Influence of thin dielectric layers on electron emission and plasma-surface contact stability

Strongly Emitting Surfaces Unable to Float below Plasma Potential

“…Thermionic emission from hot metals [15] and photoemission from sunlight exposure [16] can induce a "strong" emitted flux Γ emit that exceeds the influx of electrons from the plasma Γ ep . Secondary emission coefficients γ ≡ Γ emit /Γ ep can exceed unity for metal surfaces [17] at high plasma temperatures while dielectric surfaces or oxide film coated electrodes [18] have significantly higher emission yields which can enable γ > 1 at more modest temperatures. For any emission type when γ > 1, the zero current condition cannot be met unless some of the emission is returned to the wall by a barrier field (Γ eret > 0).…”

Emission properties of the plasma faced materials covered with thin films

“…For example, sheath potential for tokamaks varies from few to hundreds volts, but electric field strength depending on plasma density and electron temperature can be as high as 10 7 V/m [1]. Additional electron emission into plasma can change sheath potential, thus influencing plasma faced materials (PFM) by sputtering, and in some cases leads to the plasma-surface instabilities [2]. The increase of the surface emissivity correlates also with the probability of unipolar arcs ignition [3] resulting in plasma pollution with impurities.…”

Prebreakdown Currents From Tungsten Samples Covered With Thin Films

“…As far as there are no strong evidences that the reason of such currents is classical field emission due to field enhancement on sharp surface relief, it was named as low-field emission (LFE) [2]. Electron emission into plasma can change sheath potential, thus influencing plasma facing materials by sputtering, and in some cases leads to the plasma-surface instabilities [3]. The total current of LFE from the wall in general is much lower, compared with that of secondary ion-electron emission, but local current density can be rather high as electrons emit from small emission sites.…”