Generation and Radiation of Waves at Combined Frequencies in a Warm Inhomogeneous Plasma Layer

We investigate generation and radiation of waves at combined frequencies from an arbitrary inhomogeneous, isotropic plasma layer, when electromagnetic waves are obliquely incident on it to interact with a surface wave at the plasma boundary. This interaction has been described for both S- and P-polarized waves. We consider a warm plasma layer with thickness very small as compared to the wavelengths of oscillations. It is shown that generated waves are strongly amplified, compared to cold plasma, when phase velocities of generated waves approaches the electron thermal velocity. Waves are not emitted when P-polarized waves are incident perpendicular onto the plasma layer.     

[Russian Text Ignored]

The stability of the electromagnetic plasma confinement by powerful external s-polarized pump waves is considered. The parametric excitation of standing electromagnetic waves along the plasma boundary with frequencies close to the frequency of the pump wave leads to a periodic density modulation of the plasma boundary. The density disturbances along the direction of the external wave field are connected to the excitation of transverse p-polarized surface waves while the modulation in the direction perpendicular to the pump field are created by the parametric interaction between the external wave and s-polarized trapped leaking oscillations. Only when the leaking waves are excited the scale length of the modulation is larger than half the free space wave length of the incident radiation.     

Resonant interaction of the Langmuir wave with quasi-stationary plasma electrons

Evolution of the Langmuir wave in quasi-stationary plasma is considered. A consistent solution to the closed system of the Vlasov–Poisson equations is obtained in the adiabatic approximation. Dispersion of the wave evolving in the electron distribution tail with variable electron concentration or plasma temperature is described. It is established that the plasma oscillation energy increases with decreasing electron concentration or increasing temperature. After plasma regains its initial state, the wave parameters also restore their initial values. That is, the wave evolution in the quasi-stationary plasma is a completely reversible process.     

Radiation reflection by a plasma with electron temperature anisotropy

The reflection of a test electromagnetic wave normally impinging on a plasma surface is investigated within the formalism of the surface impedance. The plasma is assumed to possess an anisotropic two-temperature bi-Maxwellian electron velocity distribution function. The linearly polarized impinging wave during reflection transforms into an elliptically polarized one, the degree of ellipticity depending on the electron temperature anisotropy. Polarization modifications of the reflected wave are particularly important in the conditions of the anomalous skin-effect, when the influence of the wave magnetic field on the electron kinetics in the skin layer is strong. Relations are reported connecting the reflected wave basic parameters to those of the reflecting plasma surface, making possible, through the experimental determination of the reflected wave characteristics, to find the plasma electron concentration and the two effective temperatures. Received 21 May 2002 / Received in final form 21 August 2002 Published online 6 November 2002 RID="a" ID="a"e-mail: zarcone@unipa.it     

Study on the polarization properties of electromagnetic waves with arbitrary magnetic declination in magnetized plasma

The polarization characteristics of oblique incidence electromagnetic waves in magnetized cold plasma layer are studied using the finite-difference time-domain (FDTD) method based on the trapezoidal recursive convolution (TRC) technology. The TRC-FDTD formulations are derived in detail and are confirmed by computing the reflection and transmission coefficients for the elliptically polarized wave through a magnetized cold plasma slab with arbitrary magnetic direction. Particularly, when the propagation direction of the EM wave is parallel to the magnetic direction, the right-circular-polarized and left-circular-polarized wave should be considered. When the propagation direction of the EM wave is perpendicular to the magnetic direction, the ordinary polarized wave and extraordinary polarized wave should be considered.     

Turbulent Boundary Layer Control via a Streamwise Travelling Wave Induced by an External Force

Turbulent boundary layer control via a streamwise travelling wave is investigated based on direct numerical simulation of an incompressible turbulent channel flow. The streamwise travelling wave is induced on one side wall of the channel by a spanwise external force, e.g., Lorenz force, which is con~ned in the viscous sublayer. As the control strategy used in this study has never been examined, we pay our attention to its efficiency of drag control. It is revealed that the propagating direction of the travelling wave, i.e., the downstream or upstream propagating direction with respect to the streamwise flow, has an important role on the drag control, leading to a significant drag reduction or enhancement for the parameters considered. The coherent structures of turbulent boundary layer are altered and the underlying mechanisms are analysed. The results obtained provide physical insight into the understanding of turbulent boundary layer control.     

