4.3 Emission of Ions and Neutrals
59
Fig. 4.11 Temperature dependence of carbon sputtering by H + and D + ions (reprinted with
permission from [22])
yield increases again. The radiation-enhanced sublimation is separately discussed in
the next section. Different from the physical sputtering, the chemical sputtering of
C shows flux dependence as shown in Fig. 4.12 [23]. With the increasing ion flux,
the chemical sputtering yields significantly decrease, and in reactor condition with
the flux of around 10
24 m
−2 s
−1 , more than one order of magnitude less than those
for lower flux. The reason for the reduction is not fully understood, yet.
The chemical stability of the C-H bond is also the cause of the H incorporation
in deposited C layers of high H retention. Similar to C, Be is chemically sputtered
by H plasma making BeH 2 [24] and erosion of Be is not small. Correspondingly,
deposited Be layers retain a large amount of H making BeH 2 . Erosion and deposition
of Be are quite important in JET [25] because Be is used as the first wall. However,