Physik

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Final report on the DFG Heisenberg project "Quantum Gravity from String Theory"
(Hannover : Technische Informationsbibliothek, 2025) Plauschinn, Erik
[No abstract available]
Report on the DFG Project "Approaching Superconductor/Cold Atom Hybrid Quantum Circuits " (KL 930_16-1)
(Hannover : Technische Informationsbibliothek, 2025-06-10) Kleiner, Reinhold; Fortagh, Jozsef
The project was intended to realize superconductor-cold atom hybrid systems, whereby the atoms are captured and manipulated in the vicinity of a superconducting chip. The chip should operate in the millikelvin range, which enables both subsystems to operate in the quantum regime. The vision: In this geometry, the methods of solid-state physics and quantum optics can be combined to manipulate the atoms or solid-state circuits directly or via photons from microwave to optical frequencies. If successful, the hybrid system offers unique opportunities to study the coupling between macroscopic objects (superconducting qubits, resonators) and natural atoms. In the context of quantum information, one can imagine a hybrid in which the superconducting circuit acts as a processor and the atoms act as a quantum memory. Cold atoms coupled to superconducting resonators could also enable the realization of new quantum gates. At the start of the project, a cold atom/superconductor setup was already in operation, working at a bath temperature of 4.2 K. Measurement systems were also available that allow superconducting microwave structures to be examined in the absence of cold atoms. In these setups, concepts for atom-superconductor coupling could be developed and tested. On the atom side, the focus was on Rydberg atoms, which offer a variety of resonant transitions and enable strong electrical dipole coupling to the superconducting devices. On the superconducting side, chips have been developed that contain resonators optimized for coupling to Rydberg atoms and can be combined with structures to trap the atoms. Originally, magnetic trap structures were considered but, as the project progressed, the focus went to optical dipole traps. A UHV millikelvin system was also available at the start of the project and was designed to incorporate the superconductor-cold atom hybrid systems. In this cryostat, part of the transport path for capturing the atoms and transporting them magnetically to the millikelvin level was demonstrated as part of a previous project. The transport path was completely rebuilt in the project, including the necessary laser and microwave components. A suitable superconducting chip was also installed at the end of the transport path. Unfortunately, the construction of the coil system required for transport turned out to be considerably more difficult than expected and took until the end of the project. The system is now functional, although the actual experiments on the coupled superconductor-cold atom hybrid systems are reserved for further projects.
Report on the DFG project "Optimization of different strategies for designing an energy harvester based on spin-torque diodes" (BE 2464/21-1)
(Hannover : Technische Informationsbibliothek, 2025-06-04) Berkov, Dmitry
Steadily growing demand for cheap and green energy has caused a rapid development of so called ‘energy harvesting’ devices for producing dc-power from the ambient microwave radiation from various sources like TV and mobile-phone networks, Wi-Fi routers etc. The energy density of this radiation ranges from 1 to 1000 nW/cm^2, so that corresponding technology could be successfully used by low-power applications (digital thermometer, smoke detectors, some sensors in medicine etc.). The main goal of this project was the optimization of various designs for energy harvesters based on spin-torque-diodes (STDs), i.e. devices where dc-voltage is generated when an ac-current flows through a magnetic tunnel junction (MTJ). Using computer simulations, we have studied and optimized three main types of MTJ-based nanodevices: (i) ‘standard’ MTJ nanopillars of the resonant type employing quasi-homogeneous in-plane magnetization oscillations; (ii) MTJs in the out-of-plane precession regime for broadband rectification and (iii) multilayer stacks with the in-plane shape designed for oscillation of domains walls. As the results of this project we have determined optimal geometric and magnetic parameters for all three kinds of spin-torque-based energy harvesters listed above, and predicted corresponding maximal rectification efficiencies in ambient conditions.
