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To minimise radiation losses, plasmas in nuclear fusion reactors have to be kept as free as possible of high-Z impurities. For this reason, the walls of TEXTOR and other tokamaks are coated with a thin layer of amorphous carbon (a-C:H) or amorphous carbon/boron (a-C/B:H). The sputtering behaviour of such layers was investigated under bombardment with Ar(+) ions with energy of 1.5 keV and current of only a few mA/cm(2), the aim being to determine the velocity distribution of the sputtered atoms and the sputtering yield in the vacuum ultra-violet range, by the use of laser induced fluorescence. The velocity distribution and absolute density of sputtered H (121.6 nm), D (121.5 nm), B (249.8 nm) and O (130.2 nm) were measured. The velocity distribution corresponds approximately to a Thompson distribution. The Thompson distributions matched best at surface energies of 1.0 eV in H and D atoms, 7.6 in C atoms and 5.6 in B atoms. The velocity distribution of atomic oxygen matched the Thompson distribution poorly. A good match was obtained when two different Thompson distributions were used for surface energies of 0.5 eV and 4.8 eV respectively. With a-C:H layers, and to a much greater degree with a-C/B:H layers, the sputtering yield for H atoms showed marked dependence on the particle flux.

Additional information

Authors: PASCH E, Forschungszentrum Jülich GmbH, Institut für Plasmaphysik (DE)
Bibliographic Reference: Report: Jül-2647 DE (1992) 120 pp.
Availability: Available from Zentralbibliothek der Forschungszentrum Jülich GmbH, Postfach 1913, 5170 Jülich (DE)
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