Single exposure 3D imaging of dusty plasma clusters

We have worked out the details of a single camera, single exposure method to perform three-dimensional imaging of a finite particle cluster. The procedure is based on the plenoptic imaging principle and utilizes a commercial Lytro light field still camera. We demonstrate the capabilities of our technique on a single layer particle cluster in a dusty plasma, where the camera is aligned inclined at a small angle to the particle layer. The reconstruction of the third coordinate (depth) is found to be accurate and even shadowing particles can be identified.Comment: 6 pages, 7 figures. Submitted to Rev. Sci. Inst

Bistable solutions for the electron energy distribution function in electron swarms in xenon: a comparison between the results of rst-principles particle simulations and conventional Boltzmann equation analysis

At low reduced electric elds the electron energy distribution function in heavy noble gases can take two distinct shapes. This ‘bistability effect’—in which electron–electron (Coulomb) collisions play an essential role—is analyzed here for Xe with a Boltzmann equation approach and with a rst principles particle simulation method. The solution of the Boltzmann equation adopts the usual approximations of (i) searching for the distribution function in the form of two terms (‘two-term approximation’), (ii) neglecting the Coulomb part of the collision integral for the anisotropic part of the distribution function, (iii) treating Coulomb collisions as binary events, and (iv) truncating the range of the electron–electron interaction beyond a characteristic distance. The particle-based simulation method avoids these approximations: the many-body interactions within the electron gas with a true (un-truncated) Coulomb potential are described by a molecular dynamics algorithm, while the collisions between electrons and the background gas atoms are treated with Monte Carlo simulation. We nd a good general agreement between the results of the two techniques, which con rms, to a certain extent, the approximations used in the solution of the Boltzmann equation. The differences observed between the results are believed to originate from these approximations and from the presence of statistical noise in the particle simulations

On the metastability of the hexatic phase during the melting of two-dimensional charged particle solids

For two-dimensional many-particle systems first-order, second-order, single step continuous, as well as two-step continuous (KTHNY-like) melting transitions have been found in previous studies. Recent computer simulations, using particle numbers in the $\geq 10^5$ range, as well as a few experimental studies, tend to support the two-step scenario, where the solid and liquid phases are separated by a third, so called hexatic phase. We have performed molecular dynamics simulations on Yukawa (Debye-H\"uckel) systems at conditions earlier predicted to belong to the hexatic phase. Our simulation studies on the time needed for the equilibration of the systems conclude that the hexatic phase is metastable and disappears in the limit of long times. We also show that simply increasing the particle number in particle simulations does not necessarily result in more accurate conclusions regarding the existence of the hexatic phase. The increase of the system size has to be accompanied with the increase of the simulation time to ensure properly thermalized conditions.Comment: 6 pages, 4 figure

Erősen csatolt sokrészecske-rendszerek szimulációja = Simulation of strongly coupled many-body systems

A pályázat keretében végzett munkánk a gázkisülés- és plazmafizika egyes rendszereinek, illetve alapjelenségeinek megértését célozta, nagyrészt a numerikus modellezés és szimulációk által nyújtott lehetőségek kihasználásával. A modellezés mellett kísérleteket is végeztünk eredményeink ellenőrzésére, valamint számítási eredményeinket együttműködő csoportok elméleti, illetve kísérleti eredményeivel hasonlítottuk össze. Az alacsonyhőmérsékletű egyenáramú gázkisülések területén vizsgáltuk az elektronok kinetikáját egyszerű gázokban és komplex gázelegyekben (pl. levegő), elemeztük a gázkisülések modelljeinek korlátait, hiányosságait, foglalkoztunk különleges, pl. gyors atomok általi gerjesztési folyamatokkal. Rádiófrekvenciás gerjesztésű gázkisülésekben vizsgáltuk az elektronok fűtési folyamatait, a plazma tér- és időbeli fejlődésének sajátosságait, valamint annak lehetőségeit, hogy az elektródákra érkező ionok fluxusa és energiája miként állítható be egymástól függetlenül (ez a plazmaalapú megmunkálási folyamatok szempontjából elsődleges fontosságú). A komplex plazmák (erősen csatolt sokrészecske-rendszerek) kutatása területén Yukawa és Coulomb kölcsönhatási poteciállal jellemezhető rendszerek leírásával foglalkoztunk. Kétdimenziós rendszerekre vizsgáltuk a szilárd-folyadék fázisátalakulást, a transzportjellemzőket és a kollektív gerjesztéseket. | In the framework of the project our aim had been to understand the characteristics of systems in gas discharge and plasma physics, by means of numerical modeling and simulations. In order to confirm our modeling results we have also carried out experimental studies, or have cross-checked our results with experimental and/or theoretical results of collaborating partners. In our studies of low pressure steady state gas discharges we have examined the electron kinetics in simple gases as well as in complex gas mixtures (like air), we have clarified the limitations of gas discharge models, and investigated special (e.g. fast atom impact) excitation processes in gas discharges. We have studied the electron heating mechanisms in radiofrequency-excited discharges, the peculiarities of spatial and temporal development of discharge characteristics and the possibilities of the independent control of the energy and flux of ions reaching the surface of the electrodes. These properties play a key role in the plasma-suface interaction in processing plasmas. In the field of strongly coupled plasmas we have investigated many-particle systems characterized by Coulomb and Yukawa interaction potentials. We have studied the solid-liquid phase transition, transport processes, as well as collective excitations in two-dimensional systems

Plazmafizikai sokrészecske-rendszerek modellezése = Modeling of many particle systems in plasma physics

