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

Kraus, Brian; Raitses, Yevgeny

doi:10.1063/1.5018335

Cited by 31 publications

(27 citation statements)

References 38 publications

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“…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.…”

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confidence: 99%

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

Campanell

Johnson

2019

Phys. Rev. Lett.

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Contemporary models of bounded plasmas assume that the target plasma electron temperature far exceeds the temperature of the cold electrons emitted from the target, Temit. We show that when the sheath facing a collisional plasma becomes inverted, the target plasma electron temperature has to equal Temit even if the upstream plasma is hotter by orders of magnitude. This extreme cooling effect can alter the plasma properties and the heat transmission to thermionically emitting surfaces in many applications. It also opens a possibility of using thermionic divertor plates to induce detachment in tokamaks.

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confidence: 99%

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

Campanell

Johnson

2019

Phys. Rev. Lett.

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“…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.…”

Section: Electron Emissive Probes (Eep) 21 Basics Of Electron Emissive Probes (Eep)mentioning

confidence: 99%

“…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 .…”

Section: Electron Emissive Probes (Eep) 21 Basics Of Electron Emissive Probes (Eep)mentioning

confidence: 99%

Plasma potential probes for hot plasmas

et al. 2019

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Plasma probes are well established diagnostic tools. They are not complicated, relatively easy to construct and to handle. The easiest and fastest accessible parameter is their floating potential. However, the floating potential of a cold probe is not very significant. Much more important and relevant is the plasma potential. But in most types of plasmas, consisting mainly of electrons and only positive ions, the floating potential is more negative than the plasma potential by a factor proportional to the electron temperature. Obviously this is due to the much higher mobility of the electrons. We present a review on probes whose floating potential is close to or ideally equal to the plasma potential. Such probes we name Plasma Potential Probes (PPP) and they can either be Electron Emissive Probes (EEP) or so-called Electron Screening Probes (EPS). These probes make it possible to measure the plasma potential directly and thus with high temporal resolution. An EEP compensates the plasma electron current by an electron emission current from the probe into the plasma, thereby rendering the current-voltage characteristic symmetric with respect to the plasma potential and shifting the floating potential towards the plasma potential. Only the simplest case of an EEP floating exactly on the plasma potential is discussed here in which case no sheath is present around the probe. An ESP, principally operable only in strong magnetic fields, screens off most of the plasma electron current from the probe collector, taking advantage of the fact that the gyro radius of electrons is usually much smaller than that of the ions. Also in this case we obtain a symmetric current-voltage characteristic and a shift of the probe's floating potential towards the plasma potential. We have developed strong and robust EEPs and two types of ESPs, called BUnker Probes (BUP), for the use in the Scrape-Off Layer (SOL) of Medium-Size Tokamaks (MST), and other types of strongly magnetized hot plasmas. These probes are presented in detail.

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“…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.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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The collection and emission of electrons from a spherical body in the Space-Charge Limited (SCL) regime are investigated. When a Virtual Cathode (VC) in the potential profile around the body is present, the barrier in the effective potential energy of electrons is assumed to be located near the position of the minimum of the VC potential, for both collected and emitted electrons. This assumption is confirmed to be reasonable in the case of a double Yukawa potential profile and allows the SCL cross-section for electron collection and the emitted electron's trapped-passing boundary to be written in a simple way. An expression for the collection current for Maxwellian electrons is derived and is shown to recover the classical Orbital Motion Limited (OML) theory when the VC vanishes. Using the same assumptions, an expression for the thermionic emission current in the SCL regime is also obtained and comparisons with the OML þ theory are made. Finally, an expression for the dust electric charge in the SCL regime is derived and shown to give drastically different results when compared to the commonly used formula (obtained from a Yukawa potential profile). Consequences in the framework of dust in tokamak plasmas are discussed.

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

Cited by 31 publications

References 38 publications

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

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

Plasma potential probes for hot plasmas

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

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