Double layers in a modestly collisional electronegative discharge

The effect of ion-neutral collisions on the structure and ion flux emanatin g from a steady-state, planar discharge with two negative components is inv estigated. The positive ion component is modelled as a cold fluid subject t o constant-mobility collisions, while the electrons and negative ions obey Boltzmann relations. The model includes the collisionless limit. When the n egative ions are sufficiently cold three types of discharge structures are found. For small negative ion concentrations or high collisionality, the di scharge is 'stratified', with an electronegative core and an electropositiv e edge. For the opposite conditions, the discharge is 'uniform' with the ne gative ion density remaining significant at the edge of the plasma. Between these cases lies the special case of a double-layer-stratified discharge, where quasi-neutrality is violated at the edge of the electronegative core. Double-layer-stratified solutions are robust in that they persist for mode rate collisionality. Numerical solutions for finite non-neutrality verify t hat the plasma flux varies continuously with collisionality, although the d erivative of the flux with respect to collisionality is discontinuous when the discharge structure changes from uniform to stratified. Double-layer so lutions are found when the flux predicted for the plasma approximation is d ouble-valued and the flux associated with the smaller plasma edge potential is less than that associated with the larger edge potential. A comparison with numerical non-neutral solutions confirms that the flux is correctly pr edicted using the plasma approximation when the larger value of the flux is taken in the two-solution regime.