MPE - Plasmakristall Publications 2000

2000

Nonlinear vertical oscillations of a particle in a sheath of a rf discharge,
A. V. Ivlev, R. Suetterlin, V. Steinberg, M. Zuzic and G. Morfill,
submitted to Physical Review Letters (2000)

Abstract. Vertical oscillations of single particle in the sheath of a low-pressure radio-frequency discharge are experimentally investigated. It is shown that the oscillations become strongly nonlinear and secondary harmonics are generated as the amplitude increases. The theory of anharmonic oscillations provides a good qualitative description of the data and gives estimates for the first two anharmonic terms in an expansion of the sheath potential around the particle equilibrium.

Vertical pairing of identical particles suspended in the plasma sheath,
V. Steinberg, R. Suetterlin, A. V. Ivlev, and G. Morfill,
submitted to Physical Review Letters (2000)

Abstract. It is experimentally shown that vertical pairing of two identical particles suspended in the sheath of a radio-frequency (rf) discharge can occur as the rf power is decreased. It is demonstrated that the pairing is related to a symmetry breaking transition and develops in two stages: (i) continuous transition from a horizontal particle configuration to some vertical displacement, and (ii) discontinuous transition to the final vertical pairing. The transitions are reversible, as can be shown by increasing the power. The pairing can be explained by considering both the Coulomb particle interaction and the wake potential caused by ion flow in the sheath.

Levitation of cylindrical particles in the sheath of an RF plasma,
Annaratone M. Khrapak AG. Ivlev AV. Söllner G. Bryant P. Sütterlin R. Konopka U. Yoshino K. Zuzic M. Thomas HM. Morfill GE.,
submitted to Phys. Rev. E, 2000

Three-dimensional strongly-coupled plasma crystal under gravity conditions,
Zuzic M. Goree J. Ivlev AV. Morfill GE. Thomas HM. Rothermel H. Konopka U. Sütterlin R. Goldbeck DD.,
Phys. Rev. Lett., 2000, 85 (19), 4064-4067, Nov. 2000

Abstract. Experiments were carried out to investigate a three-dimensional plasma crystal. A method of determining the three dimensional (3D) positions of each individual dust particle has been developed. The analyzed crystal volume consisted of about 2*10⁴ particles in 19 horizontal planes. The investigation of local crystal structures and the measurement of the 3D pair correlation function show that "domains" of fcc and hcp lattices coexist in the crystal. Other structures, in particular the theoretically predicted bcc lattice, were not observed.

Detection of stochastic waves in plasma monolayer crystals from video images,
Melandso F. Bjerkmo A., Morfill G. Thomas H. Zuzic M.,
Physics of Plasmas, 7(11), 4368-4378, Nov. 2000

Abstract. The motion of dust particles confined in plasma monolayer crystals is analyzed from video images, under conditions dominated by dust-neutral collisions. In these crystals, dust-neutral collisions will act as a random driving force, exciting phonons with a stochastic nature. The phonons are investigated using standard statistical tools, including both single- and multiparticle correlation functions. Single-particle correlations as obtained from the velocity autocorrelation function yield oscillations in a very narrow frequency band. Similar behaviors have previously been reported for strongly coupled one-component plasmas, and for trapped Brownian particles. Spatial correlations in the crystal lattice are studied from multiparticle correlation functions, suggesting an average wavelength slightly larger than the dimension of the crystal. Throughout the crystal, the dust velocity amplitude and polarization vary significantly, with the main variation in the radial direction out of the crystal center. This suggests the observed wave feature is a standing wave with a stochastic amplitude, dominated by its lowest eigenfrequency.

