Mierk Schwabe

Fluid-solid phase transitions in three-dimensional complex plasmas under microgravity conditions

Phase behavior of large three-dimensional (3D) complex plasma systems under microgravity conditions onboard the International Space Station is investigated. The neutral gas pressure is used as a control parameter to trigger phase changes. Detailed analysis of structural properties and evaluation of three different melting-freezing indicators reveal that complex plasmas can exhibit melting by increasing the gas pressure. Theoretical estimates of complex plasma parameters allow us to identify main factors responsible for the observed behavior. The location of phase states of the investigated systems on a relevant equilibrium phase diagram is estimated. Important differences between the melting process of 3D complex plasmas under microgravity conditions and that of flat 2D complex plasma crystals in ground based experiments are discussed.

Auto-oscillations in complex plasmas

Experimental results on an auto-oscillatory pattern observed in a complex plasma are presented. The experiments are performed with an argon plasma, which is produced under microgravity conditions using a capacitively coupled rf discharge at low power and gas pressure. The observed intense wave activity in the complex plasma cloud correlates well with the low-frequency modulation of the discharge voltage and current and is initiated by periodic void contractions. Particle migrations forced by the waves are of long-range repulsive and attractive character.

Simulating the dynamics of complex plasmas

Complex plasmas are low-temperature plasmas that contain micrometer-size particles in addition to the neutral gas particles and the ions and electrons that make up the plasma. The microparticles interact strongly and display a wealth of collective effects. Here we report on linked numerical simulations that reproduce many of the experimental results of complex plasmas. We model a capacitively coupled plasma with a fluid code written for the commercial package comsol. The output of this model is used to calculate forces on microparticles. The microparticles are modeled using the molecular dynamics package lammps, which we extended to include the forces from the plasma. Using this method, we are able to reproduce void formation, the separation of particles of different sizes into layers, lane formation, vortex formation, and other effects.

Direct measurement of the speed of sound in a complex plasma under microgravity conditions

We present a direct measurement of the speed of sound in a three-dimensional complex plasma —a room-temperature plasma that contains micrometer-sized particles as fourth component. In order to obtain an undisturbed system, the setup was placed under microgravity conditions on board the International Space Station. The speed of sound was measured with the help of Mach cones excited by a supersonic probe particle moving through the extended particle cloud at Mach numbers M3. We use the Mach cone relation to infer the particle charge and compare with that predicted by standard theories. In addition, we compare our results with a numerical simulation. In both experiment and simulation, we observe a double Mach cone structure.

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.

Mach cones in a three-dimensional complex plasma

A three-dimensional hydrodynamic model has been applied to study the Mach cones in a three-dimensional complex plasma. Numerical results for the velocity distribution of dust particles showed the presence of compressional-wave Mach cones. The compressional Mach cones were excited when subjected to supersonic excitations. It was found that multi-cone structures became a single cone when the discharge pressure was increased. The experiment of Mach cones in a three-dimensional complex plasma under microgravity conditions on board the International Space Station was also reported. A single compressional-wave Mach cone in a three-dimensional complex plasma was observed and could also be obtained from our hydrodynamic model.

Complex Plasmas in Strong Magnetic Field Environments

To complete our picture of general complex plasmas, experiments under the influence of high magnetic fields have been carried out in a radio frequency (rf) discharge with and without embedded micro‐particles. The influence of the strong magnetic field on the plasma with respect to its homogeneity as well as on the isotropy of the particle interaction was studied. We observed a filamentation of the plasma at low pressures and low powers even in the absence of particles. The plasma filaments moved around — traced by embedded particles — and suddenly changed to a crystalline like arrangement.

Convection in a dusty radio-frequency plasma under the influence of a thermal gradient

Gas convection is a common phenomenon in systems under atmospheric pressure conditions with a temperature gradient. Under low pressure conditions, convection can be induced by creep flows along a surface. This has important applications in fluid physics as well as in low pressure plasmas in which a temperature gradient is present. Here, we visualize the gas dynamics in a system with and without a plasma using microparticles as tracers. Two types of gas convection have been identified from the particle motion, i.e. free (Rayleigh–Benard) convection at high gas pressures, and convection induced by thermal creep at low pressures. The gas flow profile detected using the microparticles is compared with that obtained in a simulation using the direct simulation Monte Carlo method.

Autowaves in a dc complex plasma confined behind a de Laval nozzle

Experiments to explore stability conditions and topology of a dense microparticle cloud supported against gravity by a gas flow were carried out. By using a nozzle-shaped glass insert within the glass tube of a dc discharge plasma chamber a weakly ionized gas flow through a de Laval nozzle was produced. The experiments were performed using neon gas at a pressure of 100?Pa and melamine-formaldehyde particles with a diameter of 3.43??m. The capturing and stable global confining of the particles behind the nozzle in the plasma were demonstrated. The particles inside the cloud behaved as a single convection cell inhomogeneously structured along the nozzle axis in a tube-like manner. The pulsed acceleration localized in the very head of the cloud mediated by collective plasma-particle interactions and the resulting wave pattern were studied in detail.

Measurement of the speed of sound by observation of the Mach cones in a complex plasma under microgravity conditions

We report the first observation of the Mach cones excited by a larger microparticle (projectile) moving through a cloud of smaller microparticles (dust) in a complex plasma with neon as a buffer gas under microgravity conditions. A collective motion of the dust particles occurs as propagation of the contact discontinuity. The corresponding speed of sound was measured by a special method of the Mach cone visualization. The measurement results are incompatible with the theory of ion acoustic waves. The estimate for the pressure in a strongly coupled Coulomb system and a scaling law for the complex plasma make it possible to derive an evaluation for the speed of sound, which is in a reasonable agreement with the experiments in complex plasmas.