Abstracts of tutorial lectures |
Tutorial: Dusty plasmas under microgravityOliver Arp
Microgravity conditions offer unique opportunities for investigating structural and dynamical phenomena of spatially extended three-dimensional dusty plasmas. The interpenetration of spatially extended three-dimensional dust clouds and the plasma within a large volume gives rise to a complex interaction that leads to the emergence of new features which are not known from one- or two-dimensional systems. In the absence of gravity comparatively weak plasma-induced forces in the bulk plasma like the electric field force and forces that are exerted by flowing ions become important and govern the behavior of the confined dust cloud. The resulting complex force balance is responsible, e.g., for the formation of a dust-free void region, the alignment of particles in chains, and the excitation of self-excited dust-density waves. For a correct interpretation of dynamical phenomena in three-dimensional dusty plasmas a profound understanding of the elastic properties of such systems is required. Important information is obtained from experiments and simulations that use fast dust projectiles to apply a moving point-like distortion to a three-dimensional dust cloud. A prominent instability of three-dimensional dusty plasmas under microgravity are self-excited dust-density waves. They emerge spontaneously at low gas pressures and are driven by streaming ions. Recent experiments showed that the resulting complex wave pattern with its spatially varying frequency and wave length cannot be explained by a linear wave that propagates through a passive medium. Rather, the phenomenon can be described by partial synchronization of neighboring regions in a system of mutually coupled non-linear oscillators. The observation of coherent wave activity in spatially separated regions of the discharge suggests the existence of an additional long-range synchronization mechanism. The simultaneous investigation of the dust cloud, the plasma glow and global discharge parameters show a complex interaction between the plasma and the dust in the complete discharge, which may even change the topology of the dust cloud dramatically. Refined studies and simulations reveal that the observed wave phenomenon is the product of a self-organized system consisting of the dust cloud, the plasma, and the external circuit of the discharge. Tutorial: Charging of dust grains: Electron surface effectsFranz-Xaver Bronold
It is well-known that dust grains immersed in an ionized gas collect electrons more efficiently than ions because the influx of electrons outruns the influx of ions. Hence, the floating potential acquired by a dust grain is negative with respect to the plasma potential. The calculation of the floating potential or charge of a spherical dust grain in an ionized gas has a long history. It is one of the classic problems in the physics of bounded low-temperature plasmas. Considering the grain as a floating electric probe whose diameter is much smaller than the spatial extension of the surrounding sheath, the charge of the grain is orbital-motion limited, and two main assumptions have been typically invoked for its calculation: (i) collisions in the sheath of the grain are negligible and (ii) any electron and ion coming from the plasma that hits the grain is absorbed. By now it is well-accepted that assumption (i) is unwarranted for the ions because ion-neutral charge-exchange collisions increase the ion influx and thereby substantially reduce the grain charge as compared to the collisionless result. The idealized treatment (ii) of the grain surface however is still invoked, an exception being approaches which include secondary electron and/or photo-electron emission. But even here it is assumed that low-energy electrons hitting the grain are absorbed with probability one, and once they are absorbed, they remain inside the grain forever. In reality, however, a low-energy electron may have a change to get reflected or to be only temporarily bound. The perfect absorber model leaves no room for the description of this type of electron microphysics. We recently questioned therefore the perfect absorber model, treating the interaction of electrons with planar plasma walls in terms of an electron physisorption process, characterized by an electron sticking coefficient and an electron desorption time. We also inquired in which states electrons making up the wall charge reside. In this tutorial I will discuss a method to incorporate electron physisorption into orbital-motion limited calculations of the grain charge. To work out the consequences of electron surface effects as clearly as possible, I will present results only for a grain in a quiescent, isotropic plasma. How the electron microphysics at the surface interferes with charge-exchange corrections to the ion flux or the possibility of a directed flow of plasma particles will be discussed at a later stage. Tutorial: Nanodust in magnetized plasmasFranko Greiner