Influence of charge variation on particle oscillations in the plasma sheath,
A. V. Ivlev, U. Konopka, and G. Morfill,
Physical Review E 62(2), 2730-2744, Aug. 2000

Abstract. The theory of dust particle oscillations in the plasma sheath is presented, taking into account particle charging kinetics and neutral gas friction. Effects of "regular" and stochastic charge variations are considered. It is shown that whilst regular variations generally enhance the damping of horizontally propagating dust lattice waves, they can also cause an instability in the vertical oscillations of single particles. The stochastic charge variations, if sufficiently strong, result in exponential growth of the mean energy of both types of oscillations.

Measurement of the interaction potential of microspheres in the sheath of a rf discharge,
Konopka U, Morfill GE, Ratke L.,
Physical Review Letters. 84(5):891-894, 2000 Jan 31.

Abstract. The interaction potential of two microspheres that are levitated in the sheath region of a radio frequency (rf) argon discharge is studied experimentally by analyzing their trajectories during head-on collisions. It is shown that the interaction parallel to the sheath boundary can be described by a screened Coulomb potential. Thus, values for an effective charge and a screening length can be obtained. The horizontal part of the interaction potential has been determined for several plasma conditions. There is no evidence for an attractive part in the potential within the accuracy of the present measurements and the given plasma conditions. [References: 27]

Low-frequency dispersion properties of plasmas with variable-charge impurities,
Ostrikov KN. Vladimirov SV. Yu MY. Morfill GE.,
Physics of Plasmas 7(2):461-465, 2000 Feb.

Abstract. A theory of low-frequency dust-acoustic waves in low-temperature collisional plasmas containing variable-charge impurities is presented. Physical processes such as dust-charge relaxation, ionization-recombination of the electrons and ions, electron and ion elastic collisions with neutrals and dusts, as well as charging collisions with the dusts, are taken into account. Inclusion of these processes allows a balance of the plasma particles and thus a self-consistent determination of the stationary state of the unperturbed plasma. The generalized dispersion relation describing the propagation and damping of the dust acoustic waves is derived and analyzed. (C) 2000 American Institute of Physics. [S1070-664X(00)02302-8]. [References: 35]

Rigid and differential plasma crystal rotation induced by magnetic fields,
Konopka U. Samsonov D. Ivlev AV. Goree J. Steinberg V. Morfill GE.,
Physical Review E 61(2):1890-1898, 2000 Feb.

Abstract. Observations show that plasma crystals, suspended in the sheath of a radio-frequency discharge, rotate under the influence of a vertical magnetic field. Depending on the discharge conditions, two different cases are observed: a rigid-body rotation (al the particles move with a constant angular velocity) and sheared rotation (the angular velocity of particles has a radial distribution). When the discharge voltage is increased sufficiently, the particles may even reverse their direction of motion. A simple analytical model is used to explain qualitatively the mechanism of the observed particle motion and its dependence on the confining potential and discharge conditions. The model takes into account electrostatic, ion drag, neutral drag, and effective interparticle interaction forces. For the special case of rigid-body rotation, the confining potential is reconstructed. Using data for the radial dependence of particle rotation velocity, the shear stresses are estimated. The critical shear stress at which shear-induced melting occurs is used to roughly estimate the shear elastic modulus of the plasma crystal. The latter is also used to estimate the viscosity contribution due to elasticity in the plasma liquid. Further development is suggested in order to quantitatively implement these ideas. [References: 37]

Acoustic modes in a collisional dusty plasma: Effect of the charge variation,
Ivlev AV. Morfill G.,
Physics of Plasmas 7(4):1094-1102, 2000 Apr.

Abstract. The dispersion relation for the acoustic modes in a collisional dusty plasma is derived, taking into account the particle charge variation and ionization. Short and long wavelengths are considered separately. It is shown that the coupling between the short-wavelength branches is weak, and for sufficiently small dust fraction they can be treated independently, as in the case of a fixed charge. In the long-wavelength limit, the dust-ion acoustic (DIA) and dust acoustic (DA) branches interact "through" the dust charging (DCh) branch, unlike the direct coupling obtained in the absence of charge variations. (C) 2000 American Institute of Physics. [S1070-664X(00)00904-6]. [References: 18]