Lithium As Plasma Facing Component for Magnetic Fusion Research
Authors: Masayuki Ono
Abstract:
The use of lithium in magnetic fusion confinement experiments started in the 1990's
in order to improve tokamak plasma performance as a low-recycling plasma-facing
component (PFC). Lithium is the lightest alkali metal and it is highly chemically reactive
with relevant ion species in fusion plasmas including hydrogen, deuterium, tritium,
carbon, and oxygen. Because of the reactive properties, lithium can provide strong
pumping for those ions. It was indeed a spectacular success in TFTR where a very small
amount (~ 0.02 gram) of lithium coating of the PFCs resulted in the fusion power output
to improve by nearly a factor of two. The plasma confinement also improved by a factor
of two. This success was attributed to the reduced recycling of cold gas surrounding the
fusion plasma due to highly reactive lithium on the wall. The plasma confinement and
performance improvements have since been confirmed in a large number of fusion
devices with various magnetic configurations including CDX-U/LTX (US), CPD (Japan),
HT-7 (China), EAST (China), FTU (Italy), NSTX (US), T-10, T-11M (Russia), TJ-II
(Spain), and RFX (Italy). Additionally, lithium was shown to broaden the plasma
pressure profile in NSTX, which is advantageous in achieving high performance H-mode
operation for tokamak reactors. It is also noted that even with significant applications (up
to 1,000 grams in NSTX) of lithium on PFCs, very little contamination (< 0.1%) of
lithium fraction in main fusion plasma core was observed even during high confinement
modes. The lithium therefore appears to be a highly desirable material to be used as a
plasma PFC material from the magnetic fusion plasma performance and operational point
of view. An exciting development in recent years is the growing realization of lithium as
a potential solution to solve the exceptionally challenging need to handle the fusion
reactor divertor heat flux, which could reach 60 MW/m2 . By placing the liquid lithium
(LL) surface in the path of the main divertor heat flux (divertor strike point), the lithium
is evaporated from the surface. The evaporated lithium is quickly ionized by the plasma
and the ionized lithium ions can provide a strongly radiative layer of plasma ("radiative
mantle"), thus could significantly reduce the heat flux to the divertor strike point
surfaces, thus protecting the divertor surface. The protective effects of LL have been
observed in many experiments and test stands. As a possible reactor divertor candidate, a
closed LL divertor system is described. Finally, it is noted that the lithium applications as
a PFC can be quite flexible and broad. The lithium application should be quite
compatible with various divertor configurations, and it can be also applied to protecting
the presently envisioned tungsten based solid PFC surfaces such as the ones for ITER.
Lithium based PFCs therefore have the exciting prospect of providing a cost effective flexible means to improve the fusion reactor performance, while providing a practical
solution to the highly challenging divertor heat handling issue confronting the steadystate
magnetic fusion reactors.
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Submitted to: Nova Scientific Publications, Inc. (August 2012)
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