Removal of carbon films by oxidation in narrow gaps: Thermo-oxidation and plasma-assisted studies

Abstract

a b s t r a c tThe removal of hard amorphous hydrogenated carbon (a-C:H) films from narrow gaps simulating the macro-brush structures present in controlled fusion devices has been investigated. Films with a thickness of 50-150 nm were generated through plasma-assisted chemical vapor deposition (PACVD) in glow discharges of CH 4 /He on Si and stainless steel plates. The deposited plates were then arranged to form sandwich structures building narrow gaps and were subjected to erosion by exposure to O 2 /He plasmas and to thermal oxidation by O 2 and by a NO 2 /N 2 (1:1) mixture. In the plasma etching experiments, the deposited layers were only partially removed by the plasma at the side wall gap surfaces, but were efficiently removed at the bottom of the gap. In the thermo-oxidation experiments, the deposited films were effectively and homogeneously removed with oxygen at 670 K and with the NO 2 /N 2 mixture at T > 570 K.

Long-term carbon transport and fuel retention in gaps of the main toroidal limiter in TEXTOR

“…Gaps between castellation cells serve, however, as traps for the migrating impurities and co-deposited fuel, contributing to the safety problem of the tritium retention [1]. The efficiency of the fuel removal methods is restricted in gaps due to their geometry [2][3][4]. A dedicated campaign to study the mechanisms of fuel retention was performed in the Tore Supra tokamak, revealing the details of the asymmetry of the deposition in toroidal and poloidal gaps [5] and peculiarities of the co-deposition in gaps [6].…”

Carbon removal from tile gap structures with oxygen glow discharges

“…Tritium retention in Next Step Devices and removal techniques were recently reviewed by Counsell et al [1]. Except for thermo-oxidation, which proofed to be successful also for narrow gaps [4] for most of the presently known cleaning methods [5][6][7][8][9] only data for flat surfaces are available and it is not yet known if they are also efficient on structured surfaces. Therefore, assessing the efficiency of cleaning methods to remove co-deposited layers from tile gap structures is a critical issue for the current ITER design.…”

Surface effects on the diagnostic of carbon/nitrogen low‐pressure plasmas studied by differentially pumped mass spectrometry

“…Stainless steel [21,22] T = 330 K NH B = 0.13; also at +200 V a-C:N(H) [18,23] T = 300 K NH 2 B = 0.60-0.37; also at +200 V a-C:N(H) [18,24] NH 3 /CH 4 30-50% B = 0.20; B = 0.30 at +200 V a-C:N(H) [18,24] NH 3 /CH 4 75% O· B = 0.17 Stainless steel [22] T = 330 K O· s = 0.1 a-C:H [25] H· s = 0.01 a-C:H [26,27] H· γ = 0.03 Stainless steel [28] T = 321 K H · N · NH · NH 2 · s = 1 on free site Stainless steel [29] H + H(s) → H 2 γ = 1.5 × 10…”