R. Kompaneets

Potential around a charged dust particle in a collisional sheath

By employing a self-consistent kinetic approach, an analytical expression is derived for the potential of a test charge in a weakly ionized plasma with ion drift. The drift is assumed to be due to an external electric field, with the velocity being mobility-limited and much larger than the thermal velocity of neutrals. The derived expression is proven to be in excellent agreement with the measurements by Konopka et al. [Phys. Rev. Lett. 84, 891 (2000)] performed in the sheath region of a rf discharge.

Design of new binary interaction classes in complex plasmas

A method is proposed to “design” binary interactions between charged “dust” microparticles in weakly ionized plasmas by applying external ac electric fields of various polarizations. The physical idea of the method is that the applied field induces ion flow which distorts the Debye spheres of the compensating plasma charges surrounding microparticles and thus changes the interparticle interactions. To investigate the resulting interactions, we use a self-consistent kinetic description for ions with the Bhatnagar–Gross–Krook ion-neutral collision integral. The analysis shows a variety of interaction classes including positive and negative dipolar interactions, triaxially anisotropic interactions, and isotropic interactions of molecular type (i.e., consisting of a repulsion at short distances and an attraction at larger distances). This technique will make it possible to use three-dimensional complex plasmas as realistic model systems for studying the atomistic dynamics of fluids, including liquid-vapor pha...

Wakes in complex plasmas: A self-consistent kinetic theory

In ground-based experiments with complex (dusty) plasmas, charged microparticles are levitated against gravity by an electric field, which also drives ion flow in the parent gas. Existing analytical approaches to describe the electrostatic interaction between microparticles in such conditions generally ignore the field and ion-neutral collisions, assuming free ion flow with a certain approximation for the ion velocity distribution function (usually a shifted Maxwellian). We provide a comprehensive analysis of our previously proposed self-consistent kinetic theory including the field, ion-neutral collisions, and the corresponding ion velocity distribution. We focus on various limiting cases and demonstrate how the interplay of these factors results in different forms of the shielding potential.

Surface plasmon polaritons in a semi-bounded degenerate plasma: Role of spatial dispersion and collisions

Surface plasmon polaritons (SPPs) in a semi-bounded degenerate plasma (e.g., a metal) are studied using the quasiclassical mean-field kinetic model, taking into account the spatial dispersion of the plasma (due to quantum degeneracy of electrons) and electron-ion (electron-lattice, for metals) collisions. SPP dispersion and damping are obtained in both retarded (ω/kz∼c) and non-retarded (ω/kz≪c) regions, as well as in between. It is shown that the plasma spatial dispersion significantly affects the properties of SPPs, especially at short wavelengths (less than the collisionless skin depth, λ ≲ c/ωpe). Namely, the collisionless (Landau) damping of SPPs (due to spatial dispersion) is comparable to the purely collisional (Ohmic) damping (due to electron-lattice collisions) in a wide range of SPP wavelengths, e.g., from λ∼20 nm to λ∼0.8 nm for SPP in gold at T = 293 K and from λ∼400 nm to λ∼0.7 nm for SPPs in gold at T = 100 K. The spatial dispersion is also shown to affect, in a qualitative way, the dispersi...

Dust clusters with non-Hamiltonian particle dynamics

The modes of clusters formed by two or three charged dust particles in a plasma are analyzed. The non-Hamiltonian dynamics of the particles is taken into account, which includes (i) nonreciprocal interaction forces due to wake effects and (ii) spatial variations of the particle charge and shielding parameters. It is shown that these effects can trigger an oscillatory instability under realistic experimental conditions. An experiment is suggested to observe this instability.

Dust-lattice waves: Role of charge variations and anisotropy of dust-dust interaction

Dust-lattice waves are studied in the framework of the one-dimensional particle string model. The dust-dust interaction potential is assumed to have an arbitrary dependence on the vertical and horizontal coordinates, which allows to take into account the wake field effects. Both the vertical and horizontal charge variations are also included into the model. The model yields the coupling between the vertical and horizontal (longitudinal) modes: the coupling coefficient is the sum of six terms, each caused by a different physical mechanism. It is shown that the coupling can trigger the resonance oscillatory instability, which has been already observed in experiments. It is also shown that a nonoscillatory instability can appear at small wave numbers due to the coupling.

Wakes in inhomogeneous plasmas

The Debye shielding of a charge immersed in a flowing plasma is an old classic problem. It has been given renewed attention in the last two decades in view of experiments with complex plasmas, where charged dust particles are often levitated in a region with strong ion flow. Efforts to describe the shielding of the dust particles in such conditions have been focused on the homogeneous plasma approximation, which ignores the substantial inhomogeneity of the levitation region. We address the role of the plasma inhomogeneity by rigorously calculating the point charge potential in the collisionless Bohm sheath. We demonstrate that the inhomogeneity can dramatically modify the wake, making it nonoscillatory and weaker.

Interparticle attraction in 2D complex plasmas

Complex (dusty) plasmas allow experimental studies of various physical processes occurring in classical liquids and solids by directly observing individual microparticles. A major problem is that the interaction between microparticles is generally not molecularlike. In this Letter, we propose how to achieve a molecularlike interaction potential in laboratory 2D complex plasmas. We argue that this principal aim can be achieved by using relatively small microparticles and properly adjusting discharge parameters. If experimentally confirmed, this will make it possible to employ complex plasmas as a model system with an interaction potential resembling that of conventional liquids.

Stability and size of particle pairs in complex plasmas

Particle pairing in a complex plasma was experimentally studied with the emphasis on pair spatial extent and stability. Micron-size particles were suspended in the (pre)sheath area above the lower electrode in a capacitively coupled radio-frequency discharge in argon. They formed vertical pairs due to the ion wakes created by the flow of ions past particles. We discuss the confinement mechanism for the lower particle, resulting from a combination of the wake field and the field of non-uniform sheath. A model of particle pairs is proposed, which provides good description for the dependence of pair size and stability on experimental parameters.

Mode coupling in two-dimensional plasma crystals: Role of the wake model

The theory of mode-coupling instability in 2D plasma crystal is combined with a self-consistent model of plasma wakes. The wake model is based on the solution of a kinetic equation for ions, providing realistic representation of their kinetics for the sheath environment. Furthermore, the self-consistent approach allows us to express the interparticle interaction via experimentally measurable parameters. It is suggested that distinct features of dispersion relations predicted by different wake models can be identified experimentally.