Cold Inhomogeneous Plasma Research Articles

Influence of a Uniform Magnetic Field on the Generation of Strong Small-Scale Magnetic Fields during the Injection of a Plasma with Hot Electrons into an Inhomogeneous Cold Plasma Layer

Influence of a Uniform Magnetic Field on the Generation of Strong Small-Scale Magnetic Fields during the Injection of a Plasma with Hot Electrons into an Inhomogeneous Cold Plasma Layer

Nonlinear interaction of the inhomogeneous electron beam with magnetized inhomogeneous cold plasma

We study the nonlinear interactions of an inhomogeneous cold beam of electrons of arbitrary density with an inhomogeneous cold plasma in the presence of an external static magnetic field. The formation of waves of double frequency at the inlet of the beam into the plasma is considered. Far enough from the plasma boundary inwards, the external static magnetic field may increase the amplitude of the fundamental waves, and the electric field component of waves with double frequency will remain stronger than that of the basic frequency. In addition, it is found that the inhomogeneity of electron beams has more effect on the generation of the second harmonic wave.

Bright terahertz (THz) generation by frequency mixing of dichromatic lasers in inhomogeneous cold plasma: Scaling of THz field

We report a theoretical model for bright, radially polarized terahertz (THz) generation based on difference frequency generation in periodic density plasmas. An initial phase difference between two lasers is incorporated in our model. It is observed that the THz field significantly varies with the initial phase difference. It is also found that the THz field and efficiency depend on the periodic plasma density structure parameters (like amplitude nγ and wave vector γ). Our investigations reveal that close to the phase matching condition, and optimized values of laser and plasma parameters, peak THz fields ∼ 15 GV/m can be obtained for the laser field ∼ 5×1010 V/m. We also found that the THz field distribution can be controlled with laser field profile parameters. The conversion efficiency of ∼0.01 can be achieved by optimizing the laser field profile and plasma parameters. In our model, high field and radially polarized THz can be obtained to meet the demands of THz-matter interactions, nonlinear THz spectroscopy, imaging, etc. Radially polarized THz field is also useful to penetrate deeply into the layers inside the skin with less risk of collateral damage and thereby improved safety and efficacy of treatment.

Phase-mixing of large amplitude electron oscillations in a cold inhomogeneous plasma

Phase-mixing of large amplitude non-relativistic electron oscillations around an inhomogeneous background of massive ions has been studied in a cold plasma. For our purpose, a space periodic but time independent ion density profile along with a perturbation in the electron density is considered. An exact space-time dependent solution is presented in the parametric form by using Lagrangian coordinates. An inhomogeneity in the ion density causes the characteristic plasma frequency to acquire spatial dependency, leading to phase-mixing and thus breaking of excited oscillations at arbitrary amplitudes. The effects of finite amplitude electron density perturbation on the process of phase-mixing have also been discussed.

Phase mixing of upper hybrid oscillations in a cold inhomogeneous plasma placed in an inhomogeneous magnetic field

We study phase mixing/wave breaking phenomena of upper hybrid modes in a cold inhomogeneous plasma placed in an inhomogeneous magnetic field. Inhomogeneities both in the background ion density and magnetic field profile are treated as periodic in space but independent in time. The Lagrangian fluid description is employed to obtain an exact solution of this fully nonlinear problem. It is demonstrated that the upper hybrid modes, excited by an initial local charge imbalance, break via phase mixing, induced by the inhomogeneities. It is also shown that it is possible to avoid phase mixing in excited upper hybrid oscillations in an inhomogeneous plasma containing a finite amplitude ion density fluctuation. The choice of external magnetic field is shown to have a key role in avoiding phase mixing in such oscillations. The relevance of our investigation regarding the particle acceleration in an inhomogeneous plasma has also been discussed.

