Fuel removal from tile gaps with oxygen discharges: reactivity of neutrals

Abstract

Carbon removal in so-called remote areas, where no energetic species can reach the surface, is investigated with low-temperature glow discharges in pure oxygen. Plasma-deposited amorphous hydrogenated carbon thin films are used as a model system for redeposited films. Erosion measurements are performed on flat substrates as well as on two different 3D test structures. One design consists of 19 mm deep gaps with widths ranging from 0.5 to 4 mm to simulate ITER tile gaps. A second design consists of a flat box-like structure where particles can enter only through a narrow slit. Measurements are performed for substrate temperatures ranging from 290 to 580 K. Erosion rates follow an Arrhenius-type dependence on substrate temperature with an effective activation energy of 0.25 eV. While at room temperature the surface loss probability of the dominant eroding species is 50 % it becomes smaller at elevated temperatures. At elevated temperatures the films are not only removed faster but simultaneously erosion penetrates deeper into the gaps.

Erosion of tungsten-doped amorphous carbon films in oxygen plasma

“…This is not too surprising since an oxygen plasma is a source for species with a much higher reactivity than stable ground state oxygen molecules. Secondly, this value is also significantly lower than that found for oxygen glow discharge removal of plasma-deposited amorphous hydrogenated carbon (a-C:H) films in plasma-shaded areas (0.25 eV [12]). Thirdly, the 1.8 % a-C:W films has a higher apparent activation energy than the pure a-C film and shows, therefore, a larger resilience against oxidation than the pure C film.…”

“…For example, ASDEX Upgrade showed that removal is very effective even at room temperature and similar results were obtained in TEXTOR and HT-7 for elevated temperatures [4][5][6][7][8]. Schwarz-Selinger et al [11,12] have recently shown that oxygen plasmas are very efficient for removal of hydrocarbon films which are a good model system for re-deposited carbon films in fusion devices. This is true not only for plasma exposed areas [11], but at elevated surface temperatures (> 400 K) also for areas which have no direct line of sight to the cleaning discharge such as, e.g., in tile gaps.…”

“…A quantitative analysis of the raw data is at present not possible because not only the optical constants but also the layer structure of the surface layer of the films change during the erosion and the corresponding film thicknesses and the optical constants of the different layers are presently unknown. Furthermore, amorphous carbon material prepared by magnetron sputtering of a graphite target consists of nano-graphite particles dispersed in an amorphous carbon matrix [12,25,26], and therefore it is strongly absorbing at the laser wavelength of 623.8 nm. As a consequence, time-resolved erosion rates could not be deduced from real-time ellipsometry measurements.…”

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].…”

Tritium inventory control during ITER operation under carbon plasma-facing components by nitrogen-based plasma chemistry: a review

“…Also, increasing the substrate temperature to 200 • C or the current density did not convey any improvement of the cleaning rate in this region. Taking into account the expected enhancement of erosion rate by neutral species with temperature [47], channelling of the impinging ions by the local electric field self-generated within the gap [48] appears as the leading mechanism for the removal of these hard-to-reach deposits, in contradiction with our previous conclusions on this subject [40]. Therefore, plasma techniques implying the presence of energetic ions, such as dc GDs, not only would lead to enhanced rates by synergistic ion-radical effects but they could be strictly required if deposition at the gap bottom becomes an issue in a fusion reactor.…”