B. P. Pandey

Plasma sheath in the presence of an oblique magnetic field

In this work the structure of a plasma sheath in the presence of an oblique magnetic field is investigated. The effect of plasma magnetization, plasma ionization and plasma–neutral collisions on the sheath is examined for different orientations of the magnetic field. It is shown that the width of the plasma sheath is dependent not only on the collision frequencies and the plasma magnetization but also on the angle of magnetic field orientation. The size of the sheath layer decreases with the increase in the angle between the magnetic field and wall. The grain inside the sheath acquires a more positive charge when the magnetic field is perpendicular to the wall in comparison with the parallel to the wall case. This results in the different levitational equilibrium of the grain. The implication of the results for measuring the sheath characteristics is discussed.

Parametric Instability in Dark Molecular Clouds

The present work investigates the parametric instability of parallel propagating circularly polarized Alfven (pump) waves in a weakly ionized molecular cloud. It is shown that the relative drift between the plasma particles gives rise to the Hall effect, resulting in the modified pump wave characteristics. Although the linearized fluid equations with periodic coefficients are difficult to solve analytically, it is shown that a linear transformation can remove the periodic dependence. The resulting linearized equations with constant coefficients are used to derive an algebraic dispersion relation. The growth rate of the parametric instability is a sensitive function to the amplitude of the pump wave as well as to the ratio of the pump and the modified dust-cyclotron frequencies. The instability is insensitive to the plasma β. The results are applied to the molecular clouds.

The stability of the mesospheric plasma layer

The presence of micron and sub-micron size dust in the Earth’s summer mesopause are a possible cause of electron density depletion. Whereas electrons in this weakly ionized and weakly magnetized layer are frozen in the magnetic field, the ions and dust are highly diffusive. This relative drift between the plasma particles will cause a current in the medium. The presence of such a current can destabilize the plasma layer with a growth rate of the order of Alfven frequency. Since required current density for the onset of this instability is on the order of J≳0.03A/m2, it is quite unlikely that such a strong current is present in the mesosphere. However, owing to the prevailing ambiguity of measurements, the existence of such a current is not completely ruled out.

Parametric instability in a collisional dusty plasma

The present work investigates the parametric instability of parallel propagating circularly polarized Alfven (pump) waves in a collisional dusty plasma. It is demonstrated that the relative drift between the charged dust and the electrons and ions gives rise to the Hall effect resulting in the modified pump wave characteristics. Although the linearized fluid equations with periodic coefficients are difficult to solve analytically, it is shown that a linear transformation can remove the periodic dependence. The resulting linearized equations with constant coefficients are used to derive an algebraic dispersion relation. The growth rate of the parametric instability is a sensitive function of the amplitude of the pump wave as well as to the ratio of the pump and the dust-cyclotron frequencies. The instability is insensitive to the plasma-beta parameter. The possible application of the result in the astrophysical context is discussed.

Shear instability in magnetized, collisional dusty plasmas

The shear instability of magnetized, collisional dusty plasma is investigated in the present work. It is demonstrated that the relative drift between the charged dust and magnetised electrons and ions which give rise to the Hall effect is crucial to this instability. Although the nature of present shear instability is similar to the Kelvin-Helmholtz instability, the role of magnetic field in the present case is important in destabilising waves. The maximum growth rate of the instability is proportional only to the shear gradient and is independent of the ambient magnetic field strength. Most unstable wavenumber is a function of ambient dust parameters.

Nonlinear waves in collisional dusty plasma

The nonlinear wave propagation in collisional dusty plasma is investigated in the present work. When the electrons and ions are highly magnetized, i.e., when the Lorentz force acting on the plasma particles dominates its collision with the dust and charged dust remains weakly magnetized, the drift between the plasma and dust particles may significantly modify the wave characteristics of the medium. It is shown that when electrons and ions move approximately with the same bulk velocity, the large amplitude waves can be easily excited in such a collisional dusty medium and they can be described by derivative nonlinear Schrodinger equation. It is quite possible that the soliton solutions of the nonlinear equation may be useful in explaining the parsec scale structures in the astrophysical plasmas.

