Negative Plasma Potential Relative to Electron-emitting Surfaces
Author: M.D. Campanell
Abstract:
Most works on plasma-wall interaction predict that under strong electron emission, a nonmonotonic ¨space charge limited¨ (SCL) sheath forms where plasma potential is positive relative to the wall. We show a
fundamentally different sheath structure is possible where the potential monotonically increases towards a
positively charged wall that is shielded by a single layer of negative charge. No ion-accelerating presheath
exists in the plasma and the ion wall flux is zero. An analytical solution of the ¨inverse sheath¨ regime is
demonstrated for a general plasma-wall system where the plasma electrons and emitted electrons are
Maxwellian with different temperatures. Implications of the inverse sheath effect are (a) the plasma potential is
negative, (b) ion sputtering vanishes, (c) no charge is lost at the wall, (d) the electron energy flux is thermal.
We predict that the inverse sheath is more likely than the SCL sheath to appear in practice under strong
emission. To test the prediction, a plasma bounded by strongly emitting walls is simulated. It is found that
inverse sheaths form and ions are confined in the plasma. Our model differs from past PIC simulation studies
of emission which contain an artificial source sheath that accelerates ions to the wall.
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To be presented at: APS 2013
To be published in: Physical Review Letters E
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Download PPPL-4949 (pdf 1.6 MB 11 pp)
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