Effects Of Ion-neutral Collisions Research Articles

On the effect of ion-neutral collisions on dust grain screening in a low-pressure gas discharge plasma

The effect of ion-neutral collisions on charging of micrometer-sized dust grains immersed in a low-pressure argon discharge plasma is studied on the basis of the Vlasov-Bhatnagar-Gross-Krook kinetic equations. The equations are solved numerically using the method described in our previous work [I. L. Semenov et al., Phys. Plasmas 18, 103707 (2011)]. A modified version of the numerical method is proposed to reduce the required computational time. Numerical calculations are carried out for typical plasma parameters used in laboratory investigations of dusty plasma. On the basis of the obtained results, the influence of collisions on the ion flux and grain charge is analyzed. A comparison of our results with those obtained using different analytical models proposed earlier is presented. In addition, applicability of simple kinetic models describing the influence of collisions on the electric potential around a dust grain [S. A. Khrapak et al., Phys. Rev. Lett. 100, 225003 (2008); A. G. Zagorodny et al. Ukr. J. Phys. 54, 1089 (2009)] is examined. The influence of ion-neutral collisions on the distribution of plasma macroparameters near the grain surface is also demonstrated.

Stochastic dynamics of charge fluctuations in dusty plasma: A non-Markovian approach

Dust particles in typical laboratory plasma become charged largely by collecting electrons and/or ions. Most of the theoretical studies in dusty plasma assume that the grain charge remains constant even though it fluctuates due to the discrete nature of the charge. The rates of ions and electrons absorption depend on the grain charge, hence its temporal evolution. Stochastic charging model based on the standard Langevin equation assumes that the underlying process is Markovian. In this work, the memory effect in dust charging dynamics is incorporated using fractional calculus formalism. The resulting fractional Langevin equation is solved to obtain the amplitude and correlation function for the dust charge fluctuation. It is shown that the effects of ion-neutral collisions can be interpreted in phenomenological sense through the nonlocal fractional order derivative.

Modeling of plasma turbulence and transport in the Large Plasma Device

Numerical simulation of plasma turbulence in the Large Plasma Device (LAPD) [W. Gekelman, H. Pfister, Z. Lucky et al., Rev. Sci. Instrum. 62, 2875 (1991)] is presented. The model, implemented in the BOUndary Turbulence code [M. Umansky, X. Xu, B. Dudson et al., Contrib. Plasma Phys. 180, 887 (2009)], includes three-dimensional (3D) collisional fluid equations for plasma density, electron parallel momentum, and current continuity, and also includes the effects of ion-neutral collisions. In nonlinear simulations using measured LAPD density profiles but assuming constant temperature profile for simplicity, self-consistent evolution of instabilities and nonlinearly generated zonal flows results in a saturated turbulent state. Comparisons of these simulations with measurements in LAPD plasmas reveal good qualitative and reasonable quantitative agreement, in particular in frequency spectrum, spatial correlation, and amplitude probability distribution function of density fluctuations. For comparison with LAPD measurements, the plasma density profile in simulations is maintained either by direct azimuthal averaging on each time step, or by adding particle source/sink function. The inferred source/sink values are consistent with the estimated ionization source and parallel losses in LAPD. These simulations lay the groundwork for more a comprehensive effort to test fluid turbulence simulation against LAPD data.

3D Hybrid Simulation of Pure Magnetic Sail Including Ion-Neutral Collision Effect in Laboratory

Magnetic Sail is a propellantless propulsion system proposed for an interplanetary space flight. The propulsive force is produced by the interaction between the magnetic field artificially generated by a hoop coil equipped with the magnetic sail and the solar wind. Three-dimensional hybrid particle-in-cell simulations are performed to reproduce the plasma flow structure around the magnetic sail and to measure the propulsive force of the magnetic sail with two different coil currents. We report the characteristics of the magnetosphere, such as the profile of the magnetic field and the predicted thrust values obtained by simulations, which agree well with laboratory experiments, when simulations are carried out by considering the ion-neutral collision effect. The hybrid particle-in-cell simulation carried out without considering the collisional effect gave a thrust value of 2.3∼3.5 N, which can be applied to the thrust evaluation of the magnetic sail in a magnetosphere with size of 250∼300 km in a collisionless interplanetary space.

