Andrey Lipaev

Compressional waves in complex (dusty) plasmas under microgravity conditions

Complex plasmas consist of electrons, ions and charged microparticles, with typical charge-to-mass ratios 1:10−5:10−13. The interest in these systems has grown explosively, because they can be investigated at the kinetic level (the microparticles). However, on Earth the supporting forces (against gravity) are of the same order as the electrostatic interparticle forces—and hence only strongly compressed systems can be investigated. Under microgravity conditions these “body forces” are a factor 102 smaller which allows the experimental investigation of weakly compressed three-dimensional complex plasmas. One way to study these systems is by the controlled excitation of low-frequency compressional waves. The first such experiments, conducted with the PKE-Nefedov laboratory on the International Space Station is reported. The waves were excited by modulating the voltage on the rf electrodes. By varying the modulation frequency the dispersion relation was measured. The results are compared with existing theoret...

Structural properties of 3D complex plasmas: experiments versus simulations

In the paper we discuss recent observations of the three-dimensional plasma crystals created onboard the International Space Station under microgravity conditions. By using the special scanning technique and the image analysis we could retrieve 3D particle positions. We apply the bond-order parameter method to identify the different lattice types of the plasma crystals. It has been shown that observed plasma crystals contain numerous hcp and fcc clusters. Evidence of a small amount of bcc clusters is also reported. Molecular dynamics simulations of crystallization of a simple Yukawa system reproduce remarkably well all peculiarities of the local properties of observed plasma crystals.

Comprehensive experimental study of heartbeat oscillations observed under microgravity conditions in the PK-3 Plus laboratory on board the International Space Station

Heartbeat oscillations in complex plasmas with a broad range of fundamental frequencies are observed and studied. The experiments are performed with monodisperse microparticles of different diameters in argon as well as in neon plasmas. The oscillation frequency increases with increasing rf power and neutral gas pressure. At the lower frequencies, oscillations are strongly nonlinear. The microparticle pulsations, the variation of the electrical discharge parameters and the spatially resolved changes in the plasma glow are proven to be strongly correlated. Heartbeat oscillation dynamics is associated with global confinement modes.

Assessing particle kinematics via template matching algorithms

Template matching algorithms represent a viable tool to locate particles in optical images. A crucial factor of the performance of these methods is the choice of the similarity measure. Recently, it was shown in [Gao and Helgeson, Opt. Express 22 (2014)] that the correlation coefficient (CC) leads to good results. Here, we introduce the mutual information (MI) as a nonlinear similarity measure and compare the performance of the MI and the CC for different noise scenarios. It turns out that the mutual information leads to superior results in the case of signal dependent noise. We propose a novel approach to estimate the velocity of particles which is applicable in imaging scenarios where the particles appear elongated due to their movement. By designing a bank of anisotropic templates supposed to fit the elongation of the particles we are able to reliably estimate their velocity and direction of motion out of a single image.

Heartbeat instability under microgravity conditions observed in the PK-3 plus laboratory

In many experiments that were performed with the PK-3 Plus setup on board of the International Space Station the so called heartbeat instability could be observed. Under microgravity conditions the microparticles in a complex plasma arrange themselves in a vast cloud that spreads nearly all over the available inter-electrode space. In the middle of the plasma chamber a void is often formed. The void is completely free of particles. Under certain conditions the complex plasma becomes unstable and rhythmically pulsates in the radial direction. In given experiments the instability has been observed in a wide parameter range. Measurements where performed with MF particles of different diameters from 6.81mum to 15 mum in argon as well as in neon plasma at different discharge powers. The gas pressure varies between 8Pa and 100Pa. The frequency of the observed oscillation ranges from 0.8Hz to 7Hz. At the lower frequencies oscillations are strongly nonlinear. The oscillation frequency increases linearly with plasma power and with the neutral gas pressure. The correlation of the particle motion and the recorded plasma parameters is discussed.