Analysis of Tamm's problem on charge radiation at its uniform motion over a finite trajectory

The main results of Tamm's problem on radiation produced by an electron moving uniformly along a finite rectilinear section of its trajectory in a transparent medium are analysed. The radiation is compared with the bremsstrahlung occurring at an instant charge acceleration with allowance for dielectric properties of the medium. The Tamm formula is shown to describe the resulting radiation from interference of two bremsstrahlungs caused by double change in the particle velocity in the same and opposite way. This mechanism can be another approach to explanation of the Vavilov-Cerenkov effect.Finally, the authors express their deep gratitude to Professor B. M. Bolotovskii for valuable discussions and coments, and to A. P. Kobzev for discussions that initiated this paper.     

半透明梯度折射率介质内辐射熵传递方程及其数值模拟

在非相干辐射条件下,基于Planck光谱辐射熵强度定义,导出半透明梯度折射率介质内光谱辐射熵传递方程,以及局部辐射熵产率理论表达式.基于离散坐标法对辐射熵传递方程进行数值求解.以一维半透明梯度介质平板为例,对辐射熵方程及其算法进行验证.平板整体无因次辐射熵产的计算结果与宏观热力学定律的结果一致.     

Passage of monochromatic radiation through a resonance radiation absorbing medium

The problem of passage of monochromatic radiation pulses through a one-dimensional medium containing several types of nonuniformly distributed absorption centers with coincident energy levels are considered. The physical picture associated with the effect of a brightening wave arising in an optically dense absorbing medium is analyzed. The investigated relations represent a generalization of Bouguer's law for the case of powerful fluxes of monochromatic radiation with allowance for absorption saturation.     

Spontaneous radiation of molecules in open cavities

Within the framework of classical electrodynamics, a formula is derived for the spontaneous radiation rate of molecules and atoms in an arbitrary open cavity in the weak coupling approximation with allowance made for radiation absorption or amplification by the cavity material. The formula agrees well with microdroplet luminescence data. The effect of suppression of the spontaneous resonance radiation rate by the active laser medium is predicted.     

Localization of radiation in two-dimensional random media of finite extent

The localization of electromagnetic waves in a two-dimensional random medium composed of strong scatterers is studied theoretically. It is shown that an allowance for the spatial radiation intensity distribution inside the medium, along with an analysis of the distance dependence of the transmission coefficient, is needed to reveal the localization states in media of finite extent.     

Combustion Wave Stability in Transition through the Interface of Gasless Systems

In this paper, using mathematical modeling, we study combustion wave stability in transition through the interface of gasless systems. The effect of a gas layer separating two chemically active gasless layers on the combustion wave stability was studied at all stages of the transition. Using the criteria obtained, we estimate the stability conditions of the transition combustion wave. The nonstationary dynamics of the combustion wave transition through the gas gap is studied with allowance for competing mechanisms of heat transfer, such as conductive and radiant transfer. We analyze the effect of radiation heat transfer in the gas gap on the characteristics and stability of the transient combustion process. The failure region of the igniter combustion wave is determined through the approach to the ignition system, while estimates of temperature and heat flux at the interface of the systems are given with respect to the time of the igniter combustion completion under conditions of dominant conductive and radiant heat transfer.     

Three-dimensional vector radiative transfer in a semi-infinite, Rayleigh scattering medium exposed to spatially varying polarized radiation: generalized reflection matrix

Three-dimensional vector radiative transfer in a semi-infinite medium exposed to spatially varying, polarized radiation is studied. The problem is to determine the generalized reflection matrix for a multiple scattering medium characterized by a 4×4 scattering matrix. A double integral transform is used to convert the three-dimensional vector radiative transfer equation to a one-dimensional form, and a modified Ambarzumian's method is then applied to derive a nonlinear integral equation for the generalized reflection matrix. The spatially varying backscattered radiation for an arbitrarily polarized incident beam can be found from the generalized reflection matrix. For Rayleigh scattering and normal incidence and emergence, the generalized reflection matrix is shown to have five non-zero elements. Benchmark results for these five elements are presented and compared to asymptotic results. When the incident radiation is polarized, the vector approach used in this study correctly predicts three-dimensional behavior, while the scalar approach does not. When the incident radiation is unpolarized, both the vector and scalar approaches predict a two-dimensional distribution of the intensity, but the error in the scalar prediction can be as high as 20%.     

