Hannover 2016 – scientific programme

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P: Fachverband Plasmaphysik

P 9: Theory and Modelling I

P 9.5: Talk

Tuesday, March 1, 2016, 15:55–16:10, b305

What is the correct Bohm criterion? — •Tsanko Vaskov Tsankov and Uwe Czarnetzki — Institute for Plasma and Atomic Physics, Ruhr-University Bochum, 44780 Germany

The Bohm criterion is a fundamental concept in plasma physics that relates the ion and the electron characteristics at the sheath edge. Since its introduction, the concept has seen various extensions to account for arbitrary distributions of the particles, for negative ions or multiple positive ion species. However, the classical kinetic version of this criterion inevitably suffers from mathematical difficulties if at the sheath edge ions with zero velocities are present. This is the typical case when, e.g., charge exchange collisions are present.

Here, using the Boltzmann equation for the ions it will be shown, that these difficulties do not exist. Furthermore, the correct treatment leads to additional terms that account for collisionality and geometrical expansion. More importantly, the whole concept of a Bohm criterion as a defining condition for the position of the sheath edge is questioned.