Kinetic model for a low-pressure discharge with negative ions

A computational model of a low-pressure discharge having a negative-ion com ponent is developed. Many existing models for this type of discharge consid er collisionless positive ions and two negative species each obeying Boltzm ann relations. Our aim is to relax the Boltzmann-negative-ion assumption an d we use a more realistic model with kinetic positive ions, kinetic negativ e ions and Boltzmann electrons. Positive and negative ions are created unif ormly 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 wi th particle-in-cell (PIC) ions. The negative-ion distribution function is f ound 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 approx imate to 5-20 The computed positive ion flux exiting the discharge agrees a pproximately with those calculated from Boltzmann-negative-ion models when the negative-ion temperature is accounted for correctly. It has been predic ted that three electronegative discharge structures can exist: uniform, str atified and double-layer stratified. All three structures are observed in o ur model. In particular, a double-layer stratified discharge is observed wh en the effective negative-ion temperature is sufficiently low, in qualitati ve agreement with Boltzmann-negative-ion models.