Grains In Plasma Research Articles

Interacting dust grains in complex plasmas: Ion wake formation and the electric potential

Dust grains have been used as minimally invasive probes to determine plasma parameters including the plasma density, temperature, and electric field in a plasma discharge. However, the dust grains in a plasma generate local potential disturbances due to the collection of charge and the subsequent electrostatic interactions between the dust and charged plasma particles. Dust grains in close proximity to one another exhibit interesting non-reciprocal interactions and self-organize into structures such as one-dimensional filamentary chains, two-dimensional “zigzags,” and three-dimensional helices, among others. The formation of these structures suggests that although the dust grains may be less invasive than traditional plasma probes, the disturbance to the local plasma environment introduced by dust grains is non-trivial. Commonly used analytic forms of the electric potential describing complex plasmas have failed to resolve the near-dust region, and as a result are insufficient to provide insight about the formation of complex dust structures. Here, we use an N-body simulation to compute the electric potential from ion densities near various dust grain configurations. We provide an alternative description to the standard analytic model for the electric potential of dust and ion wakes based on a Gaussian shaped cloud of ions. The electric potential obtained from simulations is used to identify minimum energy configurations for two and three dust grains. It is further demonstrated that the minimum potential region identified for N dust grains and their associated ion wakes does not predict the minimum-energy configuration of N + 1 dust grains.

On the force exerted on a non-spherical asymmetric dust grain from homogeneous, stationary, isotropic, non-magnetized plasma

Whereas the conventional wisdom suggests that the force between non-magnetized homogeneous, stationary, isotropic plasma, and the dust grain is only possible for the case of relative plasma–grain velocity, it is shown that stationary non-spherical asymmetric dust grain immersed in stationary, non-magnetized, isotropic plasma can experience a force caused by the grain–plasma interactions. The component of the force due to scattering of plasma particles in the limit of infinite Debye length is considered analytically. Both the particle scattering and absorption force components are modeled numerically in the limits of infinite and finite Debye length using a newly developed 2D3V Aspherical Particle-in-Cell code. The code simulates interactions of dust grain of selected non-spherical asymmetric shape with plasmas using dust shape conforming coordinates. The simulations confirm the existence of the force on non-spherical asymmetric grain in stationary non-magnetized plasma and show that the plasma screening effects can lead to reversal of the force direction.

Effect of external central force on charged grains in plasma

Molecular dynamics simulation is used to investigate the evolution of an initially homogeneous distribution of charged grains with Maxwellian velocities in the presence of effective grain-grain interaction and external central force. The grain trajectories are tracked until the system reaches a quasistationary asymptotic state. It is found that, depending on the initial parameters, the final state can have several interesting configurations, including that of a fractured lattice, a collection of grain clusters, as well as a single crystal-like sphere. Moreover, in the evolution, the time needed by each grain or grain cluster to reach its final location can be very different. The results may be useful as a guide for studies on self organization of chemical and biological systems.

Effect of external fields on pattern formation of charged grains in plasma

AbstractTwo‐dimensional cluster and pattern formation by charged grains in plasma under external harmonic force and transverse magnetic field is investigated using molecular dynamics simulation. It is found that, as a result of the Yukawa‐like grain‐grain interaction and the external fields, the asymptotic quasistationary state of an initially homogeneous and Maxwellian velocity distributed grain system can be a disk cluster with the grain trajectories having flowery zonal patterns.

On the Role of Electron Quantum Tunneling in Charging of Dust Grains in Complex Plasma

The role of the quantum tunneling effect in the electron accretion current onto a negatively charged grain immersed in isotropic plasma is analyzed, within the quasiclassic approximation, for different plasma electron distribution functions, plasma parameters, and grain sizes. It is shown that the contribution of the quantum tunneling into the grain charging is small (negligible) for relatively large (micron-sized) dust grains in plasmas with electron temperatures of the order of a few eV, but becomes important for nano-sized dust grains (tens to hundreds nm in diameter) in cold and ultracold plasmas (electron temperatures ~ tens to hundreds of Kelvin degrees), especially in plasmas with depleted high-energy "tails" in the electron energy distribution.

Screening of Absorbing Grains in Plasma with a Flow

The electrostatic shielding of a charged absorbing object (dust grain) in a flowing collisionless plasma is investigated by usingthe linearized kinetic equation for plasma ions with a point-sink term accounting for the ion absorption on the object. The effect ofabsorption on the attractive part of the dust grain potential distribution in plasma is investigated. For subthermal ion flows, the attractive part of the grain potential in the direction perpendicular to the ion flow can be significantly reduced or completely destroyed, depending on the absorption rate (proportional to the surface area of a grain). For superthermal ion flows, however, the effect of absorption on the grain attraction in the direction perpendicular to the ion flow is shown to be exponentially weak. It is thus argued that, in the limit of superthermal ion flow, the effect of absorption on the grain potential distribution in plasma can be safely ignored for typical grain sizes relevant to complex plasmas.

