The Impact Of Lithium Wall Coatings On NSTX Discharges And The Engineering Of The Lithium Tokamak eXperiment (LTX) Page: 6 of 24
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The effect of lithium coatings on PFCs has been a primary focus of recent NSTX research. Such
surfaces are expected to improve plasma performance by reducing recycling and suppressing
oxygen impurities. This has been observed in other magnetic confinement devices when PFCs
were coated with lithium, including TFTR , T-11 , FT-U  and the stellarator TJ-II . A
particularly salient result was achieved in the CDX-U device, where a record enhancement in the
confinement time of Ohmically-heated plasmas was observed in discharges with a toroidal liquid
lithium limiter [7,8].
The follow-on device to the CDX-U liquid lithium limiter experiments is the Lithium Tokamak
eXperiment - LTX. The LTX will be somewhat larger than CDX-U (Ro = 0.4 m, a=0.26 m, x =
1.6), but like CDX-U it will operate with a limited, rather than a diverted, discharge. However,
LTX is designed to employ a thin-film liquid lithium wall covering 90% of the plasma-facing
area (5 m2), whereas in CDX-U the liquid lithium limiter covered only 5% (0.2 m2) of the last
closed flux surface. In LTX, the maximum plasma current will be increased to 400kA, with a 50
msec flattop. The toroidal field will also be increased to 3.4 kG. LTX has has begun plasma
operation, and is presently scheduled to begin operations with liquid lithium walls early in 2010.
The research goal of the LTX program is to produce tokamak discharges with very low global
recycling, and determine the consequences for transport and stability of operating in this limit.
Many engineering features of LTX have been developed to accommodate the use of liquid
lithium as the primary PFC.
2. Lithium Coating Techniques and Their Effects in NSTX
The NSTX has two LiThium EvaporatoRs (LITERs), which are lithium ovens mounted on the
upper dome of the vacuum vessel (Figure 1) . They are separated toroidally by about 1500,
and have apertures that point downwards to permit lithium deposition on the PFCs in the lower
divertor region. Lithium evaporation rates up to about 80 mg/min have been achieved for periods
between 5 and 15 minutes. Since the LITERs cannot be cooled rapidly enough to stop
evaporation during discharges, shutters have been installed to interrupt the emission of lithium.
Surface conditioning with lithium coatings has allowed both boronization , which used to be
conducted after about two weeks of plasma operation, and helium glow-discharge cleaning
(HeGDC) between tokamak discharges to be largely discontinued. The effect of the lithium
coating, however, appears to be significant for only one or two discharges. Lithium applied after
the last discharge on a given day still does improve the performance of the first plasma the next
morning. This may mean that it is plasma-surface interactions, and not reactions between the
lithium and the residual gas in the vacuum chamber, that determines how active the lithium
Modifications to the characteristics of NSTX H-mode plasmas due to lithium coatings have been
observed since the beginning of LITER operation. Effects included a lowering in deuterium
recycling, and the line emission from the divertor region of low-ionization states of oxygen and
carbon decreased. These changes were accompanied by a significant improvement in the energy
confinement time with lithium . At the same time, however, the average Zeff and the total
power radiated from the plasma both increase. The rise in the Zeff appears to be related to
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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/m1/6/: accessed March 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.