Hybrid Particle-in-Cell Simulations of Magnetic Sail in Laboratory Experiment

Magnetic sail is a propellantless propulsion system proposed for an interplanetary space flight. The propulsive force is produced by the interaction between the magnetic field artificially generated by a hoop coil equipped with the magnetic sail and the solar wind. Three-dimensional hybrid particle-in-cell simulations are performed to elucidate the plasma flow structure around the magnetic sail and to measure the propulsive force of the magnetic sail. We report the characteristics of the magnetosphere, such as the profile of the magnetic field, the thickness of the magnetopause current layer, and the predicted thrust value obtained by simulations, which agree well with laboratory experiment when simulations are carried out by considering the ion-neutral collision effect. The hybrid particle-in-cell simulation carried out without considering the collisional effect gave a thrust value of 3.5 N, which can be applied to the thrust evaluation of the magnetic sail in a magnetosphere with size of 300 km in a collisionless interplanetary space.

Time damping of non-adiabatic MHD waves in an unbounded partially ionised prominence plasma

Aims. We study the joint effect of ion-neutral collisions and thermal mechanisms on the damping of MHD waves in a partially ionised prominence plasma. Methods. Thermal conduction, radiation, and heating were included in the energy equation of the one-fluid MHD set of equations we derived earlier for an adiabatic, partially ionised plasma. Then, assuming small perturbations, these equations were linearised and the dispersion relation for magnetoacoustic, Alfven, and thermal waves obtained. Results. Compared with the non-adiabatic, fully ionised, and the adiabatic, partially ionised cases, the main result is that there is an increase in the efficiency of the damping of magnetoacoustic waves in prominence oscillations within the observed range of wavelengths for the magnetoacoustic waves. On the other hand, the Alfven wave is only damped by the ion-neutral collision mechanism, and this damping becomes important for almost neutral plasmas. Conclusions. The coupling of non-adiabatic mechanisms with the ion-neutral collisions mechanism offers a more complete model for the damping of magnetoacoustic waves with those values compatible with those observed in prominence oscillations. Furthermore, the ion-neutral collisions mechanism is able to damp the Alfven wave in an efficient way.

A note on the binary interaction potential in complex (dusty) plasmas

The effect of ion-neutral collisions on the potential of interaction between a pair of negatively charged grains in isotropic weakly collisional plasmas is investigated. Two interaction mechanisms are considered: Electric repulsion between like-charged grains and attraction due to the so-called “ion shadow” effect. It is demonstrated that in the presence of collisions, both interaction potentials exhibit Coulomb-like ∝r−1 decay at large distances. A necessary condition for the existence of long-range attraction is derived. The obtained results are then used to reconsider the possibility of liquid-vapor critical point occurrence in complex plasmas.

Chromospheric magnetic reconnection: two-fluid simulations of coalescing current loops

Aims: To investigate magnetic reconnection rates during the coalescence of two current loops in the solar chromosphere, by altering the neutral-hydrogen to proton density ratio, ioniziation/recombination coefficients, collision frequency and relative helicity of the loops. Methods: 2.5D numerical simulations of the chromosphere were conducted using a newly developed two-fluid (ion-neutral) numerical code. Developed from the Artificial Wind scheme, the numerical code includes the effects of ion-neutral collisions, ionization/recombination, thermal/resistive diffusivity and collisional/resistive heating. Results: It was found that the rates of magnetic reconnection strongly depend on the neutral-hydrogen to proton density ratio; increasing the density ratio by a thousand-fold decreased the rate of magnetic reconnection by twenty-fold. This result implies that magnetic reconnection proceeds significantly faster in the upper chromosphere, where the density of ions (protons) and neutral-hydrogen is comparable, than in the lower chromosphere where the density of neutral-hydrogen is over a thousand times the ion density. The inclusion of ionization/recombination, an important physics effect in the chromosphere, increases the total reconnected magnetic flux, but does not alter the rate of magnetic reconnection. Reductions in the ion-neutral collision frequency, result in small increases to the rates of magnetic reconnection. The relative helicity of the two current loops was not observed to have any significant effect on the rates of magnetic reconnection. Conclusions: The magnetic reconnection rates of coalescing current loops are strongly affected by the inclusion of neutral-hydrogen particles.

