Study of “source sheath” problem in PIC/MC simulation: Spherical geometry

Trunec, David; Zikán, Petr; Wagner, J.; Bonaventura, Zdeněk

doi:10.1063/1.4984990

Cited by 3 publications

(3 citation statements)

References 17 publications

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“…However, this is not correct and it leads to a false electron concentration decrease further from the boundary through which the electrons are injected into the computational domain. This problem and its removal have been studied in our previous paper [21].…”

Section: Plasma Modelmentioning

confidence: 99%

PIC/MC calculation of current–voltage characteristic of emissive probe

Jilek¹,

Caloud²,

Zikán³

et al. 2022

Plasma Sources Sci. Technol.

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Two numerical models were used to study the function of an emissive (electron-emitting) probe—the PIC/MC model and the thermal model. The PIC/MC model was used to calculate the I–V characteristics of the emissive probe. The calculations were focussed on the determination of the floating potential of the probe, which increases with increasing probe temperature. It was found that the floating potential can reach the value of the plasma potential, and it can even be higher than the plasma potential. The dependence of the floating potential on probe temperature is linear in the vicinity of the plasma potential, and the slope of this dependence changes when the floating potential equals the plasma potential. The potential profiles near the probe were also calculated and it was found that the space charge effects can be neglected for the plasma parameters studied (electron density 1014–1016 m−3, electron temperature 2500–40 000 K, probe temperature up to 2500 K). The thermal model was used for the calculation of the dependence of probe temperature and potential profiles on DC current passing through the probe (and heating the probe). The thermal model was based on the heat equation, which was solved using the finite element method. Finally, the results from both the above-mentioned models were combined to obtain the dependencies of floating potential on probe heating DC current.

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Section: Plasma Modelmentioning

confidence: 99%

PIC/MC calculation of current–voltage characteristic of emissive probe

Jilek¹,

Caloud²,

Zikán³

et al. 2022

Plasma Sources Sci. Technol.

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“…Many past computational studies of hot cathode sheaths simulated a collisionless domain between a cathode and a source boundary where the plasma ions and electrons were injected [15,16]. Such boundary injection models suffer from source sheath problems [42,43] and cannot capture the key influences of the presheaths, resistivity field and anode sheath.…”

Section: Simulation Studymentioning

confidence: 99%

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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“…The Maxwellian velocity distribution was assumed for electron and ion velocities in the unperturbed plasma, with the electron and ion temperatures T e and T i , respectively. For the case of ion current to a spherical probe, it is possible to derive a self-consistent boundary condition [25] that allows performing the simulations with low value of r d while connecting particle densities smoothly to values from analytical asymptotic formulae. Unfortunately, this approach is not feasible for the case of cylindrical probes, as the analytical asymptotic formulae are not known.…”

Section: Pic/mc Modelmentioning

confidence: 99%

Particle‐in‐cell/Monte Carlo simulation of electron and ion currents to cylindrical Langmuir probe

Zikán

Farkaš

Trunec

et al. 2018

Contributions to Plasma Physics

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Electron and ion currents to a cylindrical Langmuir (electrostatic) probe were calculated using the particle-in-cell/Monte Carlo (PIC/MC) self-consistent simulation for a neutral gas in the pressure range 2-3,000 Pa. The simulation enables us to calculate the probe currents even at high neutral gas pressures when the collisions of collected charged particles with neutral gas particles near the probe are important. The main aim of this paper is the calculation of probe currents at such high gas pressures and the comparison of the results with experimentally measured probe currents. Simulations were performed for two cases: (a) probes with varying radii in a non-thermal plasma with high electron temperature at low neutral gas pressure of 2 Pa (in order to verify the correctness of our simulations), and (b) probe with the radius of 10 m in the afterglow plasma with low electron temperature and a higher neutral gas pressure (up to 3,000 Pa). The electron probe currents obtained in case (a) show good agreement with those predicted by the orbital motion limited current (OMLC) theory for probes with radii up to 100 m for the given plasma conditions. At larger probe radii and/or at higher probe voltages, the OMLC theory incorrectly predicts too high an electron probe current for the plasma parameters studied. Additionally, a formula describing the spatial dependence of the electron density in the presheath in the collisionless case is derived. The simulation at higher neutral gas pressures, i.e. case (b), shows a decrease of the electron probe current with increasing gas pressure and the creation of a large presheath around the probe. The simulated electron probe currents are compared with those of measurements by other authors, and the differences are discussed. KEYWORDS Langmuir probe, PIC/MC simulation INTRODUCTIONParticle-in-cell/Monte Carlo (PIC/MC) simulation provides the most accurate and reliable tool for the calculation of electron and ion currents to a Langmuir (electrostatic) probe. The calculation of probe currents (probe theory) is necessary for the determination of electron and ion densities, electron temperatures, and electron energy distribution functions from probe measurements. Most of the analytical calculations of probe currents have been carried out under the assumption of a collisionless movement of charged particles around the probe [1] and can, therefore, be applied without error only to low-pressure plasmas.There is no widely accepted theory of probe currents for medium-pressure and high-pressure plasmas where the charged particles do collide with neutral particles in the sheath and presheath around the probe. Therefore, the probe measurements cannot be used for such plasmas. There exist three groups of collisional probe theories.

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Study of “source sheath” problem in PIC/MC simulation: Spherical geometry

Cited by 3 publications

References 17 publications

PIC/MC calculation of current–voltage characteristic of emissive probe

PIC/MC calculation of current–voltage characteristic of emissive probe

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

Particle‐in‐cell/Monte Carlo simulation of electron and ion currents to cylindrical Langmuir probe

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