Stability characteristic of hypersonic flow over a blunt wedge under freestream pulse wave

To investigate the stability characteristic of hypersonic flow under the action of a freestream pulse wave, a high-order finite difference method was employed to do direction numerical simulation (DNS) of hypersonic unsteady flow over an 8° half-wedge-angle blunt wedge with freestream slow acoustic wave. The evolution of disturbance wave modes in the boundary layer under a pulse wave and a continuous wave are compared, and the wall temperature effect on the hypersonic boundary layer stability for a pulse wave disturbance is discussed. Results show that, both for a pulse wave and a continuous wave in freestream, the disturbance waves inside the nose boundary layer are mainly a fundamental mode; the Fourier amplitude of pressure disturbance mode in the boundary layer for a pulse wave is far less than that for a continuous wave, and the band frequency of the former is wider than that of the latter. All disturbance modes decay rapidly along the streamwise in the nose boundary layer. In the non-nose boundary layer, the dominant mode is transferred from fundamental mode into second harmonic. The transformation of dominant mode for a pulse wave appears much earlier than that for a continuous wave. Different frequency disturbance modes present different changes along streamline in the boundary layer, and the frequency band narrows around the second harmonic mode along the streamwise. Keen competition and the transformation of energy exist among different modes in the boundary layer. Wall temperature modifies the stability characteristic of the hypersonic boundary layer, which presents little effect on the development of fundamental modes and cooling wall could accelerates the growth of the high frequency mode as well as the dominant mode transformation.     

Electromagnetic Reflection of Conductive Plane Covered with Magnetized Inhomogeneous Plasma

This paper presents numerical results in the form of graphs of the power reflection coefficients for electromagnetic signals normally incident upon a conductive plane covered with a layer of plasma. The plasma's electron density varies only in the direction perpendicular to the plane. A non-uniform external magnetic field, which is oriented in a direction paralleled to the conductor's surface and has a gradient normal to this surface, is applied to the plasma. Parameters considered in the computation cover a relatively wide range and the functional dependence of the power reflection coefficients on these parameters are studied.     

Collisionless absorption in an overdense plasma with anisotropic electron distribution function

Collisionless absorption of linearly polarized electromagnetic wave in a plasma with anisotropic bi-Maxwellian electron velocity distribution is investigated. Due to the wave magnetic field influence on the electron kinetics in the skin layer, the wave absorption is found to significantly depend on the degree of the electron temperature anisotropy. Depending on the value of the skin layer anomaly parameter, and on the electron temperature anisotropy degree, the conditions are found when a significant decrease or increase of the collisionless absorption is expected. Received 25 January 2002     

Transmission of microwaves through magnetoactive plasma

The possibility of transmission of microwaves through the plasma layer with an electron concentration of more than critical value is considered. The problem is solved by consideration of the interaction of a microwave with the plane layer of magnetoactive plasma. The problem is formulated by such a way as to obtain the estimation of the greatest lower bound of microwave transmission coefficient through the plasma layer. The results of numerical parametric investigations are applied for the transparency conditions of the shock wave plasma in space communication. The possibility of stable (without interruptions) microwave communication through the shock wave plasma is demonstrated     

Pair annihilation effects on dust ion acoustic surface wave in semi-bounded magnetized pair–ion–dust plasmas

The electron–positron pair annihilation effects on the dust ion acoustic surface wave are investigated in semi-bounded magnetized electron–positron–ion–dust plasmas. The dispersion relation of the low frequency dust ion acoustic surface wave is obtained by the plasma dielectric function with the specular reflection boundary condition. The results show that the frequency of the dust ion acoustic surface wave is found to be increased with increasing the annihilation of the electron–positron pair. In addition, the group velocity of the dust ion acoustic surface wave is also found to be increased with the annihilation of the electron–positron pair.     

Radiative transfer in a layer of magnetized plasma with random irregularities

The problem of radio wave reflection from an optically thick plane uniform layer of magnetized plasma is considered in the present work. The plasma electron density irregularities are described by a spatial spectrum of arbitrary form. The small-angle scattering approximation in invariant ray coordinates is proposed as a technique for the analytical investigation of the radiation transfer equation. The approximate solution describing the spatial and angular distribution of radiation reflected from a plasma layer is obtained. The solution obtained is investigated numerically for the case of ionospheric radio wave propagation. Two effects occur as a consequence of multiple scattering: a change in the reflected signal intensity and an anomalous refraction.     

Boundary graphene layer effect on surface plasmon oscillations in a quantum plasma half-space

The effect of graphene on unique features of surface plasmon-polariton excitations near the interface of vacuum and quantum plasma half-space is explored using a quantum hydrodynamic model including the Fermi electron temperature and the quantum Bohm potential together with the full set of Maxwell equations.It is found that graphene as a conductive layer significantly modifies the propagation properties of surface waves by making a change on the corresponding wave dispersion relation.It is shown that the presence of graphene layer on the interface of vacuum and plasma leads to a blue-shift in the surface Plasmon frequency.The results of present study must be contributed to the modern electronic investigations.     