The stability of weakly ionized collisional dusty plasma in the presence of flow

The stability of weakly ionized and magnetized plasma in the presence of transverse (to the magnetic field) neutral wind is investigated in the present work. The collisional coupling of ambient background flow to the magnetized plasma gives rise to an electric field. In the presence of charged unmagnetized dust, electrostatic fluctuations in such plasma become unstable, with the growth rate dependent on the plasma thermal speed as well as on the dust charge and collision frequencies. This instability is similar to the Farley-Buneman instability. However, unlike Farley-Buneman, where the growth rate is directly dependent on the background flow, this dependence in the present case is only indirect. It is shown that this instability can grow over few seconds in the Earths lower ionosphere and thus could play an important role in the structure formation.

The acoustic instabilities in magnetized collisional dusty plasmas

The present work investigates the wave propagation in collisional dusty plasmas in the presence of electric and magnetic field. It is shown that the dust ion-acoustic waves may become unstable to the reactive instability whereas dust-acoustic waves may suffer from both reactive and dissipative instabilities. If the wave phase speed is smaller than the plasma drift speed, the instability is of reactive type whereas in the opposite case, the instability becomes dissipative in nature. Plasma in the vicinity of dust may also become unstable to reactive instability with the instability sensitive to the dust material: dielectric dust may considerably quench this instability. This has implications for the dust charging and the use of dust as a probe in the plasma sheath.

Surface waves in the magnetized, collisional dusty plasmas

The properties of the low frequency surface waves in inhomogeneous, magnetized collisional complex dusty plasma are investigated in this work. The inhomogeneity is modelled by the two distinct regions of the dusty medium with different dust densities. The external magnetic field is assumed to be oriented along the interface dividing the two medium. It is shown that the collisional momentum exchange that is responsible for the relative drift between the plasma particles affects the propagation of the surface waves in the complex plasma via the Hall drift of the magnetic fluctuations. The propagation properties of the sausage and kink waves depend not only on the grain charge and size distribution but also on the ambient plasma thermal conditions.

Dust in near-wall magnetized plasma

The experimental work on dust in near-wall plasmas has attracted considerable interest in the last few years. Dust particles can carry both negative as well as positive charge depending upon the ambient plasma conditions. The charged dusts not only levitate over a negatively charged wall due to balance between the gravitational and electrostatic forces but also move under the action of fields and flows in the plasma. The controlled experiment with massive dust grains in the plasma allows us to investigate the spatial and temporal scales inaccessible by probe techniques. The micrometre-sized dust has been utilized as a diagnostic tool to investigate the plasma edge characteristics. Dust occurs in almost every plasma device and interactions of plasmas with near-wall impurities and/or dust significantly affects the efficiency and lifetime of such devices. Often presence of the magnetic field is inevitable in all such investigations and, therefore, it is important to study the effect of such a field on the near-wall plasma. Here, the charge on the dust, plasma potential, and plasma density in near wall region are calculated self-consistently. The electrons are assumed non-Boltzmannian and the effect of electron magnetization and electron-atom collisions on the dust charge is calculated in a self-consistent fashion. For various plasma magnetization parameters viz. the ratio of the electron and ion cyclotron frequencies to their respective collision frequencies, plasma - atom and ionization frequencies, the evolution of the plasma potential and density in the near wall region is investigated. It was found that the profiles of the plasma parameters change significantly not only when the ion cyclotron frequency is increased by an order of magnitude but also by the angle of the field orientation. When the magnetic field is perpendicular to the wall, grains of smaller size stay deeper inside the sheath in comparison with the case when the field is directed parallel to the wall. With the increase in the ion-magnetization level, and the angle between the field and the wall, the plasma flow velocity increases. The grains acquire less negative charge, so grains of larger size can stay inside the near wall plasma.