92
5 Erosion and Deposition and Their Influence on Plasma …
hardly seen. Accordingly, W deposition at plasma shadowed area and remote area
was not significant. It should be noted that if the W deposits included impurities like
C and O, a significant amount of H should be retained as discussed in Sect. 9.4 in
Chap. 9.
In the case of Be, owing to chemical sputtering making BeH 2 , its erosion and
deposition are similar to those of carbon. In JET ILW (ITER-Like Wall), Be dominates in the deposits, and deposition at plasma shadowed area is appreciable, of
which detail is described in Chap. 8.
Erosion by surface melting is another concern for the metallic wall. Surface
melting layers move following the electromagnetic field and often leave as droplets.
Adhesion of the droplets on PFS or surfaces of components in a reactor is not strong
enough so that the droplets easily become dust and move around during the following
discharges. Because the dusts made of metals contain neutron-activated elements
together with T, they are much hazardous and concerned with safety compared to
dust made of carbon.
5.4 Summary
Erosion and deposition behavior/mechanism is quite different between PFMs,
hydride-making, and non-hydride-making elements. For carbon wall, chemical
erosion through hydrocarbon formation results in a significant amount of erosion
which shortens the lifetime of the C-wall and its deposits retain a large amount of
hydrogen. On the other hand, small erosion of W appreciably reduces deposits and
small hydrogen solubility in W also reduces hydrogen retention. That is the main
reason for the selection of W as divertor tiles of ITER.
The above conclusion is based on the data obtained in present tokamaks which are
operated at RT or ambient temperature much below the operation temperature of a
reactor. Since high temperature use of C and W would change erosion and deposition
scheme and hydrogen retention as well, data accumulation under higher temperature
are awaited for the selection of PFM (see Chap. 10).
Although the deposits retain a large amount of hydrogen to be the main cause
of T inventory in a reactor, higher temperature reactor operation could change the
retention scheme which is discussed separately in Chap. 8 considering temperature
effects.
Owing to the prompt deposition, sides of plasma-facing armor tiles or facing to
gaps of the tiles are mostly deposited. In addition, eroded materials from C-wall
(mostly hydrocarbons) travel a long distance to be deposited at a far remote area
like pumping duct or even penetrating into pumps but mostly in the line of sight
from the plasma. In this respect, installation of collector plates heatable to very high
temperature at possible deposition locations on plasma shadowed or remote areas
could be one of the important methods to reduce T inventory.
5 Erosion and Deposition and Their Influence on Plasma …
hardly seen. Accordingly, W deposition at plasma shadowed area and remote area
was not significant. It should be noted that if the W deposits included impurities like
C and O, a significant amount of H should be retained as discussed in Sect. 9.4 in
Chap. 9.
In the case of Be, owing to chemical sputtering making BeH 2 , its erosion and
deposition are similar to those of carbon. In JET ILW (ITER-Like Wall), Be dominates in the deposits, and deposition at plasma shadowed area is appreciable, of
which detail is described in Chap. 8.
Erosion by surface melting is another concern for the metallic wall. Surface
melting layers move following the electromagnetic field and often leave as droplets.
Adhesion of the droplets on PFS or surfaces of components in a reactor is not strong
enough so that the droplets easily become dust and move around during the following
discharges. Because the dusts made of metals contain neutron-activated elements
together with T, they are much hazardous and concerned with safety compared to
dust made of carbon.
5.4 Summary
Erosion and deposition behavior/mechanism is quite different between PFMs,
hydride-making, and non-hydride-making elements. For carbon wall, chemical
erosion through hydrocarbon formation results in a significant amount of erosion
which shortens the lifetime of the C-wall and its deposits retain a large amount of
hydrogen. On the other hand, small erosion of W appreciably reduces deposits and
small hydrogen solubility in W also reduces hydrogen retention. That is the main
reason for the selection of W as divertor tiles of ITER.
The above conclusion is based on the data obtained in present tokamaks which are
operated at RT or ambient temperature much below the operation temperature of a
reactor. Since high temperature use of C and W would change erosion and deposition
scheme and hydrogen retention as well, data accumulation under higher temperature
are awaited for the selection of PFM (see Chap. 10).
Although the deposits retain a large amount of hydrogen to be the main cause
of T inventory in a reactor, higher temperature reactor operation could change the
retention scheme which is discussed separately in Chap. 8 considering temperature
effects.
Owing to the prompt deposition, sides of plasma-facing armor tiles or facing to
gaps of the tiles are mostly deposited. In addition, eroded materials from C-wall
(mostly hydrocarbons) travel a long distance to be deposited at a far remote area
like pumping duct or even penetrating into pumps but mostly in the line of sight
from the plasma. In this respect, installation of collector plates heatable to very high
temperature at possible deposition locations on plasma shadowed or remote areas
could be one of the important methods to reduce T inventory.