The effects of ion-ion Coulomb collisions on the plasma presheath

The effects of ion-ion Coulomb collisions on the presheath region of an unmagnetized plasma are investigated using both kinetic and fluid treatments. In the kinetic treatment, ion collisions are included in the analysis through a Bhatnagar-Gross-Krook (BGK) collision term. An equation describing the electrostatic potential variation in the presheath region is derived and solved numerically for the range of collsionless to moderately collisional plasmas. Ion-ion Coulomb collisions are modeled by a form of the BGK collision operator which conserves particles, momentum and energy. The results obtained for moderate collisionality show that ion-ion collisions have little effect on the presheath. Ion-neutral charge exchange collisions are modeled by conserving only particles in a collision, since momentum and energy are transferred to the neutral species. In this case the presheath is affected by collisions and shows an increased potential drop. Ion-ion collisional effects are also investigated using fluid equations which are valid in collisionless and collisional limits. Effects of neutral gas-plasma interactions are included through the source terms in the fluid equations. The effects of ion-ion collisions are included through the closure conditions. The collisionless equations are solved numerically and the results compared to previous kinetic results. This comparison shows that the fluid equations are a reasonable approach to calculating presheath profiles. Results obtained from the collisional fluid equations show that ion-ion Coulomb collisions have a negligible effect on the plasma presheath. These results are in agreement with the results of the kinetic treatment