Use the Wall boundary condition when the plasma is in contact with a solid surface. This also allows for secondary and thermionic emission effects from the solid surfaces. Use the Wall boundary condition to describe how the electrons interact with the wall

Enter a value for the Reflection coefficient re (dimensionless) on the selected boundaries. This must be a number between 0 and 1.

The Include migration effects checkbox should be checked if the electron flux to the wall is due to the electric field. This is usually not the case because the plasma potential is positive with respect to the walls, so the electron flux to the wall due to the electric field is zero. In models where the electrostatic field changes very rapidly (like with dielectric barrier discharges for example), the electron flux to the wall can be due to the electric field, in which case this option should be activated.

The Use wall for electron density checkbox is selected by default and deactivates the wall boundary condition for the electron density only. When activated, the flux of electrons lost to the wall is self-consistently computed based on the electron temperature and electron density adjacent to the wall. When this is deactivated, the flux term is no longer applied for the corresponding equation, and the software will impose a zero flux boundary condition for that dependent variable.

Select or enter a value for the Secondary emission flux (SI unit: 1/m2·s)). The secondary emission flux can either be a user-defined expression or come from another physics interface, typically The Heavy Species Transport Interface. When using this boundary condition in the Plasma interface, this setting is not editable, and the secondary emission flux comes from the surface reactions.

Select a Thermal emission model to define the Thermal emission flux (SI unit: 1/m2·s)) on the wall boundary. It describes the flux of electrons being emitted from the wall due to thermal emission, which typically occurs when the wall is at a very high temperature. By default User defined model is selected, and the value of the flux can be directly specified (SI unit: 1/m2·s)). Alternatively, Richardson’s law can also be selected as the Thermal emission model, and then the Correction factor λR, the Work function Φ, and the Surface temperature Ts need to be specified for the Richardson’s equation shown as follows:

where JR is the current density, qe is the elementary charge, Ts is the electrode temperature, kB is the Boltzmann constant, and A0 is a constant defined as follows:

The Use wall for electron energy checkbox is selected by default and deactivates the wall boundary condition for the electron energy. This is useful in order to use the wall condition for the electron density but impose the value of the mean electron energy on the same boundaries.

Select or enter a value for the Secondary emission energy flux, which can either be a user-defined expression or come from another physics interface, typically The Heavy Species Transport Interface. When using this boundary condition in the Plasma interface, this setting is not editable, and the secondary emission energy flux comes from the surface reactions.

Select or enter a value for the Mean thermionic energy. If electrons are being emitted from the wall due to thermionic emission then enter an expression for the mean energy of the emitted electrons. The mean electron energy of the emitted electrons is usually a function of the work function of the surface.