Sita Sundar

Relativistic electromagnetic flat top solitons and their stability

The inclusion of ion response in the study of relativistically intense electromagnetic laser pulse propagation in plasma yields certain distinct varieties of single peak solitonic structures. A flat top slow moving structure (for which the various fields have a flat profile over a wide spatial range) is one such solution. A detailed characterization of these solutions along with the eigen spectrum of their formation in the parameter space has been presented. The evolution of this particular solution is studied in detail with the help of a coupled fluid Maxwell set of equations. The study shows that the flat top solution is unstable. The instability is characterized as the backward Brillouin instability for which the electron quiver velocity plays the role of the effective temperature.

Role of natural length and time scales on shear driven two-dimensional electron magnetohydrodynamic instability

The electron magnetohydrodynamic (EMHD) model represents an incompressible electron fluid flow against a static neutralizing background ion species. In contrast to hydrodynamic fluid models the EMHD model contains intrinsic length (the electron skin depth) and time scale (the whistler period). The paper discusses the role of skin depth and the existence of whistler waves on a prominent fluid instability, namely, the velocity shear driven Kelvin–Helmholtz instability in the context of two-dimensional EMHD. Numerical simulations are also carried out to understand the role played by the whistler waves in the nonlinear saturated regime of the instability.

Impact of collisions on the dust wake potential with Maxwellian and non-Maxwellian ions

This work examines the formation of wake fields caused by ions streaming around a charged dust particle, using three-dimensional particle-in-cell (PIC) simulations with charge-neutral collisions included. The influence of an external driving electric field, which leads to a non-Maxwellian distribution of ions, is investigated in detail. The wake features formed for non-Maxwellian ions exhibit significant deviations from those observed within the model of a shifted Maxwellian distribution. The dependence of the peak amplitude and position of the wake potential upon the degree of collisionality is analyzed for a wide range of streaming velocities (Mach numbers). In contrast to a shifted Maxwellian distribution of ions, the drift-driven non-Maxwellian distribution exhibits an increase of the wake amplitude of the first attractive peak with increase in collisionality for high streaming velocities. At very low Mach numbers, collision-induced amplification is observed for Maxwellian as well as non-Maxwellian distributions.This work examines the formation of wake fields caused by ions streaming around a charged dust particle, using three-dimensional particle-in-cell simulations with charge-neutral collisions included. The influence of an external driving electric field, which leads to a non-Maxwellian distribution of ions, is investigated in detail. The wake features formed for non-Maxwellian ions exhibit significant deviations from those observed within the model of a shifted Maxwellian distribution. The dependence of the peak amplitude and position of the wake potential upon the degree of collisionality is analyzed for a wide range of streaming velocities (Mach numbers). In contrast to a shifted Maxwellian distribution of ions, the drift-driven non-Maxwellian distribution exhibits an increase in the wake amplitude of the first attractive peak with an increase in collisionality for high streaming velocities. At very low Mach numbers, collision-induced amplification is observed for Maxwellian and non-Maxwellian distributions.

Dynamic anisotropy in MHD turbulence induced by mean magnetic field

In this paper, we study the development of anisotropy in strong MHD turbulence in the presence of a large scale magnetic field B 0 by analyzing the results of direct numerical simulations. Our results show that the developed anisotropy among the different components of the velocity and magnetic field is a direct outcome of the inverse cascade of energy of the perpendicular velocity components u? and a forward cascade of the energy of the parallel component u k . The inverse cascade develops for a strong B0, where the flow exhibits a strong vortical structure by the suppression of fluctuations along the magnetic field. Both the inverse and the forward cascade are examined in detail by investigating the anisotropic energy spectra, the energy fluxes, and the shell to shell energy transfers among different scales.

Free energy source for flow shear driven instabilities in electron-magnetohydrodynamics

The paper discusses the free energy source for the flow shear driven instability in the context of electron-magnetohydrodynamic (EMHD) system. In EMHD as the electron flow velocity also corresponds to the current in the system, the flow shear driven instability has often been identified both as the fluid Kelvin–Helmholtz (KH)-like mode and as the current gradient driven sausage and kinklike modes. It has been shown here that the free energy source for the flow shear driven instability is the kinetic energy of the electron flow and the instability is essentially a fluid KH mode. The manuscript also provides interpretations for certain characteristic features, such as existence of a threshold wavenumber along the flow direction, the order of magnitude estimation of the growth rate, etc., from physical considerations.

Electron velocity shear driven instability in relativistic regime

The electron magnetohydrodynamics model has been generalized to incorporate relativistic effects. The model is then employed to study the instability associated with sheared electron velocity flow in the relativistic regime. The instability has features similar to the conventional velocity shear driven Kelvin–Helmholtz-like mode [A. Das and P. Kaw, Phys. Plasmas 8, 4518 (2001)] in the weakly relativistic regime. However, in the strongly relativistic regime the instability shows certain distinct characteristics. The threshold value of the wave number is found to be considerably higher than the inverse shear width of the equilibrium velocity profile in this regime. Thus, the unstable domain of the wave-number space is considerably wider in this case. Also the mode does not remain purely growing but acquires a real frequency even for an antisymmetric velocity profile. These features of the mode have been understood by realizing that in the strongly relativistic regime the relativistic mass factor γ0 for the ...

Weakly relativistic electromagnetic solitons in warm plasmas

For slowly propagating electromagnetic solitons, validity of the cold plasma model is addressed using a more realistic model involving effects arising due to temperature as well as ion dynamics. Small amplitude single peak structures which are quasineutral are studied, and different regions of existence of bright and dark classes of solitons are delineated. Influence of temperature on spectral characteristics of the solitary structures is presented.

Perturbative analysis of sheared flow Kelvin–Helmholtz instability in a weakly relativistic magnetized electron fluid

In the interaction of intense lasers with matter/plasma, energetic electrons having relativistic energies get created. These energetic electrons can often have sheared flow profiles as they propagate through the plasma medium. In an earlier study [Phys. Plasmas 17, 022101 (2010)], it was shown that a relativistic sheared electron flow modifies the growth rate and threshold condition of the conventional Kelvin—Helmholtz instability. A perturbative analytic treatment for the case of weakly relativistic regime has been provided here. It provides good agreement with the numerical results obtained earlier.