title: Existence of subsonic plasma sheaths.
abstract: The location of the plasma sheath edge, where quasineutrality is broken, is rigorously derived by using a kinetic description of the plasma. It is shown that sheaths can exist with arbitrarily small ion velocity at the sheath edge, thus violating the Bohm criterion, V(i)=c(s) at the sheath edge. Bohm's criterion is recovered in the case of large enough ion current through the wall, and it is found to be a reasonable approximation in floating potential conditions. However, in the case of a predominant electron current through the wall, Bohm's criterion is not able to describe the sheath-edge transition. The analytical results are supported by numerical simulations performed with a fully kinetic particle-in-cell code modeling a source-driven, weakly collisional plasma, bound between two absorbing walls.
authors.affiliation: Centre de Recherches en Physique des Plasmas-École Polytechnique Fédérale de Lausanne, Association EURATOM-Confédération Suisse, CH-1015 Lausanne, Switzerland. joaquim.loizu@epfl.ch
authors.forename: Joaquim
authors.initials: J
authors.lastname: Loizu
authors.forename: Paolo
authors.initials: P
authors.lastname: Ricci
authors.forename: Christian
authors.initials: C
authors.lastname: Theiler