Amita Das

Secondary instability in drift wave turbulence as a mechanism for zonal flow and avalanche formation

The article reports on recent developments in the theory of secondary instability in drift-ion temperature gradient turbulence. Specifically, the article explores secondary instability as a mechanism for zonal flow generation, transport barrier dynamics and avalanche formation. These in turn are related to the space-time statistics of the drift wave induced flux, the scaling of transport with collisionality and β, and the spatio-temporal evolution of transport barriers.

Whistlerization and anisotropy in two-dimensional electron magnetohydrodynamic turbulence

A detailed numerical simulation to understand the turbulent state of the decaying two-dimensional electron magnetohydrodynamics is presented. It is observed that the evolved spectrum is comprised of a collection of random eddies and a gas of whistler waves, the latter constituting the normal oscillatory modes of such a model. The whistlerization of the turbulent spectra has been quantified by novel diagnostics. In this work, results are presented only in the regime where the spatial excitation scales are longer than the electron skin depth. Simulations suggest that spectra at short scales are comparatively more whistlerized. The long scale field merely acts as the ambient field along which whistler waves propagate. It is also observed that, in the presence of an external magnetic field, the power spectrum acquires a distinct directional dependence. This anisotropy is dominant at short scales. It is shown that such an anisotropy at short scales results from a cascade mechanism governed by the interacting whistlers waves.

Direct observation of turbulent magnetic fields in hot, dense laser produced plasmas

Turbulence in fluids is a ubiquitous, fascinating, and complex natural phenomenon that is not yet fully understood. Unraveling turbulence in high density, high temperature plasmas is an even bigger challenge because of the importance of electromagnetic forces and the typically violent environments. Fascinating and novel behavior of hot dense matter has so far been only indirectly inferred because of the enormous difficulties of making observations on such matter. Here, we present direct evidence of turbulence in giant magnetic fields created in an overdense, hot plasma by relativistic intensity (1018W/cm2) femtosecond laser pulses. We have obtained magneto-optic polarigrams at femtosecond time intervals, simultaneously with micrometer spatial resolution. The spatial profiles of the magnetic field show randomness and their k spectra exhibit a power law along with certain well defined peaks at scales shorter than skin depth. Detailed two-dimensional particle-in-cell simulations delineate the underlying interaction between forward currents of relativistic energy “hot” electrons created by the laser pulse and “cold” return currents of thermal electrons induced in the target. Our results are not only fundamentally interesting but should also arouse interest on the role of magnetic turbulence induced resistivity in the context of fast ignition of laser fusion, and the possibility of experimentally simulating such structures with respect to the sun and other stellar environments.

Nonlocal sausage-like instability of current channels in electron magnetohydrodynamics

The question of stability of electron current channels in a plasma to rapidly growing modes on electron time scales is of considerable interest in a wide range of physical problems, e.g., fast ignitor concept of laser fusion, plasma opening switches, Z pinches, etc. Earlier studies have demonstrated the instability of such current channels to kink modes. However, the stability to sausage-like modes has not been investigated so far. In this paper such an investigation has been carried out and it is shown that the current channels are unstable to sausage-like modes with parallel scale lengths longer than the current channel width. These instabilities are closely related to the Kelvin–Helmholtz (K-H) instability of a sheared electron layer.

Formation of a density blob and its dynamics in the edge and the scrape-off layer of a tokamak plasma

Formation of a density blob and its motion in the edge and scrape-off layer (SOL) of a tokamak plasma have been simulated using two-dimensional, two-field, fluid model equations. The simulation results show that density blobs form in the edge or in the edge-to-SOL transition region where the poloidal velocity shear is maximum. From the numerical data, a condition for density blob formation has been obtained. Dynamics of the detached blob in the edge and SOL regions has been studied. It is observed that not all the blobs that form in the edge or edge-to-SOL transition region are capable of ejection deep into the SOL. A condition for their ejection is also discussed. Radial particle transport associated with the blobs in the SOL has been calculated. It is found that about 60% of the total radial particle flux is carried out by these blobs.