A new and fascinating area of dusty plasmas are magnetized dusty plasmas. The introduction of a magnetic field leads to new phenomena, like formation of three-dimensional toroidal dust structures, magnetic field induced rotation of planar clusters, modification of ion wakes and 'filamentation' of the discharge. In our nanodust experiments we focus on the investigation of extended nanodust clouds in magnetized argon-actetylene plasmas in a rf driven parallel plate reactor. Nanodust has the major advantage that gravitation can be neglected for grains smaller than 200nm. In this tutorial we want to focus on two aspects of nanodust experiments in magnetized plasmas: Nanodust confinement and nanodust diagnostics. Depending on the strength of the magnetic field and the discharge parameters, electrons, ions and nanodust are magnetized and the discharge conditions change. The magnetization of the plasma species leads to the modification of the dust confinement and changes the interaction of the dust particles. In comparison to unmagnetized plasmas, magnetic fields alter the plasma potential and density profiles in a way, which weakens the confinement. This is shown by the increasing frequency of the growth instability at medium magnetic fields ( B < 100mT, B perpendicular to the electrode surface). At higher magnetic fields ( 500mT ) more sophisticated electrode structures have to be used to successfully produce and confine nanodust clouds. The nanodust clouds show interesting dynamics, like density waves and spectacular dust ejections perpendicular to the magnetic and electric fields. However, the small size of the grains deprives us of one of the main diagnostics of dusty plasma: video microscopy of single dust grains. New diagnostics are desirable. One new option is spatio-temporal resolved Mie Ellipsometry. We will discuss the main physical an technical challenges. In addition, a robust plasma diagnostic would be helpful. Electrostatic probes an other techniques, like resonance cones will be discussed. Tutorial: Dust particle wakes in flowing plasmasIan H. Hutchinson
The presence of a dust grain perturbs the plasma around it. The interaction between grains depends entirely upon this plasma perturbation, which therefore constitutes the foundation of all dusty plasma physics. In many situations, the plasma ions have a systematic flow velocity past the grain in addition to their thermal velocity. This flow, which in sheaths can exceed the ion-acoustic speed, breaks the spherical symmetry that is presumed by the classic theories of probe-plasma interaction, and requires two-dimensional and (for more than one grain or magnetized plasma) three-dimensional analyses. The most important effect is well understood qualitatively. It is that ions are focussed by attraction to the grain and cause a localized trailing region of enhanced ion density and potential. This enhanced potential is sometimes sufficient to overcome the mutual repulsion of charged grains and align them by attraction. Although this explanation is convincing and qualitatively correct, quantitative calculations of the wake and interparticle dynamics are only recently becoming available. Linearized-response kinetic-theory treatments of the flow of plasma past a charge have, since the 1960s, provided analytic expressions for the wake of an elementary point charge and the resulting plasma ion drag force on it. However, dust grains often have sufficient charge that they are not well represented by a linearized calculation. In any case, the linearized response calculations require numerical evaluation of difficult complex integrals which even today is not routine. Large-scale computation using Particle-In-Cell codes (several of which have been specifically developed for this purpose) are a very natural way in which to solve rigorously for the plasma wake. They are exploring the important quantitative effects of non-linearity and resolving long-standing uncertainties about the relative importance of potential and particle distribution perturbations in dust grain dynamics. Tutorial: Growth of dust particlesEva Kovacevic
The observation of dust particles has a long history in physics – their role in
laboratory and cosmic plasmas was discussed early on by, for example, Irving
Langmuir, Lyman Spitzer and Hannes Alfvén, the pioneers of plasma physics in the
20th century. In his speech in 1924, Langmuir described the "profound effects" he
observed in an arc discharge when minute droplets of tungsten vapor were sputtered
from the cathode into the plasma. And although it was clear already in the 20ties that
dust particles in plasmas have “profound effects”, the further, rapid explosion of
investigations in this field had to wait for about 50 years, for new technologies,
diagnostic methods and applications.
