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Title: Presheath structure of a dust-contaminated plasma

Abstract

The presheath structure of a dust-contaminated plasma is studied by taking into account the electron-impact ionization, the plasma loss due to the capture of electrons and ions by the dust grains, the ion-dust collisions, as well as the dust charge variations. It is shown that at the edge of the presheath with the sheath, there is a critical ion Mach number (the generalized Bohm criterion modified by the dust) which is exactly the same as that obtained from collisionless sheath model. On the other hand, depending on different equilibrium states of the bulk plasma, different connections between the bulk plasma and the presheath will be inferred. When the electron-impact ionization exactly balances the plasma loss, there is a smooth transition from the bulk plasma to the presheath, and both the ion velocity and the gradients of the velocity and density vanish at the edge of the presheath with the bulk plasma. When the plasma loss exceeds the ionization, the presheath starts at the point where the ion velocity equals the ambipolar diffusion velocity, i.e., the presheath profile is connected to the diffusion profile of the bulk plasma. When the ionization exceeds the plasma loss, the bulk plasma-presheath edge appears wheremore » the gradient of the ion density is zero but the gradient of the ion velocity is nonzero. For the plasma loss exceeding or being less than the ionization, numerical results reveal that the change of the dust density can result in opposite effects on the presheath profiles (shortening or broadening of the presheath region)« less

Authors:
;  [1]
  1. CAS Key Laboratory of Basic Plasma Physics, University of Science and Technology of China, Hefei, Anhui 230026 (China) and Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026 (China)
Publication Date:
OSTI Identifier:
20782472
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 13; Journal Issue: 1; Other Information: DOI: 10.1063/1.2158142; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; AMBIPOLAR DIFFUSION; BOHM CRITERION; BOUNDARY LAYERS; CAPTURE; CHARGED-PARTICLE TRANSPORT; DENSITY; DUSTS; ELECTRON COLLISIONS; ELECTRONS; EQUILIBRIUM; ION COLLISIONS; ION DENSITY; IONIZATION; IONS; MACH NUMBER; PLASMA; PLASMA DENSITY; PLASMA SHEATH; VARIATIONS