Berry effect in unmagnetized inhomogeneous cold plasmas

The propagation of electromagnetic waves in an unmagnetized weakly inhomogeneous cold plasma is examined. We show that the inhomogeneity induces a gauge connection term in wave equation, which gives rise to Berry effects in the dynamics of polarized rays in the post geometric optics approximation. The polarization plane of a plane polarized ray rotates as a result of the geometric Berry phase, which is the Rytov rotation. Also, the Berry curvature causes the optical Hall effect, according to which, rays of left/right circular polarization deflect oppositely to produce a spin current directed across the direction of propagation.

ALOHA: an Advanced LOwer Hybrid Antenna coupling code

The Advanced LOwer Hybrid Antenna (ALOHA) code, has been developed to improve the modelling of the coupling of lower hybrid (LH) waves from the antenna to a cold inhomogeneous plasma while keeping a fast tool. In contrast to the previous code Slow Wave ANtenna (SWAN) (that only described the interaction of the slow wave between the waveguides and the plasma in a 1D model), the equations are now solved in 2D including the contribution of both the slow and fast waves, with a low computational cost. This approach is completed either by a full-wave computation of the antenna that takes into account its detailed geometry or by a mode-matching code dedicated to multijunctions modelling, which is convenient in preliminary design phases. Moreover, ALOHA can treat more realistic scrape-off layers in front of the antenna, by using a two-layer electron density profile. The ALOHA code has been compared with experimental results from Tore Supra LH antennas of different geometries, as well as benchmarked against other LH coupling codes, with very good results. Once validated, ALOHA has been used as a support for the design of COMPASS and ITER LH antennas and has shown to be a fast and reliable tool for LH antenna design.

Study and Optimization of Plasma-Based Radar Cross Section Reduction Using Three-Dimensional Computations

The radar cross section (RCS) of a flat plate covered with a cold collisional inhomogeneous plasma has been studied using a 3-D finite-difference time-domain (FDTD) method for electromagnetics. Two problems have been considered. In problem 1, using experimentally reported plasma density profiles, we have observed some interesting features in the bistatic RCS and provided simple physical interpretations for some of these features. The simulations confirm that a plasma shroud can successfully be used for reducing the RCS of a flat plate at almost all scattering angles, although the RCS could increase at some other angles. This is a novel extension of the FDTD method for the calculation of the bistatic RCS of an object shielded by a nonuniform collisional plasma. Problem 2 involves an optimization study for the input power required to achieve a desired RCS reduction (RCSR), examining a variety of plasma density levels and spatial profiles. For this optimization study, we have considered a helium plasma produced by a high-energy electron beam. We find that the maximum achievable reduction increases monotonically with power up to an optimum point, beyond which the RCSR decreases, finally showing some tendency to saturate. This is of practical importance and indicates the usefulness of FDTD simulations in identifying the optimal point. Furthermore, at a given power level, there can be a considerable scatter in the RCSR achievable. This is because various combinations of the plasma parameters, differing considerably in their RCSR abilities, could require the same power to sustain them. Simulations would be of great use in helping to identify the best profiles to be used for a given input power level.

Electron acceleration in a rectangular waveguide filled with unmagnetized inhomogeneous cold plasma

AbstractThis paper deals with the study of propagation of electromagnetic wave in a rectangular waveguide filled with an inhomogeneous plasma in which electron density varies linearly in a transverse direction to the mode propagation. A transcendental equation in ω (microwave frequency) is obtained that governs the mode propagation. In addition, an attempt is made to examine the effect of density inhomogeneity on the energy gain acquired by the electron (electron bunch) when it is injected in the waveguide along the direction of the mode propagation. On the basis of angle of deflection of the electron motion we optimize the microwave parameters so that the electron does not strike with the waveguide walls. Conditions have been discussed for achieving larger energy gain. The plasma density inhomogeneity is found to play a crucial role on the cutoff frequency, fields and dispersion relation of the TE10 mode as well as on the acceleration gradient in the waveguide.

Interaction of a relativistic soliton with a nonuniform plasma.

