Plasma processing techniques for tritium inventory control in fusion research

Abstract

Some techniques with a long tradition in the plasma technology field have already been successfully applied to research in plasma-wall interactions of fusion devices. They have produced important advances in the control of particle and energy exhaust. In this paper, the possible application of these techniques to the problem of tritium inventory control in fusion reactors with carbon-based plasma facing materials, as in ITER, is proposed. It is based on a critical analysis of relevant information obtained in the field of hard CN film deposition and consists of the use of chemical scavengers for the inhibition of tritium-rich carbon-film formation in hidden areas of the divertor. The practical implementation of the technique, however, requires a detailed knowledge of the physio-chemical processes involved, and, to date, experiments in cold and divertor plasmas have been performed. Very recent experiments in the ASDEX Upgrade device have shown that the injection of nitrogen in the sub-divertor region can lead to a drastic decrease in the level of deposited material with no significant effects in the performance of the main plasma. This and other findings are interpreted in the light of recent results from laboratory and divertor plasma experiments and the extrapolation to new divertor scenarios is discussed.

On the Mechanism of Nitrogenated Carbon Film Deposition by PACVD in the Presence of Nitrogen. A Cryo‐Trapping Assisted Mass Spectrometric Study

“…However, the conductance of a 2mm hole in molecular regime for acetylene is ∼0.36 ls -1 . This value is in very good agreement with the calibration factor for acetylene under low pressures, given by Equation 5.…”

“…Again, a plasma was initiated over the fresh carbonized walls, but this time methane was removed from the gas phase and only a N 2 /H 2 (20:80) mixture was used. This kind of plasma has been recently proposed for the removal of carbon contaminants in fusion devices [5] due to the lack of oxygen in their composition (highly efficient for the task but with deleterious side effects on the rest of the components) and their high etching rate. Separate experiments have yielded erosion rates up to 3.6 nm min -1 of laboratory-produced, hard a-C:H films.…”

“…At the same time, a fairly broad range of conditions exists under which little or no film deposition of hydrogenated carbon or carbon nitride, in any of their possible structures, takes place. [5] Interestingly, this problem was also detected in the hot-filament CVD of the films under relatively high pressures, and for film precursors other than methane/nitrogen (e.g., ammonia or HCN) for the N component, and several kinds of hydrocarbons for the C component. [6] Underlying these observations is the fact that nitrogen bonding with itself or with carbon is highly stable, and usually leads to the formation of volatile products.…”

Growth and erosion of amorphous carbon (a-C:H) films by low-temperature laboratory plasmas containing H and N mixtures

“…Preliminary laboratory experiments on plasmaassisted deposition of amorphous hydrocarbon (a-C:H) films in rf and dc low-pressure plasmas by Mutsukura [2], Vandentop et al [3], and Tabares et al [1] have reported on considerable reduction of film growth from methane plasmas if NO or N 2 are admixed to the gas phase. It has been hypothesized that this is due to the scavenging effect of N 2 and NO [1,[4][5][6]. Scavenger particles are supposed to transform reactive species into non-reactive ones, thereby reducing the deposition rate.…”

The scavenger effect – Does it work?

“…In the literature different diagnostics were applied to study the interaction between nitrogen and hydrogen containing mixtures. In fact, one can find a lot of information for totally different experimental conditions: catalyser based HCN production at atmospheric pressure [3] , production of synthetic materials and gas phase reactions in microwave plasmas [4,5], low temperature plasma studies [6,7,12,13] as well as experiments in fusion experiments [8]. The spectrum of intermediate species and final products covers nearly all possibilities for C x H y N z (for x = 1,2,.. y =1…6 , z =1,2).…”