Internal sheaths in electronegative discharges

In three component electronegative discharges a parameter regime can be found in which the positive ions reach the local ion sound velocity at a position where the negative ion density may be significant compared to the electron density. For this regime a quasineutral electronegative core breaks down and a space charge region forms. Solutions in the space charge region are obtained in collisionless and collisional cases, neglecting ionization and positive-negative ion recombination. The structure of the non-neutral region is shown to vary significantly with the ratio of the negative ion and electron densities at the core edge, the ratio of ion and electron temperatures, and the ratio of the electron Debye length to the ion mean free path. If the first ratio is not too large the non-neutral region displays potential oscillations on the electron Debye length spatial scale, which damp away on the scale of the ion-neutral mean free path. The non-neutral region then terminates within the plasma. The change in electric potential across this region is several times the negative ion temperature, which is sufficient to confine the negative ions to the core. The non-neutral region merges with a quasineutral halo containing essentially only positive ions and electrons. If the negative ion density is sufficiently high compared to the electron density the electropositive halo disappears and the non-neutral region extends from the ion sound velocity threshold to the wall. For intermediate values of the negative ion density, a space charge double layer forms between the electronegative and electropositive regions. (C) 1999 American Institute of Physics. [S0021-8979(99)02418-4].