DFG final project report: Lattice QCD investigation of a b-bar b-bar u d tetraquark resonance
(Hannover : Technische Informationsbibliothek, 2025) Wagner, Marc
Quarks typically appear in pairs, known as mesons, or in triplets, known as baryons, with protons and neutrons as well-known representatives of the latter. However, there are also exotic combinations of four quarks, known as tetraquarks, which have gained significant interest, particularly in recent years. This interest arises from the fact that they can be both detected in modern accelerator experiments and increasingly well understood and precisely predicted on a theoretical level with modern numerical calculations. The goal of this project was to study the existence and properties of specific tetraquark systems, consisting of two heavy antiquarks and two light quarks, based on first principles quantum chromodynamics, using numerical lattice field theory calculations. Investigating a $\bar b \bar b u d$ tetraquark resonance with quantum numbers $I(J^P) = 0(1^-)$ proved to be particularly challenging. Within the scope of this project, it was understood through the Born-Oppenheimer approximation that the tetraquark resonance is not located slightly above the lower $B B$ threshold, as previously expected, but considerably higher, above the $B^\ast B^\ast$ threshold. Final results with full lattice QCD calculations beyond the Born-Oppenheimer approximation have yet to be achieved, as this requires the numerical solution of a complex two-channel scattering problem involving a $B B$ and a $B^\ast B^\ast$ channel. However, essential technical steps for such a future computation have been implemented. For the two $\bar b \bar c u d$ tetraquark systems with quantum numbers $I(J^P) = 0(0^+)$ and $I(J^P) = 0(1^+)$, a rigorous finite-volume scattering analysis based on full lattice QCD computations was performed for the first time. This led to the prediction of a weakly bound but stable tetraquark for each of the two systems, as well as a tetraquark resonance approximately $100 \, \text{MeV}$ above the lowest meson-meson threshold ($B D$ or $B^\ast D$, respectively).
Topological transport control of colloidal particles
(Hannover : Technische Informationsbibliothek, 2025) de las Heras, Daniel; Fischer, Thomas M.
We have studied experimentally and with computer simulations the transport of magnetic particles on top of magnetic patterns. The motion is driven by either a modulation loop of the orientation of a uniform external magnetic ﬁeld or by a drift force. The application of an adiabatic modulation loop of the direction of an external magnetic ﬁeld to magnetic colloids or macroscopic magnetic particles on a periodic pattern oﬀers unprecedented control over the motion and assembly of such colloids or particles. The motion is topologically protected since only those loops that wind around special orientations of the external ﬁeld induce particle transport. The set of winding numbers around the special orientations is the topological invariant that protects the motion. The colloidal or macroscopic particles are sorted into topological classes and the transport of each class can be controlled independently and simultaneously with the other topological classes. The use of non-periodic patterns facilitates the transport of identical colloidal particles independently and simultaneously. The complexity of the loop can be imprinted in either the pattern or the modulation loop. In twisted magnetic patterns high mobility peaks of non-topologically driven particles emerge at non generic magic angles, but these mo- bility peaks in contrast to topologically driven systems are very fragile and can be easily destroyed via the analogue of an Anderson transition.
DFG final report for the Walter Benjamin Programme : Stationary and time-dependent radiative heat transfer with cylindrical waveguides
(Hannover : Technische Informationsbibliothek, 2025) Asheichyk, Kiryl
Following the progress in development of micro- and nanodevices, the physics of thermal radiation at these small length scales has been studied intensively in the current century. It requires more sophisticated theories and obeys different laws compared to radiative heat exchange at large scales, for example, between the Sun and the Earth. Yet main questions remain similar: How to improve the efficiency of radiative heat transfer, suppress or enhance the cooling rate of an object, thermally isolate a part of a system? Another important and only recently emerged research area concerns systems subject to nonstationary conditions, such as time-dependent temperatures or material properties of the objects, where the heat fluxes can depend on time and show fundamentally different properties compared to the stationary case. In this project, we addressed the aforementioned questions for the stationary heat transfer between two nanoparticles placed close to a nanowire or inside a cylindrical cavity. In the former case, the heat transfer was shown to decay logarithmically with the interparticle distance, thus greatly outperforming the transfer without an additional object or in the presence of objects of other shapes. In the latter case, a cylindrical cavity can largely suppress or resonantly enhance the heat flow, depending on its radius. If we consider that one particle starts to radiate at a certain time, the heat flux to another one is no longer stationary, which requires an extension of the existing theoretical formalisms. Making this extension, we derived a formula for the time-dependent flux, numerically demonstrating it for two isolated nanoparticles, where picosecond-scale oscillatory exponential relaxation to the stationary value was observed. In addition to the aforementioned studies of radiative heat flow through vacuum, we also developed a theory for computing this flow inside media, and derived the corresponding thermal conductivity tensor for an arbitrary object. In agreement with recent experiments, it was found that the radiative part of conductivity of a nanosheet can be larger compared to the phononic part due to the electromagnetic surface waves contribution. The developed theoretical frameworks can be used to further advance the subject area, while the found results may be relevant for practical applications, such as thermal microscopy, thermal logic, or nanomedicine.
