Strongly emissive plasma-facing material under space-charge limited regime: Application to emissive probes

Cavalier, J.; Lemoine, N.; Bousselin, Guillaume; Plihon, Nicolas; Ledig, J

doi:10.1063/1.4973557

Cited by 12 publications

(20 citation statements)

References 23 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…When object scale lengths are small compared to the plasma Debye length λ D or (for magnetized plasmas) the Larmor radius ρ L , emitted electrons experience orbital motion effects [10], which lengthen the trajectories of trapped emitted electrons between emission and surface reabsorption. These two-dimensional effects have been shown to not only reduce the magnitude of the potential dip [11], but also to build up the potential of floating emitting surfaces [12][13][14][15]. When electron emission from a small object sufficiently overwhelms the incoming plasma current, much of the sheath remains negative, but the surface floats above φ P [16] as in Fig.…”

mentioning

confidence: 99%

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

Kraus

Raitses

2018

Physics of Plasmas

View full text Add to dashboard Cite

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)

show abstract

mentioning

confidence: 99%

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

Kraus

Raitses

2018

Physics of Plasmas

View full text Add to dashboard Cite

show abstract

“…From that we determine the spatial distribution of plasma density Np(x) to be linear in x. (9) It may be surprising that the ions were not considered in deriving Np(x). In the inverse mode, the ions are just a confined background of positive charge whose purpose is to neutralize whatever profile the electrons take.…”

Section: B the Collisional Inverse Modementioning

confidence: 99%

“…This is crucial for understanding the current flow, power dissipation and electrode erosion for any plasma application that relies on hot cathodes. Examples include thermionic discharges [5,6], thermionic converters [7], thermionic tethers [8], emissive probes [9], arc cutting [10], torches [11] and electron transpiration cooling [12].…”

Section: Introductionmentioning

confidence: 99%

“…He argued that when the thermionic emission reaches a critical intensity a VC begins to form, as in vacuum. Many other authors including Prewett and Allen (1976) [14], Takamura et al (2004) [15], Gyergyek and Čerček (2007) [16], Din (2013) [17], Pekker and Hussary (2015) [18], and [9] extended Langmuir's model. Their solutions known as "space-charge limited" (SCL) sheaths are used when predicting how plasmas interact with hot cathodes in current-limited conditions.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Alternative model of space-charge-limited thermionic current flow through a plasma

Campanell

2018

Phys. Rev. E

View full text Add to dashboard Cite

It is widely assumed that thermionic current flow through a plasma is limited by a "space-charge-limited" (SCL) cathode sheath that consumes the hot cathode's negative bias and accelerates upstream ions into the cathode. Here, we formulate a fundamentally different current-limited mode. In the "inverse" mode, the potentials of both electrodes are above the plasma potential, so that the plasma ions are confined. The bias is consumed by the anode sheath. There is no potential gradient in the neutral plasma region from resistivity or presheath. The inverse cathode sheath pulls some thermoelectrons back to the cathode, thereby limiting the circuit current. Thermoelectrons entering the zero-field plasma region that undergo collisions may also be sent back to the cathode, further attenuating the circuit current. In planar geometry, the plasma density is shown to vary linearly across the electrode gap. A continuum kinetic planar plasma diode simulation model is set up to compare the properties of current modes with classical, conventional SCL, and inverse cathode sheaths. SCL modes can exist only if charge-exchange collisions are turned off in the potential well of the virtual cathode to prevent ion trapping. With the collisions, the current-limited equilibrium must be inverse. Inverse operating modes should therefore be present or possible in many plasma devices that rely on hot cathodes. Evidence from past experiments is discussed. The inverse mode may offer opportunities to minimize sputtering and power consumption that were not previously explored due to the common assumption of SCL sheaths.

show abstract

“…Part of the major difficulties are related to its kinetic nature. However, the abundant number of applications, such as emissive probes [1][2][3] and the charging of dusty-grain 4,5 and spacecraft, 6 stimulates theoretical investigations on this topic. The progress on the probe theory can also substantially advance novel applications like low-work-function tethers for deorbiting space debris.…”

Section: Introductionmentioning

confidence: 99%

Kinetic features of collisionless sheaths around polarized cylindrical emitters from the orbital motion theory

Sánchez‐Arriaga

2017

Physics of Plasmas

View full text Add to dashboard Cite

The kinetic features of the sheath around a cylindrical emitter immersed in collisionless plasma at rest are analysed. After finding self-consistently the electric potential by applying the Orbital Motion Theory to the Vlasov-Poisson system, the local distribution functions are reconstructed and the radial profiles of important macroscopic quantities (plasma densities, currents, and temperatures) are then computed. It is found that there can only be three kinds of holes that are bound by three different boundaries-two related to the constraints from orbital effects and the other due to the electric potential barrier. The results are presented for three regimes: negative probe bias with monotonic and non-monotonic potential and positive probe bias with non-monotonic potential. To understand the variation of macroscopic-quantity radial profiles, three diagrams are presented for kinetic features: the l-diagram for the integration domains of the two orbital invariants, the effective potential, and the local distribution function. The envelope in the l-diagram is crucial to identify different orbital behaviours, which can be used as a guideline for analytical analyses and serve as one of the criteria to refine the mesh used in numerical calculations.

show abstract

Strongly emissive plasma-facing material under space-charge limited regime: Application to emissive probes

Cited by 12 publications

References 23 publications

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

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

Alternative model of space-charge-limited thermionic current flow through a plasma

Kinetic features of collisionless sheaths around polarized cylindrical emitters from the orbital motion theory

Contact Info

Product

Resources

About