Jiri Pavlu

Mass-loss rate for MF resin microspheres

This paper deals with the influence of experimental conditions on properties of melamine formaldehyde resin particles. Motivation for this study was the fact that many laboratories and experimenters use these particles as samples for dust plasma investigations. We have found that the mass of these grains decreases during long-time exposure in vacuum. This decrease proceeds much faster if the temperature of grains is increased.

Secondary Emission From Glass Grains: Comparison of the Model and Experiment

The surface potential of dust grains immersed in surrounding plasma results from the balance of many charging processes such as photoemission, electron/ion attachments, and secondary and field emissions. Since hot electrons are often present in space as well as laboratory plasmas, the understanding of the secondary electron (SE) emission process for small dust grains is of great interest because their size effects modify well-known characteristics of large samples. This paper compares the measured surface potential of SiO2 spherical dust grains with the results of the Monte Carlo model of secondary emission developed for metallic samples. It was found that 1) the model can be used for description of the secondary emission process from cosmic dust, 2) the backscattering of primary beam electrons is the most important factor for the charging of small grains, and 3) the actual value of the surface grain potential is given by the energy spectrum of SEs

Influence of Charging Conditions on Field Ion Emission From Dust Grains

This paper deals with field ion emission (FIE) of spherical gold grains of micrometer sizes. We have analyzed the discharging characteristics measured under different conditions on one gold grain and suggested that the process limiting the discharging current is probably a diffusion of primary ions toward the grain surface. In order to confirm this hypothesis, we concentrated on the influence of the charging conditions (ion beam energy, ion species) on the discharging current. The investigations show that the ion field emission is a very complex process in which the charging conditions can change the discharging rate by an order of magnitude for a given electric field. We found that the total dose of the ions impacting the dust grain is an important parameter of the FIE. Our possible explanation is based on the implantation of the charging ions and their diffusion toward the surface of the dust grain

The Sputtering of Dust Grains: Aspects of Experimental Observations

Dust grains are sputtered at every environment containing energetic ions (i.e., ions with energies of several kiloelectronvolts). In the laboratory, only the beam experiments would fulfil these conditions; however, in the space, ions of these energies can be found even in the solar wind. It was suggested that the sputtering is one of the most important destruction processes of micrometer-sized dust grains, and on the other hand, it would be a source of heavy species in the interplanetary medium. We simulate the space environment by trapping the dust grains in an electrodynamic quadrupole trap and by influencing them by the ion beam with a variable energy up to 5 keV. The grains are charged to high surface potentials, and thus, a strong electric field near the surface can affect the sputtering rate. The finite size and the small curvature radius of grains play an important role in the quantification of sputtering efficiency. We propose a simple sputtering model for spherical grains and compare its predictions with measurements. An interpretation of the preliminary results obtained on gold microspheres bombarded by argon ions indicates that not only the grain mass but also the grain shape is changing in the course of our experiment. We suggest that similar effects can occur in the space if the dust is exposed to collimated ion beams

Lunar surface and dust grain potentials during the earth's magnetosphere crossing

Interaction between the lunar surface and the solar UV radiation and surrounding plasma environment leads to its charging by different processes like photoemission, collection of charged particles, ...

Dust as a Gas Carrier

Dust in space can collect particles from surrounding plasma and transport them over long distances. Release of the implanted particles can then change the mass composition in a particular place of the space. The depth of ion penetration into the dust body strongly depends on an initial mutual energy and differs with ion species as well as with the grain composition. The same holds for diffusion constant of implanted ions (already neutralized) exiting back to the free space. We have used our measurements of the release of Ar ions implanted into glassy carbon dust grains for determination of the diffusion coefficient. Our calculations provide the limits for the amount of gas that can be dissolved in the grain as well as its release rate. We discuss the influence of the dust sputtering and dust temperature on the aforementioned quantities.

