Removal of codeposited layers by ECR discharge cleaning

Citation
W. Jacob et al., Removal of codeposited layers by ECR discharge cleaning, J NUCL MAT, 269, 1999, pp. 552-556
Citations number
11
Categorie Soggetti
Apllied Physucs/Condensed Matter/Materiales Science","Nuclear Emgineering
Journal title
JOURNAL OF NUCLEAR MATERIALS
ISSN journal
00223115 → ACNP
Volume
269
Year of publication
1999
Pages
552 - 556
Database
ISI
SICI code
0022-3115(199903)269:<552:ROCLBE>2.0.ZU;2-9
Abstract
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.