Jakub Vaverka

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.

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, ...

Detection of meteoroid hypervelocity impacts on the Cluster spacecraft: First results

We present the first study of dust impact events on one of the Earth-orbiting Cluster satellites. The events were identified in the measurements of the wide band data (WBD) instrument on board the ...

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.

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.

Relation of Charging History to Field Ion Emission From Gold and Carbon Dust

A study of dust-grain charging plays a very important role in the understanding of complex (dusty) plasma. The dust grains are charged by several different processes (e.g., electron and ion attachments, secondary electron emission, photoemission, and field electron and ion emissions), and their charge significantly influences the surrounding plasma. Our laboratory experiment based on an electrodynamic quadrupole trap allows us to investigate some of these processes on a single isolated dust grain which can be trapped and influenced with different agents for a very long time (days). In this paper, we focused on the determination of the relation between charging conditions and the field-emission mechanism because this emission limits positive charges that dust grains can acquire due to photoemission, secondary emission, or ion attachment. The field-ion-emission process is based on the field ionization of the atoms that crosses a critical distance from the grain surface. We have found that the sources of these atoms are either the surrounding gas or the ions implanted into the grain and leaving it due to diffusion. The diffusion can be described by two time constants differing by an order of magnitude. We used two sets of dust grains: gold and amorphous carbon. The experimental results are confirmed by a simple model.

Potential of Earth Orbiting Spacecraft Influenced by Meteoroid Hypervelocity Impacts

Detection of hypervelocity impacts on a spacecraft body using electric field instruments has been established as a new method for monitoring of dust grains in our solar system. Voyager, WIND, Cassini, and STEREO spacecraft have shown that this technique can be a complementary method to conventional dust detectors. This approach uses fast short time changes in the spacecraft potential generated by hypervelocity dust impacts, which can be detected by monopole electric field instruments as a pulse in the measured electric field. The shape and the duration of the pulse strongly depend on parameters of the ambient plasma environment. This fact is very important for Earth orbiting spacecraft crossing various regions of the Earth’s magnetosphere where the concentration and the temperature of plasma particles change significantly. We present the numerical simulations of spacecraft charging focused on changes in the spacecraft potential generated by dust impacts in various locations of the Earth’s magnetosphere. We show that identical dust impacts generate significantly larger pulses in regions with lower electron density. We discuss the influence of the photoelectron distribution for dust impact detections showing that a small amount of energetic photoelectrons significantly increases the potential of the spacecraft body and the pulse duration. We also show that the active spacecraft potential control (ASPOC) instrument onboard the cluster spacecraft strongly reduces the amplitude and the duration of the pulse resulting in difficulties of dust detection when ASPOC is ON. Simulation of dust impacts is compared with pulses detected by the Earth orbiting cluster spacecraft in the last part of Section III.

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.

Low frequency oscillatory flow signatures and high-speed flows in the Earth's magnetotail†

Using plasma sheet data from Cluster 1 spacecraft from 2001 till 2011, we statistically investigate oscillatory signatures in the plasma bulk flow. These periodic oscillations are compared to high- ...

PLASMA-F experiment: Three years of on-orbit operation

Composition and tasks of the PLAZMA-F experiment onboard the SPEKTR-R satellite are described in this paper. A record high time resolution is a feature of solar wind plasma flux and energetic particle flux measurements. It allowed detecting a number of new and significant properties and parameters.