Thermally activated reaction–diffusion-controlled chemical bulk reactions of gases and solids

• We model the reaction–diffusion system of solid–gas reactions.
• We reveal two limits, the diffusion and the reaction-limited regimes.
• Comparisons of the model with experimental data show good agreement.
• The method offers a possibility to remove plasma deposits in situ in the gas phase.
• In nuclear fusion the method can solve the hydrogen retention issue of carbon.

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.