By using a relativistic fluid model, a nonlinear theory for the propagation of an intense laser pulse in an inhomogeneous cold plasma is developed. Assuming that the radiation spot size is larger than the plasma wavelength, we derive an envelope equation for the momentum of the electron fluid, taking into account relativistic electron mass variation and finite amplitude electron density perturbations that are driven by the relativistic ponderomotive force of light. Localized solutions of the envelope equation are discussed from an energy integral containing an effective potential. Numerical results for envelope solitons are obtained in a quasistationary approximation. The dependency of these localized solutions on the amplitude and the group velocity of the laser pulse is discussed. Also derived is an equation that governs the dynamics of the pulse center.

Poloidal Rotation Induced by Injecting Lower Hybrid Waves in Tokamak Plasma Edge

The poloidal rotation of the magnetized edge plasma in tokamak driven by the ponderomotive force which is generated by injecting lower hybrid wave(LHW) electric field has been studied. The LHW is launched from a waveguide in the plasma edge, and by Brambilla's grill theory, analytic expressions for the wave electric field in the slab model of an inhomogeneous cold plasma have been derived. It is shown that a strong wave electric field will be generated in the plasma edge by injecting LH wave of the power in MW magnitude, and this electric field will induce a poloidal rotation with a sheared poloidal velocity.

Second-harmonic generation by intense lasers in inhomogeneous plasmas.

We study the second-harmonic generation by laser interaction with a cold inhomogeneous underdense plasma, using a perturbative approach to solve the coupled set of Maxwell fluid, and momentum equations. The laser field is assumed to be Gaussian with diffractive effects included. The solution of the second-order inhomogeneous wave equation is obtained from the Green's function formalism in the paraxial approximation. We show that the total power generated is proportional to the square of the laser intensity and independent of the plasma density in the low-density limit. Depending on the laser beam waist and on the plasma density profile, the second-harmonic generation can be highly efficient. \textcopyright{} 1996 The American Physical Society.

Electrostatic Surface Wave Resonances for an Inhomogeneous Plasma Cylinder

Two solutions of the Laplace differential equation for an inhomogeneous cold plasma cylinder, whose radial electron density profile is considered parabolic and parametrically variable, are presented and discussed. In the first case (a smooth profile) the equation obtained is the confluent hypergeometric differential equation, also called the Kummer equation or the Pochhammer-Baines equation. The solution is expressed in terms of Kummer (or confluent hypergeometric) functions. In the second case, which is a more general case, the series solution to the same equation is achieved by the method of Frobenius. The surface resonance conditions are investigated and the profile-dependent frequency spectrum calculated when an harmonic electric field is incident perpendicularly to the cylinder axis of the plasma considered as collisionless and opaque. In particular, the effects of boundary different media in which the plasma cylinder is embedded are estimated.

Langmuir oscillations in a cold inhomogeneous plasma.

One-dimensional nonlinear Langmuir oscillations in a cold inhomogeneous plasma are considered. The spatial distribution of the amplitude of oscillations is set by the initial disturbance and can be arbitrary, i.e., it is not in any way connected with the inhomogeneity of the plasma. Taking place at small spatial gradients of the initial disturbance because of a growing with time phase shift of oscillations of different electrons, the effect of formation of narrow density peaks is illustrated. The density peaks always move in the direction of a decrease of ion concentration. In this movement their amplitude can grow or decrease depending on the direction of the initial disturbance gradient. A number of electrons, forming density peaks, reduce with a diminution of plasma inhomogeneity but the peak amplitudes always increase unlimitedly with time up to the self-intersection of electron trajectories. An equation, making it possible to determine the instant of trajectory intersection in case of small plasma inhomogeneity of arbitrary kind and arbitrary gradients of the initial disturbance, is obtained. It is shown by a numerical calculation that the analytical relations obtained are not essentially changed if one takes into account the nonlinearity of corresponding differential equations.

