lower plasma current than 500 kA
~
20 E o
0
I
I
K
I
200
400
600
800
1000
Ip (kA) Fig. 5. Temperature increment of the third tile during each discharge. (o) and ( x ) indicate normal and disruptive discharges, respectively.
283
H. Yokomizo et al. / Limiter damage due to ooer-heat load
/
~
Primary Limiter Heat Deposition
~
Armor T i l e s (T~:21~r~,t ~J~ Backup Lirniter
:~0
31
_ ~
____Total240 kJ"
Fig. 6. Integrated energy balance of a beam-heated discharge without a disruption. Plasma parameters are: Ip = 800 kA, discharge duration of 1 s, injected beam power of 2 MW, beam duration of 180 ms.
discharge duration of a disruptive discharge, total input energy of which is less (about 20%) than that of a normal discharge. Another possible reason is the heat loss which is due to the dropping of small fragments of the tile after the disruption. Thermocouples installed in the armor tiles show that the temperature increments for the disruptive discharges are similar or slightly lower than that for the normal discharges. This suggests that a heat deposition profile to the materials around the plasma during a disruptive discharge is quite similar to that during a normal discharge. Integrated energy flow of a beam-heated discharge without disruption ( l p = 800 kA) is shown in fig. 6. Heat depositions to the primary limiter, armor tiles and back-up limiters are obtained from the heat analysis using the temperature increments measured by thermocouples with the assumption of a homogeneous temperature profile of inside tiles. The radiation loss energy is the integrated value obtained by bolometric measurement. The integrated input energy to the plasma is estimated by summing up the neutral beam injected
Heat Flux Ohmic 0.06 03 0.8 Q8 0.4 0.2
(kW/crrF) N BI 0.5 1.2 2.3 2.1 2.0 0.9
Fig. 7. Heat fluxes to six tiles of the primary limiter during ohmic and beam heating phases.
energy and the ohmically heated energy. About 25% of the integrated input energy deposits onto the primary limiter. Heat fluxes to the primary limiter during the ohmic heating phase and the beam heating phase are estimated by comparing the thermocouple data of two successive discharges with and without a neutral beam injection. Fig. 7 shows the heat flux to six tiles of the primary limiter. The heat flux to the third tile is 0.8 k W / c m 2 during the ohmic heating phase with an ohmic input power of 1.1 MW. It increases to 2.3 k W / c m 2 during the neutral beam heating phase with an input power of 2.8 MW. It is assumed that the heat flux homogeneously deposits to one third of the whole surface area of the primary limiter [7]. A thermal calculation shows that the temperature of the third tile reaches 1300°C after a beam injection as shown in fig. 8. Once the plasma becomes disruptive, then the stored energy of the plasma is released to the limiter and to the wall within a short time. The heat flux to the primary limiter during the disruption is estimated to be 600 k W / c m 2 when 50% of the stored thermal energy (100 k J) is assumed to deposit onto the limiter area of 10 cm x 40 cm with the disruption time of 0.2 ms [8]. This heat flux is large enough to make the surface temperature reach the sublimation temperature. It is interesting that the disruption occurs at the 5th or 7th discharge after successive discharges with a similar type of operation condition as shown in fig. 4. The
1400 1200 A
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200
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I
0
2oo
I
I
I
400 600 800 Time (msec)
looo
Fig. 8. Numerical simulation of surface temperature of the Poco graphite.
284
H. Yokomizo et aL / Limiter damage due to ooer-heat load
base temperature of the limiter tiles are high (more than 300 o C) and almost the same values for these successive discharges. If there is a small area of low heat conductivity compared with that of the bulk graphite (i.e., the microcracks and the runaway damages on the limiter surface [5]), it is possible that the thermal energy in this area might accumulate during the sequential discharges and its surface temperature might become higher than indicated by the thermocouple measurement. In another aspect, it is also possible that the thermal fatigue of the graphite tile might accumulate during the sequential discharges. These kinds of conditions seem to have some correlations to create a tile crack and to produce a major disruption.
