Beams of fast electrons in a cold electron background play a key role in the generation of a magnetic field in the wake of an ultrashort ultraintense laser pulse propagating in an underdense plasma. Here we study the linear and nonlinear evolution of the electromagnetic beam-plasma instability in a collisionless inhomogeneous plasma by using a set of two-fluid electron equations in the nonrelativistic and relativistic regimes. We show the characteristic spatial structures in the current and magnetic field distributions that are generated by this instability. These structures can be used as a signature of the physical mechanism at play in the analysis of the numerical and experimental results of the laser-plasma interaction. [S1063-651X(98)05412-9]
The linear dispersion relation and the spatial structure of high frequency instabilities, with a mixed tearing-bending character is studied. These instabilities are driven by the electron velocity gradient in a collisionless electron plasma moving against a background of immobile neutralizing ions in an inhomogeneous magnetic field. As the angle between the perturbations and the magnetic field lines is varied, perturbations change from the tearing type (parallel propagation) to the bending type (perpendicular propagation). (C) 1999 American Institute of Physics. [S1070-664X(99)01406-8]
The intermittent nature of energy dissipation in two-dimensional electron-magnetohydrodynamic turbulence is investigated by means of high resolution direct numerical simulations. It is found that, when the main contribution to the energy is given by the magnetic field, dissipation is mostly concentrated on one-dimensional filaments. As a consequence, the multifractal spectrum has a simple form which can be approximately described in terms of a bifractal model. ͓S1063-651X͑99͒01803-6͔PACS number͑s͒: 47.27.EqThe statistical theory of three-dimensional fully developed hydrodynamic turbulence relies on one outstanding issue: the nonlinear transfer of energy from large to small scales ͓1,2͔. The energy flux is constant over the intermediate scales of the inertial range, but does not need to be homogeneous in space. Moreover, experiments in fluid turbulence indicate that the self-similarity of the energy dissipation distribution is broken by the presence of small-scale structures in the flow. Recent direct numerical simulations revealed the presence of filaments and other dissipative structures candidate as physical sources of intermittency ͓3͔.It is therefore interesting to look for two-dimensional turbulent systems sharing these same features. Actually many of them exhibit a reversed energy flux, from the small scales to the larger ones, as is the case of two-dimensional ͑2D͒ Navier-Stokes turbulence ͓4-7͔, Hasegawa-Mima turbulence ͓8͔, or its geophysical counterpart equivalent barotropic turbulence ͓9͔. In this framework 2D electronmagnetohydrodynamic ͑EMHD͒ turbulence deserves special attention, beyond its modeling applications, since it has been shown to display, for the freely decaying case, a forward energy cascade á la Richardson-Kolmogorov ͓10͔.EMHD equations are a fluid dynamical model for a cold electron plasma, moving in a uniform charge-neutralizing background of stationary ions. In recent years this model has received considerable interest for its relation to inertially confined plasma and to laser-plasma interactions, but the comparison with experimental results is limited by the fact that plasma which evolve according to EMHD equations are usually short-lived.In the 2D case, the velocity and magnetic field are z independent, and can be expressed in terms of the stream function and the magnetic flux function according to v ϭ(Ϫץ y ץ, x ,Ϫ⌬) and Bϭ(Ϫץ y ץ, x ,). 2 )͔ 1/2 is the ratio of the inertial electron length scale to the integral scale L. The density of the number of electrons n is assumed to be uniform according to the incompressibility of the velocity field "•vϭ0. The generalized dissipation operators correspond to resistivity for ϭ1 and to electron viscosity for ϭ2.In the ideal limit ϭ0, Eqs. ͑1͒ and ͑2͒ conserve the total energy ͑kinetic plus magnetic͒For finite dissipation, EMHD exhibits a direct energy cascade from large to small scale, which suggests an analogy with 3D hydrodynamic turbulence. We thus expect, for a sufficiently small dissipation coefficient, a constant energy flux in...
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