Nikhil Chakrabarti

Nonlinear excitation of geodesic acoustic modes by drift waves

In this paper, two mode-coupling analyses for the nonlinear excitation of the geodesic acoustic modes (GAMs) in tokamak plasmas by drift waves are presented. The first approach is a coherent parametric process, which leads to a three-wave resonant interaction. This investigation allows for the drift waves and the GAMs to have comparable scales. The second approach uses the wave-kinetic equations for the drift waves, which then couples to the GAMs. This requires that the GAM scale length be large compared to the wave packet associated with the drift waves. The resonance conditions for these two cases lead to specific predictions of the radial wave number of the excited GAMs.

Viscosity gradient-driven instability of 'shear mode' in a strongly coupled plasma

The influence of the viscosity gradient (due to shear flow) on low-frequency collective modes in a strongly coupled dusty plasma is analyzed. It is shown that for a well-known viscoelastic plasma model, the velocity shear-dependent viscosity leads to instability of the shear mode. The inhomogeneous viscous force and velocity shear coupling supply the free energy for the instability. The combined strength of the shear flow and viscosity gradient dominates over any stabilizing force and makes the shear mode unstable. The implications of this novel instability and its applications are briefly described.

Linear and nonlinear electrostatic modes in a strongly coupled quantum plasma

The properties of linear and nonlinear electrostatic waves in a strongly coupled electron-ion quantum plasma are investigated. In this study, the inertialess electrons are degenerate, while non-degenerate inertial ions are strongly correlated. The ion dynamics is governed by the continuity and the generalized viscoelastic momentum equations. The quantum forces associated with the quantum statistical pressure and the quantum recoil effect act on the degenerate electron fluid, whereas strong ion correlation effects are embedded in generalized viscoelastic momentum equation through the viscoelastic relaxation of ion correlations and ion fluid shear viscosities. Hence, the spectra of linear electrostatic modes are significantly affected by the strong ion coupling effect. In the weakly nonlinear limit, due to ion-ion correlations, the quantum plasma supports a dispersive shock wave, the dynamics of which is governed by the Korteweg-de Vries Burgers’ equation. For a particular value of the quantum recoil effect...

Jeans instability in a viscoelastic fluid

The well known Jeans instability is studied for a viscoelastic gravitational fluid using generalized hydrodynamic equations of motions. It is found that the threshold for the onset of instability appears at higher wavelengths in a viscoelastic medium. Elastic effects playing a role similar to thermal pressure are found to lower the growth rate of the gravitational instability. Such features may manifest themselves in matter constituting dense astrophysical objects.

Nonlinear behavior of electron acoustic waves in an un-magnetized plasma

The nonlinear electron acoustic wave, which is found in the earth’s magnetosphere by satellite observations, is studied analytically by Lagrangian fluid description. The basic linear mode is observed in a two temperature electron species plasma where ions form stationary charge neutral background. We have obtained nonlinear description of this mode, which depends on both time and space. A possible solution shows a soliton like structure, which is localized in space, and the amplitude increases with time in the absence of dispersion. Small dispersive correction, however, shows spread of the solution in space. This method can be generalized to study the nonlinear behavior of a general class of multispecies plasma.

Nonlinear lower-hybrid oscillations in cold plasma

In a fluid description nonlinear lower-hybrid oscillation have been studied in a cold quasineutral magnetized plasma using Lagrangian variables. An exact analytical solution with nontrivial space and time dependence is obtained. The solution demonstrates that under well defined initial and boundary conditions the amplitude of the oscillations increases due to nonlinearity and then comes back to its initial condition again. These solutions indicate a class of nonlinear transient structures in magnetized plasma.

Nonlinear interaction of electron plasma waves with electron acoustic waves in plasmas

An analysis of interaction between two temperature electron species in the presence of static neutralizing ion background is presented. It is shown that electron plasma waves can nonlinearly interact with electron acoustic wave in a time scale much longer than ωp−1, where ωp is electron plasma frequency. A set of coupled nonlinear differential equations is shown to exist in such a scenario. Propagating soliton solutions are demonstrated from these equations.

Geodesic acoustic modes excited by finite beta drift waves

Presented in this paper is a mode-coupling analysis for the nonlinear excitation of the geodesic acoustic modes (GAMs) in tokamak plasmas by finite beta drift waves. The finite beta effects give rise to a strong stabilizing influence on the parametric excitation process. The dominant finite beta effect is the combination of the Maxwell stress, which has a tendency to cancel the primary drive from the Reynolds stress, and the finite beta modification of the drift waves. The zonal magnetic field is also excited at the GAM frequency. However, it does not contribute to the overall stability of the three-wave process for parameters of relevance to the edge region of tokamaks.

Nonlinear evolution of ion-acoustic waves in unmagnetized plasma

In a fluid description large amplitude electrostatic ion-acoustic waves have been studied in an unmagnetized plasma using Lagrangian variables. We obtained solutions for ion-acoustic waves with nontrivial space and time dependence. The non-dispersive solutions demonstrate that under well defined initial and boundary conditions the amplitude of the solutions decreases indicating a new class of nonlinear solutions that lead to short lived structures.

Shear wave vortex solution in a strongly coupled dusty plasma

The properties of electrostatic transverse shear waves in a strongly coupled dusty plasma are examined using the nonlinear version of the generalized hydrodynamic equation. In the kinetic limit, it is shown that strongly coupled plasmas support localized dipolar vortexlike solutions with amplitude modulated periodically.