The investigation of particle formation in plasmas is nowadays of great interest in
various branches of physics and technology - in astrophysics [1, 2] in fusion
processes [3, 4], and in processing plasmas [5, 6, 7]. The generation of particles
ranging from nanometer to micrometer size scale was observed in several kinds of
discharge types: in inductively and capacitively coupled RF-discharges, in microwave
discharges and in DC-glow discharges... The phenomenon of dust particle growth
involves both: plasma physics and plasma chemistry; reaching further in the fields of
material analysis, chemistry, and solid state physics, thus being highly
interdisciplinary.
The intriguing questions in this field (and thus the subject of this presentation)
concern: Tutorial: Optical diagnostics of dusty plasmasAndre Melzer The diagnostic methods applied in dusty plasmas are in many cases based on optical imaging. These methods include the direct imaging of the particles in scattered light by video microscopy since the temporal and spatial scales of dust particle motion are ideally suited for (high-speed) video cameras. Single-camera or stereoscopic multi-camera setups can be employed to extract the particle positions and trajectories in two or three dimensions. Alternatively, image velocimetry can be applied to extract the motion within different cells of the dust cloud. Optical methods also include the imaging of the plasma emission. The overall intensity of the plasma emission and/or the relative intensity of different emission lines is generally influenced by the presence of the dust. The spatially resolved imaging of the plasma emission allows to relate dust and plasma properties. In this tutorial, the fundamental ideas behind these approaches are developed and specific applications are discussed. Tutorial: Dust particles in trapsJiri Pavlu A role of dust in the space environment is of increasing interest due to a number of missions (e.g., Rosetta, Cassini) that provide or will provide investigations of the properties and global dynamics of charged dust grains. On the other hand, the fast development of fusion devices with magnetic confinement brought new issues in the plasma-surface interaction. While there is a number of dusty plasma reviews, the papers introducing experimental techniques for investigation of dust grains under well-defined laboratory conditions (for both space and industrial applications) are limited. The main purpose of these experiments is to study particular charging/discharging processes on space-related as well as on elemental materials. Several laboratory experiments have been carried out to study the parameters of grain ensembles within a plasma, however, experiments at single grains are still a challenge. After its invention in the 1950's, an electrodynamic quadrupole trap and/or its different modifications quickly proved to be an extremely powerful tool for the experimental investigation of a wide range of phenomena. This contribution presents a digest of trapping techniques and discuss their dis/advantages and area of use because every design has its phenomenological and technological limits and also the studied processes have different requirements. Tutorial: Lunar dustZoltan SternovskyAerospace Engineering Sciences University of Colorado,Boulder, USA The lunar surface is an excellent place to study the complex interaction of plasmas with dust and dusty surfaces in space. The solar UV radiation and the solar wind charge the surface and differential charging due to light/shadow boundaries may locally enhance the electrostatic field intensity. The electrostatic forces were suggested to mobilize dust on the lunar surface, for which there are some indications from the Apollo era, however, electrostatic dust mobilization remains an outstanding issue. The lunar surface is also exposed to the continual bombardment by interplanetary dust particles with an estimated influx of mass at about 5×10e3 kg/day, and impact speeds > 2 km/s. The micrometeoroid impacts generate high-velocity secondary ejecta particles, which can reach high altitudes, and thus a permanently present dust exosphere is expected about the Moon. In the last decade, there has been a revived interest in the Moon, and the recent space missions greatly contributed to the understanding of its interaction with the solar wind plasma on the global scale. The upcoming LADEE (Lunar Atmosphere and Dust Environment Explorer) mission will be launched in 2013 and carry an in-situ and a remote sensing instruments dedicated to the mapping of the lunar dust environment from orbit. Under development are instruments designed to detect and characterize low velocity dust for future lunar lander missions. In the meantime, a series of laboratory experiments were devoted to the simulation of the conditions of the near-surface lunar environment to study the charging and mobilization of dust on surfaces. In these experiments, dust mobilization is observed under specific circumstances. |