Self-focusing of laser radiation in cluster plasma

The self-focusing of laser radiation in plasma with ionized gaseous clusters is studied both analytically and numerically. An electrodynamic model is proposed for cluster plasma in a field of ultrashort laser pulse. The radiation self-action dynamics are studied using the equation for wave-field envelope with allowance for the electronic nonlinearity of the expanded plasma bunches and the group-velocity dispersion in a nanodispersive medium. It is shown that, for a laser power exceeding the self-focusing critical power, the wave-field self-compression occurs in a medium with dispersion of any type (normal, anomalous, or combined). Due to the strong dependence of the characteristic nonlinear field on the size of ionized cluster, the corresponding processes develop faster than in a homogeneous medium and give rise to the ultrashort pulses.     

The use of the galerkin method for radiation transfer in an anisotropically scattering slab with reflecting boundaries

Radiation transfer in an absorbing, emitting, anisotropically scattering, plane-parallel medium with diffusely reflecting boundaries is solved by application of the Galerkin method. With this approach, the radiation heat flux, angular distribution of radiation intensity, and the divergence of the radiation heat flux anywhere in the medium can be determined highly accurately. For optical thicknesses up to about 10, exact results are also readily obtainable if sufficient number of terms are considered in the expansion. Numerical results are presented for representative cases.     

Influence of the nonlocal nature of fluctuations on coherent effects in multiple scattering

We solve the Bethe-Salpeter equation that describes radiation transfer in highly inhomogeneous media with an anisotropic scattering with allowance for the contributions of Legendre polynomials of the zeroth, first, and second degrees. An analytical expression for the radiation-transfer propagator is derived. We show that as the average value of the second-degree Lagrange polynomial increases, the region where the diffusion approximation is valid shifts toward large distances. Within this approach we calculate the coherent backscattering intensity and study the effect of higher-order moments on the angular dependence of this intensity. Finally, we show that it is possible to experimentally detect the coherent backscattering peak in the critical region. Zh. éksp. Teor. Fiz. 113, 2022–2033 (June 1998)     

A theoretical model of dissolution and hydrate formation processes in shock waves

A theoretical model for the processes of dissolution and hydrate formation behind a shock wave in a gas-liquid medium with allowance for convective and molecular gas diffusion in the liquid and convective and conductive heat transfer caused by heat release at the interphase boundary due to dissolution and hydrate formation is proposed. A comparison of the model calculations with experimental data is made.     

吸收-透射界面下正弦折射率介质内辐射换热

对具有吸收-透射性边界面的梯度折射率半透明介质层,建立了介质内热辐射传递与边界面辐射换热的数理模型,并采用数值弯曲光线跟踪法求解介质内的热辐射传递。通过数值模拟,分析了正弦折射率下,边界面的反射特性、吸收率以及介质层光学厚度对介质内热辐射平衡温度场及热流分布的影响。结果表明,边界面的反射特性与吸收率对介质内辐射换热均有重要影响,吸收率的影响与边界面反射特性、介质层光学厚度及环境条件相关,呈现特征不同的作用。     

A fundamental-source-function formulation of radiative transfer and the resulting fundamental reciprocity relations

An important problem in radiative transfer is finding the radiative fields produced by various illuminations (both external and internal) of a plane-parallel, inhomogeneous, absorbing, emitting, and anisotropically-scattering finite medium. One approach to a solution is to find the source function, which represents the rate of production of scattered radiation per unit volume and solid angle, at each point in the medium. The present study develops the existence of a Green's function, called the fundamental source function, which separates the optical properties of the medium from the driving illumination. Radiative linearity then allows the representation of all possible source functions as convolutions of the illumination with the fundamental source function. Parametric differentiation (invariant imbedding) is used to replace the governing linear integral equation for the fundamental source function with a set of differential equations appropriate for numerical integration. This approach for finding the fundamental source function leads naturally to the introduction of fundamental scattering and transmission functions. Our inclusion of anisotropic internal illumination (sources) allows us to develop four new reciprocity relations involving these functions. The reciprocity relations state general equivalences between an internally and an externally driven medium and thus greatly reduce the complexity of radiative transfer.     

