Emissions From Nonconducting Negatively Charged Dust Grains

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 grain's 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.

Field emission characteristics of gold dust grains

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.

Secondary emission from dust grains with a surface layer: comparison between experimental and model results

This study aims to understand how secondary electron emissions influence the behaviour of dusty plasma sheaths. The analysis is carried out within the framework of a non-extensive Tsallis distribution in the presence of dynamically charged dust particles and an oblique external magnetic field. Two main types of secondary emission are considered: those emitted by the dynamically charged dust grains, which are treated using the Sternglass model, and those emitted from the wall bounding the sheath. These are accelerated within the sheath by its electric potential, and modeled using a half-Maxwellian distribution. The objective of this study is to improve our understanding of the mechanisms that govern complex plasma sheaths. It is observed that the combined effect of these secondary emissions significantly alters the properties of the sheath, particularly the dynamics of the dust, the sheath potential, and the density distribution of charged species. This influence is particularly pronounced at lower values of the non-extensive parameter q of the primary electrons.

The head-on collision of dust acoustic solitary waves in a four-component unmagnetized dusty plasma with Boltzmann distributed electrons, nonthermal ions, and negatively charged dust grains as well as positively charged dust grains is investigated using the extended Poincaré-Lighthill-Kuo method. The effects of the ratio of electron temperature to ion temperature and the ratio of the positively charged dust grains mass to the negatively charged dust grains mass on the phase shift are studied. It is found that the presence of nonthermal ions plays a significant role on the collision of dust acoustic solitary waves. This study would be useful for investigations of plasma behavior in different regions of space, viz., cometary tails, upper and lower mesosphere, Jupiter’s magnetosphere, etc.

An ion beam propagating through a magnetized potassium plasma cylinder having negatively charged dust grains drives electrostatic ion-acoustic waves to instability via Cerenkov interaction. The phase velocity of sound wave increases with the relative density of negatively charged dust grains. The unstable wave frequencies and the growth rate increase, with the relative density of negatively charged dust grains. The growth rate of the unstable mode scales as one-third power of the beam density. The real part of frequency of the unstable mode increases with the beam energy and scales as almost the one-half power of the beam energy.

Supposing that the constituents of dusty plasmas are inertialess electrons, inertialess ions, and an admixture of negatively and positively charged inertial dust grains, and employing the nonlinear multi-fluid dusty plasma equations and Poisson's equation, we have derived dynamical equations for small amplitude nonlinear dust-acoustic waves, as well as the energy integral of a classical particle with a modified Sagdeev potential for large amplitude disturbances. The energy integral is analyzed both analytically and numerically to demonstrate the existence of rarefactive and compressive electrostatic potentials which travel faster than the effective dust-acoustic velocity. Compressive dust-acoustic potential exist only when there is a significant fraction of positively charged dust grains. Furthermore, the four-fluid dusty plasma system also admit double layers.

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.

Collisions of ions with charged dust grains are important for the propagation of low frequency waves such as dust acoustic waves and dust ion-acoustic waves. The collision cross-sectional area of charged dust grains depends on the velocity of an ion beam. The collision cross-sectional area of charged dust grains with beam ions is measured. It is compared with the geometrical cross-sectional area of the grain. The experiment is performed in a dusty double-plasma device with glass beads of 8.9 μm in average diameter. The ion beam current and energy are measured with a directional retarding potential analyzer. It is observed that, when dust density inside the system is increased, the beam current ratio is reduced. From the reduction of the ion beam current, the effective cross-sectional area of the dust particle is estimated as a function of the beam energy.

The charging of dust grains in astrophysical environments has been investigated with the assumption these grains are homogeneous spheres. However, there is evidence which suggests many grains in astrophysical environments are irregularly-shaped aggregates. Recent studies have shown that aggregates acquire higher charge-to-mass ratios due to their complex structures, which in turn may alter their subsequent dynamics and evolution. In this paper, the charging of aggregates is examined including secondary electron emission and photoemission in addition to primary plasma currents. The results show that the equilibrium charge on aggregates can differ markedly from spherical grains with the same mass, but that the charge can be estimated for a given environment based on structural characteristics of the grain. The "small particle effect" due to secondary electron emission is also important for determining the charge of micron-sized aggregates consisting of nano-sized particles.

The interaction of charged dust grains with nonlinear vortical structures in the Earth's atmosphere is analyzed. Certain aspects of the atmosphere−ionosphere interaction, in particular, mechanisms for the appearance of dust grains at ionospheric altitudes, are discussed. It is shown that, at certain altitudes, there are regions in the wavenumber space in which conditions leading to the excitation of acoustic−gravity waves are satisfied. The interaction of nonlinear acoustic−gravity waves with dust grains of meteoric origin at ionospheric altitudes, which leads to the mixing and redistribution of dust grains over the region where vortices exist, is investigated. The possibility of formation of vertical and horizontal dust flows in dusty ionospheric plasma as a result of modulational instability is analyzed. The dynamics of dust grains in dust devils frequently arising in the atmosphere above well-heated surfaces is modeled. The vortical structure of such a dust devil is characterized by a reduced pressure in the center, which facilitates the lifting of small dust grains from the surface. The formulated model is used to calculate the trajectories of dust grains in dust devils with allowance for the influence of the electric field generated in the vortex by colliding dust grains. The calculations show that dust devils play an important role in the transport of dust grains.

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 transfer of kinetic energy between cosmic rays and charged dust grains is investigated. The time required for heating of the dust grains due to Coulomb interactions is estimated and compared with the heating time due to ionisation losses by the cosmic rays. Heating of dust grains by cosmic rays is examined in connection with the FIR–radio correlation in galaxies: even for the most favorable conditions, including a high energy density in low-energy, ≲ 100 keV, cosmic rays, the conclusions are equivocal.

Results are presented from Monte Carlo calculations of the electric charge of dust grains in a plasma produced during the slowing down of the radioactive decay products of californium nuclei in neon. The dust grain charging is explained for the first time as being due to the drift of electrons and ions in an external electric field. It is shown that the charges of the grains depend on their coordinates and strongly fluctuate with time. The time-averaged grain charges agree with the experimental data obtained on ordered liquidlike dust structures in a nuclear-track plasma. The time-averaged dust grain charges are used to carry out computer modeling of the formation of dynamic vortex structures observed in experiments. Evidence is obtained of the fact that the electrostatic forces experienced by the dust grains are potential in character.