Formation of stable inverse sheath in ion–ion plasma by strong negative ion emission

Zhang, Zhe; Wu, Bihu; Yang, Shali; Zhang, Ya; Chen, Dezhi; Fan, Mingwu; Jiang, Wei

doi:10.1088/1361-6595/aac070

Cited by 9 publications

(14 citation statements)

References 36 publications

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“…1(a). Inverse sheath formation was recently shown to trigger temperature saturation in Hall thrusters [14], mode transitions in thermionic discharges [15], and ion-ion plasma formation in negative ion sources [13]. Inverse sheath theory also explains why emissive probes often float above the plasma potential [5].…”

mentioning

confidence: 98%

“…Recent works [12,13] find that SCL sheaths are destroyed by ion trapping in the virtual cathode (VC) and the equilibrium strongly emitting sheath must be inverse, see Fig. 1(a).…”

mentioning

confidence: 99%

“…[1] produced stable SCL's while simulations of H-emission with CX collisions by Zhang et al [13] yielded an inverse regime. SCL equilibria here are sustained by excluding the CX collisions and Scharge in the VC potential well.…”

mentioning

confidence: 99%

“…Recent studies show that CX collisions make cold ions accumulate in the VC, causing the sheath to invert [12]. This is why collisionless simulations of emitting divertor sheaths by Komm et al [1] produced stable SCL's while simulations of H-emission with CX collisions by Zhang et al [13] yielded an inverse regime. SCL equilibria here are sustained by excluding the CX collisions and Scharge in the VC potential well.…”

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confidence: 99%

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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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mentioning

confidence: 98%

“…Recent works [12,13] find that SCL sheaths are destroyed by ion trapping in the virtual cathode (VC) and the equilibrium strongly emitting sheath must be inverse, see Fig. 1(a).…”

mentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

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

Campanell

Johnson

2019

Phys. Rev. Lett.

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“…More recent studies suggested that the SCL sheath cannot remain stable if cold ions are generated by charge exchange collisions in the sheath [11]. The emissive sheath is difficult to be directly accessed by conventional probe diagnostics due to its small size, so sheath theories are frequently validated against numerical simulations [12][13][14].…”

Section: Introductionmentioning

confidence: 99%

Vlasov simulation of the emissive plasma sheath with energy-dependent secondary emission coefficient and improved modeling for dielectric charging effects

et al. 2022

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A one-dimensional Vlasov–Poisson simulation code is employed to investigate the plasma sheath considering electron-induced secondary electron emission (SEE) and backscattering. The SEE coefficient is commonly treated as constant in a range of plasma simulations; here, an improved SEE model of a charged dielectric wall is constructed, which includes the wall charging effect on the SEE coefficient and the energy dependency of the SEE coefficient. Pertinent algorithms to implement the previously mentioned SEE model in plasma simulation are studied in detail. It is found that the SEE coefficient increases with the amount of negative wall charges, which in turn reduces the emissive sheath potential. With an energy-dependent SEE coefficient, the sheath potential is a nonlinear function of the plasma electron temperature, as opposed to the linear relation predicted by the classic emissive sheath theory. Simulation combining both wall-charging effect and SEE coefficient’ energy dependency suggests that the space-charged limited sheath is formed at high plasma electron temperature levels, where both sheath potential and surface charging saturate. Additionally, different algorithms to implement the backscattering in the kinetic simulation are tested and compared. Converting backscattered electrons to secondary electrons via an effective SEE coefficient barely affects the sheath properties. The simulation results are shown to be commensurate with the upgraded sheath theory predictions.

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Possible mitigation of tokamak plasma–surface interactions using thermionic divertor plates with inverse sheaths

Campanell

2020

Physics of Plasmas

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Recent studies demonstrate that when a plasma-facing surface emits a sufficient flux of electrons, it will form an inverse sheath. Here, we consider a possibility of using thermionic target plates with inverse sheaths as an innovative divertor operating scenario. We derive an electron heat flux boundary condition for inverse sheaths and show that for given power exhaust into a tokamak scrape-off-layer, an inverse sheath leads to a much lower target plasma electron temperature than a conventional sheath. Low enough target plasma temperatures for radiative divertor detachment could therefore be achieved using inverse sheaths instead of the usual need to inject neutral atoms that compromise the core plasma. Other advantages of inverse sheath detachment over conventional sheath operating scenarios include (a) ion impact energies are as low as possible, minimizing sputtering and tritium implantation, (b) surface recombination heat flux is reduced due to ion flux reduction, and (c) arcs are inhibited due to the sign of the surface electric field. This paper outlines the basic properties of inverse sheath detachment and considers the feasibility of implementation. We offer recommendations for future modeling efforts needed to better understand the effects of thermionic emission in tokamaks and whether inverse sheaths present a viable divertor solution.

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Formation of stable inverse sheath in ion–ion plasma by strong negative ion emission

Cited by 9 publications

References 36 publications

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

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

Vlasov simulation of the emissive plasma sheath with energy-dependent secondary emission coefficient and improved modeling for dielectric charging effects

Possible mitigation of tokamak plasma–surface interactions using thermionic divertor plates with inverse sheaths

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