Diffusionless motion of high-absorptivity domains along the laser beam axis

A new mechanism of motion of high-absorptivity domains under the action of Gaussian beams without allowance for diffusion of the material characteristics along the direction of beam propagation is studied by computer simulation. The essence of this mechanism is the dependence of the absorption factor growth rate on the radiation intensity. As a result, high-absorptivity domains may drift from heavily to weakly irradiated regions of the medium.     

Local field effects and stimulated multimode scattering of resonance radiation in a two-level medium

Scattering of resonance radiation in a dense two-level medium is considered with allowance for the local field effects. The system is studied in the framework of the mean-field approximation for a modified hierarchy of the Bogoliubov-Born-Green-Kirkwood-Yvon equations for reduced density matrices of an ensemble of atoms and modes of a quantized electromagnetic field. The local field correction is consistently derived from the initial Hamiltonian of the system. The problem of scattering of a short rectangular pulse from a quasi-point sample is considered numerically and theoretically. The possibility of a multimode spectrum of scattered radiation with frequencies multiple of the Rabi frequency and with other intermediate frequencies is demonstrated. The relative intensities of spectral lines are determined.     

Features of the optical response of dielectric crystals with incommensurate phases

The constitutive relation describing the optical response of dielectric crystals in incommensurately modulated phases is derived for the plane-wave range of modulation. The symmetry properties of modulation corrections to the permittivity tensor of such crystals are analyzed with allowance for spatial dispersion. These properties are dictated by the Onsager principle for a nonabsorbing medium and by the mesoscopic periodicity of the medium. It is shown that the basic results are consistent with the general conventional approach in crystal optics of spatially inhomogeneous media.     

Experimental studies of passive correlation tomography of incoherent acoustic sources in the megahertz frequency band

Correlation properties of thermal acoustic radiation in the megahertz frequency band are studied experimentally. The amplitudes of correlation peaks measured at the sum of the delays that correspond to the direct signal propagation between the piezoelectric transducers are found to depend on the relative transducer positions. This effect is explained by a possible difference between the receive patterns of the transducers and the patterns of their intrinsic thermal radiation. The correlation of signals received from a spatial region heated above the ambient medium is measured. The correlation between the received signals is found to depend on the time of heating of the emitter’s working medium. Numerical calculation of temperature distributions and the allowance for the decorrelation effect due to the spatial extent of the source provide a quantitative agreement between the theoretical and experimental results. Estimates of the spatial resolution achievable with the given experimental configuration show that it is close to the coherence length of the received radiation.     

On the gravitational motion of a nonuniformly heated solid particle in a gaseous medium

The steady motion of a nonuniformly heated spherical aerosol particle through a viscous gaseous medium is theoretically studied in the Stokes approximation. It is assumed that the mean temperature of the particle surface may differ appreciably from the ambient temperature. The solution of gasdynamic equations yields an analytical expression for the drag of the medium and the gravitational fall velocity of the nonuniformly heated spherical solid particle with allowance for the temperature dependence of the density of the medium and molecular transfer coefficients (viscosity and thermal conductivity). Numerical estimates show that heating of the particle surface considerably influences the drag force and gravitational fall velocity.     

Radiation field of a dipole situated near an interface of two media

The electromagnetic-field distributions and angular and total radiation intensities of a point electric dipole situated near a semibounded plasmalike medium are obtained for specular diffuse, and random reflection of plasma charged particles from the interface. It is shown that the dipole's radiation field is completely determined by the Fresnel coefficient. of electromagnetic-wave reflection from a plasma half-space with allowance for mechanism of interaction of plasma-system charged particles with the boundary surface. The frequency spectra of the dipole's radiation intensity are studied in detail in a cold-plasma approximation.Institute of Theoretical Physics, Academy of Sciences of Ukraine. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 35, No. 6/7, pp. 540–567, June–July, 1992.