Comment on ‘‘Alfvén ‘resonance’ reconsidered: Exact equations for wave propagation across a cold inhomogeneous plasma’’ [Phys. Plasmas 1, 3523 (1994)

In a recent paper Bellan [P. M. Bellan, Phys. Plasmas 1, 3523 (1994)] criticizes magnetohydrodynamics for not being an accurate model of plasma behavior at low frequencies and leading to the incorrect prediction of singularities associated with the Alfvén wave resonances. Grave errors of the paper having to do with misunderstanding of what is a physical model and with misrepresentation of the original contributions to the field are exposed.

Response to Comments on ‘‘Alfvén ‘resonance’ reconsidered: Exact equations for wave propagation across a cold inhomogeneous plasma’’ [Phys. Plasmas 1, 3523 (1994)

We agree with both Goedbloed and Lifschitz (whom we will refer to as GL) and Ruderman, Goossens and Zhelyazkov (RGZ) that the apparent violation of quasi-neutrality in Ref. is not a true problem and rescind this particular argument against MHD. However, the rest of the discussion in our paper is independent of this argument regarding quasi-neutrality, and as discussed below, the conclusions in Ref. about the validity of MHD, about compressibility versus incompressibility, and about Alfvén resonance remain valid.

Alfvén ‘resonance’ reconsidered: Exact equations for wave propagation across a cold inhomogeneous plasma

Previous discussions of Alfvén wave propagation across an inhomogeneous plasma predicted that shear Alfvén waves become singular (resonant) at the ω=kzvA layer and that there is a strong wave absorption at this layer giving localized ion heating. In this paper the three standard derivations of the Alfvén ‘resonance’ (incompressible magnetohydrodynamics, compressible magnetohydrodynamics, and two-fluid) are re-examined and shown to have errors and be mutually inconsistent. Exact two-fluid differential equations for waves propagating across a cold inhomogeneous plasma are derived; these show that waves in an ideal cold plasma do not become ‘resonant’ at the Alfvén layer so that there is no wave absorption or localized heating. These equations also show that the real ‘shear’ Alfvén wave differs in substance from both the ideal MHD and earlier two-fluid predictions and, in the low density, high field region away from the ω=kzvA layer, is actually a quasielectrostatic resonance cone mode. For ω≪ωci and ky=0, the ω=kzvA layer turns out to be a cutoff (reflecting) layer for both the ‘shear’ and compressional modes (and not a resonance layer). For finite ω/ωci and ky=0 this layer becomes a region of wave inaccessibility. For ω≪ωci and finite ky there is strong coupling between shear and compressional modes, but still no resonance.

The development of discrete active auroral forms

Using analytical and numerical methods, it is shown that all the basic forms of the active arcs can be described within the framework of a nonlinear Alfven wave model. The auroral forms are then considered as nonlinear two-dimensional Alfven vortex structures that are uniform along the ambient magnetic field. They can exist in a cold inhomogeneous plasma, e.g., the Earth's ionosphere. Large-scale spirals and folds can recover the initial form of a typical arc. In the case of a small solitary disturbance of the current density or a sharp bending of the current sheets, vortex chains are formed in the spirals and the folds. >

Resonance absorption of energy by a cold inhomogeneous plasma

The active and reactive power resonantly absorbed by a linear passive system is calculated in terms of its continuum eigenfunctions and the spatial–temporal spectral density of the external force. An application to the resonance absorption of electromagnetic radiation by a cold inhomogeneous plasma is given.

Full-wave calculations of the O-X mode conversion process

A two-point boundary-value problem has been formulated that describes the conversion between ordinary (O) and extraordinary (X) wave modes in a cold inhomogeneous plasma. Numerical solutions to this problem have been obtained for various values of the WKB parameter k0L; where k0 is the vacuum wavenumber and L the density-gradient scale length. The results are compared with three different theoretical expressions for the O-X mode conversion efficiency derived by others in the WKB limit of k0 L ≫ l. Most of the results presented in this paper are obtained for a collisionless plasma with finite density near the plasma cut-off density. However, some examples are also given of wave propagation from vacuum. In these examples, collision effects are added to the equations in order to remove the singularity otherwise present at the position of the upper hybrid resonance layer.