Problems of Dust Grains Charging to Negative Potentials

Field emission characteristics of gold dust grains

The modified Simon-Hoh Instability (MSHI) in an inhomogeneous plasma in presence of charged dust grains is investigated where the ions and dust are unmagnetized but electrons are strongly magnetized. Our dispersion relation matches exactly to the dispersion equation (18) of Sakawa et al.15, when we withdraw the contribution of dust grains from our expression. The growth rates of MSHI in presence of dust grains, shows either comparable or lower than that of dust free plasma.

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.

Physics and applications of secondary electron emission: Bruining, H.; Pergamon Press Ltd. London, and McGraw-Hill Co., New York, xii + 178 pp. 130 figures. 25s. net or $5

Analytical model of secondary electron emission yield in electron beam irradiated insulators

Ion beam effects on dust grains: 2—Influence of charging history

Microwave (mw) breakdown of atmospheric pressure microgaps is studied by a one-dimensional Particle-in-Cell Monte Carlo Collisions numerical model. The effect of both field electron emission and secondary electron emission (due to electron impact, ion impact, and primary electron reflection) from surfaces on the breakdown process is considered. For conditions where field emission is the dominant electron emission mechanism from the electrode surfaces, it is found that the breakdown voltage of mw microdischarge coincides with the breakdown voltage of direct-current (dc) microdischarge. When microdischarge properties are controlled by both field and secondary electron emission, breakdown voltage of mw microdischarge exceeds that of dc microdischarge. When microdischarge is controlled only by secondary electron emission, breakdown voltage of mw microdischarge is smaller than that of dc microdischarge. It is shown that if the interelectrode gap exceeds some critical value, mw microdischarge can be ignited only by electrons initially seeded within the gap volume. In addition, the influence of electron reflection and secondary emission due to electron impact is studied.

The heliosphere is a “bubble” of plasma that forms around the Sun through a pressure balance between the outflowing solar wind and the interstellar medium. The Sun is currently traversing the local interstellar medium at a relative velocity of approximately 26 km s−1. Due to the Sun’s motion, interstellar dust grains present in the interstellar medium are transported through the heliosphere’s boundary, from the upwind direction.Dust grains in a space environment are subject to a variety of charging mechanisms, which result in an overall equilibrium charge on their surface. In the interstellar medium and in the solar wind, the primary charging mechanisms are plasma collection, secondary electron emission, and photoelectric emission. The charge acquired by a dust grain depends on several factors, including the size, composition, and structure of the dust grain itself, as well as on the characteristics of the surrounding environment.The trajectories of charged dust grains are influenced by the magnetic field in the environment they are moving through due to the emerging Lorentz force. When approaching the heliosphere, the interstellar magnetic field starts to get disturbed by the solar wind (heliospheric) magnetic field. The amount of trajectory deflection an inflowing interstellar dust grain experiences depends on its charge-to-mass ratio. Consequently, not all interstellar dust grains enter the solar system.We discuss the dust charging with a particular focus on the influence of the space environment conditions that are expected at different locations throughout the heliosphere, including the boundary regions and including short-term and long-term variations of the environmental conditions due to the solar activity. Using these results, we show the influence of heliospheric properties on the dust grain trajectories at the heliospheric interface in specific. The results will help to understand the physical processes occurring at the boundary of the heliosphere.

A set of fluid equations is obtained for a dusty plasma, with dust charge and mass distributions, interacting with ambient neutral dust through inelastic collision processes involving dust grain charging and discharging and secondary electron emission from the dust grains. These are obtained as appropriate moments of the Boltzmann–Vlasov equations for the positively and negatively charged dust components, neutral dust component, electrons, and ions. Also obtained are the charge and mass moments of these equations, which bring out a new term involving the charge dispersion in the equation for the phase space charge density. The inelastic collision process involving dust grain charging and discharging and secondary electron emission provides a coupling between the positive and negatively charged dust components and the neutral dust component. The source/sink terms in the equations of continuity, momentum balance, and heat equations then are the proper terms obtained systematically from the Boltzmann–Vlasov equations, as against some ad hoc formulations that have been used earlier. The set of equations so obtained should prove useful in analyzing the various dusty plasma phenomena: waves, instabilities, and nonlinear processes in space and astrophysical scenarios.

Secondary electron (SE) emission (SEE) from material surfaces is a frequent phenomenon in space and vacuum environments. SEE modulation is important since it governs the performance of some devices such as electronic multipliers or induces some detrimental effects such as multipactors. Surface coating has been reported to modulate SEE effectively, whereas SEE behaviors of coating structures are not clearly understood yet, and the appropriate theory to describe SEE characteristics quantitatively for coating structures is less developed so far. Here, we have prepared four alumina coatings possessing various thicknesses to research the SEE characteristics of coating structures and have shown how the coating thickness affects the SEE behaviors. Besides, by considering coating/substrate as an ideal double-layer structure, we have derived several equations to describe the producing, transmitting, and escaping processes of excited inner SEs and finally constructed a unidimensional SEE model for double-layer structures. The model is applicable to reveal the dependence of true SE yield (TSEY) on the top and bottom layers’ physical properties and estimate TSEY proportions contributed by the top and bottom layers at random energy points. By employing the concept, SEE characteristics of Al2O3/Si, MgO/Si, and TiO2/Si double-layer structures have been quantitatively interpreted. Moreover, the abnormal SEY curve with a double-hump shape, which is induced by the peak position distinction of SiO2/Si structures, can also be explained. This work is of great significance to comprehend TSEY modulating regularities of various double-layer structures applied in surface engineering.

