Electronegative Discharges
Electronegative plasmas are plasmas that contain negative ions. Negative ions are mainly created by electron dissociative attachment (for example, e+Cl2=>Cl+Cl-). This reaction tends to be very effective at low electron energies and can reduce the electrons in a discharge to a point that an ion-ion discharge is obtained. The transport and volume creation/destruction mechanisms tend to be more complex than electropositive plasmas in many aspects.
In electronegative discharges negative ions are well confined by the ambipolar electric field and losses by transport are very small. This means that to achieve a steady-state volume losses need to be included for negative ions. The mechanisms by which negative ions are lost depend on the gas mixture and pressure and they are: mutual recombination with positive ions (for example, Cl-+Cl+=>2C or Cl-+Ar+=>Cl+Ar), detachment in collisions with excited or neutral atoms or molecules (for example, Cl-+Cl=>Cl2+e or Cl-+Cl2=>Cl+Cl2+e), and electron-impact detachment (for example, e+Cl-=>Cl+2e).
In electronegative discharges it is often possible to identify two spatial regions using the electronegativity (ratio of the negative ion density to the electron density): (i) one in the core of the discharge (the electronegative core) with high electronegativity where the dominant charge species are positive and negative ions; (ii) and the other close to the boundaries (electropositive edges) where the dominant charged species are electron and positive ions. In the transition between these two regions the negative ion density drops abruptly causing a chock-like phenomena. This transition needs to be well resolved spatially. If not, oscillations can be seen in the negative ion density and the model might not converge. Some strategies to deal with this involve increasing the negative ions diffusion and thy are:
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Increase the negative ion temperature of about 0.3 eV. The transition from the electronegative core of the plasma to the electropositive sheath region can be very abrupt and a higher ion temperature makes the transport numerical easier. The ion temperature is defined in the section Mobility and Diffusivity Expressions in the species Settings. By default the ion temperature is the gas temperature.
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Enable Isotropic diffusion for ions in the Inconsistent Stabilization section (the stabilization sections are visible when Stabilization is selected in Show More Options). This option adds artificial diffusion to all ions and helps smoothing the sharp transition of the negative ion density between the electropositive edge and the electronegative core, and also increase the density of the negative ions in the electropositive edge effectively increasing its losses by transport. This option should be used very carefully since completely wrong results can be obtained if too much diffusion is used (the tuning parameter for ions should not be larger than 0.1). A useful strategy is to start with a large Tuning parameter for ions (for example, 0.5) and ramp it down using an Auxiliary sweep.