Laser envelope solitons in cold overdense plasmas

Some questions pertaining to the existence and nature of one-dimensional envelope pulse solitons propagating into an overdense plasma are examined by a numerical investigation of the relativistic cold plasma equations. Finite amplitude single hump solitons with significant density cavitation are obtained for both immobile and mobile ions. For the immobile ion case the eigenvalue spectrum has a continuum nature and there is a smooth transition from standing single pulse solitons to moving solitons. A composite spectrum of moving multipeak solitons is also obtained and approximate analytical estimates of their amplitudes are provided.

Theory of two-dimensional mean field electron magnetohydrodynamics

Theoretical studies of mean field electrodynamics for diffusive processes in the electron magnetohydrodynamic (EMHD) model is presented. In contrast to magnetohydrodynamics (MHD), the evolution of the magnetic field here is governed by a nonlinear equation in magnetic field variables. A detailed description of diffusive processes in two dimensions are presented in this paper. In particular, it has been shown analytically that the turbulent magnetic field diffusivity is suppressed from naive quasilinear estimates. It is shown that for complete whistlerization of the spectrum, turbulent diffusivity vanishes. The question of whistlerization of the turbulent spectrum is investigated numerically, and a reasonable tendency towards whistlerization is observed. Numerical studies also show suppression of magnetic field diffusivity in accordance with analytical estimates.

Weakly relativistic one-dimensional laser pulse envelope solitons in a warm plasma

A class of exact one-dimensional solutions of coupled nonlinear equations describing the propagation of a weakly relativistic circularly polarized electromagnetic pulse in a warm, collisionless and unbounded plasma is presented. The solutions investigated are in the form of a slowly moving dark or bright envelope soliton with a propagation velocity comparable to the thermal speed of the particles. For such a slowly propagating entity, the modulational envelope is strongly modified by the effects arising due to ion inertia as well as by the thermal effects of both ions and electrons. Different regions of existence of dark and bright solitons have been identified. The analysis carried out here is restricted to nearly quasi-neutral dynamics where the second derivative term in the Poisson equation plays a subsidiary role. Under this approximation, the eigenvalue problem has continuum solutions and one can establish the nonlinear relationship between the group velocity of the soliton and the amplitude and frequency of the light pulse.

Simulation of plasma transport by coherent structures in scrape-off-layer tokamak plasmas

The formation of coherent structures by two-dimensional interchange turbulence in the scrape-off layer (SOL) of tokamak plasmas and their subsequent contribution to anomalous plasma transport has been studied in recent years using electron continuity and current balance equations. In this paper, it is demonstrated that the inclusion of electron energy equation in the simulations changes the nature of coherent structures in a significant manner and gives results which are in better agreement with experiments. Specifically, it is observed that radial potential gradients are established which give a poloidal elongation and movement to the structures. Only during the radial transport events do the structures get significantly extended in the radial direction giving radial velocities of order 1 km/s. Sometimes detachment of density structures from the main plasma is observed. These detached structures either decay into the background plasma or are transported out from the SOL. The simulated particle flux and its statistical properties also are discussed.

Nonlinear electron magnetohydrodynamic simulations of sausage-like instability of current channels

The stability of current channels to fast electron magnetohydrodynamic modes is a topic of great interest in several frontier areas of plasma research, e.g., fast ignitor concept of laser fusion, fast Z pinches, plasma opening switches, current channels at the center of fast magnetic reconnection region, etc. This paper deals with a detailed fluid simulation study of linear and nonlinear aspects of the velocity shear modes in electron current channels in a two dimensional geometry. Simulation results clearly show the development of sausage-like structures (kink structures, which are intrinsically three-dimensional excitations, are ruled out in the present simulations) which grow linearly and eventually saturate by nonlinear effects. An analytic understanding of the nonlinear saturation mechanism is also provided.