Berlin 2014 – wissenschaftliches Programm

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P: Fachverband Plasmaphysik

P 13: Poster Session - Diagnostics

P 13.6: Poster

Dienstag, 18. März 2014, 16:30–18:30, SPA Foyer

Ion energy distribution functions and the problem of the Bohm criterion for multi-component plasmas — •Tsanko Vaskov Tsankov and Uwe Czarnetzki — Institute for Plasma and Atomic Physics, Ruhr-University Bochum, 44780 Germany

The energy distributions of the ions reaching the walls contain information on a number of fundamental plasma properties and hold the key to solving the problem for the ion velocities at the sheath edge in a multi-ion-species plasma. The Bohm criterion in such plasmas gives a relation between the velocities of the different ion species at the sheath edge, but does not specify them uniquely. Several theories exist that try to resolve this problem [1,2]. Laser induced fluorescence measurements [3] seem to support one of the theories. However, for convenience the measurements are done in front of a sheath with a bias, which basically expels all the electrons from the sheath.

In this work mass-resolved ion energy distribution functions (IEDF) are combined with probe measurements in an attempt to unravel the mystery for the individual ion velocities at the sheath edge. The results are compared with the predictions of existing theories and of a new solution of the Bohm criterion, which minimizes the electron flux through a floating sheath. Furthermore, it is shown that the IEDFs contain information about the ion mobilities. The obtained values in various noble gases agree well with the data in the literature.

[1] R N Franklin, J. Phys. D: Appl. Phys. 33 (2000) 3186

[2] S D Baalrud, C C Hegna, Phys. Plasmas 18 (2011) 023505

[3] N Hershkowitz, C-S Yip, G D Severn, Ibid. 057102