**DESIGN OF THE PLASMA FACING COMPONENTS FOR THE NATIONAL SPHERICAL
TOKAMAK EXPERIMENT (NSTX)**[^1]

P. Goranson^2^, G. Barnes^1^, J. Chrzanowski^1^, P. Heitzenroeder^1^, B.
Nelson^2^, C. Neumeyer^1^, J. Ping^2^

^1^Princeton Plasma Physics Laboratory, P.O. Box 451, Princeton, NJ
08502

^2^Oak Ridge National Laboratory, Oak Ridge, TN 37831

ABSTRACT

The NSTX device plasma facing components \[PFC\] consist of inboard
divertors, outboard divertors, primary passive plates, secondary passive
plates, a center stack casing \[CSC\], and the heating/cooling fluid
distribution system. The PFC surfaces are protected by 3584 individually
mounted carbon tiles. Surfaces exposed to high heat flux and/or high
loads utilize composite carbon-carbon graphite and the remainder utilize
less costly ATJ graphite. A variety of diagnostics are incorporated into
the PFCs including thermocouples, Langmuir probes, Mirnov coils, and
Rogowski coils. The NSTX device is designed to be operated in a pulse
mode of five seconds on followed by five minutes off. Its PFC components
are also required to be baked out to 350 C. During operation the PFC
tiles are permitted to ramp up thermally and then cool sufficiently
between shots to prevent ratcheting during subsequent shots. The CSC
tiles are required to be thermally isolated from the CSC so that the
primary heat loss is radiation to the other PFC components. The other
PFCs are thermally coupled to water cooled plates by conductive gaskets.
Special mounts are required which permit thermal expansion and can
withstand disruption loads while maintaining thermal contact.

The tiles and mounts for the CSC are required to fall within a total
radial space allotment of only 14 mm. A unique design for mounting
graphite tiles to the CSC was developed which utilizes drift \[shear\]
pins and Inconel brackets. Installation is accomplished via hidden
fasteners accessed through very small holes in the tile faces. Analyses
of the CSC mounting structure were performed and pull tests were
performed on assemblies which simulated the attachment geometry in an
attempt to determine the ultimate

strength of the configuration and the mechanism of failure.

INTRODUCTION

The PFC components are shown in figure 1. The PFC surfaces are required
to be baked to 350 C while maintaining the vacuum vessel at 100 C. The
CSC is constructed of Inconel 625 and contains a layer of
Microtherm\[1\] insulation to limit heat transfer to the central
solenoid. Coolant channels are provided to carry away the resulting
small heat load. All other portions of the CSC and other PFCs are cooled
by conduction. The horizontal ring at the top and bottom of the CSC
serve as Inner Divertors.

The Outer Divertor mounting structure consists of coolant traced copper
plates attached to stainless steel grill structures. The attachments use
belleville washers to permit expansion during bake out and operation.
The inner and outer Passive Plates are also mounted on coolant traced
copper plates but the attachment scheme is much more elaborate. The
plates are mounted to U shaped ears on the Passive Plate electrical
jumpers which flex elastically during thermal expansion. The jumper
mounts are attached via a system of pinned and slotted brackets, again
designed for thermal expansion. The CSC tiles are contour machined but
the Outer Divertor and Passive Plates are machined flat \[faceted\].
![](media/image1.wmf){width="3.0194444444444444in" height="3.725in"}

Two composite materials were considered during the development program ;
FMI's Coarse Weave 4D carbon-carbon composite and product 865-19-4, a
carbon-carbon composite developed by Allied Signal\[AS\]. The FMI
product was favored for its perceived higher strength and more isotropic
structure but was it was also costly and its exclusive use would have
exceeded the NSTX PFC budget allotment. The development program proved
the adequacy of the AS material, in fact showed it to be superior in
strength when oriented in an optimal direction to the grain and it was
adopted in lieu of the FMI material.

CSC DESIGN

The baseline rail tile design is shown in figure 2. The rail tile
assembly is attached to the CSC by Inconel weld studs and captured allen
head nuts. The nuts are tightened through small \[3.8 mm\] access holes
in the face of the tiles. The design utilizes four, 3.2 mm diameter,
Inconel drift pins to retain the tile. A step in the rail tile permits
lapping adjacent \[cover\] tiles and serves to retain them, thereby
halving the number of mounting studs required. Wave springs under the
cover tiles serve multiple functions; they pre-load the cover tiles to
the rail tiles, cushion them against disruption loads, and permit
thermal expansion. Radiation shields are used behind the cover tiles to
prevent excessive heat transfer to the center stack column. Inner and
Outer

DIVERTOR AND PASSIVE PLATE DESIGN

The envelope for divertors and passive plates is 2.54 cm which permits a
more conventional mounting scheme. The tiles are attached by captive
button head screws and Inconel T-bars to structural copper plates.
Belleville washers under the screw heads permit thermal expansion. See
figure 3. A composite graphite gasket \[Grafoil\] \[2\] assures good
thermal contact with the copper. The T-bar permits thermal expansion in
longitudinal and transverse directions while distributing the clamp up
load into the gasket. The attachment screws are accessed through a small
\[ 4.6 mm \] hole in the tile face.

DIAGNOSTICS![](media/image2.wmf){width="3.05625in" height="1.49375in"}

A large number of diagnostics are built into the graphite tiles. Space
does not permit going into detail about the individual designs but they
are designed to mount in typical manner to conventional tiles and the
leads are routed in miters or channel provided along the edge of the
tiles.

