Mechanical Design of the NSTX Liquid Lithium Divertor Page: 4 of 8
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relatively poor thermal diffusivity of the stainless steel and
lithium, the top two layers must be as thin as possible, in order
to minimize the front surface temperature rise.
Electric cartridge heaters (Fig. 3) are used to heat the plate
to its initial operating temperature. Each quadrant uses twelve
500W heaters, for a total power of 6kW. This provides almost
twice the power needed to balance radiation losses at 200C,
and allows the heating time for the LLD to be within reason.
Thermal analyses showed that, if one heater were to fail, the
temperature drop between heaters would not be excessive.
Temperature control is accomplished by feedback from eight
thermocouples in each quadrant. Each heater is equipped with
an integral thermocouple, which is used in control circuits that
protect the heater from excessive temperatures.Figure 2. Side View of LLD and Divertor Tiles
Although the area covered by the LLD is an annulus, the
LLD must, as a practical matter, be segmented so that it can be
installed in the vacuum vessel. The LLD is thus made in four
segments, each of which subtends an angle of 82.5deg
toroidally. Graphite tiles occupy the space between LLD
quadrants. In addition to serving as a proper interface between
segments, these tiles will be equipped with various diagnostics
The LLD will be made from a copper substrate, with a thin
layer of stainless steel bonded to the plasma face, and a thin
layer of porous molybdenum plasma sprayed onto the stainless
steel. The molybdenum layer retains the lithium and causes it
to wet the surface, rather than bead up. A heating and cooling
system will be required to maintain the lithium within its
narrow operating temperature range. Four insulating supports at
the corners of each quadrant, along with a conducting plug at
the center, locate the divertor segment, accommodate thermal
expansion, and react the forces caused by eddy currents during
a plasma disruption.
III. DESIGN DESCRIPTION
The fundamental challenge associated with the LLD is that
of keeping the temperature of the lithium within a narrow
operating range. The melting point of lithium is 200 degrees
Celsius, and at 400C the vapor pressure begins to increase
rapidly, causing unacceptable outgassing. Given that the LLD
is in the divertor region, where heat fluxes on the order of
1000W/cm2, the problem of maintaining the lithium
temperature within this operating range is daunting.
The use of a thick copper plate as a substrate provides good
thermal diffusivity. Scoping calculations during the conceptual
design phase indicated that a thickness of 0.875in [2.22cm]
would, in conjunction with sweeping the strike point of the
plasma, keep the front surface temperature within an acceptable
operating range. A thin, 0.010in [0.25mm] thick layer of
stainless steel is bonded to the plasma side of the copper plate
to provide a baffler between the lithium and the copper.
Finally, a thin --0.015in [.4mm] layer of porous molybdenum is
plasma sprayed onto the stainless steel. Because of theFigure 3. Cartridge Heater and Support Locations on LLD Plate
Radiation cooling is almost, but not sufficient, to return the
LLD plates to their initial temperature between pulses. A gas
cooling system was chosen for the plate. Using forced helium
or nitrogen in a 0.375in [10mm] diameter tube, the LLD
quadrant is returned to its initial operating temperature in
between pulses. Each quadrant has its own independent cooling
line.
The thickness of the liles on the existing outboard divertor
is l.Oin [25.4mm]. The thickness of the LLD plate, and the
need for some clearance underneath, requires that it be located
iS50in [38.1mm] above the copper plates that are underneath
the divertor tiles. The replacement tiles for the row 1
[innermost] locations are thus thicker than the original row 1
tiles, in order to match the height of the LLD. The tiles for the
row 3 locations are made with a profile that blends to the LLD
height at one end, and matches the unchanged row 4 tiles at the
other.
The LLD must be maintained in its position, but significant
differential thermal expansion between it and the outboard
divertor structure must be accommodated. The change in length
of the outboard edge of the LLD, if the entire plate reached
400C, would be roughly 0.2Oin [5mm]. Assuming that a
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R. Ellis, R. Kaita, H. Kugel, G. Paluzzi, M. Viola and R. Nygren. Mechanical Design of the NSTX Liquid Lithium Divertor, article, February 19, 2009; Princeton, New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc928639/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.