80
5 Erosion and Deposition and Their Influence on Plasma …
deposited layers except Be, hydrogen retention is much less than that in C-deposited
layers, and they are crystallized. However, metallic deposited layers often contain
a large amount of impurities of O and C and show amorphous-like structure that
contains a large amount of D+T because O and C in metals trap hydrogen.
In the following, the formation of the deposited layers is described separately
for carbon wall and metallic wall focusing basic process for the formation of the
deposited layers. In Chap. 8 is revisited erosion and deposition observed in large
tokamaks together with the formation of dust mostly resulting from the exfoliation
of the deposited layers.
5.3.1 Carbon Wall
5.3.1.1 Deposition on Plasma-Facing Surface
Erosion occurs at plasma-facing surface, while deposition at various places such as
plasma-facing surface, area on the line of sight from plasma, plasma shadowed
area, and remote area from plasma. Detailed observation of deposited layers in
Fig. 5.5 gives the information on how deposited layers were formed in sequential 40 discharges in JT-60 in which most of the plasma-facing surface was covered
by graphite tiles [5]. The deposited layers were mostly made of carbon and showed
layered structure with each layer corresponding to a discharge indicated in the left.
Compared to limiter discharges, NBI heated divertor discharges gave thicker layers
showing columnar structure because of their higher power load resulting in higher
erosion and deposition and temperature rise. Even disruption added a layer including
small black dots probably corresponding to particles or dust exfoliated from PFS.
Deposited layers retain a significant amount of hydrogen because during deposition hydrogen is incorporated in them and after deposition, the layers are exposed
to boundary plasma and residual fuel gas. Since the temperature of plasma shadowed area and remote area is lower than that of PFS, hydrogen concentration in the
deposited layers at these areas is much larger than that in the deposited layers on
PFS and eroded area.
On JET divertor CFC tiles used in the DTE campaign with DT discharges,
deposited profiles in divertor area well corresponded to T retention profiles as seen
in Fig. 5.6. The figure shows the T profiles and the photos of JET divertor tiles
(numbered from BN 1 to BN 10) [6, 7]. The T profiles were quite consistent with
the C-deposited profiles seen in the photos. The numbers in the divertor structure in
the center are the amount of T in mg retained in each CFC tile. The highest retention
was observed in the deposited layers on the louvers of the pumping duct which were
not directly exposed to plasma but exposed to neutral or gas flow pumped out. The
high T retention also appeared at the bottom of the BN 4 tile which was shadowed
from plasma by BN 3 and heavily deposited. Inhomogeneity in the T profile on the
shadowed area was caused by the exfoliation of the deposited layers very similar
to the T profile of TFTR tile in Fig. 5.7 [8]. The lowest T retention appeared at the
5 Erosion and Deposition and Their Influence on Plasma …
deposited layers except Be, hydrogen retention is much less than that in C-deposited
layers, and they are crystallized. However, metallic deposited layers often contain
a large amount of impurities of O and C and show amorphous-like structure that
contains a large amount of D+T because O and C in metals trap hydrogen.
In the following, the formation of the deposited layers is described separately
for carbon wall and metallic wall focusing basic process for the formation of the
deposited layers. In Chap. 8 is revisited erosion and deposition observed in large
tokamaks together with the formation of dust mostly resulting from the exfoliation
of the deposited layers.
5.3.1 Carbon Wall
5.3.1.1 Deposition on Plasma-Facing Surface
Erosion occurs at plasma-facing surface, while deposition at various places such as
plasma-facing surface, area on the line of sight from plasma, plasma shadowed
area, and remote area from plasma. Detailed observation of deposited layers in
Fig. 5.5 gives the information on how deposited layers were formed in sequential 40 discharges in JT-60 in which most of the plasma-facing surface was covered
by graphite tiles [5]. The deposited layers were mostly made of carbon and showed
layered structure with each layer corresponding to a discharge indicated in the left.
Compared to limiter discharges, NBI heated divertor discharges gave thicker layers
showing columnar structure because of their higher power load resulting in higher
erosion and deposition and temperature rise. Even disruption added a layer including
small black dots probably corresponding to particles or dust exfoliated from PFS.
Deposited layers retain a significant amount of hydrogen because during deposition hydrogen is incorporated in them and after deposition, the layers are exposed
to boundary plasma and residual fuel gas. Since the temperature of plasma shadowed area and remote area is lower than that of PFS, hydrogen concentration in the
deposited layers at these areas is much larger than that in the deposited layers on
PFS and eroded area.
On JET divertor CFC tiles used in the DTE campaign with DT discharges,
deposited profiles in divertor area well corresponded to T retention profiles as seen
in Fig. 5.6. The figure shows the T profiles and the photos of JET divertor tiles
(numbered from BN 1 to BN 10) [6, 7]. The T profiles were quite consistent with
the C-deposited profiles seen in the photos. The numbers in the divertor structure in
the center are the amount of T in mg retained in each CFC tile. The highest retention
was observed in the deposited layers on the louvers of the pumping duct which were
not directly exposed to plasma but exposed to neutral or gas flow pumped out. The
high T retention also appeared at the bottom of the BN 4 tile which was shadowed
from plasma by BN 3 and heavily deposited. Inhomogeneity in the T profile on the
shadowed area was caused by the exfoliation of the deposited layers very similar
to the T profile of TFTR tile in Fig. 5.7 [8]. The lowest T retention appeared at the