Simulating Plasma-Induced Hall Thruster Wall Erosion With a Two-Dimensional Hybrid Model

A 2-D radial-axial (r-z) hybrid fluid/particle-in-cell (PIC) model has been developed to model energetic particle-induced channel-wall erosion in coaxial Hall discharge plasma thrusters. The discharge model geometry corresponds to that of a so-called stationary plasma thruster with an extended dielectric channel, and the computational domain extends from the anode at the base of this channel through the channel interior and into the near-field plume region. A model of the wall-erosion process has been added to the simulation in order to assess thruster degradation due to ion and energetic-neutral-induced sputtering of the channel walls. The effect of ion-neutral collisions, including momentum and charge-exchange collisions, on the erosion process is examined. These models are used to simulate the long-term wall-erosion history. For the specific Hall-thruster-configuration modeled, collisions were found to have less than a 10% effect on wall erosion. The erosion rate is seen to decrease with time, in good agreement with experimental measurements of long-term erosion in similar thrusters, resulting in a wall recession of as much as 2 mm after 4000 h of simulated operation.

Particle charge in the bulk of gas discharges

An experimental determination of particle charge in a bulk dc discharge plasma covering a wide range of neutral gas pressures, was recently reported [S. Ratynskaia, Phys. Rev. Lett. 93, 085001 (2004)]. The charges obtained were several times smaller than the predictions of collisionless orbital motion limited theory. This discrepancy was attributed to the effect of ion-neutral collisions. In the present paper a more detailed description of this experiment is provided and additional experimental results obtained with particles of different sizes are reported. The measurements are compared with molecular dynamics simulations of particle charging for conditions similar to those of the experiment, with other available experimental data on particle charge in the bulk of gas discharges, and with a simple analytical model accounting for ion-neutral collisions. All the considered evidence indicates that ion-neutral collisions represent a very important factor, which significantly affects (reduces) the particle charge under typical discharge conditions.

Effect of ion neutral collisions on the ion and neutral velocity distribution on negatively biased electrodes

In the present work, a model is proposed that can estimate ion and neutral energy distributions at negatively biased electrodes (substrates), assuming charge exchange as the dominant ion neutral collision. The model is applicable over a wide pressure range, and can be used to compute the ratio of ion flux to neutral flux, net momentum transfer from plasma to substrate, etc. The ratio of the sheath thickness to the collision mean free path is a crucial parameter determining the ion and neutral velocity distribution The model predicts that at high pressure (∼few Torr), neutral flux and momentum transfer is dominant for negative electrode bias (∼1 kV). The model can be used for plasma nitriding, plasma immersion ion implantation, plasma etching, etc., to predict ion and neutral bombardment effects on the electrode surface.

Numerical investigation of interface region flows in mass spectrometers: ion transport

The transport of free ions through highly under-expanded jet flows of neutral gases and in the presence of applied electric fields is investigated by continuum-based numerical simulations. In particular, numerical results are described that are relevant to ion flows occurring in the interface region of mass spectrometer systems. A five-moment mathematical model and parallel multi-block numerical solution procedure are developed for predicting the ion transport. The model incorporates the effects of ion–neutral collision processes and is used in conjunction with a Navier–Stokes model and flow solver for the neutral gas to examine the key features of the ion motion. The influences of the neutral gas flow, electric field, and flow field geometry on ion mobility are all carefully assessed. Several ions of varying mass and charge are considered, and the relative importance of competing effects (i.e. electric field and ion–neutral collision effects) is discussed. The capability of controlling the charged particle motions through a combination of directed neutral flow and applied electric field is demonstrated for high-speed, hypersonic jet flows.

