J. Plasma Fusion Res. SERIES, Vol.3 (2000) 243-245

Development of Mechanical Mounting Scheme for PlasmaFacing Components of SST-1 Tokamak
KHIRWADKAR Sameer*, D. CHENNA Reddy, CHOUDHARY Paritosh, JACOB Saji, PRAKASH Ravi,SANTRA Prosenjit and SINHA Prabhakar
Institute for Plasma Research, Bhat, Gandhinagar-382428, INDIA(Received: 19 January 2000 / Accepted: ll May 2000)

Abstract
Steady-state Superconducting Tokamak (SST-1) is being designed for 1000 seconds of continuous
operation with total input power up to 1.0 MW. The tokamak will have a D-shaped plasma with double
null divertor. Each toroidal module of inboard as well as outboard divertor will have graphite tiles
mechanically mounted on a metallic backplate made of high strength copper alloy. The poloidal crosssection of divertor plates has been designed such that the average heat flux on graphite tiles is less than
0.6 MWm2 which is nearly the limit on incident heat flux for mechanically attached graphite tiles forplasma facing components in tokamaks. As a first step towards development of plasma facing
components, preliminary experiments are conducted with a test mockup to develop the mechanicalmounting scheme for attachment of graphite tiles on actively cooled copper backplate. This paper
describe the results of these experiments.

Keywords:
SST-I tokamak, plasma facing components, divertor plate, mechanical mounting scheme, heat removal
scheme1. Introduction

have graphite or C-C composite tiles of approximate
size 75 x 75 x 25 mm3 mechanically attached to about
25 mm thick backplate made of copper alloy (Cu-Cr-Zr)
with flexible graphite sheet tsed at copper-graphite
interface to reduce thermal contact resistance. Steeltubes will be brazed (or welded) to the backplate forcooling as well as baking purpose. In steady-state
operation, divertor plates will be receiving maximum

Steadystate Superconducting Tokamak (SSZ-1) isbeing designed to operate for 1000 sec with a D-shaped
plasma having double null divertor configuration andtotal input power of 1.0 MW. Plasma having major
radius (Rs) of l. I m and minor radius (a) of 0.2 m will
be generated to carry 220 kA current in presence of

toroidal magnetic field of 3.0 T (at R = Ro). Plasmaequilibria under consideration will have elongation (r),triangularity (d) and internal inductance (li ) in the rangeof 1.7 - 1.9,0.4 - 0.7 and 0.75 - 1.40 respectively. A
typical equilibrium will have r=1.8 and /i = 1.0 andheat load. Therefore, poloidal inclination of divertor
plates has been optimized so as to intercept maximum
area of scrape off layer (SOL) while maintaining
average heatflux on carbon tile to be less than 0.65
MWm2. The poloidal location of strike point of SOL on

will be symmetric about the midplane.
Plasma Facing Components (PFCs) of SST-l

top/bottom divertor plate has been fixed for allequilibria. Thus, seperatrix of SOL for all equilibria

mainly consists of Divertor, Limiter, Passive Stabilizerand Baffle plates [], Typical assembly of PFCs will

@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corresponding author's e-mail: sameer@ plasma.ernet.in
243

Khirwadkar S. et al., Development of Mechanical Mounting Scheme for Plasma Facing Components of SST-1 Tokamak

under consideration will intercept the top/bottomdivertor plate at the predetermined strike point location.
Experimental as well as computational study is

being carried out to develop mechanical mounting
scheme for plasma facing components of SST- I . Resultsof some preliminary experiments conducted in thisregard are presented here.

2. Assembly of Test Mockup
Test mockup consists of nine graphite tiles of size
50 x 50 x 25 mm3 arranged in the form of 3 x 3 matrix.
Each tile is centrally bolted to a copper backplate of size

150 x 150 x 18 mm3 by using steel fastners. Thebackplate is actively cooled by water flowing through 8mm ID copper tube brazed to the backplate as shown in
Fig. l. The steel fastners consists of a cylindrical nutbarof diameter 12 mm & length 50 mm having threadedhole for M6 size bolt at its center, M6 size bolt of length
75 mm and disc springs. The nutbar inserted in thegraphite tile is tightened with an M6 size bolt as shown

Fig. 1 Assembly of Test Mockup showing nine graphitetiles (50 x mm 50 x mm x 25 mm) mechanicallymounted on a copper backplate (150 x mm150 xmm x 18 mm) having U-shaped copper tube (8mm lD) brazed to it for active cooling.

inFig.2.
Compliant Layer i.e. flexible graphite sheet(GRAFOIL@) having 0.? mm thickness is inserted
between graphite tile and copper backplate to reduce
thermal contact conductance. The M6 size bolt istightened with 4 Nm of torque to apply pressure on thetile as well as graphite sheet.

