194
10 Selection of Plasma-Facing Materials
Benefit of C is possibility of in situ repairing, with using CVD process. The surface
of C subjected to such high temperatures is mostly graphitized and more or less no
appreciable difference in the origin of the carbon materials.
The mechanical fixing ensures easy replacement of failed tiles by remote handling
and periodic break for system maintenance gives opportunity for routine replacements of the tiles. Still concerns remain; how to fix the carbon armor tiles to PFC
mechanically, tolerance of the mechanical fixing to surface temperature escalation
due to its poor thermal contact, and any effects caused by irradiation. It should be
mentioned that less activation of C by neutron irradiation than any other candidate plasma-facing metals (except liquid metals) makes system maintenance safer
in aspect of radiation exposure of workers.
Still improvement of thermal contact coefficient for the mechanical fixing of the
armor tiles to a heat sink material is needed. It totally depends on the method of
fixing. Utilization of thin armor tiles is required not only for thermal conductance
but also stress relieving caused by neutron irradiation dimensional change. Higher
temperature operation is favored owing to significantly reduction of the neutron
irradiation effects although data on neutron irradiation of CFC at elevated temperature
is scarce.
10.4 Liquid PFM
Lithium-based liquid surface as a candidate PFM has gained deserved attention in
the last decade [6–8]. Table 10.3 compares solid and liquid as PFM [8]. Notable
advantages have resulted from the use of Li in various applications including thin-film
coatings on metallic substrates, lithium-graphite surfaces, Li based alloys, and others.
There are two main challenges to the use of lithium-based surfaces as viable PFM.
One is their narrow operating temperature window. At temperatures just above the
melting point of Li (e.g. 180°C), non-linear erosion ensues and sputtering increases
toward unity. The eroded Li atoms by sputtering are mostly ions and the remaining
neutrals quickly ionized due to their low ionization potential. This fact leads to
effective screening of eroded Li material from the core plasma at all temperatures.
For liquid Li, therefore, macroscopic emission of material due to off-normal events
such as ELMs or similar events can be problematic and thus the second challenge
using Li as PFS [6]. Still difficulty remains in establishing a liquid cooling system
including hazardous radioactivity and unstable chemical nature of Li in atmosphere.
10 Selection of Plasma-Facing Materials
Benefit of C is possibility of in situ repairing, with using CVD process. The surface
of C subjected to such high temperatures is mostly graphitized and more or less no
appreciable difference in the origin of the carbon materials.
The mechanical fixing ensures easy replacement of failed tiles by remote handling
and periodic break for system maintenance gives opportunity for routine replacements of the tiles. Still concerns remain; how to fix the carbon armor tiles to PFC
mechanically, tolerance of the mechanical fixing to surface temperature escalation
due to its poor thermal contact, and any effects caused by irradiation. It should be
mentioned that less activation of C by neutron irradiation than any other candidate plasma-facing metals (except liquid metals) makes system maintenance safer
in aspect of radiation exposure of workers.
Still improvement of thermal contact coefficient for the mechanical fixing of the
armor tiles to a heat sink material is needed. It totally depends on the method of
fixing. Utilization of thin armor tiles is required not only for thermal conductance
but also stress relieving caused by neutron irradiation dimensional change. Higher
temperature operation is favored owing to significantly reduction of the neutron
irradiation effects although data on neutron irradiation of CFC at elevated temperature
is scarce.
10.4 Liquid PFM
Lithium-based liquid surface as a candidate PFM has gained deserved attention in
the last decade [6–8]. Table 10.3 compares solid and liquid as PFM [8]. Notable
advantages have resulted from the use of Li in various applications including thin-film
coatings on metallic substrates, lithium-graphite surfaces, Li based alloys, and others.
There are two main challenges to the use of lithium-based surfaces as viable PFM.
One is their narrow operating temperature window. At temperatures just above the
melting point of Li (e.g. 180°C), non-linear erosion ensues and sputtering increases
toward unity. The eroded Li atoms by sputtering are mostly ions and the remaining
neutrals quickly ionized due to their low ionization potential. This fact leads to
effective screening of eroded Li material from the core plasma at all temperatures.
For liquid Li, therefore, macroscopic emission of material due to off-normal events
such as ELMs or similar events can be problematic and thus the second challenge
using Li as PFS [6]. Still difficulty remains in establishing a liquid cooling system
including hazardous radioactivity and unstable chemical nature of Li in atmosphere.