5. Discussion
The TV picture showed the large eruption at the disruption and photographs revealed cracks of the limiter tiles. Two scenarios are considered in this phenomenon: (a) increase of limiter surface temperature ~ increase of impurity influx ~ major disruption ~ tile crack, (b) increase of limiter surface temperature--, tile crack --* increase of impurity influx ~ major disruption. There is little doubt that the increase of the impurity influx triggers the major disruption. At first, a mechanism of the impurity creation which must explain a sudden and rapid increase of the impurity influx is discussed. Experimental data of the chemical sputtering on targets of TiC [9] and graphite [10] show that the sputtering ratio is almost constant or smoothly increases with the rising of the target temperature. No drastic increase is expected at the high target temperature up to 2300%. The blistering rate is expected to be low when the target temperature is high
POWER FLOW
cz ~.~
BARRE
NUT HOLE
,, . . . . J~,,,
u~
Fig. 9. Cross section of the limiter tile and schematics of the produced in a chord through the tile. Tensile stress at the barrel nut hole is three times higher than the value shown in this picture.
stress
Table 1 Maximum stress in a tile of primary limiter (kg/mm 2)
Steady state Disruption Strength ~
Tensile stress at hole
Compression stress at surface
5 5 5.6
11 19 20 (steady state) 29 (disruption)
" Catalog value of Poco graphite AXF-5Q.
and the incident particle is hydrogen [11]. The evaporation rate smoothly increase~ with a nearly exponential function as the surface temperature becomes high so that a sudden and rapid increase of the evaporation is not expected. The scenario (a) has no appropriate mechanism to explain the impurity behaviour. A one-dimensional stress calculation based on the analytical formula [12] was carried out in order to examine the mechanism of the tile crack. The stress profile along a chord through a limiter tile is shown in fig. 9. The limiter surface has a maximum value of the compression stress. This value is low to reach a compression strength of the Poco graphite as shown in table 1. The barrel nut hole is located in the region of the maximum tensile stress as shown in fig. 9. The tensile stress at the hole edge is comparable to the tensile strength. This tensile stress is suspected to be a primary cause of the tile crack during a disruption and also during a steady state. If the limiter tile suddenly cracks for the first incident during a steady state, then the rapid increase of the impurity influx is easily explained and a following major disrupt'ion is also understood. Presently, the scenario (b) seems more probable. The high occurrence of disruptions have been observed in other series of experiments on Doublet III under the following conditions, (a) after sequential discharges with especially high plasma current (around 1 MA), and (b) after a runaway discharge. These disruptions are suspected to be triggered by an enhanced impurity influx from a hot spot which is created on the limiter surface by previous discharges. Any pronounced damage of the limiter, however, had not been observed in these discharges except a spotty damage due to a runaway discharge [13]. The disruption caused by a high heat deposition onto a small area is not unusual and its occurrence will happen often in future devices because they will have high power heatings and high power productions of fusion energy. This
H. Yokomizo et al. / Limiter damage due to over-heat load
kind of disruption, however, is easily prevented by adjusting the plasma position a n d the plasma shape to maximize the plasma surface which makes contact to the limiters. The m e a s u r e m e n t of the heat depositions to the materials a r o u n d the plasma is one usual method in o r d e r to know the status of the plasma operation and will become an i m p o r t a n t tool in future devices for the interlock in order to avoid a hazardous plasma operation.
6. Conclusion Major disruptions a c c o m p a n i e d with the limiter damages have been observed during the high power heating phase (2 M W neutral b e a m injection) in D o u b let III. The analysis suggests that the limiter tiles are cracked by the thermal stress due to the over-heat load (2.3 k W / c m 2 ) . Major disruptions are triggered by the e n h a n c e d impurity influx following to the tile cracks. T h e base t e m p e r a t u r e of the limiter tile is over 3 0 0 ° C at these disruptions although the tile is m o u n t e d to a water-cooled metal structure. T h e base temperature m o n i t o r i n g the limiter tiles is one of the useful methods to know the heat deposition and to avoid heat concentration o n t o a small area.