Long-time evolution of charged grains in plasma under harmonic external force and after being withdrawn

Evolution of the charged grains in a two-dimensional dusty plasma under a spatially harmonic external force, in particular, their long-time behaviors after the force has been withdrawn, is studied by using molecular dynamics simulation. Under an external force and a grain–grain interaction force, initially homogeneously distributed grains can reach a quasi-stationary state in the form of a disk crystal. After the external force is withdrawn, the disk moves initially with its size and shape nearly unchanged until it rapidly stops moving, and eventually the disk grain rotates like a vortex. The time needed to reach the final state increases with the strength of the initial external force increasing.

A machine learning based Bayesian optimization solution to non-linear responses in dusty plasmas

Nonlinear frequency response analysis is a widely used method for determining system dynamics in the presence of nonlinearities. In dusty plasmas, the plasma–grain interaction (e.g. grain charging fluctuations) can be characterized by a single-particle non-linear response analysis, while grain–grain non-linear interactions can be determined by a multi-particle non-linear response analysis. Here a machine learning-based method to determine the equation of motion in the non-linear response analysis for dust particles in plasmas is presented. Searching the parameter space in a Bayesian manner allows an efficient optimization of the parameters needed to match simulated non-linear response curves to experimentally measured non-linear response curves.

Excitation of Trivelpiece–Gould mode in a magnetized plasma having dust by relativistic electron beam?

Excitation of various modes by a beam propagation through a partially or fully ionized plasma has been a subject of great interest. Trivelpiece–Gould (TG) mode are the electrostatic waves having frequency in between the ion plasma frequency and electron cyclotron frequency. In the investigations, scientists have included dust grains in the plasma. We have noticed that in a theoretical investigation, the speed of electron beam propagating through a dusty plasma has been taken relativistic. Since the inclusion of dust grains in plasma and consideration of relativistic speed of electron beam could be a challenging task for theoretical treatment, we have looked into the details of such work and have found that for such situation, the dispersion relation is so complicated that its analysis is not feasible. We have found that earlier work has several discrepancies.

Electron emission from electrically isolated spheres

The ever-decreasing size of electron sources, many of which are now on the nanometer scale, has prompted several recent theoretical studies of electron emission from highly curved surfaces. These studies have naturally focused on emission from grounded tips and spheres, as is appropriate for most nanoelectronic devices, and have found significant corrections from the planar theories. However numerous examples of nongrounded nanoscale electron emitters exist in the form of hot and/or charged dust, droplets and aerosols in a variety of industrial, laboratory, and natural environments. In this paper, the field-induced and thermal emission currents from electrically isolated spherical surfaces are considered by using both approximate series expansions and numerical calculations of the barrier form correction factors within the framework of the Murphy–Good emission theory. The resulting expressions are used to find the floating potentials of dust grains in plasmas and significant corrections to the planar theory, which continues to be in widespread usage, are found.

On the formation process of charged clusters

The clustering process of charged grains often resembles a formation stage of colloidal and spongy matter, as well as some astrophysical objects. In this paper, molecular dynamics simulation is used to simulate the formation process of clusters of massive charged grains in plasmas. It is found that, from an initially uniform distribution of grains with Maxwellian velocity distribution, a statistically stationary system of clusters, each with different dynamic as well as thermodynamic characteristics, can form. The dependence of the asymptotic, of the final, state of the cluster system on the initial temperature and density of the grains is discussed.

Interaction of energetic electrons with dust whistler-mode waves in magnetospheric dusty plasmas

In this Letter, a new conceptual approach has been presented to investigate the interaction of energetic electrons with dust whistler-mode waves in magnetospheric dusty (complex) plasmas. Dust whistler-mode waves generated in the presence of charged dust grains in the magnetized dusty plasma, can scatter the launched electrons into the loss-cone leading to precipitation into the upper atmosphere which is an important loss process in the radiation belts and provides a major source of energy for the diffuse and pulsating aurora. To study the scattered electrons and chaotic regions, a Hamiltonian model of the electron-dust wave interaction has been employed in the magnetospheric plasma by considering the launched electron beam self-fields. Numerical simulations indicate that an electron beam interacting with the whistler-mode wave can easily trigger chaos in the dust-free plasma, while in the presence of dust charged grains in the plasma, the chaotic regions are quenched to some extent in the magnetosphere. Consequently, the rate of scattered electrons into the loss-cone reduces for the regions that the dust grains are present.