Berechnung, Messung und Kontrolle der Energiebarrieren und der lichtinduzierten Kinetik des ASi-Sii-Defektes
(Hannover : Technische Informationsbibliothek, 2025-04-09) Lauer, Kevin
Die Siliziumtechnologie hat umfangreiche Anwendungsmöglichkeiten, die sich im gegenwärtigen Alltag überall wiederfinden. Anwendungen wie Computer, Smartphones, Sensoren, Detektoren, Solarzellen und viele mehr sind nur möglich geworden durch jahrzehntelange Erforschung des Siliziums und der nötigen technologischen Prozesse. Nichtsdestotrotz gibt es noch immer unverstandene Phänomene und Mechanismen, speziell im Bereich der Defekte und der Degradation. Ein besseres Verständnis ist notwendig, da Defekte zum einen notwendig für die Funktionsweise von Bauelementen sind, sich aber auch negativ auswirken können. Das Projekt untersuchte eine besondere Kategorie von Defekten, die einen Akzeptor und interstitielles Silizium involvieren, so genannte ASi-Sii-Defekte. Sie tragen vermutlich maßgeblich zur licht-induzierten Degradation von Solarzellen und Detektorbauelemente bei. Konkret wurden in diesem Projekt die Energiebarrieren zwischen verschiedenen Defektzuständen der ASi-Sii-Defekte, die teilweise erst identifiziert werden mussten, in enger Zusammenarbeit von Experiment und Theorie erforscht und bestimmt. Die Barrierenhöhen haben einen direkten Einfluss auf die Defektkinetik und -stabilität, weshalb ihre Kenntnis essentiell ist. Gelingt es, die Barrierenhöhen gezielt zu beeinflussen (z.B. durch Wahl des Akzeptors), so können Bauelementeigenschaften ggf. gezielt eingestellt werden, z.B. zur Effizienzsteigerung oder Lebenszeitverlängerung von Solarzellen. Zudem erscheint es reizvoll künftig zu untersuchen, inwieweit Defekte aus dieser Kategorie als Qubit Verwendung finden können und somit für die Silizium-basierte Quantentechnologie interessant sind. Das Projekt hat erfolgreich einen Beitrag zum grundlegenden Verständnis der ASi-Sii-Defekte geleistet und legt den Grundstein für die weitere Erforschung dieser interessanten Defektkategorie.
Collaborative Research Center 917 : Resistively Switching Chalcogenides for Future Electronics : Structure, Kinetics, and Device Scalability : Final report : 2019/2-2020-2021-2022-2023/1
(Hannover : Technische Informationsbibliothek, 2025-03-25) Wuttig, Matthias; Waser, Rainer; Dronskowski, Richard; Dittmann, Regina; Simon, Ulrich; Mayer, Joachim
The goal of SFB 917 has been the development of novel nanoswitches that can be reproducibly and reversibly changed between two states on very short time and length scales. Such nanoswitches can enable new storage and memory devices as well as neuro-inspired architectures for information technology. In the third and final funding period of SFB 917 we have witnessed and contributed to three major trends. The exponential growth in the demand for data storage and processing has continued. Hardware improvements are therefore urgently needed to meet the increased demands for data storage and processing as well as the related increase in energy consumption. SFB 917 strives to realize novel storage devices by exploiting the full potential of chalcogenide-based nanoswitches. Yet, it has become increasingly clear in the last few years, that improvements in device performance alone are insufficient to deal with the exponential growth mentioned above. The advance of Large Language Models (LLMs) like ChatGPT and related software has produced a further increase in data processing related energy consumption. This is a major challenge considering the expected further increase of professional and private usage of such machine learning tools. To minimize the related energy consumption, particularly energy-efficient software and hardware developments are mandatory. Within SFB9917, we have this intensified our efforts to work on effects related to a reduction in energy consumption. While energy-efficient devices can help, an improvement in hardware architecture offers significantly more leverage. We have thus explored the potential of chalcogenide based nano-switches in neuro-inspired computer architectures. To this end, several new large-scale research projects have been initiated (NeuroSys and NeuroTec), which extend our research on these devices in increasingly more complex architectures, offering new opportunities to harvest the findings of SFB 917 in new applications. To tailor chalcogenide-based nano-switches, major advances in instrumentation as well as an in-depth understanding of the origin of underlying phenomena and unconventional properties in these materials have been mandatory. Challenges included the characterization of switching in these materials on nanosecond time and nanometer length scales. Sophisticated tools have been built and utilized. Understanding unconventional properties are required building a bridge between concepts of inorganic chemistry and material properties leading to novel treasure maps which help to identify and tailor chalcogenides for specific applications. These successes are described in detail in the present report. To demonstrate that these findings are also relevant for industry, close cooperation with industrial partners has been established to ensure that the findings made within SFB 917 can also be implemented on the shortest possible time scales.
