Floating potential of emitting surfaces in plasmas with respect to the space potential

Abstract

The potential difference between a floating emitting surface and the plasma surrounding it has been described by several sheath models, including the space-charge-limited sheath, the electron sheath with high emission current, and the inverse sheath produced by charge-exchange ion trapping. Our measurements reveal that each of these models has its own regime of validity. We determine the potential of an emissive filament relative to the plasma potential, emphasizing variations in emitted current density and neutral particle density. The potential of a filament in a diffuse plasma is first shown to vanish, consistent with the electron sheath model and increasing electron emission. In a denser plasma with ample neutral pressure, the floating filament potential is positive, as predicted by a derived ion trapping condition. Lastly, the filament floated negatively in a third plasma, where flowing ions and electrons and nonnegligible electric fields may have disrupted ion trapping. Depending on the regime chosen, emitting surfaces can float positively or negatively with respect to the plasma potential.Any solid surface in contact with a plasma is surrounded by the sheath, a potential structure that controls particle and energy transport between the plasma and the surface [1]. Sheath structure is complicated when the surface emits an electron current, which can be caused by impinging radiation or plasma particles. Emissive sheaths are present in divertors [2] and scrape-off layers [3] in magnetic fusion devices, around dust grains in laboratory [4] and astrophysical [5] plasmas, around satellites [6], in RF plasma processing devices [7] and around plasma probes [8]. In all of these cases, the interplay between emitted and background plasmas determines the structure of sheath that forms. Predicting which structure exists is essential for understanding the heat and charge flux to the surface.A surface emits a normalized currentĴ = J emit /J e in a background electron current J e . WhenĴ = 0, mobile plasma electrons charge the surface negatively so that its potential is negative with respect to the plasma potential φ P (in this work, all surface potentials called positive r/r 0 −3 −2 −1 0 ∆φ [V] T e [eV] (a)

Thermionic Cooling of the Target Plasma to a Sub-eV Temperature

“…No simulation studies have analyzed how emission feedback cools plasma electrons. Experimental studies [5,18,19] have focused on measuring sheath properties, so the cooling of the plasma interior is not as well understood. Our new code is designed to study the cooling effect at a fundamental level.…”

Plasma potential probes for hot plasmas

“…As it has recently become more and more obvious that in future large tokamaks such as ITER [80] the plasma heat flux onto Plasma Facing Components (PFC) can be as high 60 MW m −2 , a strong heating of the divertor plates up to white glow and consequent electron emission cannot be completely ruled out. This has renewed the interest in the interaction of plasmas with emissive walls [81][82][83] since, if the entire divertor becomes emissive, this will obviously have a drastic effect on the entire tokamak plasma.…”

“…Of course this derivation is simplified insofar as we have neglected the possible formation of space charges in front of an EEP [76,77,83,84]. Especially if there is a strong mismatch between the temperatures T em of the emitted electrons and of the plasma electrons T e , several experiments [33,34], theoretical [59,60,65] and numerical investigations [84] have indicated that the floating potential even of a very strongly emitting electrode in a plasma could always remain below Φ pl .…”

Electron collection and thermionic emission from a spherical dust grain in the space-charge limited regime

“…It was also recently demonstrated that an inverse sheath structure can appear near planar surfaces with electron emission yields exceeding unity and when collisions are important. [22][23][24] Again, orbital motion effects are not taken into account. Moreover, since a dust grain potential satisfies the floating condition, its electron emission yield remains below unity.…”