Theory of the plasma-sheath transition and the Bohm criterion
- Ruhr-Univ. Bochum (Germany). Inst. fuer Theoretische Physik
In typical boundary layer problems a quasineutral plasma is shielded from a negative absorbing wall by a positive space charge region (sheath) with a characteristic extension of several electron Debye lengths {lambda}{sub D}. In the usual case {lambda}{sub D} {much_lt} L--where L is the smallest competing characteristic length relevant for the boundary layer--the formation of a positive space charge is impeded by ion wall losses, and a positive sheath can exist only if the Bohm criterion is fulfilled. In its simplest form it requires that the ions enter the sheath region with a velocity exceeding the ion sound velocity. Consequently, the ions must be preaccelerated by a non-shielded residual field in the quasineutral presheath region with an extension L {much_gt} {lambda}{sub D}. Apart from field and inertia, the presheath ion acceleration is necessarily governed by (at least) one of the following processes: (a) geometric current concentration; (b) collisional ion friction; (c) ionization; (d) magnetic deflection of the ion orbits (L = ion gyro radius). The sheath edge separating the sheath and presheath regions in the limiting case {lambda}{sub D}/L {yields} 0 is (usually) defined by a field singularity. This singularity is related to the marginal (equality) form of the Bohm criterion and may be interpreted in terms of the ion acoustic sound barrier. The simple picture of the plasma-sheath transition sketched so far must be supplemented or modified in many cases. These and related problems concerning the plasma-sheath transition are discussed coherently on the basis of a general review and of recent results.
- OSTI ID:
- 51756
- Report Number(s):
- CONF-940604-; ISBN 0-7803-2006-9; TRN: 95:012043
- Resource Relation:
- Conference: 1994 Institute of Electrical and Electronic Engineers (IEEE) international conference on plasma science, Santa Fe, NM (United States), 6-8 Jun 1994; Other Information: PBD: 1994; Related Information: Is Part Of IEEE conference record -- Abstracts; PB: 252 p.
- Country of Publication:
- United States
- Language:
- English
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