Complex Chemistries
Plasma chemistries with many species, excited states and reactions are considered complex. Some approaches to help in the implementation of complex plasma chemistries are given below.
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When preparing a new plasma chemistry, 0D and 1D models should be used to take advantages of the fast computational times.
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When there are species from different elements (for example, an Ar/O2 plasma) the Inflow or the Species Group feature should be used to facilitate mass conservation for each of the chemical elements. The Inflow constrains species at a boundary and is easier to obtain converge than when using the Species Group. The Inflow feature can be used as a strategy to ensure mass conservation even if there is no flow in the system.
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If a given species is suspected to be problematic, in that it may be confined to the plasma core, or have very steep gradients, or be negatively charged, use a Species Group Constraint feature to constrain its mass fraction to the initial value. The results obtained with the species constrained can then be used as initial condition to a separate study.
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Start with a simple chemistry (species and reactions) and add complexity. A possible approach to implement an Ar/O2 plasma chemistry would be: (i) solve for Ar only; (ii) add O2 excitation electron impact reactions which only change the mean electron energy and do not create any other species. Use the Species Group feature for O2 to facilitate the mass conservation of the system. When other O2 and O states are introduced in the chemistry they should be added to the Species Group feature; (iii) add O2 ionization and ion losses to the walls; (iv) add other excited states and their corresponding reactions; (v) add O2 dissociation and all chemistry associated with O including surface recombination; (vi) add electron attachment reactions and ion-ion recombination.