Acknowledgement The authors would like to express their sincere gratitude to Doublet 11I operation, diagnostics and neutral b e a m groups for their fine supports. They would like to express their appreciation to Drs. N. Fujisawa and T. A b e for their fruitful discussion. They would also like to t h a n k Drs. T. Ohkawa, S. Mori, M. Yoshikawa for their encouragement. This work was authorized by a cooperative agreem e n t between the J a p a n Atomic Energy Research Institute and the United States d e p a r t m e n t of energy u n d e r D O E contract No. D E - A T 0 3 - 8 0 S F l l 5 1 2 .
285
References [1] INTOR GROUP, International tokamak reactor: Phase one, Rep. Int. Tokamak Reactor Workshop Vienna, 1980/1981, int. Atomic energy Agency, Vienna (1982). [2] For example, IAEA Syrup. Current Disruption in Toroidal Devices, Garching, 1979 Max Plank Inst. Plasmaphysik, Garching, Rep. IPP 3/51 (1979). [3] R.W. Callis, Doublet Ill baseline description, General Atomic Company Rep. GA-A13996 (1976). [4] D.L. Sevier, P.W. Trester, G. Hopkins, T.E. McKelvey, T.S. Taylor, Performance of TiC-coated graphite in electron beam tests and Doublet IlI operation, General Atomic Company Rep. GA-AI6384 (1981). [5] T. McKelvey, T. Taylor, P. Trester, E. Reis, R. Gallix, P. Galdos, E. Johnson, F. Puhn, D. Doll. L. Sevier, H. Yokomizo, M. Nishikawa, A. Kitsunezaki, in Fusion Technology (Proc. 12th Symp. Julich, 1982) 511. [6] W. Engelhart, O. Kluber, K . Lackner, S. Sesnic, IAEA Syrup. Current Disruption in Toroidal Devices, Garching, 1979 Max Plank inst. Plasmaphysik, Garching, Rep. IPP 3/51 (1979) A6. [7] T. Taylor, N. Brooks, K. Ioki, J. Nucl. Mater. l l l & l 1 2 (1982) 569. [8] T. Taylor, Limiter heat fluxes during plasma disruptions, General Atomic Company Rep. GA-AI6816 (1981) 27. [9] R. Yamada, K. Nakamura, M. Saido, J. Nucl. Mater. l l l & l 1 2 (1982) 744. [10] J. Roth, J. Bohdansky, K.L. Wilson, J. Nucl. Mater. l l l & l 1 2 (1982) 775. [11] M. Saidoh, R. Yamada, K. Nakamura, J. Nucl. Mater. l l l & l 1 2 (1982) 848. [12] S. Taira, Thermal stress and thermal fatigue, The Nikkan Kogyo Shinbun, Tokyo (1974) 22. [13] M. Nishikawa, H. Yokomizo, A. Kitsunezaki, T.E. McKelvey, T.S. Taylor, D. Doll, N. Brooks, R. Seraydarian, Graphite limiter damage due to runaway electron and abrupt lp termination in Doublet III, 6th Internat. Conf. on Plasma Surface Interactions in Controlled Fusion Devices, J. Nucl. Mater. 128&129.
LIMITER D A M A G E D U E TO O V E R - H E A T
279
LOAD IN DOUBLET
! I i TOKAMAK
H. Y O K O M I Z O , M. K A S A I *, T. T A Y L O R **, R. C A L L I S , **, D. D O L L **, H. A I K A W A , A. K I T S U N E Z A K I , S. K O N O S H I M A , T. M A T S U D A , M. N A G A M I a n d M. S H I M A D A Japan Atomic Energy Research Institute, Tokai, Ibaraki, Japan
Received 1 September 1983
Two tiles of the primary limiter, which are composed of TiC coated Poco graphite, are heavily cracked during neutral beam heating experiment (beam power of 2 MW). These damages are considered to be caused by the thermal stress due to the over-heat load (2.3 kW/cm2). The major disruption occurs after a rapid increase of the impurity influx following to the tile cracks. A base temperature of the limiter tile is over 300 o C at the time of this accident. These kind of accidents are avoidable with a careful monitoring of the limiter temperature.
