Unified Bohm criterion

Recent decades have seen research into the conditions necessary for the formation of the monotonic potential shape in the sheath, appearing at the plasma boundaries like walls, in fluid, and kinetic approximations separately. Although either of these approaches yields a formulation commonly known as...

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Bibliographic Details
Published in:Physics of plasmas 2015-09, Vol.22 (9)
Main Authors: Kos, L., Tskhakaya, D. D., Jelić, N.
Format: Article
Language:English
Subjects:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY
BOHM CRITERION
BOUNDARY LAYERS
DISTRIBUTION FUNCTIONS
Ion distribution
KINETICS
PLASMA
Plasma physics
PLASMA SHEATH
POTENTIALS
SHAPE
Sheaths
WALLS
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Summary:Recent decades have seen research into the conditions necessary for the formation of the monotonic potential shape in the sheath, appearing at the plasma boundaries like walls, in fluid, and kinetic approximations separately. Although either of these approaches yields a formulation commonly known as the much-acclaimed Bohm criterion (BC), the respective results involve essentially different physical quantities that describe the ion gas behavior. In the fluid approach, such a quantity is clearly identified as the ion directional velocity. In the kinetic approach, the ion behavior is formulated via a quantity (the squared inverse velocity averaged by the ion distribution function) without any clear physical significance, which is, moreover, impractical. In the present paper, we try to explain this difference by deriving a condition called here the Unified Bohm Criterion, which combines an advanced fluid model with an upgraded explicit kinetic formula in a new form of the BC. By introducing a generalized polytropic coefficient function, the unified BC can be interpreted in a form that holds, irrespective of whether the ions are described kinetically or in the fluid approximation.
ISSN:1070-664X
1089-7674
DOI:10.1063/1.4930207