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In 2003 the ROSETTA space mission to Comet 46P/Wirtanen will be launched by the European Space Agency (ESA). On board of the spacecraft there will be a Lander platform, which opens for the first time the possibility to carry out “in situ” measurements on a comet surface. The ROSETTA Lander experiment CASSE aims to investigate the outermost surface of the comet by means of transmitting, receiving and even passively monitoring acoustic waves in a frequency range from a few hundred to several kilohertz. In particular, by transmitting and recording compression and shear waves, the mechanical properties of the cometary surface will be investigated. To guarantee sufficient ground contact in any foreseeable surface topography, and in any composition of the material to be encountered, e.g. dust, sand, ice and mixtures of these materials, the acoustic transmitters (actuators), stacked piezoceramics, and triaxial commercial piezoelectric accelerometers as receivers, will be integrated in each of the three feet of the ROSETTA Lander. The Lander feet thus act as antennas for the transmitters and receivers. By switching different actuators and accelerometers, an analysis of the cometary surface material will become possible by direct foot to foot transmission of the acoustic signals. Further, an in-depth sounding will allow the detection of layered structures or embedded local inhomogeneities. The ground contact of the Lander will be strengthened by a harpoon providing a fixation force up to at least 5 N per foot. This paper describes the first design of the Lander feet incorporating piezoceramic composite stacks as sound transmitters and receivers, and the experiments to test their efficiency. In the first part of this paper, the results of acoustic wave propagation experiments in ice, sand and olivine dust, performed in the laboratory are reported. These experiments verify that a signal strength of at least 20 dB above noise can be expected for acoustic wave propagation in cometary analogue materials. The second part of the paper deals with the outdoor experiments, performed with sensors integrated into the Lander feet prototypes and placed in a sand filled construction container. The experiments serve to estimate the minimal range of detectability of the sounding signals.

Publisher Summary This chapter describes the experiments and simulations for two kinds of monolayer plasma crystals. In a monolayer with a small number of particles, ranging from 1–19, the microscopic structure of very small crystals is measured. The crystals have concentric rings. For certain numbers of particles, corresponding to “closed-shells” in the outermost ring, the configurations are the same as for hard spheres. Yukawa molecular-dynamics simulations accurately reproduce the experimental results. By adding far more particles (∼10,000) and operating at higher power, Mach cones are studies. These are V-shaped shocks created by supersonic objects, in which most particles are arranged in a monolayer, with a hexagonal lattice in a horizontal plane. Beneath the lattice plane, a sphere moves faster than the lattice sound speed. The resulting Mach cones are double, first compressional, then rarefactive, due to the strongly-coupled crystalline state. Molecular dynamics simulations using a Yukawa potential also show multiple Mach cones.

Observations show that plasma crystals, suspended in the sheath of a radio-frequency discharge, rotate under the influence of a vertical magnetic field. Depending on the discharge conditions, two different cases are observed: a rigid-body rotation (all the particles move with a constant angular velocity) and sheared rotation (the angular velocity of particles has a radial distribution). When the discharge voltage is increased sufficiently, the particles may even reverse their direction of motion. A simple analytical model is used to explain qualitatively the mechanism of the observed particle motion and its dependence on the confining potential and discharge conditions. The model takes into account electrostatic, ion drag, neutral drag, and effective interparticle interaction forces. For the special case of rigid-body rotation, the confining potential is reconstructed. Using data for the radial dependence of particle rotation velocity, the shear stresses are estimated. The critical shear stress at which shear-induced melting occurs is used to roughly estimate the shear elastic modulus of the plasma crystal. The latter is also used to estimate the viscosity contribution due to elasticity in the plasma liquid. Further development is suggested in order to quantitatively implement these ideas.

If we assume that the interaction potential of a dust particle in a plasma is of a Yukawa type, then we can determine the particle charge and the screening length from its trajectory during central collisions. We present a theoretical approach taking into account, that the particles are horizontally confined and that friction may influence the trajectories. In addition we show former results (1) based on this theory in the limit of negligible friction and confinement forces.