1. Introduction The tokamak device seems to be one of the promising candidates for the fusion reactors [1]. The tokamak, however, has a dangerous phenomena regarding the machine safety during operation. The heat deposition onto the structure of the first wall and the limiter are of concern for the machine's safe operation. Especially, if during a major disruption, the major part of the stored energy is released within a short period and the heat flux onto the first wall exceeds the allowable value of the materials. Plasma operation which avoids major disruptions will be of the upmost importance in future devices. The main features of disruptions are well studied, however, causes of disruptions are less understood because of the variety of observations [2]. In general, the major disruption is triggered by the current related or the density-related instability. In the initial phase of the disruption, some M H D instabilities are usually observed to grow. In this paper, one type of disruption is discussed, which is correlated with the high heat flux to the primary limiter. Major disruptions were observed during neutral beam heating phase (beam power of 2 MW) in Doublet Ill. Two limiter tiles were broken at these disruptions. Plasmas had a small minor radius which was limited by the primary limiter. Since the heat deposition con* On leave from Mitsubishi Atomic Power Industry, Japan. ** GA Technologies, Inc. San Diego, California, USA.
centrated in a small area on the primary limiter, the base temperature of the limiter tile became high although the backing plate of the limiter was water-cooled. Impurity influx suddenly increased and the perturbations of soft X-ray signals propagated from the outer region to the central region. Thereafter a current disruption occured. Mirnov coil did not show any enhanced fluctuation until the plasma current started to decrease. Plasma TV showed that a large eruption occurred on the limiter surface after the disruption. Section 2 describes the experimental conditions of the plasma configuration and the plasma diagnostics. Section 3 shows time behaviours obtained from several diagnostics during a disruption and section 4 presents the results of thermocouple measurement of the limiter tiles and their analysis. Section 5 discusses the mechanisms of the tile crack and the disruption. Section 6 presents the conclusion.
2. Experimental apparatus D O U B L E T III [3] has a capability of making several kinds of plasma shapes. The present experiment was carried out in the following conditions: a major radius of 1.45 m, a minor radius of 0.36 m (so called a small minor radius plasma), an elongation of 1.2, a toroidal field of 24 kG, and neutral beam heating power of 2 MW. One poloidal limiter, which is called as the primary limiter, is located on the outer-side vessel wall at one toroidal angle. It is composed of six graphite tiles in a
0 1 6 7 - 8 9 9 x / 8 4 / $ 0 3 . 0 0 © Elsevier S c i e n c e P u b l i s h e r s B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g D i v i s i o n )
280
H. Yokomizo et aL / Limiter damage due to over-heat load
75 cm x 18 cm array m o u n t e d to a water-cooled metal structure with barrel nuts [4,5]. The graphite tiles are Poco A X F - 5 Q coated with 20 /~m of TiC. The heat deposition due to the conduction and convection losses of the plasma energy was concentrated in a small area of the primary limiter because the plasma surface was in contact only with the primary limiter as shown in fig. 1. Plasma diagnostics used in the present study are summarized as follows; TiXX(254 ,~) line radiation was measured by V U V spectrometer a n d CIII(4649 ,~,) and H , ( 6 5 6 3 A) line radiations were measured by photodiodes with b a n d - p a s s filters. Their time resolutions are 0.4 ms which are limited by the sampling time of digitizers. A n array of 21 silicon barrier diodes was used to measure the soft X-ray flux emitted along horizontal chords with a sampling time of 0.2 ms. The spacial resolution is 3 cm a r o u n d the plasma center. T h e vertical distribution of the radiation power was measured by a 21-channel bolometer array. One M i r n o v coil was located inside the v a c u u m vessel and measured M H D fluctuations up to several 10 kHz recorded with a sampling time of 25 p,s. A tangential TV observed the interaction between the plasma surface a n d the primary limiter with a s t a n d a r d speed of 16 ms. M a n y thermocouples were installed in the tiles of the primary limiter, back-up limiters a n d a r m o r tiles. Their temperatures were automatically recorded at the time of 10 s before a n d after each discharge. The increment of the temperature before a n d after a discharge provides the information of the integrated heat deposition during one discharge. Photographs of the inside v a c u u m vessel were taken from outside through the same window as the tangential T V m e a s u r e m e n t on a m a i n t e n a n c e day.