Citation Formats

Li Yangfang, and Ma, J.X.. Presheath structure of a dust-contaminated plasma. United States: N. p., 2006. Web. doi:10.1063/1.2158142.
Li Yangfang, & Ma, J.X.. Presheath structure of a dust-contaminated plasma. United States. doi:10.1063/1.2158142.
Li Yangfang, and Ma, J.X.. Sun . "Presheath structure of a dust-contaminated plasma". United States. doi:10.1063/1.2158142.
@article{osti_20782472,
title = {Presheath structure of a dust-contaminated plasma},
author = {Li Yangfang and Ma, J.X.},
abstractNote = {The presheath structure of a dust-contaminated plasma is studied by taking into account the electron-impact ionization, the plasma loss due to the capture of electrons and ions by the dust grains, the ion-dust collisions, as well as the dust charge variations. It is shown that at the edge of the presheath with the sheath, there is a critical ion Mach number (the generalized Bohm criterion modified by the dust) which is exactly the same as that obtained from collisionless sheath model. On the other hand, depending on different equilibrium states of the bulk plasma, different connections between the bulk plasma and the presheath will be inferred. When the electron-impact ionization exactly balances the plasma loss, there is a smooth transition from the bulk plasma to the presheath, and both the ion velocity and the gradients of the velocity and density vanish at the edge of the presheath with the bulk plasma. When the plasma loss exceeds the ionization, the presheath starts at the point where the ion velocity equals the ambipolar diffusion velocity, i.e., the presheath profile is connected to the diffusion profile of the bulk plasma. When the ionization exceeds the plasma loss, the bulk plasma-presheath edge appears where the gradient of the ion density is zero but the gradient of the ion velocity is nonzero. For the plasma loss exceeding or being less than the ionization, numerical results reveal that the change of the dust density can result in opposite effects on the presheath profiles (shortening or broadening of the presheath region)},
doi = {10.1063/1.2158142},
journal = {Physics of Plasmas},
number = 1,
volume = 13,
place = {United States},
year = {Sun Jan 15 00:00:00 EST 2006},
month = {Sun Jan 15 00:00:00 EST 2006}
}
  • The effect of magnetic field on the plasma-wall transition layer is investigated using the two-fluid formulation. The quasi-neutral, near-sheath plasma (presheath) is examined, with the presence of neutral particles, and a magnetic field parallel to the confining wall. A general approach is used which takes into account the electron momentum equation, including the electric field force, the magnetic force, the pressure gradient, and the drag force. The influence of the electron to ion current ratio on the potential and velocity distribution in the near-sheath plasma is investigated. It is shown that the electron density distribution in the presheath may deviatemore » from the Boltzmann distribution normally used in previous presheath models. Even when the plasma density dependence on the potential corresponds to the Boltzmann distribution, the presheath thickness deviates from that calculated with a model based on this distribution. The potential in the presheath with respect to the plasma{endash}presheath interface can be negative or positive depending on the electron to the ion flux ratio {eta} and Hall parameter {beta}{sub i}. In the case of magnetized ions ({beta}{sub i}{gt}1) the potential distribution has a positive maximum and is always negative at the wall edge of the presheath. The value and position of the maximum depend on the parameter {eta}. In the case of unmagnetized ions ({beta}{sub i}{lt}1) the potential is positive for large {eta} and is negative for {eta}{lt}100. With large {beta}{sub i} the influence of the electrons is significant so that the presheath thickness decreases to the electron Larmor radius and has a strong dependence on the parameter {eta}. {copyright} {ital 1997 American Institute of Physics.}« less
  • Plasma flow measurements in the presheath have been performed using two types of directional electric Mach'' probes, in the PISCES facility at UCLA (J. Nucl. Mater. {bold 121}, 277 (1984)). A fast scanning versatile probe combination has been developed, which operates simultaneously as a magnetized'' Mach probe, an unmagnetized'' Mach probe (with characteristic probe size greater than and smaller than ion gyroradius, respectively), and an emissive probe. Presheaths have been investigated by inserting a small object at the center of the plasma column. Variations in plasma flow velocity, density, and potential along the presheath have been deduced by fluid andmore » kinetic theories. A comparison is made between Mach numbers obtained from the magnetized probe and the unmagnetized probe. Incorporation of shear viscosity of order {similar to}0.5{ital nm}{sub {ital i}D}{sub {perpendicular}} in the cross-field transport along the presheath seems best to model the results. The cross-field diffusivity ({ital D}{sub {perpendicular}}) is found to scale approximately proportional to {ital B}{sup {minus}1/2}, with magnitude about 4{times} larger than Bohm in the PISCES plasma. The effect of an electrical bias applied to the object on the presheath characteristics is discussed.« less
  • By employing Braginskii transport equations for ions and Boltzmann distribution for electrons in a dust-contaminated plasma with equilibrium density, temperature, and magnetic field gradients, the nonlinear set of equations are derived. New ion-temperature-gradient driven modes are obtained and various limiting cases are discussed. It is shown that the ion-temperature-gradient driven mode of drift-waves are attenuated in the presence of dust-charge fluctuations. It has been found that dust charging is always dissipative and the growth rate of various modes are damped. Furthermore, the possible stationary solution of the nonlinear mode coupling equations can be represented in the form of dipolar andmore » vortex chains type solutions. The results of the present investigation should be helpful in understanding the fluctuations and transport phenomena in magnetically confined dustcontaminated tokamak plasma.« less
  • The presheath region of an unmagnetized plasma is treated using ion fluid equations in the collisional and collisionless regimes. Effects of neutral gas--plasma interactions are included through source terms in the fluid equations. Ion--ion collisional effects are included through the closure conditions of the fluid equations. In the collisionless regime, the perpendicular ion temperature is assumed constant and independent of the parallel ion temperature. In the collisional limit, the parallel and perpendicular temperatures are assumed equal throughout the presheath. Profiles of ion density, flow velocity, parallel temperature, and energy flux obtained by integrating the fluid equations compare well to previouslymore » published kinetic results in the collisionless case. Ion--ion Coulomb collisions are shown to have a negligible effect on the ion particle and energy flux into the sheath.« less
  • The problem of a collisional plasma flowing into a perfectly absorbing wall has been investigated using a kinetic approach. The plasma is assumed to have a nonzero ion temperature and a Boltzmann distribution of electrons. Ion collisions are included in the analysis through a Bhatnagar--Gross--Krook (BGK) collision term. An equation describing the electrostatic potential variation in the presheath region is derived. This equation is solved numerically for a range of collisionalities. In addition to the potential variation in the presheath, the ion distribution function, the wall potential, and the ion particle and energy fluxes into the sheath are also calculated.more » The calculation is repeated for three different cases. In the first case ion--neutral collisions are modeled by conserving only particles in the BGK operator. Ion--ion collisions are modeled by conserving particles and momentum in the second case, and by conserving particles, momentum, and energy in the third case. These results give insight into the role of collision conservation laws in describing the plasma flow.« less