Validity of Fluid Models
The fluid model of a plasma is only valid if the mean free path of the charged particles is much smaller that the reactor characteristic dimensions. This limit occurs around 10 s of millitorr for most CCP industrial reactors. In a low collisional discharge physical mechanisms such as stochastic heating and nonlocal ionization are important to correctly describe the plasma and are not included in the COMSOL Multiphysics Plasma Module at the moment.
Numerical solutions at the collisional limit can still be obtained but caution should be used when analyze them since unphysical behaviors might be present. Also, close to the collisional limit, numerical instabilities tend to cause problems. One of the reasons for the instability is that when applying very large voltages (100s of volts) the electron density reaches very low values in the sheath and as a consequence the electron mean energy (electron energy density divided by the electron density) reaches unphysical values that make the numerical problem very difficult to solve. The
Uniform
model from the
Secondary emission model
property allows to introduce a uniform background electron source that can prevent the electron energy to reach abnormal values.
Figure 6-6:
Total current and ion current at a metal contact when using the product space and the base geometry.
