Redeposition of eroded carbon on the walls of fusion devices together with
the D-T plasma forms amorphous, hydrogenated carbon films with D-T concentr
ations ranging for deposition at a surface temperature of 300 K from 0.3 up
to 0.6, depending on the deposition conditions. The trapping of large amou
nts of hydrogen isotopes in these so-called codeposited layers may represen
t a potential safety hazard during D-T operation because large amounts of t
ritium will be retained in these layers. To erode carbon films and deplete
the T concentration an experimental investigation was performed using O-2,
D-2, H-2, H2O, and O-2/H-2, electron cyclotron resonance low-pressure disch
arges. Real-time in-situ ellipsometry was applied to measure the optical re
sponse of the layers during deposition and erosion. This allows a precise d
etermination of the optical constants and the film thickness. From these da
ta the deposition and erosion rates are calculated. The dependence of the e
rosion rates on substrate temperature, applied ion energy, gas composition
and discharge pressure for two different types of a-C:H films was measured.
In the investigated parameter range oxygen discharges showed the highest e
rosion rates of all investigated source gases. The highest rates achieved a
t 300 K with the substrate at floating potential are 3.6 mu m/h for polymer
-like, soft C:H films (H/C about 1) and 1.7 mu m/h for hard a-C:H films (WC
about 0.5). The erosion rates increase with increasing substrate temperatu
re (roughly by a factor of 3-4, 300-650 K) and with increasing ion energy (
by about a factor of 6-8, 0-200 eV). The paper presents the experimental re
sults and a discussion of the parameter dependencies and the consequences f
or the removal of codeposited layers by low-pressure discharge cleaning in
ITER. (C) 1999 Elsevier Science B.V. All rights reserved.