Numerical solutions of sheath structures in front of an electron-emitting electrode immersed in a low-density plasma

Din, Alif

doi:10.1063/1.4821829

Cited by 13 publications

(11 citation statements)

References 29 publications

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“…Thus it is negative for classical Debye sheaths. Many models of hot cathode sheaths have been constructed [14,15,16,17] where the cathode sheath potential difference is an input parameter assumed negative. But in practice only the potential of the cathode relative to the anode is constrained to be negative by a bias.…”

Section: Fundamental Aspects Of the Hot Cathode Current Modesmentioning

confidence: 99%

“…The cathode is then covered by a double layer or double sheath..." Langmuir demonstrated the current saturation in a diode model where thermoelectrons and ions passed through the gap in opposite directions. Later researchers modeling hot cathode sheaths [15,17,34] improved upon Langmuir's model and accounted for the Bohm criterion. They showed that as the emitted flux Gemit is raised in the classical sheath mode, the electric field at the cathode surface weakens and eventually vanishes.…”

Section: A Cathode Sheath Physicsmentioning

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%

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

Campanell

2018

Phys. Rev. E

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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.

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Section: Fundamental Aspects Of the Hot Cathode Current Modesmentioning

confidence: 99%

Section: A Cathode Sheath Physicsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Campanell

2018

Phys. Rev. E

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“…For the last several decades, plasma sheath problems have been extensively analysed using kinetic [1][2][3][4][5][6][7][8][9][10][11] and fluid [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] models. Tonks and Langmuir [1] were the first to consider the sheath in gas discharges, with cold ions and hot electrons, and solved the mystery of higher ion velocity in the sheath than the respective thermal velocity in the bulk plasma.…”

Section: Introductionmentioning

confidence: 99%

Plasma sheath in the presence of warm ions

Din

Adnan

Khan

2023

Contributions to Plasma Physics

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The numerical solution of a warm ion sheath for an electrode/wall in contact with a low‐density isotropic plasma is analysed for sonic and supersonic flow of ions at the sheath entrance. For this, we consider half Maxwellian electron and water bag ion velocity distribution functions at the sheath edge. The generalized kinetic Bohm criterion is used to calculate the average ion velocity at the sheath edge. A relation between electrode/wall floating potential, sheath edge potential, and ion temperature is derived from the floating electrode/wall condition. The numerical solution of potential profiles for sonic and supersonic ions is calculated with varying ion temperature at different values of sheath edge potential. The results show that for sonic ions, the sheath solution merges smoothly into presehath solution, but for supersonic ions, we have density gradients, which increase with increasing ion temperature.

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“…Electron collection and emission by an electron emitting surface have been extensively studied, 4,[14][15][16][17] especially in the case of emissive probes. 18 Yet, the emitting surface is usually assumed to be planar, which might be acceptable for millimeter probes in cold plasmas, but not in the case of dust grains where the orbital motion of charged particles plays a crucial role.…”

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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Numerical solutions of sheath structures in front of an electron-emitting electrode immersed in a low-density plasma

Cited by 13 publications

References 29 publications

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

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

Plasma sheath in the presence of warm ions

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

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