Franklin, R. N.
What significance does the Bohm criterion have in an active collisional plasma-sheath?
Journal of Physics D: Applied Physics, 35(18) pp. 2270–2273.
The known solutions joining plasma and sheath for the situation where both plasma and sheath are collisional (Blank 1968 Phys. Fluids 11 1688–98, Benilov and Franklin 2002 J. Plasma Phys. 65 163–73) are examined in the context which produces the Bohm criterion as a necessary condition for collisionless sheath formation. The conditions under which even in a collisional situation the ions might exceed the ion sound speed before reaching the wall are examined for both the constant ion collision frequency model and the constant ion mean free path model. It is concluded that under such conditions the ions remain in collisional equilibrium with the electric field and that there is no such thing as a collisionally modified Bohm criterion.
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