3. Time evolutions of diagnostics during major disruptions M a j o r disruptions occur after several successive discharges with a current of more than 700 kA. Waveforms of several plasma diagnostics during a disruption are s h o w n in fig. 2. T h e neutral beam is injected from 670 ms to 840 ms. T h e plasma is stable as with normal discharges before 820 ms. The sawtooth oscillation is normally observed d u r i n g the steady state with a repetition time of 17 ms and a last relaxation occurs at 807 ms. T h e impurity line radiations seem also the same as the normal discharges and" the total radiation power before the disruption is a b o u t 800 kW which is a b o u t 30% of the total input power.
1000 800 600
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810
820 830 840 Time (msec)
850
Fig. 2. Wave-forms of several measurements during a major disruption. (a) plasma current and one turn voltage, (b) average electron density, (c) line radiations of H,, CIII and TiXX, (d) soft X-ray diodes looking through chords of Z = 0 cm and Z = 24 cm, (e) Mirnov oscillation.
H. Yokomizo et al. / Limiter damage due to ooer-heat load
281
J
Q
1 .,0
f Fig. 3. TV pictures during a major disruption and a photograph of the primary limiter. Black points in TV pictures have no meaning. (a) at 64 ms before the disruption, (b) at the disruption. (c) at 32 ms after the disruption. (d) at 64 ms after the disruption. (e) an illustration of the tangential view of TV, (f) a photograph of the damaged third and forth tiles from the top.
At the time of 822 ms, a sudden and rapid increment is observed in CIII line radiation at first, and then T i X X signal shows the sudden increase. C a r b o n and titanium are both the intrinsic impurities from the primary limiter. The perturbations of the soft X-ray diode signals are observed to propagate from outer location into inner location with a propagation time of 1
ms, which indicates a high propagation speed of 240 m / s . The start of the outer diode perturbation is coincident with the start time of CIII perturbation. The H,, signal starts to increase at the later time (about 2 ms) than TiXX signal. The H , perturbation is followed by the electron density perturbation. Mirnov coil signal shows no enhanced oscillation until the plasma current
282
H. Yokomizo et a L /
Limiter damage due to over-heat load
starts to decrease. The internal instability does not begin at the initial phase during which the impurity influx increases. These observations suggest that the cause of the perturbation is located at the outer region or outside of the plasma. At 1 ms after the sudden increase, each soft X-ray diode signal rapidly decreases with a decay time of 3 ms. The plasma temperature is considered to be cooled at this time from the outer region to the inner region. At 5 ms after the initial perturbation, the plasma current started to decrease with a speed of - 7 5 MA/s. The decay time is about 11 ms. This decaying speed is a typical value for a current disruption in Doublet III. It seems that the strong influx of cold impurities from the primary limiter cools the plasma temperature and extinguishes the plasma current. A similar type of the disruption which is correlated with the impurity accumulation was discussed in another report [6]. But their observations were different from the present experiment because the impurity accumulation was slow and the current density profile was changed and m / n = 2/1 mode was observed to grow exponentially in the initial phase of their disruptions. The plasma TV shows that a large eruption happens on the primary limiter, which is different from usual disruptions. At the time of the disruption the TV picture becomes bright filling the entire frame. A large eruption is observed on the limiter surface in the next frame and many fragments flew about in the frames that followed as shown in fig. 3. The photographs taken on a maintenance day following the experiment reveals that damages occured on the graphite tiles of the primary limiter at the exact location where TV showed the eruption. Two tiles were broken into several pieces and some of these pieces were missing as shown in fig. 3. Although it is unclear whether this disruption created the initial damage leading to the tile cracks or unobservable damage had already existed before this major disruption, the tiles of the primary limiter were obviously damaged by this disruption.
