8.2 Erosion and Deposition
141
even in the bottom of gaps. Thus, deposition profiles on shadowed area and remote
area are quite dependent on geometries of the areas so that it is hard to describe
general picture of carbon transport (erosion and deposition) in tokamaks.
Since all tokamaks given in Table 8.1 except JT-60U which was operated above
500 K were auxiliary cooled, their base temperatures were ambient (below 400 K)
and the temperature of PFS was raised during the discharge with a different raising
rate and highest temperature with each other. Nevertheless, the difference in the
erosion/deposition rates among them were within factors. The reason was not clear.
The C content in the plasma of these tokamaks was also not so different from
each other, a few %, which must be correlated to the deposition rate. If these
erosion/deposition rates are simply piled up for 1 year, it becomes 1 × 10
28 C atoms
= 1.6 × 10
4 mol or = 1.9 × 10
5 g. That is, nearly 190 kg·y
−1 of C is eroded/deposited.
Suppose T retention rate in the deposited C is 0.1 in T/C, 9.6 kg·y
−1 of T would be
retained in the deposited C. Compared to allowable T inventory in ITER, 1 kg, this is
too large. Thus, utilization of C as PFM in ITER was excluded. However, hydrogen
retention in JT-60U operated at higher temperature was much less. Therefore, in a
reactor with PFM made of C of which base temperature of PFM should be above
700 K for power generation, T/C would be less than 0.1. Even so, 0.96 kg·y
−1 of T
would be still too high. Hence, the removal of T from PFM is mandatory. More details
of erosion/deposition and fuel retention are discussed in the following sections.
8.2.2 Metallic Wall
It is reported that the general picture of material transport in JET C-wall (CW) was
still valid in JET-ILW with Be first wall and W divertor [23, 24]. Erosion and local
deposition of Be from and on the limiters occurred during the limiter plasma phase.
During divertor plasmas, Be was eroded from the inner wall by charge exchange
neutrals and deposited at the inner divertor region. Compared to JET-CW, there was
an overall reduction of the deposition in the JET-ILW, which should be accompanied
by the reduction of net erosion from the inner main chamber wall.
Figure 8.6 shows profiles of deposition (b) and D retention (c) on divertor tiles
of JET-ILW [25]. As depicted in the figure, the deposition profiles on the divertor
tiles well correspond to D retention. The strike points showed less deposition and D
retention compared to other regions owing to the temperature rise. Fuel retention is
discussed separately in Chap. 9.
Table 8.2 are compared experimental results from long-term expose probe samples
(LTS) installed at the first wall during 2005–2009 (JET CW), 2001–2012 (JET ILW
campaign 1 (ILW1)), and 2013–2014 (ILW2) campaign [26]. In erosion of Be, chemical sputtering with the form of hydride (BeH 2 and BeD 2 ) is largely contributing [27].
In addition, less threshold energy and yield of sputtering of Be compared C, erosion
rate of Be is a little larger than C. The erosion of W is much less, more than one
order of magnitude, owing to its high threshold energy and less sputtering yield.
141
even in the bottom of gaps. Thus, deposition profiles on shadowed area and remote
area are quite dependent on geometries of the areas so that it is hard to describe
general picture of carbon transport (erosion and deposition) in tokamaks.
Since all tokamaks given in Table 8.1 except JT-60U which was operated above
500 K were auxiliary cooled, their base temperatures were ambient (below 400 K)
and the temperature of PFS was raised during the discharge with a different raising
rate and highest temperature with each other. Nevertheless, the difference in the
erosion/deposition rates among them were within factors. The reason was not clear.
The C content in the plasma of these tokamaks was also not so different from
each other, a few %, which must be correlated to the deposition rate. If these
erosion/deposition rates are simply piled up for 1 year, it becomes 1 × 10
28 C atoms
= 1.6 × 10
4 mol or = 1.9 × 10
5 g. That is, nearly 190 kg·y
−1 of C is eroded/deposited.
Suppose T retention rate in the deposited C is 0.1 in T/C, 9.6 kg·y
−1 of T would be
retained in the deposited C. Compared to allowable T inventory in ITER, 1 kg, this is
too large. Thus, utilization of C as PFM in ITER was excluded. However, hydrogen
retention in JT-60U operated at higher temperature was much less. Therefore, in a
reactor with PFM made of C of which base temperature of PFM should be above
700 K for power generation, T/C would be less than 0.1. Even so, 0.96 kg·y
−1 of T
would be still too high. Hence, the removal of T from PFM is mandatory. More details
of erosion/deposition and fuel retention are discussed in the following sections.
8.2.2 Metallic Wall
It is reported that the general picture of material transport in JET C-wall (CW) was
still valid in JET-ILW with Be first wall and W divertor [23, 24]. Erosion and local
deposition of Be from and on the limiters occurred during the limiter plasma phase.
During divertor plasmas, Be was eroded from the inner wall by charge exchange
neutrals and deposited at the inner divertor region. Compared to JET-CW, there was
an overall reduction of the deposition in the JET-ILW, which should be accompanied
by the reduction of net erosion from the inner main chamber wall.
Figure 8.6 shows profiles of deposition (b) and D retention (c) on divertor tiles
of JET-ILW [25]. As depicted in the figure, the deposition profiles on the divertor
tiles well correspond to D retention. The strike points showed less deposition and D
retention compared to other regions owing to the temperature rise. Fuel retention is
discussed separately in Chap. 9.
Table 8.2 are compared experimental results from long-term expose probe samples
(LTS) installed at the first wall during 2005–2009 (JET CW), 2001–2012 (JET ILW
campaign 1 (ILW1)), and 2013–2014 (ILW2) campaign [26]. In erosion of Be, chemical sputtering with the form of hydride (BeH 2 and BeD 2 ) is largely contributing [27].
In addition, less threshold energy and yield of sputtering of Be compared C, erosion
rate of Be is a little larger than C. The erosion of W is much less, more than one
order of magnitude, owing to its high threshold energy and less sputtering yield.