General properties of strongly coupled colloidal plasmas are briefly summarised, and their properties of being able to “condense” into a self-organised liquid and crystalline form is discussed. Both laboratory and microgravity aspects of the research into this new form of matter are described and the theoretical constraints are compared with available measurements. Finally, the phase transition solid-liquid-gaseous is investigated using measurements of the “normalised interaction cross section,” ∑σp, derived from “molecular diffusion of the colloid component.

Elastic dust particle particle interaction in an rf plasma provides a charge measurement method independent of the knowledge of plasma parameters. Assuming a screened Coulomb potential surrounding each particle with a constant charge at fixed plasma conditions, the charge and the screening length can be calculated from the trajectories of two colliding particles. With this method, we determine the charge and screening length of dust particles in the sheath of an rf discharge. The method can also be used to determine plasma parameters taking the dust particles as local probes. {copyright} {ital 1997} {ital The American Physical Society}

The controlled transport of micrometer size dust particles in a parallel-plate radio frequency discharge has been investigated. The lower stainless steel electrode consisted of 100 independently controllable electrical metal stripes. The voltage signals on these stripes were modulated, causing traveling plasma sheath distortions. Because the particles trapped in local potential wells moved according to the direction of the distortion, the transport velocity could be actively controlled by adjusting frequencies and phase shifts of the applied periodic voltage signals. To investigate the detailed principle of this transport, molecular dynamic simulations was performed to reproduce the observations with the plasma background conditions calculated by separated particle-in-cell simulations for the experimental parameters. The findings will help develop novel technologies for investigating large-scale complex plasma systems and techniques for achieving clean environments in plasma processing reactors.

Introducing a striped electrode in a large area discharge allows us to transport microparticles in the discharge in a user defined way. A directed and continuous dust transport is established by modulating the voltage signals on all individual electrodes to cause a traveling plasma sheath distortion. Particles, trapped in the potential wells and thus following the distortions, are finally removed from the central discharge region. Transport efficiency and velocity can be controlled by changing amplitude and traveling velocity of the plasma distortions.

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.

By employing a self-consistent kinetic approach, an analytical expression is derived for the potential of a test charge in a weakly ionized plasma with ion drift. The drift is assumed to be due to an external electric field, with the velocity being mobility-limited and much larger than the thermal velocity of neutrals. The derived expression is proven to be in excellent agreement with the measurements by Konopka et al. [Phys. Rev. Lett. 84, 891 (2000)] performed in the sheath region of a rf discharge.

In ground-based laboratory experiments, in experiments under microgravity conditions and in parabolic flight experiments, it is often observed that individual microparticles appear sometimes to simply pass through a strongly coupled complex plasma without affecting the fabric (the structure) of the penetrated cloud. This is surprising at first sight, because the collisional mean free path in such 'plasma crystals' is of the order of the interparticle separation. We have termed this new effect 'classical tunnelling' in analogy to its quantum mechanical counterpart. To explain this anomalous transport, a 'geometrical model for charge variation' of moving and stationary ordered particles is proposed. Theoretical predictions are in good qualitative agreement with observations and numerical simulations, and show that the phenomenon can be considered as a consequence of the non-Hamiltonian character of complex plasmas. ...It's a mystery therefore I can think whatever I want ... ...Scientists have a strange tendency to be insufficiently empirical sometimes... J Whyte www.newscientist.com

A highly efficient coagulation process of microparticles was discovered during experiments that were carried out on the International Space Station. Charged microspheres injected into a low pressure neutral gas environment form mm-sized aggregates, provided the initial number density is high enough. The coagulation occurred at a rate ∼10 4 -10 5 times faster than that usually observed with uncharged grains. Theoretical estimates show that this process is so fast that it cannot even be explained by a coagulation process with charge enhanced cross-section. The observations can, however, be described by a 'gelation phase transition'. We summarize the salient measurements, outline the theoretical description and show their compatibility.

Parabolic flights – short time µg-flights of 25 s each – are used to determine the parameter ranges for the upcoming long term experiment on complex plasmas on the International Space Station (ISS) – the Plasma Crystal-Experiment (PKE). The experimental results of approximately 170 parabolic flights conducted in 1999 are summarised. As an example, the interaction of microparticles of two different sizes in an rf-discharge is investigated on parabolic flights under microgravity conditions.