Ion-neutral Elastic Collisions Research Articles

Particle charge fluctuation and its consequences on aerosol dynamics in dusty plasmas: from continuous to discrete statistical models

In this work we investigated different approaches and models used for the estimation of the charge fluctuation of solid particles in dusty plasmas. We compared the charge distributions inferred from these models and analyzed the consequences in terms of coagulation kinetics and particle size distributions. We especially showed that taking explicitly into account the discrete nature of the charging process along with the enhancement of ion flux due to ion-neutral elastic collisions result in significantly larger values for the fractions of neutral and positively charged particles, which affects the coagulation kinetics and the predicted values of particle density. We also showed, by comparing sectional and modal approaches, that the typical size distributions obtained in the studied plasmas may be well described using a bimodal distribution with a single size nucleation mode and a Gaussian large size mode. Such a possibility enables a great simplification of the aerosol dynamic description and open up an interesting perspective as far as multidimensionnal self consistent modeling of dusty plasmas is concerned.

Surface-wave produced discharges in hydrogen: I. Self-consistent model of diffusion-controlled discharges

A fluid-plasma model of diffusion-controlled hydrogen discharges sustained in the field of propagating surface waves is presented in this study. The self-consistent description of the discharge structure achieved provides results for the inter-related variations of the discharge characteristics: the electron concentration ne, the concentrations of the three ionic species (H+, H2+ and H3+), the concentrations of the two neutral gas components (H and H2), the electron temperature, the power Θ absorbed on average by an electron, the gas temperature, the wavenumber and the space damping rate α of the wave. Wave behaviour in radially inhomogeneous collisional plasmas is taken into account because it provides the proper description of diffusion-controlled discharges. The general mechanism of nonlocal heating of the electrons in the wave field is considered. The model is extended to comparatively low gas pressures (p⩾0.2 Torr) by introducing effective mobilities of the ions in which, besides the ion–neutral elastic collisions, the production and destruction of ions by collisions are included. Based on these effective mobilities, the ambipolar diffusion coefficients of the charged particles are specified. The most important reactions that contribute—under the gas-discharge conditions considered—to the production of charged particles and hydrogen atoms as well as to the electron-energy and gas-energy balances are involved in the model. It is shown that in hydrogen discharges, the (Θ–ne)-relation, which besides the (α–ne)-relation ensures a self-consistent description of the axial structure of surface-wave-sustained plasmas, in general, stems from the dependence of Θ on the concentrations of the neutral gas components (H, H2) and their relation to the concentrations of the ions (H+, H2+ and H3+). The results obtained using the model are discussed in the context of experiments showing a peculiar behaviour of the axial structure of hydrogen discharges compared to discharges in other gases.

Improved elastic collision modeling in DEGAS 2 for low-temperature plasmas

Recent emphasis on low-temperature divertor operations has focused attention on proper treatment of neutral-elastic collisions in low-temperature environments. For like species collisions, as in D++D, quantum mechanical indistinguishability precludes differentiation of small-angle elastic scattering from resonant charge exchange for collision energies <2 eV. The current work improves the low-temperature simulation capabilities of the DEGAS 2 Monte Carlo neutral transport code [D. P. Stotler and C. F. F. Karney, Contrib. Plasma Phys. 34, 392–397 (1994)] by employing a set of quantum mechanical collision cross sections produced by Oak Ridge National Laboratory to provide a comprehensive treatment of ion-neutral elastic collisions. Ion-molecular collisions in the form of D++D2 are included for the first time. An integration technique is utilized that reduces the total collision cross section while keeping the other transport cross sections invariant. The inclusion of ion-molecular elastic collisions results in significant increases in energy exchange between background ions and neutral test species.

Numerical study of argon ions transported across the sheath in electron cyclotron resonance discharges

The transport of argon ions through the sheath as well as through the neutral region in electron cyclotron resonance discharges in argon gas is studied by means of Monte Carlo simulation, with the emphasis on the sheath potential and the characteristics of argon ions in the sheath. The evolution of the energy, velocity, and angle distributions of the ions in the sheath and the dependence of the distributions on gas pressure and substrate bias are investigated primarily concerned with collisional sheath. It is found that the effect of the pressure on the ion behavior in the sheath is not the same as in the neutral region, however, the substrate bias greatly influences the ion behavior mainly due to ion-neutral elastic collisions within the sheath. Our results also show that the ion motion in crossing the sheath tends to be parallel to the applied magnetic field, and an ion flux with good directionality can be expected by imposing on a negative voltage substrate.

