Introduction to DC Discharge Theory
Due to the complexity of coupling the electrostatic field to the transport of electrons and heavy species, the Plasma Module provides the Plasma interface for modeling DC discharges. The complicated coupling between the electron transport, heavy species transport, and electrostatic field is handled automatically by the software. Furthermore, the secondary emission flux from ion bombardment on an electrode is automatically computed and used in the boundary condition for electrons. This makes it easy to model things such as positive columns and DC glow discharges.
The physics of DC discharges can be further complicated when a strong, static magnetic field is present. This can lead to highly anisotropic transport properties for the electrons. The Plasma interface automatically computes the tensor form of the electron mobility if requested. DC discharges require reaction rates to be specified in terms of Townsend coefficients rather than rate coefficients. The main reason is that Townsend coefficients provide a better description of the physics in the cathode fall region. The discharge is sustained by secondary emission, rather than a time varying electric field. Therefore, secondary emission effects must be included in any DC discharge model otherwise the plasma simply self-extinguishes.
It is important to be aware of the different time scales at which the components of a plasma can change:
Table 6-5:
various time scales in a plasma.
Phenomena
time scale
Electromagnetic field propagation
Instantaneous
Electron energy
< 1 ns
Electron transport
nanoseconds
Ion transport
microseconds
Excited species transport
milliseconds
Neutral gas flow
10’s milliseconds