J. Pavlů

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.

Short-scale Variations of the Solar Wind Helium Abundance

Abrupt changes of the relative He abundance in the solar wind are usually attributed to encounters with boundaries dividing solar wind streams from different sources in the solar corona. This paper presents a systematic study of fast variations of the He abundance that supports the idea that a majority of these variations on short timescales (3-30 s) are generated by in-transit turbulence that is probably driven by the speed difference between the ion species. This turbulence contributes to the solar wind heating and leads to a correlation of the temperature with He abundance.

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 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.

Problems of Dust Grains Charging to Negative Potentials

The presence of dust grains is a common phenomenon in the space environment. Grains can be charged by many different processes (e.g., photoemission, attachment of electrons/ions, the secondary emission, etc.). If the grains surface potential becomes high enough, one can observe field emission of ions or electrons. We are trapping a single dust grain in a Paul trap, expose it to a low-energy electron beam, and investigate the evolution of its charge-to-mass ratio with respect to the energy of primary electron beam. We use micron-sized (D = 2–11 μm) glass grains and charge them up to -300 V of surface potentials; it corresponds to the electric field strength of about 108 V/m. Analysis of the charging/discharging processes has shown that (1) the effect of the field enhanced secondary emission is negligible in the case of insulators and (2) the effective work function for electron field emission from charged insulators is as low as ≈ 1 eV.

Linear trap with three orthogonal quadrupole fields for dust charging experiments

Investigations of charging processes on a single dust grain under controlled conditions in laboratory experiments are the unique way to understand the behavior of dust grains in complex plasma (in space, in laboratory, or in technological applications). An electrodynamic trap is often utilized for both holding a single grain and continuously measuring its charge-to-mass ratio. We propose a modified design of the linear quadrupole trap with the electrodes split into two parts; each of them being supplied by a designated source. The paper presents basic calculations and the results of the trap prototype tests. These tests have confirmed our expectations and have shown that the suggested solution is fully applicable for the dust charging experiments. The uncertainty of determination of the dust grain charge does not exceed 10(-3). The main advantages of the suggested design in comparison with other traps used for dust investigations can be summarized as: The trap (i) is more opened, thus it is suitable for a simultaneous application of the ion and electron beams and UV source; (ii) facilitates investigations of dust grains in a broader range of parameters; and (iii) allows the grain to move along the axis in a controlled way.

Secondary electron emission from Martian soil simulant

In the recent years, growing interest in dust charging physics is connected with several lander missions running on or planned to the Moon, Mars, and Mercury for a near future. In support of these missions, laboratory simulations are a potential tool to optimize in situ exploration and measurements. In the paper, we have investigated electrical properties of a Martian soil simulant prepared at the Johnson Space Center under name JSC Mars-1 using the dust charging experiment when a single dust grain is trapped in a vacuum chamber and its secondary electron emission is studied. The exposure of the grain to the electron beam revealed that the grain surface potential is low and generally determined by a mean atomic number of the grain material at a low-energy range ( 2 keV) electron energies. We discuss possible implications of the secondary electron emission for the presence of lightnings on Mars.

Electrons Emitted From Small Dust Grains: Comparison Of Sphere And Cube

Dust grains are exposed to high‐energy electrons in different plasma environments. We have developed the numerical model that provides a description of the electron‐solid (represented by grain surface) interaction. The model fits the experimental data from spherical samples of different materials rather well, however, dust grains in space are often edgy and coarse. Thus we extended grain geometry in the model to cubes as a first approximation of irregular surface.

Electric Field Influence on Secondary Emission

The paper is devoted to a detail study of charging and discharging processes of 5 μm melamine formaldehyde resin grains that could partly simulate porous aggregates observed in the space. We present unusual charging properties of these grains influenced by both low‐ and high‐energy electron beams of various current densities. The increase of the second crossover potential for highly negatively charged grains was derived from equilibrium charging characteristics.