Note on Scattering of Electromagnetic Waves by Nonlinearly Screened Dust Grains in Plasmas

AbstractAlthough most experiments in complex plasmas correspond to non‐linear grain screening, the theoretical models of non‐linear screening have not been checked experimentally. Scattering of electromagnetic waves on a single nonlinearly screened dust grain in plasmas can serve as one of effective methods for investigation of non‐linear screening. In present paper we have developed a theory for electromagnetic wave scattering that includes both the effect of coherency of scattering for long wave range and the loss of coherency with a decrease of the wave length. It is valid for arbitrary non‐linearity in screening and demonstrates that the loss of coherency occurs for wave length less then the characteristic screening length for non‐linear screening being usually substantially larger than the linear Debye screening length. The effect is illustrated numerically using the most popular model of non‐linear screening. The dependence of the cross‐section of scattering on parameter of non‐linearity in screening is calculated numerically. The loss of coherency in scattering by a single grain can serve as effective diagnostic for checking the theoretical models of non‐linear screening. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Long-range interaction between dust grains in plasma

The nature of long-range interactions between dust grains in plasma is discussed. The dust grain interaction potential within a cell model of dusty plasma is introduced. The attractive part of intergrain potential is described by multipole interaction between two electro-neutral cells. This allowed us to draw an analogy with molecular liquids where the attraction between molecules is determined by dispersion forces. The main ideas of the fluctuation theory for electrostatic field in a cell model are formulated, and the dominating contribution to the attractive part of intergrain potential is obtained.

Spontaneous formation of nonspherical water ice grains in a plasma environment

AbstractSaturn's rings, terrestrial polar mesospheric clouds, and astrophysical molecular clouds are all dusty plasma environments where tiny grains of water ice are an important constituent. Existing models typically assume that the ice grains are spherical and then invoke various arguments about the normal distribution or the power law dependence of grain number density on grain radius. Using a laboratory plasma in which water ice grains spontaneously form, we investigated the validity of the traditional assumption that these grains are spherical. We found that in certain cases at low ambient pressures, water ice grains in the laboratory dusty plasma are not spherical but instead are highly elongated, i.e., ellipsoidal. Preliminary analysis suggests that electrical forces associated with the dusty plasma environment are responsible for the nonspherical shape.

Influence of external perturbations on the interaction between grains in plasma

An experimental study of dust grain dynamics in plasma was carried out for cluster systems relaxing to their equilibrium state after the action of a laser beam. The influence of this external perturbation on the potential between interacting dust grains was analyzed. We found that the spatial asymptotes of the pair potential for perturbed and equilibrium dust systems differ. The possible reasons for this phenomenon are discussed.

On the theory of dynamics of dust grain in plasma

The dynamics of rotationally symmetric dust grains in plasma embedded in a magnetic field are of concern. The general expressions for forces and torques acting on dust are found. It is shown that dust spinning is determined by torques related to both the Lorentz force (dominant for relatively small grains) and the gyro-motion of plasma particles impinging the grain (which prevails for large grains). The stability of grain spinning is analyzed and it is shown that, for some cases (e.g., oblate spheroid), there is no stable dynamic equilibrium of grain spinning.

Fractal dust grains in plasma

Fractal dust grains of different shapes are observed in a radially confined magnetized radio frequency plasma. The fractal dimensions of the dust structures in two-dimensional (2D) horizontal dust layers are calculated, and their evolution in the dust growth process is investigated. It is found that as the dust grains grow the fractal dimension of the dust structure decreases. In addition, the fractal dimension of the center region is larger than that of the entire region in the 2D dust layer. In the initial growth stage, the small dust particulates at a high number density in a 2D layer tend to fill space as a normal surface with fractal dimension D = 2. The mechanism of the formation of fractal dust grains is discussed.

Self-confinement of finite dust clusters in isotropic plasmas

Finite two-dimensional dust clusters are systems of a small number of charged grains. The self-confinement of dust clusters in isotropic plasmas is studied using the particle-in-cell method. The energetically favorable configurations of grains in plasma are found that are due to the kinetic effects of plasma ions and electrons. The self-confinement phenomenon is attributed to the change in the plasma composition within a dust cluster resulting in grain attraction mediated by plasma ions. This is a self-consistent state of a dust cluster in which grain's repulsion is compensated by the reduced charge and floating potential on grains, overlapped ion clouds, and depleted electrons within a cluster. The common potential well is formed trapping dust clusters in the confined state. These results provide both valuable insights and a different perspective to the classical view on the formation of boundary-free dust clusters in isotropic plasmas.

Ionization enhanced ion collection by a small floating grain in plasmas

It is demonstrated that the ionization events in the vicinity of a small floating grain can increase the ion flux to its surface. In this respect, the effect of electron impact ionization is fully analogous to that of the ion-neutral resonant charge exchange collisions. Both processes create slow ion which cannot overcome electrical attraction of the grain and eventually fall onto its surface. The relative importance of ionization and ion-neutral collisions is roughly given by the ratio of the corresponding frequencies. We have evaluated this ratio for neon and argon plasmas to demonstrate that ionization enhanced ion collection can indeed be an important factor affecting grain charging in realistic experimental conditions.