Sheath criterion and boundary conditions for an electrostatic sheath

The effect of collisionality of ions on the formation of a positive sheath is investigated using a two-fluid model. It is the definition of the sheath edge that determines the sheath criterion. Bohm’s sheath criterion is based on the collisionless sheath and a quasineutral presheath. When collisions of ions in the sheath or finite space charge of presheath are taken into account, a sheath criterion can also be obtained based on the boundary conditions chosen at the sheath edge.