J. Plasma Fusion Res. SERIES, Vol.3 (2000) 246-249

Fabrication and High Heat Flux Testing of Super
Mechanically Joined Divertor Plate for LHD
KUBOTA Yusuke*, NODA Nobuaki, SATOH Sadao and MOTOJIMA Osamu
National Institute for Fusion Science, Toki 509-5292, Japan(Received: 19 January 2000 / Accepted: 29 April 2000)

Abstract
For the helical divertor plate of large helical device (LHD) in the phase II, a super mechanicallyjoined module(MJM) without a copper heat sink has been developed using a test facility ACT with a 100
kW EBS. The thermal performance of the MJM has been drastically improved by the optimizations forstructure, material, and condition comparing with those of the previous MJM with a heat sink. Thestructure of the super MJM, thermal fatigue test, and high heat flux test up to 3.75 MWm2 for steadystate operation are mainly described.

Keywords:
LHD, high heat flux, graphite, armor tile, electron beam, divertor, mechanically joined module, andthermal fatigue test.

1. Introduction

the next experimental stage (phase II) of LHD.

Mechanically joined modules (MJM) more than
1700 are now installed as the divertor plate in large
helical device (LHD) [,2] ,which began operation in

2. Experimental Test Device AGT

1998.

a test facility called ACT [3,4] has been used, as shown

The MJM mainly consists of a graphite armor tile,
copper heat sink, stainless steel (SS) backing plate, SS
cooling pipe, and graphite sheet. The module works well
as a divertor plate under heat flux lower than about 0.5
MWm2 for a steady operation. However, if the heat flux
on the armor tile exceedS the flux level. the thermalin Fig. 1. The facility consists of a 100 kw electron
beam source, vacuum pumping system, 400 L vacuum
vessel, water cooling system, and data acquisition

In order to evaluate the thermal property of MJM,system. The test sample is located on a copper plate
isolating from ground under a beam limiter and cooled
actively by cooling water flowing from the water pump
(7.5 kW) system. The thermal properties of MJM areevaluated using a fiber optical thermometer and twothermo-couples. The main parameters of ACT are
shown in Table 1.

performance of the MJM becomes worse abruptlybecause the copper heat sink is deformed by the highheat flux flowing through the armor tile from divertor
plasma. The deformation of copper comes from the highcoefficient of expansion and the poor strength attemperature above about 200"C. To prevent thedeformation of the heat sink, several materials and3. Structure of Super MJM
Fig. 2 shows a schematic view of super MJM thatconsists of a unified armor tile/heat sink, backing plate,configuration were tested and evaluated. Through thetests and evaluation, super MJM without a separate heatcooling pipe, and compliant sheet. The former two parts

sink has been developed as a divertor plate to be used in*C o rre sp ondin g autho r' s e - r4ail : Kub o ta @ LH D. nifs. acjp246@2000 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchKubota Y. et al., Fabication and High Heat Flux Testing of Super Mechanically Joined Divertor platefor LHD

Armor tiley'heat sink(C/C)@rean tiniter
Backing plate(C/C)
Super graphite sheet
SS pip{27.2 6 )Fig.2 Schematic view of super MJM with an unified
armor tile/heat sink made of C/C.

I&P&I

:

Fig.1 Cross-sectional view of a test facility ACT.

P

Table 1 Main parameters of test facility ACT
Itempafameter

Maximum beam output

100 kw

Electron beam energy

30 keV(fix)Minimum beam size
Beam irradiation area
Flow rate of cooling waterFig 3 rherma' ,"",:::::;;',,or super MJM
under a high heat flux of 5 MWm'?for 20 sec.

8mmQ

l-400 cm2-10 m/s

responses (Ts,Tu,Tl) measured using a fiber type of
optical pyrometer and two thermo-couples. Each edge ofare made of graphite or carbon carbon composites

the thermo-couples is inserted in a small hole bored sidedifferent from that of the previous MJM, which is usefulof the unified armor/treat sink. The maximum surface

for weight reduction. Moreover, as a compliant sheet, asuper graphite sheet (made by Matsushita Co. ltd.) withan excellent thermal and mechanical properties is usedinstead of carbon sheet used in the previous MJM.

temperature (Ts) reaches 1500"C at the end of heat load
under 5 MWm2. However, if carbon carbon composites

is used as the armor tile material instead of graphite, themaximum surface temperature may reduce lower than1000'c.

4. High Heat Flux Test of Super MJM
4.1 Short operation mode

4.2 Steady state operation mode

To evaluate the thermal performance of super MJM
with a unified armor tile/heat sink made of graphite andMJM under near steady high heat flux for 1000 sec,

To evaluate the thermal performance of the supera SS cooling pipe, high heat flux tests up to 5 MWm2
have been carried out for 20 sec pulse duration with arise time of 20 sec. After the test, no damage on the tile

thermal responses (Ts,Tu,Tl) were measured. As theresult, high heat flux tests up to 3.75 MWm2 have been
carried out without any apparent damage or loss in theperformance of the MJM. Fig. 4 shows the thermal

surface was observed. Fig. 3 shows the thermal
247

Kubota y. et al., Fabr\cation and High Heat Flux Testing of Super Mechanically Joined Divertor Plate

responses of the super MJM with a copper cooling pipeand super graphite sheet under a heat flux of 3.75 MW/for LHDl 200m2. As the material of the unified tile/heat sink, carbon
carbon composites (CX-2002U) with a high transverseoQH*/ soo

thermal conductivity of 390 W(mK). The temperatures(Ts,Tu,Tl) saturates nearly after 400 sec passed. Themaximum surface temperature (Ts) reaches 1100"C'A

ANN

ts

which is allowable value for armor tile of divertor plate.