400
300
~. 200 E 100
T o
o
o
8 o°
_J I I
0 0
I
{
J
I
I
I
J ~t I
I
2
3
4
5
6
Time ( Hour ) Fig. 4. Temperature history of the third tile of the primary limiter. (o) and (O) indicate the temperatures before and after discharges, respectively. Arrows shgw the temperature increments during discharges. ( x ) presents two disruptive discharges discussed in this report. and the repetition time between shots is long in order to cool down the tile temperature. Note that two disruptions occur when the base temperature of the primary limiter is higher than 300 o C. Fig. 5 shows the temperature increments during discharges with the various plasma current. The temperature increment presents the integrated heat deposition onto the tile during each discharge. As the plasma current becomes high, the heat deposition becomes large for normal discharges shown by circles. The disruptions shown by crosses occur at the high plasma current of 700-800 kA. The temperature increments of disruptive discharges are lower (407o) than normal discharges. The reason of the smaller increment is explained by a short ~80
i
i
i
•
ii
o/" */
860
•
E
//
•
.?.,-1 i
o
40
4. Thermal measurement and analysis
Fig. 4 shows a history of the temperature of the third tile from the top, which presents the highest temperature among six tiles of the primary limiter. Arrows indicate the increments of the limiter temperature during discharges. Cross marks show two disruptive discharges which are discussed in this paper. Discharges with a lower base temperature between two disruptions shown in fig. 4 have a lower plasma current than 500 kA
~
20 E o
0
I
I
K
I
200
400
600
800
1000
Ip (kA) Fig. 5. Temperature increment of the third tile during each discharge. (o) and ( x ) indicate normal and disruptive discharges, respectively.
283
H. Yokomizo et al. / Limiter damage due to ooer-heat load
/
~
Primary Limiter Heat Deposition
~
Armor T i l e s (T~:21~r~,t ~J~ Backup Lirniter
:~0
31
_ ~
____Total240 kJ"
Fig. 6. Integrated energy balance of a beam-heated discharge without a disruption. Plasma parameters are: Ip = 800 kA, discharge duration of 1 s, injected beam power of 2 MW, beam duration of 180 ms.
discharge duration of a disruptive discharge, total input energy of which is less (about 20%) than that of a normal discharge. Another possible reason is the heat loss which is due to the dropping of small fragments of the tile after the disruption. Thermocouples installed in the armor tiles show that the temperature increments for the disruptive discharges are similar or slightly lower than that for the normal discharges. This suggests that a heat deposition profile to the materials around the plasma during a disruptive discharge is quite similar to that during a normal discharge. Integrated energy flow of a beam-heated discharge without disruption ( l p = 800 kA) is shown in fig. 6. Heat depositions to the primary limiter, armor tiles and back-up limiters are obtained from the heat analysis using the temperature increments measured by thermocouples with the assumption of a homogeneous temperature profile of inside tiles. The radiation loss energy is the integrated value obtained by bolometric measurement. The integrated input energy to the plasma is estimated by summing up the neutral beam injected
Heat Flux Ohmic 0.06 03 0.8 Q8 0.4 0.2
(kW/crrF) N BI 0.5 1.2 2.3 2.1 2.0 0.9
Fig. 7. Heat fluxes to six tiles of the primary limiter during ohmic and beam heating phases.
energy and the ohmically heated energy. About 25% of the integrated input energy deposits onto the primary limiter. Heat fluxes to the primary limiter during the ohmic heating phase and the beam heating phase are estimated by comparing the thermocouple data of two successive discharges with and without a neutral beam injection. Fig. 7 shows the heat flux to six tiles of the primary limiter. The heat flux to the third tile is 0.8 k W / c m 2 during the ohmic heating phase with an ohmic input power of 1.1 MW. It increases to 2.3 k W / c m 2 during the neutral beam heating phase with an input power of 2.8 MW. It is assumed that the heat flux homogeneously deposits to one third of the whole surface area of the primary limiter [7]. A thermal calculation shows that the temperature of the third tile reaches 1300°C after a beam injection as shown in fig. 8. Once the plasma becomes disruptive, then the stored energy of the plasma is released to the limiter and to the wall within a short time. The heat flux to the primary limiter during the disruption is estimated to be 600 k W / c m 2 when 50% of the stored thermal energy (100 k J) is assumed to deposit onto the limiter area of 10 cm x 40 cm with the disruption time of 0.2 ms [8]. This heat flux is large enough to make the surface temperature reach the sublimation temperature. It is interesting that the disruption occurs at the 5th or 7th discharge after successive discharges with a similar type of operation condition as shown in fig. 4. The
1400 1200 A
I000 2o 800 E 600 400 E d
23kW/crn=
200
kW/cmz) 3
Q = 0.8 kW/cm= 0
I
0
2oo
I
I
I
400 600 800 Time (msec)
looo
Fig. 8. Numerical simulation of surface temperature of the Poco graphite.