Numerical Calculation of an Equilibrium Dust Grain Potential in Lunar Environment

The interaction of plasma particles with dust grains leads to their charging. An equilibrium grain potential depends on a plasma environment, as well as on the grain composition, size, shape, and charging history. We present results of calculations of the equilibrium potential of the grain immersed in the plasma simulating a lunar environment. In calculations, we apply a modified model of the secondary electron emission from dust grains, which takes into account grain sizes, their material, and surface roughness. Since this model describes the increase in the secondary emission yield caused by a finite dimension of the dust grain, the calculations provide a realistic estimation of the dust grain charge in the near-Earth environment. We show that the grain surface potential is a descending function of the grain size and this effect can even lead to opposite polarities of small and large grains.

Emissions from nonconducting negatively charged dust grains

Dust grains immersed into a low-temperature plasma are charged negatively because the electron attachment dominates other charging processes. However, increasing energy of impinging electrons leads to the increase of the yield of secondary emission. When this yield exceeds unity, the grain becomes charged positively. Previous laboratory experiments with electron beams have shown that the surface potential of large grains follows roughly an energy dependence of the secondary emission yield. It means that the grain potential reaches its maximum for hundreds of electronvolts of electron energy and then decreases. Model calculations reveal that the decrease of the grain potential with the electron energy can lead to the reversal of charge sign in a certain range of grain diameters. After this reversal, the grain is charged to the negative potential close to the beam energy. We use of this effect for investigations of electron field emission from nonconducting spheres (glass and melamine formaldehyde resin) with the diameter 4-11 /spl mu/m for glass and 5 and 10 /spl mu/m for resin. Single grains were trapped in a Paul trap and bombarded by the electron beam. The beam energy was changed in a range of 0.3-10 keV. The experimental results confirm the aforementioned expectations and an analysis of these results has shown that: 1) the effective work function for electron field emission from charged nonconducting materials (glass and melamine resin) could be as low as 2 eV, but it can be close to 5 eV as expected for insulating materials. The difference is connected with the charging history. 2) The effect of field dependent secondary emission limits the attainable negative potential.

Secondary Emission From Clusters Composed of Spherical Grains

Dust grains or their clusters can be frequently found in many space environments-interstellar clouds, atmospheres of planets, tails of comets, or planetary rings are only typical examples. Space dust grains are formed by various processes and their shapes are complex. These grains are exposed to electrons with different energies, and thus, they can acquire positive or negative charge during this interaction. We present a systematic study of well-defined systems-clusters consisting of different numbers of small spherical (1 μm) grains and such objects can be considered as examples of real irregularly shaped space grains. The charges acquired by investigated objects as well as their secondary emission yields are calculated using the secondary emission model. We have found that: 1) the charge and surface potential of clusters exposed to the electron beam are influenced by the number of grains and by their geometry within a particular cluster; 2) the model results are in an excellent agreement with the experiment; and 3) there is a large difference between charging of a cluster levitating in the free space and that attached to a planar surface. The calculation provides a reduction of the secondary electron emission yield of the surface covered by dust clusters by a factor of 1.5 with respect to the yield of a smooth surface.

Charging Properties of Dust Grain Clusters

The dust grains or their clusters can be frequently found in many space environments ‐ interstellar clouds, tails of comets or planetary rings are only typical examples. Being surrounded by plasma, dust grains are charged by various processes as photoemission, collection of electrons and/or ions, secondary or field emissions. The determination of a charge, which can the grain or cluster of grains gain by these processes, is very important for an estimation of the dusty plasma properties because electrostatic forces highly dominate the gravitation. However, our knowledge of charging processes is inferred from measurements on large planar samples and thus it can be applied on micron‐sized grains only with a care and their application on the clusters is under question. Our experimental set‐up allows us to trap a single dust grain or cluster of grains in an electrodynamic quadrupole and to expose it by the electron and/or ion beams. Precise measurements of a stepwise change of charge‐to‐mass ratio induced by ...