Plasma-sheath transition and ion sound speed in weakly collisional plasmas

The plasma-sheath transition in weakly collisional plasma is discussed in terms of a simple fluid model. The plasma-sheath transition region is characterized as a region where the ion flow speed is close to (N+/Ne)1/2c0 (N+ is the positive ion density, Ne the electron density, and c0 the usual Bohm speed), which can be interpreted as a local ion sound speed with non-neutrality taken into account. However, at low collisionality where one can clearly distinguish a plasma and a collisionless sheath joined by a transition region, numerical results show that (N+/Ne)1/2c0 at the point where the ion flow reaches this speed is practically identical with c0.