284
H. Yokomizo et aL / Limiter damage due to ooer-heat load
base temperature of the limiter tiles are high (more than 300 o C) and almost the same values for these successive discharges. If there is a small area of low heat conductivity compared with that of the bulk graphite (i.e., the microcracks and the runaway damages on the limiter surface [5]), it is possible that the thermal energy in this area might accumulate during the sequential discharges and its surface temperature might become higher than indicated by the thermocouple measurement. In another aspect, it is also possible that the thermal fatigue of the graphite tile might accumulate during the sequential discharges. These kinds of conditions seem to have some correlations to create a tile crack and to produce a major disruption.
5. Discussion
The TV picture showed the large eruption at the disruption and photographs revealed cracks of the limiter tiles. Two scenarios are considered in this phenomenon: (a) increase of limiter surface temperature ~ increase of impurity influx ~ major disruption ~ tile crack, (b) increase of limiter surface temperature--, tile crack --* increase of impurity influx ~ major disruption. There is little doubt that the increase of the impurity influx triggers the major disruption. At first, a mechanism of the impurity creation which must explain a sudden and rapid increase of the impurity influx is discussed. Experimental data of the chemical sputtering on targets of TiC [9] and graphite [10] show that the sputtering ratio is almost constant or smoothly increases with the rising of the target temperature. No drastic increase is expected at the high target temperature up to 2300%. The blistering rate is expected to be low when the target temperature is high
POWER FLOW
cz ~.~
BARRE
NUT HOLE
,, . . . . J~,,,
u~
Fig. 9. Cross section of the limiter tile and schematics of the produced in a chord through the tile. Tensile stress at the barrel nut hole is three times higher than the value shown in this picture.
stress
Table 1 Maximum stress in a tile of primary limiter (kg/mm 2)
Steady state Disruption Strength ~
Tensile stress at hole
Compression stress at surface
5 5 5.6
11 19 20 (steady state) 29 (disruption)
" Catalog value of Poco graphite AXF-5Q.
and the incident particle is hydrogen [11]. The evaporation rate smoothly increase~ with a nearly exponential function as the surface temperature becomes high so that a sudden and rapid increase of the evaporation is not expected. The scenario (a) has no appropriate mechanism to explain the impurity behaviour. A one-dimensional stress calculation based on the analytical formula [12] was carried out in order to examine the mechanism of the tile crack. The stress profile along a chord through a limiter tile is shown in fig. 9. The limiter surface has a maximum value of the compression stress. This value is low to reach a compression strength of the Poco graphite as shown in table 1. The barrel nut hole is located in the region of the maximum tensile stress as shown in fig. 9. The tensile stress at the hole edge is comparable to the tensile strength. This tensile stress is suspected to be a primary cause of the tile crack during a disruption and also during a steady state. If the limiter tile suddenly cracks for the first incident during a steady state, then the rapid increase of the impurity influx is easily explained and a following major disrupt'ion is also understood. Presently, the scenario (b) seems more probable. The high occurrence of disruptions have been observed in other series of experiments on Doublet III under the following conditions, (a) after sequential discharges with especially high plasma current (around 1 MA), and (b) after a runaway discharge. These disruptions are suspected to be triggered by an enhanced impurity influx from a hot spot which is created on the limiter surface by previous discharges. Any pronounced damage of the limiter, however, had not been observed in these discharges except a spotty damage due to a runaway discharge [13]. The disruption caused by a high heat deposition onto a small area is not unusual and its occurrence will happen often in future devices because they will have high power heatings and high power productions of fusion energy. This
H. Yokomizo et al. / Limiter damage due to over-heat load
kind of disruption, however, is easily prevented by adjusting the plasma position a n d the plasma shape to maximize the plasma surface which makes contact to the limiters. The m e a s u r e m e n t of the heat depositions to the materials a r o u n d the plasma is one usual method in o r d e r to know the status of the plasma operation and will become an i m p o r t a n t tool in future devices for the interlock in order to avoid a hazardous plasma operation.