Sedimentation of binary mixtures: Phase stacking and Nonequilibrium dynamics
(Hannover : Technische Informationsbibliothek, 2024-12-30) Schmidt, Matthias; de las Heras, Daniel
Based on equilibrium sedimentation path theory and the local density functional approximation, we investigated the effects of gravity on several relevant types of binary colloidal mixtures. Settled systems are represented by so-called sedimentation paths, which determine the variation of the species-resolved chemical potentials with altitude. Analysing the resulting line segments in the plane of chemical potentials of the bulk phase diagram allows one to rationalize the full equilibrium stacking phenomenology for a given system under gravity. The approach predicts theoretically the stacking sequences of colloidal rod-plate mixtures that were observed in iconic experiments by van der Kooij and Lekkerkerker. Thereby the occurrence of up to five simultaneous phase layers emerges naturally from the mere interplay of gravity and two-phase bulk coexistence, without invoking particle polydispersity. We studied the effects on equilibrium phase stacking upon varying the buoyant mass ratio of both components and our predictions are testable in experiments by systematic variation of the height of sedimentation columns. We have carried out similar sedimentation studies for: plate-spheres mixtures, mass-polydisperse systems, and hard spherocylinders. We suggest that microscopic particle properties, such as the buoyant mass, can be inferred from macroscopic measurements of layer thicknesses in phase stacking sequences. We addressed gravity-induced nonequilibrium flow and structure formation on the basis of power functional theory, adaptive Brownian dynamics computer simulations, and functional machine learning. Power functional theory allows one to rationalize and to model the nonequilibrium behaviour of many-body systems based on the one-body density and velocity field. We have used the approach to categorize systematically the different types of relevant nonequilibrium force contributions and have developed corresponding analytical gradient approximations. Neural functionals, as trained on the basis of both equilibrium and nonequilibrium computer simulation data, were shown to yield accurate predictions for structure formation and design of nonequilibrium flow. We have formulated force-based density functional theory and have demonstrated that neural density functionals outperform the best available hard sphere fundamental measure functionals. We have developed adaptive Brownian dynamics as a performant and highly stable numerical integration scheme for the temporal integration of overdamped many-body Langevin equations of motion, as demonstrated for a particle gel subject to convective sedimentation flow. We have put forward general frameworks for fluctuations of general hyperobservables, for their associated hyperforce correlation functions, and for the gauge invariance of statistical mechanics, where Noether's theorem yields exact sum rules that constrain correlations, as exemplified for ideal and for active sedimentation.
Effect of a high-voltage mesh electrode on the volume and surface characteristics of pulsed dielectric barrier discharges
(Melville, NY : American Inst. of Physics, 2020) Kettlitz, M.; van Rooij, O.; Höft, H.; Brandenburg, R.; Sobota, A.
