On the effect of absorption on multiple wave-scattering in a magnetized turbulent plasma

Multiple scattering of electromagnetic waves by a plane layer of a turbulent magnetized collision plasma is considered. The influence of the distance between both the emitter and the receiver and the layer boundaries is analysed. It is found that the width of the angular spectrum of the received radiation for sufficiently strong absorption in the plasma is greater than in the collisionless plasma; the spectral maximum is substantially displaced with respect to the direction of the source. It is shown that these effects are weakened when the emitter approaches the layer. The relationship between the spectral width and also the displacement of its maximum and the distance from the receiver to the layer boundary may be substantially non-monotonic.     

On the effect of absorption on multiple wave-scattering in a magnetized turbulent plasma

Abstract

Multiple scattering of electromagnetic waves by a plane layer of a turbulent magnetized collision plasma is considered. The influence of the distance between both the emitter and the receiver and the layer boundaries is analysed. It is found that the width of the angular spectrum of the received radiation for sufficiently strong absorption in the plasma is greater than in the collisionless plasma; the spectral maximum is substantially displaced with respect to the direction of the source. It is shown that these effects are weakened when the emitter approaches the layer. The relationship between the spectral width and also the displacement of its maximum and the distance from the receiver to the layer boundary may be substantially non-monotonic.     

Statistical characteristics of radiation of a point source scattered by a layer of turbulent collisional magnetized plasma

We consider multiple scattering of electromagnetic waves by a layer of turbulent magnetized collisional plasma. The influence of the distances from the layer's boundaries to the emitter and receiver is analyzed. We find out that, provided absorption in the plasma is strong enough, the width of the angular spectrum of the received radiation can be larger than in the collisionless plasma, and the maximum of the spectrum is substantially shifted with respect to the direction to the source. It is shown that these effects weaken as the emitter approaches the layer. The dependence of the width of the spectrum and the shift of its maximum on distance from the receiver to the layer's boundary can be substantially nonmonotonic. Lobachevsky State University, Nizhny Novgorod, Russia; Georgian Technical University, Tbilisi, Georgia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol.42, No. 4, pp. 340–354, April 1999.     

Spatial and temporal distortions of a signal during multiple scattering

A calculation procedure is described which can be used to determine the spatial and temporal distortions of a signal at the receiver input for any angular characteristic of the radiation source, receiver directional pattern, or scattering characteristic of the medium. This procedure can be used to determine the signal envelope at the receiver input with respect to time and angular coordinates. The procedure is based on a study of the solution of the nonsteady-state radiation-transport equation. An expression is given for the k-th scattering order.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 12, No. 6, pp. 70–73, June, 1969.     

On the scintillations of radiation from an extended source in a randomly inhomogeneous medium

Using the method of refractive scattering of radio waves (RSRW), we solve the problem of space correlation of scintillations of radiation from an extended source in a randomly inhomogeneous medium. General expressions are obtained for the index and radius of the space correlation of fluctuations of intensity of radiation from a source with finite angular dimensions as it propagates though a multilayered medium with refractive index fluctuations. The RSRW method is similar to the diffraction calculation of space correlation of scintillations of an extended source in a thick layer with inhomogeneities. We note that under certain conditions an increase in the number of inhomogeneous layers on the radio wave propagation path leads to a pronounced decrease in the scintillation index and an increase in the space correlation of fluctuations of intensity of received radiation from an extended source. The results obtained have a simple geometric-optical interpretation. We indicate a specific feature in determining the angular dimensions of extended radio sources using the known method of scintillations. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 5, pp. 586–593, May, 1997.     

Approximate solution for describing polarization characteristics of radiation in a Rayleigh scattering medium

An approximate analytical solution for the 4 × 4 Green’s matrix of the problem of polarized radiation transfer in a plane-parallel layer of an absorptive Rayleigh scattering medium is proposed. It permits one to perform fast estimates of angular distributions of the Stokes parameters that are created by an incident beam with an arbitrary polarization state at different levels in a layer when the layer thickness, absorption magnitude, and albedo of the underlying surface are varied. The developed solution is compared with data obtained by the numerical doubling method. The value of the scattering coefficient for a circularly polarized radiation is shown to be somewhat smaller than that for linearly polarized radiation.     

Zonal model of nonstationary self-focusing of femtosecond laser radiation in air: effective beam characteristics evolution

The generic scenario of intense femtosecond laser pulse propagation in the air from the viewpoint of evolution of its integral effective parameters (energy transfer coefficient, effective radius, effective duration, limiting angular divergence) is considered. The analysis of variation of the effective parameters along the propagation path in the single and multiply filamentation scenarios based on numerical calculations is presented. It is shown that the process of self-action of the ultrashort radiation is characterized by the formation in a medium of the nonlinearity layer, after which optical pulse propagates quasi-linearly with the limiting angular divergence that depends mainly on initial pulse power. The effective pulse temporal duration and the effective beam radius increase after the passage through the nonlinearity layer, and their values are mostly determined by the initial beam power also. The coefficient of energy transmission of femtosecond laser radiation is lower than in the linear medium and has a tendency to decrease with the increase of the pulse power.     

