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Title: The limits of the Bohm criterion in collisional plasmas

The sheath formation within a low-pressure collisional plasma is analysed by means of a two-fluid model. The Bohm criterion takes into account the effects of the electric field and the inertia of the ions. Numerical results yield that these effects contribute to the space charge formation, only, if the collisionality is lower than a relatively small threshold. It follows that a lower and an upper limit of the drift speed of the ions exist where the effects treated by Bohm can form a sheath. This interval becomes narrower as the collisionality increases and vanishes at the mentioned threshold. Above the threshold, the sheath is mainly created by collisions and the ionisation. Under these conditions, the sheath formation cannot be described by means of Bohm like criteria. In a few references, a so-called upper limit of the Bohm criterion is stated for collisional plasmas where the momentum equation of the ions is taken into account, only. However, the present paper shows that this limit results in an unrealistically steep increase of the space charge density towards the wall, and, therefore, it yields no useful limit of the Bohm velocity.
Authors:
 [1] ;  [2]
  1. Ernst-Bloch-Ring 17, 07743 Jena (Germany)
  2. Department of Applied Mathematics, University Jena, Ernst-Abbe-Platz 2, 07743 Jena (Germany)
Publication Date:
OSTI Identifier:
22410356
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 22; Journal Issue: 5; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; BOHM CRITERION; CHARGE DENSITY; COLLISIONAL PLASMA; COLLISIONS; ELECTRIC FIELDS; LANDAU LIQUID HELIUM THEORY; PRESSURE RANGE KILO PA; SPACE CHARGE