Electrical breakdown in a pulsed asymmetric dielectric barrier discharge between a glass-covered mesh electrode and a grounded metal electrode in the air at atmospheric pressure is investigated. Volume discharge forms between the metal tip and the dielectric surface and spreads over the dielectric surface. Breakdown and discharge behaviors depend on the polarity of the charged electrode covered with glass compared to the metal rod electrode. In the case of the dielectric cathode (covered mesh), volume discharge features a stronger and longer-lasting emission. Volume discharge is weaker with outstretched surface discharge developing on the opposite glass electrode sustained by the embedded mesh when the metal rod functions as a cathode. The development and spatial distribution of the surface discharge depend on the relative polarity of the dielectrics caused by the charge deposition of the preceding discharge and is independent of the polarity of the applied high voltage. The discharge emission is brighter for the metal cathode and dielectric anode than for the metal anode, with a branching discharge developing and spreading in a star-like structure along the embedded grid, while a ring-like structure was observed for the metal anode and dielectric cathode. The duty cycle influences the discharge development and properties through the effects of the gas phase and surface pre-ionization.
Microplasmas: A Review
(Oak Park, Ill. : Bentham Science Publ., 2011) Papadakis, A. P.
In this paper, a review on microplasma discharges is conducted. The different types and configurations used in microplasmas such as the Cathode Boundary Layer (CBL), Dielectric Barrier Discharge (DBD), Capillary Plasma Electrode Discharge (CPED), Inverted Square Pyramid (ISP), Square Cross Sectional Cavities (SCSC), Radio Frequency Inductively Coupled Discharge (RFIC), Radio Frequency Capacitive Coupled Discharge (RFCC), Micro-Hollow Cathode Discharge (MHCD), and microstrip technology (MS) discharges at different operating conditions are discussed. Numerical and experimental methods used for the analysis of the physics involved in these microplasmas, as well as the different construction methods used, are also described.
PKE-Nefedov*: Plasma crystal experiments on the International Space Station
([London] : IOP, 2003) Nefedov, Anatoli P.; Morfill, Gregor E.; Fortov, Vladimir E.; Thomas, Hubertus M.; Rothermel, Hermann; Hagl, Tanja; Ivlev, Alexei V.; Zuzic, Milenko; Klumov, Boris A.; Lipaev, Andrey M.; Molotkov, Vladimir I.; Petrov, Oleg F.; Gidzenko, Yuri P.; Krikalev, Sergey K.; Shepherd, William; Ivanov, Alexandr I.; Roth, Maria; Binnenbruck, Horst; Goree, John A.; Semenov, Yuri P.
The plasma crystal experiment PKE-Nefedov, the first basic science experiment on the International Space Station (ISS), was installed in February 2001 by the first permanent crew. It is designed for long-term investigations of complex plasmas under microgravity conditions. 'Complex plasmas' contain ions, electrons, neutrals and small solid particles - normally in the micrometre range. These microparticles obtain thousands of elementary charges and interact with each other via a 'screened' Coulomb potential. Complex plasmas are of special interest, because they can form liquid and crystalline states (Thomas et al 1994 Phys. Rev. Lett. 73 652-5, Chu and I 1994 Phys. Rev. Lett. 72 4009-12) and are observable at the kinetic level. In experiments on Earth the microparticles are usually suspended against gravity in strong electric fields. This creates asymmetries, stresses and pseudo-equilibrium states with sufficient free energy to readily become unstable. Under microgravity conditions the microparticles move into the bulk of the plasma (Morfill et al 1999 Phys. Rev. Lett. 83 1598), experiencing much weaker volume forces than on Earth. This allows investigations of the thermodynamics of strongly coupled plasma states under substantially stress-free conditions. In this first paper we report our results on plasma crystals, in particular the first experimental observations of bcc lattice structures.
Interaction of injected dust particles with metastable neon atoms in a radio frequency plasma
([London] : IOP, 2008) Do, H. T.; Kersten, H.; Hippler, R.
Spatial density and temperature profiles of neon metastables produced in a radio frequency (rf) discharge were investigated by means of tunable diode laser absorption spectroscopy. The experiments were performed in the PULVA1 reactor, which is designed for the study of complex (dusty) plasmas. The line averaged measured density is about 1.5 × 1015 m-3 in the bulk and drops almost linearly in the plasma sheath. The gas temperature is in the range of 370390 K. The flow of metastable atoms in the plasma sheath deduced from the spatial density distribution is dominated by the flow towards the rf electrode. The sheath length is supposed as the effective diffusion length in the plasma sheath region. This approximation was used to investigate the interaction of injected particles with the plasma. The observations and estimation provide evidence for a significant interaction between metastable atoms and powder particles which is important for energy transfer from the plasma to the particles. The power per unit area absorbed by dust particles due to the collision of metastable atoms with the dust particle surface is in the range of a few tens of mW m-2.