THERMAL STUDIES

Thermal analysis was performed on the center stack and divertor tiles. A
one dimensional, transient heat transfer program \[TILE.TEMP.1\] was
written in QUICKBASIC to derive the temperature vs. time, in graphite
tiles. The BASIC program was designed to give a rough approximation of
the thermal profile of a 1-D tile. It allowed quick turnaround and
served as a guideline during conceptual design of the tiles and, later,
as a litmus test to validate thermal analyses done with P-THERMAL\[3\]
code. The thermal response of a CSC tile is shown in figures 4, 5.

The outer divertor heat flux approached 2 kW/cm^2^ and temperatures were
found to exceed the allowable \[1200 C \] for graphite. The maximum
average heat load that could be sustained for 5 seconds was found to be
700 w/cm^2^. Studies were done to determine means of limiting the
temperature. These included alternate materials, geometry changes,
varying tile thickness, plasma sweeping, and moving the x-point
location. Details of the analyses are shown in a thermal report \[4\].

STRUCTURAL ANALYSIS

Analyses were also performed to determine the forces on the PFC
components. During disruption events a distributed load of approximately
0.7 Mpa (100 psi) was assumed to be applied to the graphite, in a
direction radially away from the central column. This load is
transmitted as a shear load through the drift pins into the rail
bracket. With a nominal 5 cm x 15 cm tile this equates to 2.07 Mpa (300
lbs) per pin. The pin diameter and edge distance are geometrically
constrained ; they must clear the tile offset and the rail return leg.
Early designs utilized a pin centerline to graphite edge distance of
5.05 mm and a 2.4 mm diameter pin; maintaining a centerline to edge
distance of at least 2 diameters as recommended by strength of material
handbooks for fasteners in pure shear. This criteria was dropped as a
result of the testing program
results.![](media/image3.wmf){width="2.904861111111111in"
height="2.8680555555555554in"}

TESTING

Nonlinear stresses occur in the tiles due to pin bending, anisotrophic
grain structure, and stress concentrations at the edge of the tile
,therefore, actual failure loads and failure modes could not be
accurately predicted without testing. The two candidate composite
graphites were evaluated in pull tests in which the variables were the
mounting pin diameter, the number of pins, and the direction of the pin
relative to the grain direction of the specimens. Specimens duplicated
actual dimensions of typical tile sections. The pull fixture is shown in
figure 6. All tile failures initiated as cracking at the edges of the
holes. Flexibility in the pin/graphite transferred the load to the ends,
resulting in stress concentration and premature failure. Increasing the
pin diameter and chamfering the hole edge to prevent localized pin
contact resulted in increased load capacity \[factor of 1.7 with AS
material\] even though the edge distance was not increased. The material
and its grain orientation determined the failure mode\[direction\] and
the ultimate load value. Only the Allied Signal material was able to
meet the design criteria with any margin of safety. A compromise pin
diameter of 3.2 mm was adapted for the rail bracket design. Details of
the testing may be found in a test report \[5\].

THERMAL HYDRAULICS

![](media/image4.wmf){width="3.0444444444444443in"
height="2.354861111111111in"}

Dowtherm A\[6\] was chosen as the medium for both bakeout and heat
removal between shots. It combines the favorable characteristics of low
vapor pressure at elevated bakeout temperatures and low viscosity, even
at the operation temperature of 38 C.

The heat losses during bakeout due to conduction into the attachment
structure and radiation to the torus wall were estimated at 5.2 to 8 kW,
depending on the surface finish of the components. More detailed thermal
analyses are documented in a report. \[7\]

![](media/image5.pct){width="469.0in" height="313.0in"}

Figure 6 Pull Fixture ![](media/image6.wmf){width="2.9944444444444445in"
height="3.5381944444444446in"}

CONCLUSIONS

Calculations done after the Peer Review determined that the magnetic
loading on the CSC will not be as high as expected and, in fact, the
higher loads never manifest as tensile loads, i.e., radially away from
the center column. As a result, the bracket and tile design is very
conservative and no other testing is deemed necessary. The reduced loads
also permitted substitution of less expensive Inconel in place of
molybdenum alloy earlier planned for the CSC wave springs and rail pins.
AS carbon material is used for the CSC rail tiles only. ATJ is used on
all other PFC surfaces and is considered adequate for early operation;
upgrade operation may require replacement of some of the divertor tiles
with the AS material. To date the AS material has not been tested for
thermal shock and it is not known whether it will perform as well as the
FMI, the material of choice for very high heat flux applications. In
view of its economic advantages this is an area which should be tested
for future fusion applications.

REFERENCES

1\. Grafoil is a registered trade mark of UCAR Carbon Co., Inc.

2\. Microtherm is a registered trademark of Microtherm International
Limited.

3\. P-THERMAL is a trademark of PDA Engineering Corp.

4\. P. Goranson, B. Nelson, "Thermal Analysis of NSTX Center Stack and

Outboard Divertor Tiles", LMES Report 11-980501-LMES-PLG-01, Nov., 1998.

5\. P. Goranson, B. Nelson, "PFC Stress Analysis and Testing", LMES
Report NSTX-CALC-11-04, Nov., 1998.

6\. Dowtherm is a registered trademark of Dow Chemical Corp.

7\. A. Brooks, "Bakeout of Vacuum Vessel and Internals", PPPL Report
13-970214-AWB-01, Feb., 1997.

[^1]: ^\*^ Work supported by U.S. Department of Energy under Contract
    DE-AC02-76CH03073