Tips for Modeling DC Discharges
There are a number of key points to consider when modeling DC discharges:
•
For atmospheric pressure DC discharges, the plasma might naturally try to arc. This typically causes the solver to terminate because it cannot reach the desired tolerances. Often this behavior is undesired, so in practical experimental setups a series RC circuit is added between the driven electrode and the power supply. This prevents arcing from occurring and results in a glow discharge. It is recommended to use the
Series RC circuit
option in the
Circuit Settings
section of the
Electric Potential
feature.
•
Townsend coefficients should be used in preference to rate coefficients if possible. Townsend coefficients give a more realistic description of cathode fall regions in DC discharges.
•
Secondary emission from a surface bombarded by ions must be included somewhere in the model in order for the discharge to be sustained.
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Although a DC discharge ultimately reaches a steady-state condition, the problem is often solved as a time-dependent problem so the plasma can naturally evolve into its equilibrium state.
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In some cases it is possible to use the stationary solver to obtain directly the stationary solution. Normally, this approach is more difficult to setup than a time-dependent one since initial conditions play an important role and some adjustment in solver settings might be needed. This initial effort is often rewarding because models tend to solve faster than a time-dependent approach and parameterization are easier. A good strategy is to use a time-dependent solver to find a solution that can be used as initial conditions for a stationary solver.
•
The initial conditions should enforce the electroneutrality constraint.