The Impact Of Lithium Wall Coatings On NSTX Discharges And The Engineering Of The Lithium Tokamak eXperiment (LTX) Page: 9 of 24
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chromium plating over nickel, and plasma-sprayed tungsten and molybdenum. Thin electroplated
nickel inhibits attack of the copper by liquid lithium, but only up to temperatures in the 400 -
500C range. The outer surface of the shell is plated with nickel, as are all other copper structures
in the tokamak. A second shell is in fabrication, which will be plasma-sprayed with a 0.1 - 0.2
mm thick coating of porous molybdenum on the inner stainless steel surface. Porous
molybdenum has been shown to retain thicker layers of lithium than can be achieved with thin
films on stainless steel. Molybdenum of 30 - 60% porosity has been tested, and a porosity of
approximately 50% has been chosen for the coating. Both the stainless steel inner surface for the
first shell, and the molybdenum surface for the second shell, will be fully coated by lithium to
inhibit sputtering of high-Z materials into the plasma.
Both views in Figure 6 also show the shell support structure, which is designed for both
mechanical and 1 kV electrical isolation of each of the four shell segments from the vacuum
vessel. Mechanical support for the shell segments is provided by four legs per segment. Each leg
extends through the upper and lower vessel flanges via a vacuum electrical break and a formed
bellows, and is supported externally off the vacuum vessel. In effect, the shell "floats" within the
vessel, with no internal electrical or mechanical contact between the shell and the vessel. This
approach avoids supporting the shell segments on internal high voltage ceramic breaks, which
would be subject to repeated mechanical shock during disruptions due to the overturning
moment on the shell segments. The approach also allows for thermal expansion of the shell;
expansion is accommodated by flexing of the long support legs. The shell itself, with support
legs, is shown in Figure 7A, along with the calculated distribution of forces during a disruption,
in Figure 7B. A photograph of the interior of the shell during a vent is shown in Figure 8.
Resistive cable heaters (not shown) are clamped onto the outer, copper surface of the shell in
order to maintain a temperature of up to 500 - 600 C. These heaters are constructed with long
cold sections at the terminating ends; all sections of the heater not in good thermal contact with
the shell are unheated. Vacuum isolation is through Swagelok fittings sealed to the tubular
heaters themselves, so that all electrical connections for the heaters are made outside the vessel,
where they are not subject to coating by thin layers of evaporated lithium. The shell segments are
individually electrically isolated through insulating supports and electrical breaks on the heater
feedthroughs in order to facilitate argon glow discharge cleaning of the inner shell surface. Heat
conduction paths from the shell to the vacuum vessel are limited to the stainless steel support
rings and legs (shown in Figure 7A); these paths are over a meter long, and have small cross-
sectional area to provide for good thermal isolation. The LTX centerstack is thermally isolated
from the shell by a 1.5 mm layer of polished stainless steel, overlying a 1.5 mm layer of silicon
bonded mica, and a 6 mm vacuum gap between the mica and the 1.5 mm thick Inconel tube,
which comprises the vacuum boundary of the centerstack. The centerstack assembly contains the
epoxy-potted Ohmic solenoid and inner toroidal field coil conductors, and is water cooled. All
coolant channels are segregated from lithium coated areas by the vacuum boundary; only gas
(helium) cooling is employed within the vacuum boundary. The vacuum chamber itself is
ANSYS analysis of the thermal performance of the shell and surrounding vacuum chamber
indicates that operation to -600 C is feasible with the present 40 kW heater set, without
additional provision for cooling. A plot of the temperature distribution over the shell, outer
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Majeski, R.; Kugel, H. & Kaita, R. The Impact Of Lithium Wall Coatings On NSTX Discharges And The Engineering Of The Lithium Tokamak eXperiment (LTX), report, March 18, 2010; Princeton, New Jersey. (https://digital.library.unt.edu/ark:/67531/metadc1013046/m1/9/: accessed April 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.