3. Pressure Distribution on Compliant Layer
The experimental studies [2] conducted elsewhere

for development of mechanically mounted first wallcomponents indicate that pressure on compliant layer

should be above 0.2 MPa for improved contact
conductance. The pressure distribution on a compliantlayer was measured by using PRESCALES@ of type
LLLW & LLW along with FPD-305 Densitometer andFPD-306 Pressure Reader manufactured by Fuji Film
Co. Ltd., Japan.
Pressure on both side of the compliant layer wasmeasured for 4 Nm torque as shown in Fig. 3. It can beseen that the pressure all over the compliant layer isabove 0.9 MPa. Moreover, pressure distribution isalmost same on either side i.e. copper side and graphiteside of the compliant layer.

Fig. 2 Mechanical mounting scheme showing M6 size

stud at center of the graphite tile mechanicallyattached (tightened) to a cylindrical nutbar (12mm dia) inserted through the thickness of tile.

about 0.6 to 0.75 MWm2.

In order to check thermal response, the test mockup
was irradiated with 2.1 kW CO2 LASER beam having
rectangular cross-section. The incident heatflux on test
mockup was controlled by operating LASER at different
power levels and the area of cross-section of beam on
test mockup is adjusted by using Zinc-Selenide lens.
The power of 2.1 kW was sufficient to irradiate one of
the nine graphite tiles having surface area of 50 mm x50 mm with heat flux of 0.75 MWm2. The laser wasoperated in continuous mode for minimum 600 seconds
during each test.

4. Thermal Response of the Test Mockup
The experimental studies [2,3] to improve heatremoval capability of mechanically mounted graphitetiles in plasma facing components has indicatedmaximum operating limit on incident heat flux to be244

Khirwadkar S. et al., Development of Mechanical Mounting Scheme for Plasma Facing Components of SST-I Tokamak

The experiment with highest available heat flux of 0.75

tpt
It

toTor

MW/m2 was repeated several times. However, nophysical damage could be observed with any componentof the test mockup.

5. Conclusions> -to-to

The experiments conducted to study thermal
response of a test mockup with mechanically mounted-lo -r0X {nnrl0--r

tographite tiles are presented in this paper. For 50 x 50 x

co25 mm3 graphite tile and 0.7 mm thick flexible graphite,

4 Nm of torque on M6 size bolt was found to be
sufficient to produce minimum pressure of 0.9 MPa on
compliant layer. Each tile of the test mockup is found towithstand several exposures to heatflux of 0.75 MWm2ztt&lOt!
tlwith surface temperature of graphite below 1000'C.

> -lg

Acknowledgements

eog-1n-s*toot fnnl --r

toThe authors are grateful to Dr. A.K. Nath and otherscientific and technical staff of CO2 LASER division at
CAT (Indore, India), for providing the LASER facility

roand help in setting up the experiment.

Fig. 3 Pressure distribution on both (copper & graphite)sides of compliant layer for 4 Nm torque on M6
size bolt at center (X = 0, Y = 0) with nutbar placed

Referenceslllalong Y-direction.

233-237, p655-659 ( I 996).

tzl M.Lipa, Proceedings of SOFT-1992.
t3l Y. Kubota, N. Noda et al., Proceedings of SOFT-

The test mockup is supplied with water at22.5'C(room temperature) flowing through a 8 mm ID copper
tube at flow rate of 12 liter per minute to generate heattransfer coefficient of approximately 17,000 Wm2K.
Temperature at the surface of the graphite tiles is1996.

measured using an Infra-Red Thermometer. Following

table shows the measured surface temperature as afunction of incident heat flux.

graphite tile

Surface Temp. ofgraphite tile(MWm)(deeree C)

0.052

356

0.066

382

0.077

413

0.084

430

Incident Heat Flux on

S. Jacob, S.S. Khirwadkar et a/., J. Nucl. Matr.

o.297

543

0.378

596

0.468

630

0.756

930

245