Kinetic model for a low-pressure discharge with negative ions

A computational model of a low-pressure discharge having a negative-ion component is developed. Many existing models for this type of discharge consider collisionless positive ions and two negative species each obeying Boltzmann relations. Our aim is to relax the Boltzmann-negative-ion assumption and we use a more realistic model with kinetic positive ions, kinetic negative ions and Boltzmann electrons. Positive and negative ions are created uniformly in the discharge at a constant rate, and lost either to the walls or via volume recombination. This model is solved using a hybrid simulation with particle-in-cell (PIC) ions. The negative-ion distribution function is found to have cold and hot components of nearly equal densities for which T-e/T-cold approximate to 100 (the creation temperature) and T-e/T-hot approximate to 5-20 The computed positive ion flux exiting the discharge agrees approximately with those calculated from Boltzmann-negative-ion models when the negative-ion temperature is accounted for correctly. It has been predicted that three electronegative discharge structures can exist: uniform, stratified and double-layer stratified. All three structures are observed in our model. In particular, a double-layer stratified discharge is observed when the effective negative-ion temperature is sufficiently low, in qualitative agreement with Boltzmann-negative-ion models.