Computational study of ITER-like dissipative divertor plasmas in the collisional limit

A two dimensional (2-D) combined edge plasma/Navier-Stokes neutral transport model is used in a computational study of dissipative divertor plasmas in the collisional limit for neutrals, with ITER-like plasma conditions and scale lengths. The neutral model contains three momentum equations that are coupled to the plasma through ionization, recombination and ion-neutral elastic collisions. The neutral transport coefficients are evaluated including both ion-neutral and neutral-neutral collisions. The structure of fully detached plasma solutions is studied for ITER plasma conditions. These detached solutions are obtained through a combination of impurity radiation, from a simplified impurity model, and neutral flow baffling. Solution sensitivities are elucidated to: (a) baffle location, (b) upstream plasma density and Lyman-α opacity, (c) volume recombination and (d) neutral-neutral collisions.

A simplified fluid model of the metallic plasma and neutral gas interaction in a multicathode spot vacuum arc

A stationary fluid model with spherical symmetry is presented to describe the interaction between metallic plasma ions with neutral gas in the outer region of a multicathode spot vacuum are operated with a neutral background gas. It is found that the neutrals penetrate into the metallic plasma with density values smaller than the initial gas density values, but higher than the metallic ion densities. The neutrals are also strongly heated during the transient expansion stage of the metallic plasma. As a consequence, the ion kinetic energy is gradually delivered to the neutral gas so that the mean free path for ion-neutral elastic collisions is larger than the visible plasma ball radius which surrounds the arc.

Characterization of a partially-ionized, detached, divertor plasma

A combined edge plasma/Navier–Stokes neutral transport model is used to characterize divertor plasma detachment, in the collisional limit for neutrals, on a simplified two-dimensional slab geometry with Alcator-C-MOD-like plasma conditions [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. The neutral model contains three momentum equations which are coupled to the plasma through ionization, recombination, and ion–neutral elastic collisions. The neutral transport coefficients are evaluated including both ion–neutral and neutral–neutral collisions. Detachment is brought about via impurity radiation using a fixed fraction impurity model. The transport model is shown to reproduce all salient features of experimentally observed detachment, such as large drops in ion saturation current and plasma heat flux at the divertor plate. The solutions are also shown to be sensitive to volume recombination. A region of relatively high toroidal neutral Mach number is observed upon detachment. Due to the high neutral densities, 75% of the Lyman α radiation is assumed trapped in the problem. A total edge radiative loss (neutrals and impurities) of approximately 75% of the power crossing the separatrix is needed to observe strong detachment on Alcator-C-MOD-like plasmas using the described model.

Interpretation of neutral distribution through spatial profile of plasma emission

Using a new optical probe technique, plasma emission and its spatial profile are monitored at various gas pressures in electron cyclotron resonance Ar plasma. The emission intensity and its spatial profile strongly depend on the neutral gas pressure. In a spatial plasma emission profile, a hollow profile appears at the chamber center in a specific pressure region. It is suggested that the neutral density depletion at the center is caused by the plasma collisional heating that takes place during ion-neutral charge exchange and ion-neutral elastic collisions.

Simulation of dense recombining divertor plasmas with a Navier–Stokes neutral transport model

A two-dimensional combined edge plasma Navier–Stokes neutral transport model is presented for the simulation of dense recombining divertor plasmas. This model includes ions, electrons, and neutral atoms which undergo Coulomb collisions, electron impact ionization, ion–neutral elastic collisions, three-body and radiative recombination, and neutral–neutral collisions. The advanced fully implicit solution algorithm is briefly described and a variety of results on a model geometry are presented. It is shown that interesting neutral flow patterns can exist and that these flows can convect significant energy. A solution that ignores neutral–neutral collisions is shown to be quantitatively different from one that includes neutral–neutral collisions. Solutions are also shown to be sensitive to the plasma opacity for Lyman α radiation.

Neutral atom and molecular transport in a gaseous divertor

The Monte Carlo code DEGAS with the addition of ion-neutral elastic collisions was used to evaluate the ability of a dynamic gaseous divertor to remove power to the side walls through charge exchange and ion-neutral elastic collisions. The geometry simulated was a louvered ITER divertor. The plasma parameters for each of the four toroidal slices simulated was taken from PLANET, a 2D plasma fluid code. Results show that only in the initial hottest region of the plasma extending into the divertor throat do neutrals carry away significant energy. Even in this region at most 0.1 MW/m 2 is removed to the side walls by neutral atoms for a pressure behind the louvers of 10 mTorr. Given the proposed area of the side walls and 30 cm of hot region extending into the divertor channel, less than 10 MW of the 200–300 MW of input power are removed. This occurs because an individual atom undergoes many charge exchange and ion elastic collisions before returning to the wall. The chance for ionization during the multiple charge exchange and elastic collisions is higher in general than the chance of exiting the plasma region.