Abstract
The chemical kinetics of the reaction of thin films with reactive gases is investigated. The removal of thin films using thermally activated solid–gas to gas reactions is a method to in-situ control deposition inventory in vacuum and plasma vessels. Significant scatter of experimental deposit removal rates at apparently similar conditions was observed in the past, highlighting the need for understanding the underlying processes. A model based on the presence of reactive gas in the films bulk and chemical kinetics is presented. The model describes the diffusion of reactive gas into the film and its chemical interaction with film constituents in the bulk using a stationary reaction–diffusion equation. This yields the reactive gas concentration and reaction rates. Diffusion and reaction rate limitations are depicted in parameter studies. Comparison with literature data on tokamak co-deposit removal results in good agreement of removal rates as a function of pressure, film thickness and temperature.
Highlights
In vacuum, films can be grown by deposition from the vapour phase
If tritium is used as fuel for the nuclear fusion reaction, its retention by this co-deposition becomes a safety and selfsufficiency issue [4,3]: as a D-T nuclear reaction consumes the radioactive isotope tritium, it needs to be bred by the emitted neutron
An analytical model of the thermo-chemical removal (TCR) process based on reaction–diffusion processes in a permeable material is developed
Summary
Films can be grown by deposition from the vapour phase. All species present in the vapour can, in principle, constitute to the growing film material inventory. In magnetic nuclear fusion devices, material in contact with the plasma is intensively sputtered, transported and deposited by interactions with the energetic plasma particles with ion impact energies ranging from some eV to some keV. The presented model is based on reaction–diffusion processes, which combines gas diffusion through the material and the reactions occurring in the material [16,17,18] This model is able to connect and extend the description of the process concerning the relation to the reactive gas pressure, surface temperature, initial material inventory, co-deposit thickness and the time evolution, allowing for comparison with experimental data
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