Double layers in a modestly collisional electronegative discharge

The effect of ion-neutral collisions on the structure and ion flux emanating from a steady-state, planar discharge with two negative components is investigated. The positive ion component is modelled as a cold fluid subject to constant-mobility collisions, while the electrons and negative ions obey Boltzmann relations. The model includes the collisionless limit. When the negative ions are sufficiently cold three types of discharge structures are found. For small negative ion concentrations or high collisionality, the discharge is `stratified', with an electronegative core and an electropositive edge. For the opposite conditions, the discharge is `uniform' with the negative ion density remaining significant at the edge of the plasma. Between these cases lies the special case of a double-layer-stratified discharge, where quasi-neutrality is violated at the edge of the electronegative core. Double-layer-stratified solutions are robust in that they persist for moderate collisionality. Numerical solutions for finite non-neutrality verify that the plasma flux varies continuously with collisionality, although the derivative of the flux with respect to collisionality is discontinuous when the discharge structure changes from uniform to stratified. Double-layer solutions are found when the flux predicted for the plasma approximation is double-valued and the flux associated with the smaller plasma edge potential is less than that associated with the larger edge potential. A comparison with numerical non-neutral solutions confirms that the flux is correctly predicted using the plasma approximation when the larger value of the flux is taken in the two-solution regime.

Ion–neutral collision effects on Alfvén surface waves

It is well established that in magnetized partially ionized plasmas, the dispersion of the shear Alfvén waves is strongly influenced by the ion–neutral collisions. In the case of inhomogeneous plasmas, the study of these collisions on Alfvén surface waves can become important, especially in understanding resonant absorption of Alfvén waves. In this paper, the dispersion equation for the surface waves in partially ionized plasma along a plasma–plasma interface is derived and it is shown that ion–neutral collisions can cause a drastic change in the Alfvén surface waves’ propagation characteristics. For the case when ion–neutral coupling is weak, the wave propagates along the interface with the natural frequency of Alfvén surface waves in the charged medium without friction. When coupling is strong, this frequency is determined by the mass densities of both ions and neutrals in both media. When the ionization fraction is low, these two frequencies can differ by several orders of magnitude. There also exists a range of frequencies, depending on the collisions, in which the surface waves do not propagate. The damping of surface waves due to ion–neutral collisions can be very small in the case of strong coupling. For weak coupling, this damping can become large due to large collision frequencies. The effect of this on the resonant absorption of surface waves is discussed briefly. The possibility of propagation of surface waves along thin plasma–plasma interfaces is also considered in the context of some astrophysical systems.

Heating of the Interstellar Diffuse Ionized Gas via the Dissipation of Turbulence

We have recently published observations that specify most of the turbulent and mean plasma characteristics for a region of the sky containing the interstellar diffuse ionized gas (DIG). These observations have provided virtually all of the information necessary to calculate the heating rate from dissipation of turbulence. We have calculated the turbulent dissipation heating rate employing two models for the interstellar turbulence. The first is a customary modeling as a superposition of magnetohydrodynamic waves. The second is a fluid-turbulence-like model based on the ideas of Higdon. This represents the first time that such calculations have been carried out with full and specific interstellar turbulence parameters. The wave model of interstellar turbulence encounters the severe difficulty that plausible estimates of heating by Landau damping exceed the radiative cooling capacity of the interstellar DIG by 3-4 orders of magnitude. Clearly interstellar turbulence does not behave like an ensemble of obliquely propagating fast magnetosonic waves. The heating rate due to two other wave dissipation mechanisms, ion-neutral collisional damping and the parametric decay instability, are comparable to the cooling capacity of the diffuse ionized medium. We find that the fluid-like turbulence model is an acceptable and realistic model of the turbulence in the interstellar medium once the effects of ion-neutral collisions are included in the model. This statement is contingent on an assumption that the dissipation of such turbulence because of Landau damping is several orders of magnitude less than that from an ensemble of obliquely propagating magnetosonic waves with the same energy density. Arguments as to why this may be the case are made in the paper. Rough parity between the turbulent heating rate and the radiative cooling rate in the DIG also depends on the hydrogen ionization fraction being in excess of 90% or on a model-dependent lower limit to the heating rate being approximately valid. We conclude that the dissipation of turbulence is capable of providing a substantial and perhaps major contribution to the energy budget of the diffuse ionized medium.