Study of Propagation of Ion Acoustic Waves in Argon Plasma
(Irvine, Calif. : Scientific Research Publ., 2010) Suryanarayana, N. S.; Kaur, Jagjeet; Dubey, Vikas
The properties of small amplitude acoustic waves (IAW) in unmagnetised plasma have been discussed in detail. An experimental set up to study the propagation of IAW in argon plasma has been descried. The speed of IAW under different conditions of discharge current and pressure has been measured from the time-of flight technique. From these measurements, electron temperatures have been calculated. The results have been compared with those obtained by single probe method, and were found to be in good agreement with each other so IAW speeds can be used to calculate plasma parameters.
Instability of shear waves in a nonuniform dusty plasma
([London] : IOP, 2003) Sorasio, G.; Shukla, P. K.; Resendes, D. P.
It is shown that shear waves in a strongly coupled dusty plasma with nonuniform plasma pressure and dust density can be destabilized. The dispersion relation as well as the instability condition have been found and investigated in detail.
A unified view of acoustic-electrostatic solitons in complex plasmas
([London] : IOP, 2003) McKenzie, J. F.; Doyle, T. B.
A fluid dynamic approach is used in a unified fully nonlinear treatment of the properties of the dust-acoustic, ion-acoustic and Langmuiracoustic solitons. The analysis, which is carried out in the wave frame of the soliton, is based on total momentum conservation and Bernoulli-like energy equations for each of the particle species in each wave type, and yields the structure equation for the 'heavy' species flow speed in each case. The heavy (cold or supersonic) species is always compressed in the soliton, requiring concomitant contraints on the potential and on the flow speed of the electrons and protons in the wave. The treatment clearly elucidates the crucial role played by the heavy species sonic point in limiting the collective species Mach number, which determines the upper limit for the existence of the soliton and its amplitude, and also shows the essentially similar nature of each soliton type. An exact solution, which highlights these characteristic properties, shows that the three acoustic solitons are in fact the same mathematical entity in different physical disguises.
Uniqueness Theorem for the Non-Local Ionization Source in Glow Discharge and Hollow Cathode
(Irvine, Calif. : Scientific Research Publ., 2012) Gorin, Vladimir V.
The paper is devoted to the proof of the uniqueness theorem for solution of the equation for the non-local ionization source in a glow discharge and a hollow cathode in general 3D geometry. The theorem is applied to wide class of electric field configurations, and to the walls of discharge volume, which have a property of incomplete absorption of the electrons. Cathode is regarded as interior singular source, which is placed arbitrarily close to the wall. The existence of solution is considered also. During the proof of the theorem many of useful structure formulae are obtained. Elements of the proof structure, which have arisen, are found to have physical sense. It makes clear physical construction of non-local electron avalanche, which builds a source of ionization in glow discharge at low pressures. Last has decisive significance to understand the hollow cathode discharge configuration and the hollow cathode effect.
Plasma interaction with microbes
([London] : IOP, 2003) Laroussi, M.; Mendis, D. A.; Rosenberg, M.
The germicidal effects of a non-equilibrium atmospheric pressure plasma generated by a novel resistive barrier discharge on representatives of the two classes of bacteria (Gram-negative and Gram-positive) are discussed. The plasma exposure, while being lethal to both bacterial classes, also produced gross structural damage in the Gram-negative E. coli while none was observed in the more structurally robust Gram-positive Bacillus subtilis. An electrophysical process involving the role of the electrostatic tension on a charged body in a plasma is invoked to explain both observations. Since the efficacy of this electrophysical process depends not only on the tensile strength of the bacterial cell wall but also on its shape and texture, the need for more experimental studies, using a wide range of bacteria belonging to various morphological groups, is suggested. Ways to further test the validity of this electrophysical lysis mechanism for Gram-negative bacteria on one hand, and also to extend its operation to the more robust Gram-positive bacteria on the other, are suggested.