4.3 Thermal fatigue test

0To ensure the durability of the super MJM with aunified tile/heat sink made of graphite and SS cooling
pipe, thermal fatigue test has been carried out under ahigh heat flux of 2 MW/m2 with a cycle of 1000 s loadand 600 s rest. The temperature (Ts,Tu,Tl) of the MJM
increase gradually with cycle number. The rise rates per
cycle are 2.1'C, 1.4"C, and l.4oC for Ts, Tu, and Tl, rrespectively. The test result indicates that the durabilityof this type of super MJM is not sufficient under such ahigh heat flux as 2 MWlm2 for the steady operation.

200600400ELAPSED

8001000lztx)

TIME(sN)

Fig. 4 Thermal responses [s,Tu,Tl) of super MJM undera high heat flux of 3.75 MWm'? for 1000 sec.

600Dgs:o.lmm(,

Des:o;2m4=F 400*

4.4 Effect of thickness of graphite sheet
The thermal performance of the MJM depends

3ooF
200

largely on the thickness and kind of graphite sheet or thepressure applied on the sheet, which is used as acompliant sheet. To find out the optimum thickness of

000.511.52

graphite sheet to be used between heat sink and coolingpipe of MJM, temperature at heat sink of the super MJM
was measured as a parameter of thickness of graphite

2.5HEAT FL[lx(Mw/B!)

Fig. 5 Effect of thickness of graphite sheeton thethermal performance of super MJM.

sheet. Fig. 5 indicates the test result performed underhigh heat fluxes of 0.5 to 2.5 MW/m2 using a super
MJM with a tile/heat sink made of graphite and SS
cooling pipe. As shown in the figure, the thickerl) oFc/ss(2 5)I)cs:02mn.

700

graphite sheet shows lower temperature comparing tothe thinner one under same heat flux. This result
indicates that the thicker one is better as a complianto(J 600

sheet between these graphite sheets.

F4.5 Comparison of thermal property for MJM

Fti 5005) Dgs:0.ltllrul

I roo
E 3oo

A comparison of the thermal performance200(temperature of heat sink vs heat flux) between several
types of MJMs under several conditions is made in Fig.

B) with s qr c@llng

't00

"ooo

6 for steady operation. As shown in the figure, theo.E

1.6

2.4

3.2

4

HEAT FLUX {Mwm'l)temperature at the heat sink of normal MJM labeled l)
IGOFC/SS rises exponentially as heat flux increases.
The allowable maximum heat flux of the MJM may beless than 0.5 MWm2. The MJM is now used as divertor
plate in LHD. The other hand, the temperature of thesuper MJM labeled 5) OCC/Cu with a heat sink made ofsuper graphite sheet increases linearly with heat flux andcarbon carbon composites, copper cooling pipe, andshows the lowest value between the MJMs. TheFig. 6 A comparison for thermal performance of several

kinds of MJMs.

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Kubota Y' et al., Fabication and High Heat Flux Testing of Super Mechanically Joined Divertor plate forLHD

comparison indicates that the thermal performance ofMJM is highly dependent on the choice of materials andconfiguration. The super MJM with an unified C/CAcknowledgements

armor tile/heat sink and Cu cooling pipe can be used for

Co. Ltd for the supply of test materials and their

the divertor plate of LHD in the next experimental phasecooperation to the fabrication of MJM. The authors also

and under steady state high heat flux more than 2 MW

wish to thank Mr.Kawamoto for his useful information
about super graphite sheets.

The authors are grateful to Toyo Tanso Co. Ltd,Matsushita Research Institute, and Japan Denzi Industry

m2.

5. Conclusion

References1) The thermal performance of the super MJM with atll A. Iiyoshi et a/., Fusion Technology, Yol.l7, 169

unified tile/heat sink made of C/C was drastically improved as compared with the normal MJM with a Cu
heat sink. This improvement may allow use of for the(l990).

[2] O.Motojima et al.,Proc. l3rh IAEA Conference on
Plasma Phys. Control. Fusion 3, 513 (1990).
t3l Y. Kubota er a/., NIFS-MEMO-13, | (1994).
t4l N. Noda er al., Proc. of the ASME Heat Transfer

divertor plate of LHD in the next phase and understeady operation condition.

2) However, thermal fatigue test result under steady

Division, HTD-Vo1.317-1, 159 (1995).

high heat flux of 2 MWm2 showed that the durability ofthe super MJM is not sufficient. To get better result

from the thermal fatigue test, more effort to optimize thestructure of MJM is required.

249