Kinetic Alfvén Modes in a Collisional Plasma

Kinetic modes of the Alfven wave in an inhomogeneous plasma are investigated by using a two-fluid theory including a finite pressure effect, the Hall effect, electron inertia and collisional effects. The dispersion relation involving the kinetic Alfven wave, the surface e lectrostatic wave and the resistive wave is derived with the terms of ion-cyclotron frequency and electron-ion collisions, and it is also concluded that the observed waves in the experiment [Phys. Rev. Lett. 64 (1990) 1247; J. Phys. Soc. Jpn. 59 (1990) 2374] are kinetic Alfven waves in a collisional plasma including ion-neutral collision effects.

Numerical Simulation of Chromospheric Spicules Driven by Weakly-Damped Alfvén Waves

AbstractWe perform a time-dependent numerical simulation of the momentum transfer from weakly-damped Alfvén waves to the solar chromospheric plasma in a spatially expanding flux tube. The momentum transfer is due to the effect of ion-neutral collisions on Alfvén waves. By solving one-dimensional and time-dependent hydrodynamic equations, we study the formation of spicules. A realistic model for the solar atmosphere and for both the transport of and momentum-transfer by the Alfvén waves is used. The temperature, density and vertical velocity of the formed structures are very similar to those reported in observational studies of spicules.

Erratum: ‘‘Ion-neutral collision effect on an Alfvén wave’’ [Phys. Rev. Lett.71, 360 (1993)

Erratum: ‘‘Ion-neutral collision effect on an Alfvén wave’’ [Phys. Rev. Lett.<b>71</b>, 360 (1993)

Ion-neutral collision effect on an Alfvén wave.

This paper reports that ion-neutral collisions in a magnetized plasma cause a drastic change in the dispersion relation of the shear Alfv\'en wave with poloidal mode number m=0, connecting to the branch of the m=+1 compressional Alfv\'en wave at frequencies below the ion-cyclotron frequency. An anomaly of the dispersion then appears on the refractive index curve and a wave packet in this frequency range undergoes strong amplitude damping and profile deformation. It is confirmed that the Kramers-Kronig relation holds for the dielectric function, estimated from both the measured refractive index and damping rate.

Effect of ion-neutral collisions on plasma flow in the edge region of stellarators

The paper presents a study of the effect of charge exchange and elastic scattering collisions between ions and neutrals on the plasma flow along the divergent magnetic field lines in the divertor region of a stellarator. In the analysis, which is kinetic, an indirect solution to the kinetic equation for the ions is obtained using a Monte Carlo description for the ion dynamics together with the assumption of Boltzmann electrons and quasi-neutrality. Ion-neutral collisions are studied in a three-dimensional velocity space using a Monte Carlo algorithm. The results show that charge exchange and elastic scattering collisions play a significant role in reducing the drop of the electrostatic potential in the divertor chamber below that obtained from collisionless calculations. This, in turn, reduces the electrostatic barrier, which may be useful in retarding the flow of high Z impurity ions from the target plate towards the confined plasma. The calculations for the ion energy distribution suggest that, owing to ion-neutral collisions, most of the impact energy of ions on the divertor plate is gained while they cross the sheath region of the plate.