The interaction between the gas phase and walls is designed with the aim to provide an interface for setting up arbitrary reaction chains with high flexibility. For meshed geometric boundaries, there is one common “wall” type, where reactions and particle sources can be attached. Additionally, it is possible to declare single-sided mesh surfaces as “membranes”. Membranes fulfill special purposes such as virtual particle sources in the simulation space, particle flow barriers or virtual surfaces for sampling of energy and angular flux distributions.

A wall is represented in the GMSH mesh file as one physical surface group. In the PIC-MC / DSMC simulation parameter file a wall is represented as a Border entry as shown in the following example:

block BORDER_TYPES: {
  # ...
 
  Border Inlet = {
    icodec =   2;                       # Codec number from meshfile
    type = "wall";                      # Alternative type: "membrane" 
    T = PAR.T0;                         # Wall temperature
    reactions: {
      add_source("O2", 1, "Pa");
    };
    # epsr = 1.0;                       # Rel. dielectric permittivity (insulator only)
    # vf = "VP*cos(2*PI*13.56e6*TIME)"; # Voltage function (conductor only) 
    # vf_coupling = "";                 # Border name of counter electrode for potential coupling    
  };
 
  # ...
};

Each Border entry has the following common parameters:

• icodec : Integer number specifying the physical surface group. This number is automatically extracted from the mesh file and should be only changed in special cases ¹⁾.
• type : String constant which can be either “wall” or “membrane”. A “wall” must be part of a 3D object of the mesh file, while the “membrane” is a virtual, single-sided (i. e. infinitesimal) surface which can be used to block orifices, insert additional particle sources or for sampling purposes.
• T : Floating point number specifying the wall temperature in K.
• reactions: { … }; : Within this block, wall reactions and particle sources can be specified. A detailed description is given below.
• epsr : Optional floating point variable specifying the dielectric permittivity of a dielectric insulating object (e. g. a glass susbtrate) in case of a plasma simulation.
• vf : Optional string variable specifying a time-dependent voltage function.
• vf_coupling : Optional string variable specifying a counter electrode within systems of floating electrodes with built-in potential difference.