Species
Use the Species node () to add additional species to the model even if they do not participate in any reactions.
Due to the overwhelming number of species that can exist in a plasma, the Flux, Inlet, Outflow, and Symmetry boundary condition subnodes are available from the context menu (right-click the Species parent node) or from the Physics toolbar, Attributes menu. This simplifies and orders the process of assigning boundary conditions for a large number of species in a model. The subnodes are available for the Heavy Species Transport and Plasma physics interfaces, but not for the Plasma, Time Periodic interface. The subnodes are also not available if the Species type is set to Electron.
Species are automatically added to the model when reactions are added.
Domain Selection
COMSOL Multiphysics automatically ensures that the Species selection is the same as the Diffusion and Migration selection.
Species Formula
Species Name
Enter basic information about the name and nature of the species. When a new species is created, the Species name field is editable. A valid name for the species must be entered, for example, o2, Ar, or Ar+. After the species name has been entered, the text field cannot be edited (as long as a valid species name is entered).
Species Type
The Species type is either Neutral, Ion, or Electron.
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Enter “e” to automatically create a species as an Electron. The Species node is renamed to include the name entered in the Species type field. Also enter the Electron Parameters.
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Enter a neutral species, for example, o2 to automatically create a species as Neutral.
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Enter a charged species, for example, Ar+ or o2- to automatically create a species as an Ion. The Species node is renamed to include the name entered in the Species type field. When the species created is an Ion, also select the Initial value from electroneutrality constraint check box to indicate that the initial mass fraction of this species is automatically computed based on the initial electron density specified in the electron species properties. This ensures that the solver can find consistent initial conditions when starting because the plasma is initially electrically neutral. Also enter the General Parameters described in the next section.
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If Neutral is selected as the Species type, enter the General Parameters described in the next section.
For Neutral select the From mass constraint check box to indicate which species should be used as the mass constraint and therefore not solved for. This is typically the species with the highest mass fraction. It should not be a species whose mass fraction is a very small value (as is the case with ionic species).
General Parameters
This section is available when the Species type under Species Formula selected is Ion or Neutral. Enter the physical properties of the species. Enter the following properties:
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Preset species data. The default value is User defined which means that the transport properties are computed from the values given in the text fields below. There is preset transport data available for the most common gases encountered in plasma processing. Selecting one of these gases fills in the correct values for the species transport properties automatically.
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Molecular weight Mw (SI unit: kg/mol) of the species. The default value for the molecular weight is 0.032. The molecular weight must be a nonzero number.
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Charge z (dimensionless) number of the species. The charge number is automatically computed from the formula specified via either a reaction or by manually creating a species.
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Potential characteristic length σ (SI unit: m). The potential characteristic length is used to compute the binary diffusion coefficients used in the Mixture-Averaged transport setting.
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Potential energy minimum e/kb (SI unit: m). This is used to compute the binary diffusion coefficients.
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Dipole moment μD (SI unit: C·m). If the dipole moment is known, enter its value and obtain a more accurate expression for the binary diffusion coefficients.
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Thermal diffusion coefficient DT (SI unit: m2/s). If thermophoretic effects are important in the model then enter a value for the thermal diffusion coefficient.
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If Neutral is selected as the Species type, enter an Initial mole fraction x0 (dimensionless). Set the initial mole fraction of the selected species in the gas mixture. If the From mass constraint check box is selected, this field is not available.
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If Ion is selected as the Species type, enter an Initial number density n0 (dimensionless) to set the initial number density of the selected species in the gas mixture. If the Initial condition from electroneutrality constraint check box is selected, this field is not available.
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Diffusion coefficient Df (SI unit: m2/s). If the Diffusion model property is set to Fick’s law then the diffusion coefficient is not computed automatically from the transport properties, its value is taken from this text field.
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If Global is selected in Diffusion model, if the Reactor type is set to Constant pressure or Constant mass, and if Ion is not selected as the Species type enter an Feed mole fraction xfeed (dimensionless) to set the mole fraction of the selected species in the feed.
Electron Parameters
This section is available when the Species type under Species Formula selected is Electron and the physics interface is Heavy Species Transport. These options are not available for the Plasma, or Plasma, Time Periodic interfaces. Enter the following properties:
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Electron density ne (SI unit: 1/m3).
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Mean electron energy e (SI unit: V).
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Initial electron density ne,0 (SI unit: 1/m3).
Species Thermodynamic Parameters
This section is available if the Calculate thermodynamic properties check box is selected under Transport Settings in the Heavy Species Transport interface.
Enter thermodynamic properties for the species using the NASA polynomial format. Any coefficients for the thermodynamic polynomials are entered into the alow,k fields and apply to the temperatures in the range Tint/lower to Tint/midpoint; coefficients entered into the ahi,k fields apply to temperatures in the range Tint/midpoint to Tint/upper range.
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Enter the Lower temperature limit Tlo, Middle temperature limit Tmid, and Upper temperature limit Thi. The defaults are 300 K, 1000 K, and 5000 K, respectively.
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Enter the Polynomial coefficients alow,k and ahi,k.
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Enter an Additional enthalpy contribution Δh (SI unit: V) as required.
Mobility and Diffusivity Expressions
This section is available when the Species type selected under Species Formula is Ion. The default specification is to compute the diffusivity and mobility from the binary diffusion coefficients and Einstein’s relation.
Select a Specification — Compute mobility and diffusivity or Specify mobility, compute diffusivity.
The Mobility is computed using Einstein’s relation by default. If Mixture averaged is selected as the Diffusion model under Transport Settings in the Heavy Species Transport interface, then the mobility is computed using:
and if the Fick’s law property is selected, then the mobility is defined as:
Select an Ion temperature — Use gas temperature or Specify ion temperature. For Specify ion temperature enter an Ion temperature Tion (SI unit: K). The default is 300 K.
Mobility Specification
This section is available if Specify mobility, compute diffusivity is selected under Mobility and Diffusivity Expressions.
Select an option from the Specify using list — Constant value, Dalgarno, High field, or Lookup table.
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For Constant value enter a value or expression for the Mobility um (SI unit: m2/(V·s)). This can be a function of any of the variables in the model.
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For Dalgarno enter a value or expression for the Polarizability α (SI unit: m3). The default values is 1.64 Å3.
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For High Field enter a value or expression for the Cross section σ (SI unit: m2). The default value is 7·10-19 m2.
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For Lookup table the mobility is then a function of the electric field. Select an option from the Mobility a function of list — Electric field or Reduced electric field. Enter values into the table or click the Load from file button () to import data from a text file. The file must contain the two columns of data, the Electric field (SI unit: V/m) or the Reduced electric field (SI unit: V·m2) as the x-data and the Ion mobility (SI unit: m2/(V·s)) or Reduced ion mobility (SI unit: 1/(V·m·s)) as the y-data.
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For Argon ion in argon, the mobility automatically is defined as a function of the reduced electric field for an argon ion in a background gas of neutral argon. The data comes from Ref. 10 with data at higher reduced electric fields from Ref. 13.
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For Helium ion in helium, the mobility automatically is defined as a function of the reduced electric field for a helium ion in a background gas of neutral helium. The data comes from Ref. 10.
The Diffusivity is computed using Einstein’s relation by default.
In Ion temperature a third option is available when Specify mobility, compute diffusivity is selected under Mobility and Diffusivity Expressions: Use local field approximation. Select Use local field approximation to automatically compute the ion temperature using the local field approximation. The local field approximation can be useful for high pressure discharges where the electric field is very high, as is the case for dielectric barrier discharges.