Ivana Richterova

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.

LUNAR DUST GRAIN CHARGING BY ELECTRON IMPACT: DEPENDENCE OF THE SURFACE POTENTIAL ON THE GRAIN SIZE

The secondary electron emission is believed to play an important role for the dust charging at and close to the lunar surface. However, our knowledge of emission properties of the dust results from model calculations and rather rare laboratory investigations. The present paper reports laboratory measurements of the surface potential on Lunar Highlands Type regolith simulants with sizes between 0.3 and 3 μm in an electron beam with energy below 700 eV. This investigation is focused on a low-energy part, i.e., ≤100 eV. We found that the equilibrium surface potential of this simulant does not depend on the grain size in our ranges of grain dimensions and the beam energies, however, it is a function of the primary electron beam energy. The measurements are confirmed by the results of the simulation model of the secondary emission from the spherical samples. Finally, we compare our results with those obtained in laboratory experiments as well as those inferred from in situ observations.

Interaction between single dust grains and ions or electrons: laboratory measurements and their consequences for the dust dynamics

The present paper reviews our latest, and brings several new, results on charging of dust grains of various materials and sizes. Charging processes of dust in space and their influence on the dust dynamics are analyzed in laboratory simulations of secondary emission, field ion and electron emissions, and dust sputtering. Single micrometre-sized grains and grain clusters are stored in a hyperbolic quadrupole field under ultra-high vacuum conditions for long time periods. The charge state of the grain and its evolution are recorded while the grain is exposed to ion or electron beams of various energies and fluxes. The influence of the secondary electron emission on the charge state is measured and compared with a computer model. Limitations on the grain charge by the field electron and ion emission are considered next. The measurements allow analyzing field emission from conductive and dielectric grains. The existence of long-lived surface states on insulating materials, which are probably responsible for the anomalous behavior of field electron emission and the low threshold of the field ion emission, is indicated. The observation of sputtering by energetic ions showing a surprising anisotropic erosion of a conductive grain is analyzed. The sputtering and the field ion emission are discussed as possible sources of the so-called pick-up ions.

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

SECONDARY EMISSION FROM NON-SPHERICAL DUST GRAINS WITH ROUGH SURFACES: APPLICATION TO LUNAR DUST

Electrons impinging on a target can release secondary electrons and/or they can be scattered out of the target. It is well established that the number of escaping electrons per primary electron depends on the target composition and dimensions, the energy, and incidence angle of the primary electrons, but there are suggestions that the targets shape and surface roughness also influence the secondary emission. We present a further modification of the model of secondary electron emission from dust grains which is applied to non-spherical grains and grains with defined surface roughness. It is shown that the non-spherical grains give rise to a larger secondary electron yield, whereas the surface roughness leads to a decrease in the yield. Moreover, these effects can be distinguished: the shape effect is prominent for high primary energies, whereas the surface roughness predominantly affects the yield at the low-energy range. The calculations use the Lunar Highlands Type NU-LHT-2M simulant as a grain material and the results are compared with previously published laboratory and in situ measurements.

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.

The influence of secondary electron emission on the floating potential of tokamak-born dust

Dust production and its transport into the core plasma is an important issue for magnetic confinement fusion. Dust grains are charged by various processes, such as the collection of plasma particles and electron emissions, and their charge influences the dynamics of the dust. This paper presents the results of calculations of the surface potential of dust grains in a Maxwellian plasma. Our calculations include the charging balance of a secondary electron emission (SEE) from the dust. The numerical model that we have used accounts for the influence of backscattered electrons and takes into account the effects of grain size, material, and it is also able to handle both spherical and non-spherical grains. We discuss the role of the SEE under tokamak conditions and show that the SEE is a leading process for the grains crossing the scrape-off layer from the edge to core plasma. The results of our calculations are relevant for materials related to fusion experiments in ITER.