Linear mechanism of surface gravity wave generation in horizontally sheared flow

An analysis is presented of a linear mechanism of surface gravity wave generation in a horizontally sheared flow in a fluid layer with free boundary. A free-surface flow of this type is found to be algebraically unstable. The development of instability leads to the formation of surface gravity waves whose amplitude grows with time according to a power law. Flow stability is analyzed by using a nonmodal approach in which the behavior of a spatial Fourier harmonic of a disturbance is considered in a semi-Lagrangian frame of reference moving with the flow. Shear-flow disturbances are divided into two classes (wave and vortex disturbances) depending on the value of potential vorticity. It is shown that vortex disturbances decay with time while the energy of wave disturbances increases indefinitely. Transformation of vortex disturbances into wave ones under strong shear is described.     

Experimental Study of Planar Langmuir Probe Characteristics in Electron Current-Carrying Magnetized Plasma

Experimental study of planar Langmuir probe characteristics in a magnetized plasma with an electron current along the direction of the magnetic field shows that the usual procedure for determination of the electron temperature and plasma density, which is applicable in a current-free magnetized plasma, gives erroneous results for these plasma parameters. When this procedure is applied on the characteristics measured at two opposite orientations of the probe collecting surface with respect to the direction of the electron drift, different values of the electron temperature are obtained. These virtual electron temperatures and corresponding plasma densities calculated from the measured ion saturation currents are higher and/or smaller than the exact local electron temperature and plasma density. Calculation of particular averages of these quantities is proposed as a possible way to obtain correct results for the local electron temperature and plasma density. These averages are used in the approximate evaluation of the electron drift velocity from the electron saturation currents measured at the two orientations of the probe collecting surface.     

Resonant interaction of an electromagnetic wave with an inhomogeneous plasma slab

Resonant interaction at oblique incidence of an electromagnetic wave on an inhomogeneous plasma slab is studied. The time evolution of this interaction is solved numerically from two-fluid equations, adiabatic equation for electron pressure and from Maxwell equations. It is shown that the electromagnetic energy of an incident wave is transformed both into the heat energy and into the energy of plasma oscillations in the direction of density gradient. The distribution of the transformed energy between the heat energy and the energy of plasma oscillations is strongly dependent on the plasma temperature. The ratio of heat energy to the energy of plasma oscillations is growing with growing temperature. The plasma oscillations are generated by magnetic induction of the penetrating wave. In a cold plasma they are generated especially in the overdense region and their frequency is equal to local plasma frequency. The electric field in the direction of plasma gradient has a form of a wave packet whose envelope reaches a maximum at resonance. The characteristic wavelength in the wave packet decreases and the amplitude of the packet increases with the time.     

Investigation of the Anode Boundary Layer of an Atmospheric Pressure Argon Arc

This paper is concerned with the thermodynamic state of the plasma in the anode region of a high intensity arc. Spectrometric studies of the boundary layer in front of a plane anode perpendicular to the axis of a wall-stablized, high-intensity arc in argon atmosphere indicate substantial deviations from LTE. Data taken at distances of 0.5 to 2 mm from the anode surface are evaluated with two different but not entirely independent non-equilibrium data reduction techniques in order to obtain radial distributions of electron temperature and electron density. The electron temperature distribution displays an off-axis peak which increases in magnitude with decreasing distance from the anode. Both electron temperatures and electron densities are considerably higher than the corresponding LTE values.     

[Russian Text Ignored]

A nonlinear theory of the interaction of an external pump field with ion-acoustic and h.f. surface waves in a plasma layer with inhomogeneous boundary regions is presented for the case, when the pump field frequency is less than the electron plasma frequency. The evolution of the parametrically excited turbulence and its stationary state are both analytically treated and numerically demonstrated. Dependent on the pump field strength, two mechanisms are to be distinguished: (i) nonlinear generation of damped ion-acoustic harmonics, (ii) cascade excitation of secondary eigenmodes by growing surface oscillations. At only slight overthreshold values, mechanism (i) is responsible for the occurence of the steady state, whereas, for higher pump field strength, the wave amplitudes as well as their saturated levels are mainly determined by mechanism (ii).     

Refraction of a cylindrical laser beam in a boundary temperature layer

Refraction of a cylindrical laser beam in the boundary layer near the surface of a heated metal spherical ball in water is investigated theoretically and experimentally. An experimental setup and typical refraction images (refractograms) are described. Refractograms calculated using the proposed geometric-optics model demonstrate the close agreement with experiment. Possibilities of reconstructing the temperature field in the boundary microlayer from experimental results are considered.