U. de Angelis

Ion plasma waves in dusty plasmas: Halley's comet

We investigate ion waves in a plasma in the presence of massive charged dust particles, a common space-plasma component now known to exist also in planetary rings and comets. We derive an equation describing low-frequency electrostatic perturbations on a non-homogeneous background, where the inhomogeneity is due to a distribution of charged grains, each surrounded by an equilibrium statistical distribution of plasma particles. This model is then applied to propose an interpretation of some recent data from the Vega and Giotto space probes to Halleys comet the increase of the low-frequency electrostatic noise (ion-acoustic waves) in the region of increased dust.

On the role of dust in the summer mesopause

Abstract We propose that dust formed at the cool summer mesopause may have optical properties very different from that measured for bulk material of ice. The smallness of the dust and possible surface impurities may lead to high photoelectric yields and low workfunctions. For such reasons the dust in the summer mesopause may, at least occasionally, be charged to substantial positive surface potentials while pure ice, with its high photoelectric workfunction, would be charged to low and negative potentials by collisions with plasma particles. The presence of ‘dressed’ dust particles, with surface potentials of some volts, can lead to enhanced radar backscatter. We also suggest that the apparent reductions in electron density (‘bite-out’), which have been observed in the radar backscatter region, can be caused by the inability of an electrostatic probe to deflect the massive dust particles. The dust density which is required by our model to explain radar backscatter and electron bite-outs is of the order of 10 cm −3 for dust of radius above 5 × 10 −6 cm.

Electromagnetic wave scattering in dusty plasmas

The cross section for transition scattering of electromagnetic waves on charged dust particles in a plasma is calculated, extending the results of a previous paper [J. Plasma Phys. 42, 429 (1989)] where the case of longitudinal waves has been considered. For the case of nonlinear screening of the charged dust by the plasma particles (i.e., ‖eφ0/Te‖ ≫ 1, where φ0 is the dust grain surface potential and Te is the electron plasma temperature), numerical and analytical results are presented, showing a significant enhancement, proportional to the square of the grain surface charge, in the cross section with respect to scattering by free electrons. The effect is independent of the sign of the charge for wavelengths larger than the Debye length.

Waves in dusty plasmas

Plasma density fluctuations, due to the presence of massive, charged dust particles, can alter the usual dispersion properties of waves as shown, in a particular case, by de Angelis et al. [1]. Here a general dispersion relation is derived, as an average over a random distribution of dust particles, and the possibility of complex solutions (wave damping) is pointed out.

Kinetic theory of dusty plasmas. I. General approach

The kinetic theory of dusty plasmas is formulated introducing the dust charge as an independent variable. The Bogoliubov–Klimontovich approach is generalized for the case where the discreetness of the dust grain distribution is described rigorously, while the electrons and ions are described by continuous kinetic equations which take into account their absorption on the highly charged dust grains. The theory is valid for dust densities larger than the critical value where the binary plasma particle collisions can be neglected with respect to the collisions with dust particles. This condition is fulfilled in most dust-plasma experiments and often in space plasmas. The discreetness in the dust distribution leads to both dust fluctuations and plasma particle fluctuations, the latter induced by the dust fluctuations. The dust charge fluctuations alter the interaction appreciably, leading to effective dust charges in interactions which depend on distance and deviate substantially from the equilibrium dust char...

Dispersion properties of dusty plasmas

The presence of charged dust particles in a plasma can change its dispersion properties. From the linear response of an equilibrium dusty plasma to the propagation of small perturbations, we find an average dielectric function є(ω, k) for the system in the case when the grain space distribution is random, and ensemble averages are taken over the random variables. For the case of high-frequency plasma waves, when e ≈ 1-ω2p/ω2 for the unperturbed case (cold plasma), we find that є becomes complex in the presence of the dust, leading to possible damping in a domain where Landau damping is usually negligible (ω » kVT).

Kinetic theory of dusty plasmas II. Dust–plasma particle collision integrals

The recent results of the kinetic theory of dusty plasmas [V. N. Tsytovich and U. de Angelis, Phys. Plasmas 6, 1093 (1999)] are analyzed to assess the importance of new qualitative effects: Inelasticity of dust–plasma particle collisions, deviations from Debye screening and from the dust equilibrium charge in the interactions, the existence of a friction force on plasma particles, and their diffusion in energy. These collective effects depend on the plasma dielectric constant modified by dust charging effects as well as on two new types of dusty plasma responses, related to the fluctuations of plasma particle currents to dust particles. Analytic expressions for the responses are derived and numerical results are given to assess their importance and dependence on dusty plasma parameters. The rate of energy and momentum transfer from plasma particles to dust particles is calculated analytically and numerical results are presented.

Hypervelocity dust impacts in FTU scrape-off layer

The first evidence of dust-impact ionization processes in the scrape-off layer of the Frascati tokamak upgrade (FTU) was reported in Nucl. Fusion 47 L5 (2007). In this work an extended data analysis (both of electrostatic probe signals and probe surface analysis) is presented, using results of empirical studies of hypervelocity impacts. Within the accuracy and limits of application of such results to tokamak plasma environment, the data are shown to be consistent with occurrence of impacts by micrometre size iron particles at velocities of the order of 10 km s −1 .

Transition scattering of waves on charged dust particles in a plasma

We consider the transition scattering of waves on charged dust particles in a plasma. The scattered power is first derived from perturbation theory, for the case of weakly charged grains, where linear screening by the plasma particles can be assumed. In the case of a highly charged grain we first determine the nonlinear screening by the plasma particles, in a model where the grain can be treated as a small spherical probe at floating potential, and then solve the nonlinear transition scattering due to the electron shielding cloud. One of the results of our calculation is that the radiation pressure on the grains can be greatly enhanced with respect to the case of usual Thomson scattering on free particles, and some possible astrophysical consequences are discussed.

Dusty plasmas in fusion devices

Recent results from the theory of dusty plasmas are considered in the parameter regime close to the walls, divertor of magnetic confinement fusion devices, in an attempt to establish the effects that can be important to plasma operation and performance. The dust charge and dust-dust interaction potential are considered and it is shown that long-range attraction of negatively charged dust particles is possible. The ion-dust scattering and drag force are calculated in the linear and nonlinear regimes, and it is shown that scattering through waves can enhance the drag force on dust particles by orders of magnitude, which can be important for dust transport and removal. The effects of charged dust on wave propagation and scattering of radiation are discussed and it is shown that an instability due to dust charge fluctuations, leading to stochastic heating of dust particles, can effectively grow for submicron dust particles.