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Author Tierno, S. P. ♦ Donoso, J. M. ♦ Domenech-Garret, J. L. ♦ Conde, L.
Source United States Department of Energy Office of Scientific and Technical Information
Content type Text
Language English
Subject Keyword PLASMA PHYSICS AND FUSION TECHNOLOGY ♦ CATHODES ♦ CURRENTS ♦ DISTURBANCES ♦ ELECTRON DENSITY ♦ ELECTRON EMISSION ♦ FLOW MODELS ♦ INTERACTIONS ♦ IONS ♦ PHASE DIAGRAMS ♦ PLASMA DENSITY ♦ PLASMA DIAGNOSTICS ♦ PLASMA POTENTIAL ♦ THERMIONIC EMISSION ♦ WALLS
Abstract The interaction between an electron emissive wall, electrically biased in a plasma, is revisited through a simple fluid model. We search for realistic conditions of the existence of a non-monotonic plasma potential profile with a virtual cathode as it is observed in several experiments. We mainly focus our attention on thermionic emission related to the operation of emissive probes for plasma diagnostics, although most conclusions also apply to other electron emission processes. An extended Bohm criterion is derived involving the ratio between the two different electron densities at the potential minimum and at the background plasma. The model allows a phase-diagram analysis, which confirms the existence of the non-monotonic potential profiles with a virtual cathode. This analysis shows that the formation of the potential well critically depends on the emitted electron current and on the velocity at the sheath edge of cold ions flowing from the bulk plasma. As a consequence, a threshold value of the governing parameter is required, in accordance to the physical nature of the electron emission process. The latter is a threshold wall temperature in the case of thermionic electrons. Experimental evidence supports our numerical calculations of this threshold temperature. Besides this, the potential well becomes deeper with increasing electron emission, retaining a fraction of the released current which limits the extent of the bulk plasma perturbation. This noninvasive property would explain the reliable measurements of plasma potential by using the floating potential method of emissive probes operating in the so-called strong emission regime.
ISSN 1070664X
Educational Use Research
Learning Resource Type Article
Publisher Date 2016-01-15
Publisher Place United States
Journal Physics of Plasmas
Volume Number 23
Issue Number 1


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