6. Conclusion Major disruptions a c c o m p a n i e d with the limiter damages have been observed during the high power heating phase (2 M W neutral b e a m injection) in D o u b let III. The analysis suggests that the limiter tiles are cracked by the thermal stress due to the over-heat load (2.3 k W / c m 2 ) . Major disruptions are triggered by the e n h a n c e d impurity influx following to the tile cracks. T h e base t e m p e r a t u r e of the limiter tile is over 3 0 0 ° C at these disruptions although the tile is m o u n t e d to a water-cooled metal structure. T h e base temperature m o n i t o r i n g the limiter tiles is one of the useful methods to know the heat deposition and to avoid heat concentration o n t o a small area.
Acknowledgement The authors would like to express their sincere gratitude to Doublet 11I operation, diagnostics and neutral b e a m groups for their fine supports. They would like to express their appreciation to Drs. N. Fujisawa and T. A b e for their fruitful discussion. They would also like to t h a n k Drs. T. Ohkawa, S. Mori, M. Yoshikawa for their encouragement. This work was authorized by a cooperative agreem e n t between the J a p a n Atomic Energy Research Institute and the United States d e p a r t m e n t of energy u n d e r D O E contract No. D E - A T 0 3 - 8 0 S F l l 5 1 2 .
285
References [1] INTOR GROUP, International tokamak reactor: Phase one, Rep. Int. Tokamak Reactor Workshop Vienna, 1980/1981, int. Atomic energy Agency, Vienna (1982). [2] For example, IAEA Syrup. Current Disruption in Toroidal Devices, Garching, 1979 Max Plank Inst. Plasmaphysik, Garching, Rep. IPP 3/51 (1979). [3] R.W. Callis, Doublet Ill baseline description, General Atomic Company Rep. GA-A13996 (1976). [4] D.L. Sevier, P.W. Trester, G. Hopkins, T.E. McKelvey, T.S. Taylor, Performance of TiC-coated graphite in electron beam tests and Doublet IlI operation, General Atomic Company Rep. GA-AI6384 (1981). [5] T. McKelvey, T. Taylor, P. Trester, E. Reis, R. Gallix, P. Galdos, E. Johnson, F. Puhn, D. Doll. L. Sevier, H. Yokomizo, M. Nishikawa, A. Kitsunezaki, in Fusion Technology (Proc. 12th Symp. Julich, 1982) 511. [6] W. Engelhart, O. Kluber, K . Lackner, S. Sesnic, IAEA Syrup. Current Disruption in Toroidal Devices, Garching, 1979 Max Plank inst. Plasmaphysik, Garching, Rep. IPP 3/51 (1979) A6. [7] T. Taylor, N. Brooks, K. Ioki, J. Nucl. Mater. l l l & l 1 2 (1982) 569. [8] T. Taylor, Limiter heat fluxes during plasma disruptions, General Atomic Company Rep. GA-AI6816 (1981) 27. [9] R. Yamada, K. Nakamura, M. Saido, J. Nucl. Mater. l l l & l 1 2 (1982) 744. [10] J. Roth, J. Bohdansky, K.L. Wilson, J. Nucl. Mater. l l l & l 1 2 (1982) 775. [11] M. Saidoh, R. Yamada, K. Nakamura, J. Nucl. Mater. l l l & l 1 2 (1982) 848. [12] S. Taira, Thermal stress and thermal fatigue, The Nikkan Kogyo Shinbun, Tokyo (1974) 22. [13] M. Nishikawa, H. Yokomizo, A. Kitsunezaki, T.E. McKelvey, T.S. Taylor, D. Doll, N. Brooks, R. Seraydarian, Graphite limiter damage due to runaway electron and abrupt lp termination in Doublet III, 6th Internat. Conf. on Plasma Surface Interactions in Controlled Fusion Devices, J. Nucl. Mater. 128&129.