Features of Radiation Fields of Some Sources in a Subluminal Flow of a Nondispersive Medium

We study radiation fields of various dipole sources as well as of a source of bisphere type moving in a nondispersive medium with velocity less than the speed of light in this medium. In particular, it is shown that in certain cases, there is a displacement of the angular pattern of a source in the direction opposite to the medium flow. Expressions for energy losses of the considered sources are obtained. It is pointed out that the radiation power of both a bisphere and a toroidal dipole increases with increasing velocity of motion of the medium more rapidly compared with the ordinary dipoles.     

An optico-acoustical gas analyzer with a driftless null point

The underlying principle of this gas analyzer is that the absorption cell, which holds the gas mixture under investigation, is wedge-shaped and situated between the radiation source and a selective opticoacoustical receiver, which, as usual, contains the gas to be determined. The cell is rotated about an axis parallel to the radiation flux which enters the receiver. As a result, the thickness of the gas layer penetrated by the radiation varies periodically, so that the radiation is subjected to a modulation whose index corresponds to the concentration of the gas being determined in the mixture. If no gas is present, the instrument indicator reads zero.     

Scattering of a Narrow Wave Beam on a Rough,Locally-Specular Surface in the Case of Pulsed Illumination

We consider scattering of a pulsed narrow wave beam on a rough surface with a locally-specular indicatrix. Analytical expressions for the average received power are obtained for normal distributions of heights and slopes of the rough surface in two cases in which the direction to the receiver is close to or strongly different from the direction of specular reflection. It is shown that in these cases, the received echo pulses have drastically different profiles determined by the source and receiver parameters, the scheme of sounding, and the variance of the heights and slopes of the rough surface. The obtained analytical expressions for the average received power agree well with the results of numerical simulations.     

Features of the Sound Field Structure in a Two-Channel Oceanic Waveguide

The results of experimental studies of the energy and angular structure of a sound field in the region of the Iberian Basin in the northeastern Atlantic are discussed. The experiments are carried out in a two-channel waveguide whose axes are located at depths of approximately 450 and 2000 m. A continuous pseudonoise signal in the frequency range 2.52–4.0 kHz is emitted. The signals are received by the omnidirectional hydrophones and, simultaneously, by a 10-m-long vertical array, which allows one to realize a narrow beam reception (～2.5°) in the vertical plane. The source and the receiver are located in a 500-m-thick layer within the upper sound channel. The field characteristics are measured in the course of a continuous change of distance from 1 to 65 km. The comparison of the experimental data with calculations shows that the sound field structure formed by the lower channel is much closer to the theoretical results than the structure formed by the upper channel. In the upper sound channel, the shadow zone manifests itself only slightly and the first convergence zone begins approximately 72–11 km nearer to the source than predicted by the calculations. The corresponding angular sound field structure is fairly pronounced in the vertical plane and bears no evidence of the random behavior that is peculiar to the fields scattered by the inhomogeneities.     

Statistical characteristics of a wave propagating through a layer with random irregularities

Statistical characteristics of a wave propagating through a layer with random irregularities are investigated by a simulation procedure. The investigation is carried out within the geometrical optics approximation in its validity range. It is shown that when the irregular layer is a long distance from the source and observer, a significant role in the formation of eikonal (phase path) fluctuations is then played by trajectory fluctuations in regions of the propagation medium, free from irregularities before and after the irregular layer. With these variations taken into account, which are neglected in conventional perturbation theory, we obtained approximate expressions for the dispersion and the correlation function of the eikonal. We investigate the behaviour of the eikonal dispersions, the angles and correlation functions of the eikonal and field for different disturbances of the medium, and for different distances of the receiver and transmitter from the layer boundaries.     

Scattering of a narrow wave beam by a randomly rough locally diffuse surface subject to pulse illumination

We consider the scattering of a narrow pulse wave beam by a randomly rough surface with a complex local scattering indicatrix. Analytical expressions are found for the mean received power for a normal distribution of heights and slopes of the surface in two cases: where the direction to the receiver is close to the direction of mirror reflection and where the direction to the receiver is very different from the direction of mirror reflection. It is shown that in these two cases the echo pulse is very different in shape and is controlled by the parameters of the source and receiver, the sounding scheme, and the variance of heights of a rough surface. The received power is strongly dependent on the width of the local scattering indicatrix, and the form of this dependence is determined by the angles of illumination and reception. The analytical expressions for the mean received power are in good agreement with the results of numerical calculations. Institute for Radioelectronics and Laser Engineering of the N. é. Bauman State Technical University of Moscow, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol.42, No.4, pp. 333–339, April 1999.     