A large amplitude modulated Gaussian electromagnetic beam propagating in a dusty plasma has been studied. The electrons are heated nonuniformly by the beam. For the nonsteady state, we obtain nonlinear current density in the presence of dust grains. This expression has been used to study the nonstationary self-focusing and resulting self-distortion of the amplitude modulated electromagnetic beam. It has been observed that the effect of dust is drastic on the modulation index in comparison to dust free plasma. A simple scaling showing the dependence of modulation index on dust grain size and charge has been presented.

In this work, we have studied the physical properties of magnetized electrostatic sheaths in presence of dust grains and monoenergetic electrons. For this, we have established a three dimensional and stationary model. The electrons and negative ions are considered in thermodynamic equilibrium. However, the positive ions, the monoenergetic electrons and the dust grains are described by the cold fluid equations. Furthermore, the dust charge is described by the orbit motion limited model (OML). The numerical results show that the presence of a magnetic field reduces the sheath thickness. On the other hand, the presence of monoenergetic electrons changes the behavior of the electrostatic sheath physical parameters. The effects of the other parameters are also analyzed and discussed.

In the coma of a comet, the dust grains emitted by the cometary nucleus are embedded in a surrounding plasma and exposed to the sunlight. Several effects (impinging charged plasma particles, photo-, secondary, thermionic, and field emission of electrons, sputtering of ions) might cause the electrostatic charging and possibly the fragmentation of some of these grains. Using in-situ-measurements of the comets P/Giacobini-Zinner (P/G-Z) and P/Halley, the electrostatic charging and fragmentation of spherical dust particles of carbon and silicate in the cometary comae is investigated theoretically. The results are compared with in-situ-measurements of dust in the coma of P/Halley.

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.

A potential of low energy (< 1keV) electron channeling contrast imaging (LE‐ECCI) by scanning electron microscopy (SEM) is demonstrated to characterize the crystallographic stacking sequence of hexagonal 4H‐SiC single crystal within the first unit cell thickness from the topmost surface (< 1nm). It is also revealed that the LE‐SEM signal intensity associated with the crystallographic orientation has enough sensitivity to even the change in Si‐atom and C‐atom (i.e. the change in surface polarity) at a primary electron energy of 0.4keV. The obtained data can be explained by an electron multiple scattering cluster (EMSC) theory where the interference of both incident and diffracted electron waves is considered within a finite size (a few nanometers) of an atomic cluster. So far, for characterizing local crystallographic orientation of bulk crystals using SEM, ECCI with a higher energy (> 10keV) of primary electron beam has been utilized in order to obtain larger interaction volume due to its deeper penetration depth [1]. The electron channeling contrast (ECC) from only a shallower surface region with lower energy has not been applied yet because of the consideration attributed to the lack of enough interaction volume in the lower energy regime. In fact, to our knowledge, the critical smallest volume (specimen minimum thickness) has not been investigated yet associated with ECC. In our previous study [2], a crystallographic orientation contrast from two bilayers thick surface layers of 4H‐SiC single crystal was observed by changing an incident LE (< 1keV) EB direction with respect to the orientation of an atomically flattered 4H‐SiC surface (Figs. 1). This contrast is considered to be the ECC in an extremely low electron energy regime. This result brings about the assumption that even the two bilayers thick interaction volume can contribute to the formation of ECC. In order to verify this assumption, in this study, crystallographic orientation dependence of SEM signal intensity was quantitatively measured on an atomically flat 4H‐SiC surface. Furthermore, in order to exploit the Z ‐information, different polar faces, Si‐terminated (0001) Si‐face, and C‐terminated (000‐1) C‐face were compared, where atomic sites of Si‐ and C‐atoms are switched in the crystal. The obtained results were compared with the EMSC theory in which the concept of short‐range periodicity is included [3]. This theoretical model is more applicable to reproduce the effect of shallower information depth of the LE‐EB, compared with a Bloch‐wave model utilized in the conventional ECCI in the higher electron energy regime. Fig.2 shows the representative SEM signal intensity from 4H‐SiC Si‐face and C‐face as a function of the sample tilting angle obtained at a primary electron energy of 0.4 keV. Although the profile shapes seem to be different at a glance, peak positions are almost the same. Considering the fact that the atomic arrangement below the surface is identical both in Si‐face and C‐face SiC, except Si‐ /C‐atomic sites in the crystal, the difference in the profile shape is ascribed to the contrast of backscattered electron yield [4]. The similar trend was also reproduced by the EMSC calculation.