5.3 Formation of Deposited Layers Made of Eroded Materials
81
Fig. 5.5 TEM observation of deposited layers on JT-60 divertor produced by around 50 discharges
with a comparison of discharge history [5]. Each discharge made a deposited layer of which thickness
is different depending on discharge modes with thicker ones for divertor discharges and thinner ones
for limiter discharges. Lower power divertor discharges made columnar structured layers remain in
the incoming direction of deposited materials, while higher power discharges changed the columnar
layers to planer structure owing to the annealing effect of their higher power load. See also (Fig. 5.8)
divertor target area in BN5, which was eroded. It should be noted that in the toroidal
direction, T retention and C deposition were also quite inhomogeneous. Because of
tile alignment to avoid edge effects as shown in the inset (bottom left), the right side
of the inner divertor at the high field side was erosion dominated (less T), while the
left side was deposition dominated. The profile is opposite on the outer divertor tiles
at the lower field side.
D-T discharges in TFTR also remained that T profiles are strongly correlated to
carbon deposition profiles as shown in Fig. 5.7 [8]. Since TFTR is a limiter tokamak,
eroded area and deposited area on the inner surface or bumper limiter are clearly
distinguished. Some part of the deposits was exfoliated showing little T retention
beneath the deposits. The sides of the eroded tile show higher T retention so as the
higher C deposition. Since the tile sides were facing tile gaps, T retention in tile gaps
in a reactor becomes a concern. However, different from TFTR, deposition in tile
gaps are not so significant in divertor tokamaks like JET and JT-60U as TFTR. The
T retention in the gap is discussed later. It is important to note that the T profile of
the eroded tile reflects characteristics of a CFC tile composed of fivers and matrix
(compare the photograph and the T profile of the eroded tile in the figure). That is
because both the amount of retained T and erosion were different between the fiver
and the matrix.
81
Fig. 5.5 TEM observation of deposited layers on JT-60 divertor produced by around 50 discharges
with a comparison of discharge history [5]. Each discharge made a deposited layer of which thickness
is different depending on discharge modes with thicker ones for divertor discharges and thinner ones
for limiter discharges. Lower power divertor discharges made columnar structured layers remain in
the incoming direction of deposited materials, while higher power discharges changed the columnar
layers to planer structure owing to the annealing effect of their higher power load. See also (Fig. 5.8)
divertor target area in BN5, which was eroded. It should be noted that in the toroidal
direction, T retention and C deposition were also quite inhomogeneous. Because of
tile alignment to avoid edge effects as shown in the inset (bottom left), the right side
of the inner divertor at the high field side was erosion dominated (less T), while the
left side was deposition dominated. The profile is opposite on the outer divertor tiles
at the lower field side.
D-T discharges in TFTR also remained that T profiles are strongly correlated to
carbon deposition profiles as shown in Fig. 5.7 [8]. Since TFTR is a limiter tokamak,
eroded area and deposited area on the inner surface or bumper limiter are clearly
distinguished. Some part of the deposits was exfoliated showing little T retention
beneath the deposits. The sides of the eroded tile show higher T retention so as the
higher C deposition. Since the tile sides were facing tile gaps, T retention in tile gaps
in a reactor becomes a concern. However, different from TFTR, deposition in tile
gaps are not so significant in divertor tokamaks like JET and JT-60U as TFTR. The
T retention in the gap is discussed later. It is important to note that the T profile of
the eroded tile reflects characteristics of a CFC tile composed of fivers and matrix
(compare the photograph and the T profile of the eroded tile in the figure). That is
because both the amount of retained T and erosion were different between the fiver
and the matrix.