Correlation of signals of thermal acoustic radiation

The spatial correlation function is measured for the pressure of thermal acoustic radiation from a source (a narrow plasticine plate) whose temperature is made both higher and lower than the temperature of the receiver. The spatial correlation function of the pressure of thermal acoustic radiation is found to be oscillatory in character. The oscillation amplitude is determined not by the absolute temperature of the source but by the temperature difference between the source and the receiver. The correlation function changes its sign when a source heated with respect to the receiver is replaced by a cooled one.     

Measurement of angular distribution of soft X‐ray radiation from thin targets in the tabletop storage ring MIRRORCLE‐20SX

The only available tabletop electron storage rings are the machines from the MIRRORCLE series. The electrons are accelerated in a microtron and injected into the storage ring. During its circulation, each electron passes through a tiny target many times, emitting a photon beam. Both the spectrum and the angular distribution of the radiation depend on the material, the thickness and the shape of the target. In this paper measured angular distributions of the radiation from several different targets in the magnetic field of the 20 MeV storage ring MIRRORCLE‐20SX are presented. The detector comprises a 3 mm × 3 mm × 8.5 µm plastic scintillator (PS) coupled to a photomultiplier by a bundle of optical fibers. The output of the photomultiplier is digitized by an IF converter. This detector is sensitive mostly to soft X‐ray radiation, and its PS is moved by a mechanical system in a plane perpendicular to the radiation axis. The measured angular distributions for Mo and Sn targets contain an annulus which is attributed to transition radiation. The angular distributions for Al, carbon nanotube and diamond‐like carbon (DLC) targets show some suppression of the radiation along the magnetic field. This is the first evidence of observation of the angular distribution of synchrotron Cherenkov radiation, which represents Cherenkov radiation in a magnetic field. The power radiated from the DLC target is estimated.     

Absorption of external thermal radiation in asymmetrically illuminated droplets

A theoretical model of thermal radiation absorption in semi-transparent droplets at the surface and inside a fuel spray is presented. Asymmetry of droplet illumination is taken into account. Results of Mie calculations of thermal radiation absorption inside large spherical droplets illuminated from a hemisphere are presented. Simple approximations for the angular and radial dependencies of the absorbed radiation power are suggested. These approximations are generalisations of the approximations suggested earlier by the authors for the case of symmetric illumination of droplets. They predict the results close to those which follow from the Mie calculations. Results of approximate calculations for a typical diesel fuel droplet at the periphery of the spray are presented. As in the case of symmetrical droplet illumination, an increased absorption of thermal radiation in the central area of the droplet is predicted. Also, at the illuminated side of the droplet, the absorption of radiation in a thin layer near the surface of the droplet is predicted, as in the case of symmetrical droplet illumination. Absorption of radiation in the central area of the droplet is related to the contribution of radiation in the spectral ranges of semi-transparency. The maximum of radiation absorption near the droplet surface is linked to the contribution of radiation in the vicinity of the diesel fuel absorption peak .     

Simulation of wave transmission and reception with phased arrays near the sea shelf bottom

The wave field excited by both single sources and a vertically directed phased transmitting array in a shallow-water waveguide with a layered elastic bottom is numerically simulated. The receivers are equispaced vertical arrays with different apertures, which are positioned at different depths in the water layer. The type of vertical distribution and the transformation of the vertical angular spectrum of pulsed signals arriving at a receiving array are demonstrated for different parameters of the medium, different carrier frequencies, and different distances from the source. It is shown that a selective angular reception ensures a better resolution of the travel-time curves of waves used for sounding the bottom. As an example, the possibility of singling out the shear wave refracted by a deep-seated bottom layer from the tone pattern of the travel-time curves is demonstrated. This possibility can be realized by means of angular scanning in the course of transmission and reception with the use of vertically oriented arrays.     

Acoustic power flux in a waveguide

The acoustic power flux that occurs in an ideal waveguide in the presence of two modes propagating in it is considered. Singular points of the saddle and vortex types are found for modes of different numbers. The regions lying near the vortex-type points and characterized by the inverse direction of the power flux (i.e., from the receiver to the source) are determined. When a low-number mode propagates together with a higher-number one, the regions, where the power flux noticeably deviates from the general propagation direction, occupy a considerable part of the longitudinal section area of the waveguide.     

Experimental study of coherent transition radiation from relativistic electrons in a dihedral angle

Angular intensity distributions for transition radiation excited by a beam of relativistic electrons in the emitter in the form of a dihedral angle are measured in the millimeter range. The angle is formed by the intersection of two conducting planes. The source of radiation is a microtron with an electron energy of 7.4 MeV. We analyze the effect of the magnitude of the dihedral angle of the emitter, the position of the electron transition point on the surface of the angle, and the direction of motion of electrons on the angular distribution of radiation intensity. It is shown that the spectral and angular distributions of radiation intensity in the dihedral angle substantially differ from analogous distributions for a particle intersecting a planar conducting surface. The possibility of using radiation to measure the energy, spatial position, and direction of motion of charges is considered.