A pályázat keretében végzett munkánk a gázkisülés- és plazmafizika egyes rendszereinek, illetve alapjelenségeinek megértését célozta, nagyrészt a numerikus modellezés és szimulációk által nyújtott lehetőségek kihasználásával. A modellezés mellett kísérleteket is végeztünk eredményeink ellenőrzésére, valamint számítási eredményeinket együttműködő csoportok elméleti, illetve kísérleti eredményeivel hasonlítottuk össze. Az alacsonyhőmérsékletű egyenáramú gázkisülések területén vizsgáltuk az elektronok kinetikáját egyszerű gázokban és komplex gázelegyekben (pl. levegő), elemeztük a gázkisülések modelljeinek korlátait, hiányosságait, foglalkoztunk különleges, pl. gyors atomok általi gerjesztési folyamatokkal. Rádiófrekvenciás gerjesztésű gázkisülésekben vizsgáltuk az elektronok fűtési folyamatait, a plazma tér- és időbeli fejlődésének sajátosságait, valamint annak lehetőségeit, hogy az elektródákra érkező ionok fluxusa és energiája miként állítható be egymástól függetlenül (ez a plazmaalapú megmunkálási folyamatok szempontjából elsődleges fontosságú). A komplex plazmák (erősen csatolt sokrészecske-rendszerek) kutatása területén Yukawa és Coulomb kölcsönhatási poteciállal jellemezhető rendszerek leírásával foglalkoztunk. Kétdimenziós rendszerekre vizsgáltuk a szilárd-folyadék fázisátalakulást, a transzportjellemzőket és a kollektív gerjesztéseket. | In the framework of the project our aim had been to understand the characteristics of systems in gas discharge and plasma physics, by means of numerical modeling and simulations. In order to confirm our modeling results we have also carried out experimental studies, or have cross-checked our results with experimental and/or theoretical results of collaborating partners. In our studies of low pressure steady state gas discharges we have examined the electron kinetics in simple gases as well as in complex gas mixtures (like air), we have clarified the limitations of gas discharge models, and investigated special (e.g. fast atom impact) excitation processes in gas discharges. We have studied the electron heating mechanisms in radiofrequency-excited discharges, the peculiarities of spatial and temporal development of discharge characteristics and the possibilities of the independent control of the energy and flux of ions reaching the surface of the electrodes. These properties play a key role in the plasma-suface interaction in processing plasmas. In the field of strongly coupled plasmas we have investigated many-particle systems characterized by Coulomb and Yukawa interaction potentials. We have studied the solid-liquid phase transition, transport processes, as well as collective excitations in two-dimensional systems

Scanning drift tube measurements of electron transport parameters in different gases: argon, synthetic air, methane and deuterium

Measurements of transport coef cients of electrons in a scanning drift tube apparatus are reported for different gases: argon, synthetic air, methane and deuterium. The experimental system allows the spatio-temporal development of the electron swarms (‘swarm maps’) to be recorded and this information, when compared with the pro les predicted by theory, makes it possible to determine the ‘time-of- ight’ transport coef cients: the bulk drift velocity, the longitudinal diffusion coef cient and the effective ionization coef cient, in a well-de ned way. From these data, the effective Townsend ionization coef cient is determined as well. The swarm maps provide, additionally, direct, unambiguous information about the hydrodynamic/ non-hydrodynamic regimes of the swarms, aiding the selection of the proper regions applicable for the determination of the transport coef cients

Effects of excitation voltage pulse shape on the characteristics of atmospheric-pressure nanosecond discharges

The characteristics of atmospheric-pressure microdischarges excited by nanosecond high-voltage pulses are investigated in helium-nitrogen mixtures, as a function of the parameters of the excitation voltage pulses. In particular, cases of single-pulse excitation as well as unipolar and bipolar double-pulse excitation are studied, at different pulse durations, voltage amplitudes, and delay times (for the case of double-pulse excitation). Our investigations are carried out with a particle-simulation code that also comprises the treatment of the VUV resonance radiation in the plasma. The simulations allow gaining insight into the plasma dynamics during and after the excitation pulse, the development and the decay of charged particle density profiles and fluxes. We find a strong dependence of the electron density of the plasma (measured at the end of the excitation pulse) on the electrical input energy into the plasma and a weak influence of the shape of the excitation pulse at the same input energy

Ground state structures of superparamagnetic 2D dusty plasma crystals

Ground state structures of finite, cylindrically confined two-dimensional Yukawa systems composed of charged superparamagnetic dust grains in an external magnetic field are investigated numerically, using molecular dynamic simulations and lattice summation methods. The ground state configuration of the system is identified using, as an approximation, the experimentally obtained shape of the horizontal confinement potential in a classical single layer dusty plasma experiment with non-magnetic grains. Results are presented for the dependence of the number density and lattice parameters of the dust layer on (1) the ratio of the magnetic dipole-dipole force to electrostatic force between the grains and (2) the orientation of the grain magnetic moment with respect to the layer.Comment: submitted to Phys. Rev.

Effect of correlations on heat transport in a magnetized strongly coupled plasma

In a classical ideal plasma, a magnetic field is known to reduce the heat conductivity perpendicular to the field, whereas it does not alter the one along the field. Here we show that, in strongly correlated plasmas that are observed at high pressure and/or low temperature, a magnetic field reduces the perpendicular heat transport much less and even enhances the parallel transport. These surprising observations are explained by the competition of kinetic, potential, and collisional contributions to the heat conductivity. Our results are based on first-principle molecular dynamics simulations of a one-component plasma