The Impact Of Lithium Wall Coatings On NSTX Discharges And The Engineering Of The Lithium Tokamak eXperiment (LTX) Page: 8 of 24
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3. Plans for NSTX
The next phase of NSTX lithium research will investigate liquid metal surfaces using a Liquid
Lithium Divertor (LLD) (Figure 5). The LLD is a toroidal conic section with its plasma-facing
surface coated with a porous molybdenum layer. Lithium will be evaporated onto the plate,
which is heated above the melting point of lithium. The resulting thin liquid layer should have a
much larger capacity for absorbing and retaining hydrogen isotopes than lithium in its solid
form. The LLD is located on the outer lower divertor plate, and at this location, predictions for
its ability to provide significant edge pumping in NSTX will be tested.
The first construction phase of LTX has now been completed, and the machine achieved first
plasma in 2008. Discharge development is underway, and installation of the remaining initial
diagnostic set, as well as implementation of a novel insulated-gate bipolar transistor (IGBT)-
based Ohmic power supply, which employs a transformer arrangement to permit independent
switching of the individual paralleled IGBTs. Both elevation and cutaway CAD views of the
tokamak are shown in Figure 6, to illustrate the essential features of the device. The poloidal
field coil set is visible in the external view of the device (Fig. 6A). The most prominent feature in
the vacuum vessel is the heated shell, which will encase 90% of the last closed flux surface of
the plasma. The field coils colored blue, red, yellow, and a new uncased internal coil comprise
the poloidal field (PF) coil set for position control and shaping. All but the orange coils (which
control vertical position and elongation) are new for LTX. Equilibrium calculations indicate that
the new PF coil set will support discharges with plasma currents of over 400 kA, with a wide
range of current profiles. Following full qualification of the power supplies, coils, and other
components, lithium operation is expected to commence in early 2010.
5. Engineering of the LTX shell
Central to the LTX concept is the heated, conformal shell, which will be coated with molten
lithium. The shell is formed of 1.5 mm thick 304L stainless steel explosively bonded to 1 cm
thick OFHC copper, and is heated with custom-length, commercial resistive cable heaters. The
shell has two toroidal breaks and two poloidal breaks (the breaks in the shell are most clearly
shown in Figure 7). The outer equatorial plane break provides toroidally continuous diagnostic
access, as well as an electrical break. The two toroidal breaks provide access for diagnostics such
as Thomson scattering and the microwave interferometer system, which require good poloidal
access. The shell is seen mounted in the vessel in Figure 6B.
The bonded stainless steel inner shell surface functions as a barrier to prevent attack of the
copper by the liquid lithium. The copper backing is primarily designed to distribute heat and
inhibit hot spot formation, either from the discrete electrical heaters or from plasma contact.
Modeling  also indicates that the conducting shell, with a time constant of 140 msec, will
have a significant effect on the plasma stability. Explosive bonding of the stainless to the copper
backing was chosen only after several alternative electroplated and plasma-sprayed surfaces
failed to protect the copper from attack by liquid lithium. Surfaces which failed include hard
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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/m1/8/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.