Collision-dominated dust sheaths and voids - Observations in micro-gravity experiments and numerical investigation of the force balance relations
([London] : IOP, 2003) Tsytovich, V. N.; Morfill, G.; Konopka, U.; Thomas, H.
Numerical solutions of stationary force balance equations are used to investigate the possible dust configurations (dust structures) in complex plasmas between two floating potential plane electrodes. The distance between electrodes is assumed to be larger than the ion-neutral mean free path and the hydrodynamic description is used. It includes the known forces operating in this limit, the ionization source and the dust charge variations. The stationary balance equations are solved both in the case of the presence of one-size dust grains and for the case of a mixture of grains with two different sizes. Recent micro-gravity experiments with single-size dust grains and two-different-size dust grains show the formation of a system of dust sheaths and dust voids between the two plane electrodes. The observed configurations of dust structures depend strongly on the gas pressure and the degree of ionization used. The numerical investigations are able to show the necessary conditions for the types of structure to be created and give their size. The size of the structures observed is larger than the ion-neutral mean free path and is of the order of magnitude of that obtained numerically. The numerical investigations give details of the spatial distributions, the dust particles, the electron/ion densities, the ion drift velocity and dust charges inside and outside different dust structures. These details have not yet been investigated experimentally and can indicate directions for further experimental work to be performed. The single-dust-sheath structure with single-size dust particles surrounded by dust free regions (dustwall-voids) and floating potential electrodesis computed. Such a structure was observed recently and the computational results are in agreement with observations. It is shown that more often a dust void in the centre is observed. It is found that a dust void in the centre region between two electrodes is formed if the ionization rate is larger than the critical ionization rate and that in the presence of the floating potential walls the central void should be surrounded by two dust sheaths. The necessary condition for this dust structure to be formed is found to be that between the sheaths and the walls there are formed two other wall-void regions. The size of the central void and the distributions of the structure parameters in the two sheaths and in the three voids are computed. The qualitative features of the structure obtained in the numerical computations correspond to those observed. The distributions of the structure parameters in the case of the two dust sheaths are quite different from that for the case of a single central sheath. The possible structures between the electrodes for the case of the presence of dust particles of two different sizes are analysed numerically. It is shown that dust particles with different sizes cannot coexist in equilibrium at the same position and that the regions with different size dust particles must be separated in space. This conclusion is in agreement with most observations performed so far. It is illustrated numerically that for the case where the central void is present the dust particles of larger size form a separate dust sheath which should be located at larger distances from the centre than that for the smaller dust particles. This result also coincides qualitatively with the observations. Computations for the distributions of the parameters in the larger size dust sheath were performed both in the case where the central part is occupied by a dust sheath with smaller size dust particles and for the case where in the central part there exists a dust void surrounded by dust sheaths with smaller size dust particles. The size of the dust void between the sheaths with different size dust particles is calculated and shown to be small as compared to the sheath thickness. In the sheath with larger size dust particles the distribution of dust and plasma parameters differs qualitatively from that of the first dust sheath with smaller size dust particles. The stability of the stationary structures both with respect to excitation of dust convection cells and with respect to oscillations of dust void size is discussed.
Formation and behaviour of dust particle clouds in a radio-frequency discharge: Results in the laboratory and under microgravity conditions
([London] : IOP, 2003) Mikikian, M.; Boufendi, L.; Bouchoule, A.; Thomas, H. M.; Morfill, G. E.; Nefedov, A. P.; Fortov, V. E.; PKE-Nefedov Team
In this paper we report the first observation on submicron dust particle clouds grown in a radio-frequency sputtering discharge under microgravity conditions. These results have been obtained in the PKE-Nefedov (Plasma Kristall Experiment) chamber in the framework of a French-German-Russian programme. A dust-free region, usually called the 'void', is observed in the laboratory and under microgravity conditions even with submicron particles. In this region, successive generations of particles can be grown, leading to the coexistence of particles with various sizes. Each generation of particles constitutes a cloud separated from the others by a definite sheath. Dynamics of these clouds have been investigated showing vortex-like motions or independent behaviour of small heaps of particles, emphasizing both attractive and repulsive effects between dust clouds. As these particles drastically influence the plasma properties, the growth kinetics is followed through the evolution of the discharge current.

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