What is Bohm's criterion and the pre-sheath concept?

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appsci
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Hi,
I do not know if this is the right section to post my question. Anyway here I go..
Can anyone give a brief explanation of Bohm's criterion, that is the concept of pre-sheath.
Thank you.
 

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Astronomy news on Phys.org
the characteristics of presheath under the influence of negative ions have been investigated. An emissive probe is used to measure presheath potential drop in front of an ion-rich sheath. Presheath potential drop and the drift speed of positive ions are found to decrease with increasing negative ion concentration. The numerical results of the theory for sheath formation in multicomponent plasma is discussed and compared with experimental results.

Keywords: Presheath; Negative ions; Bohm speed