Hard undulator radiation in an inverse medium

X-ray radiation from fast electrons in undulators filled with an inverse medium is studied. A formula for the spectral density of the number of photons is derived. The intensity zeroth harmonics, as well as the intensity of radiation in zero undulator field, are described by the Tamm-Frank formula for Cherenkov radiation. It is shown that the spectral density of emitted photons in a wavelength range of λ ? 0.4–2.0 Å in such a medium can be increased by four orders of magnitude as compared to the radiation intensity in a vacuum undulator. The energy of emitted electrons in the former case must be in an interval of 5-2 GeV, while electrons with an energy of 14-6 GeV are required in vacuum undulators.

     

X-ray synchrotron radiation in medium

Energy and angular distributions of X-ray synchrotron radiation produced by an ultra relativistic electron moving in a medium are discussed. Calculations show that the medium suppresses strongly the yield of the radiation for the electron Lorentz factor smaller than some cut-off value depending on the medium electron density and magnetic field applied.     

Interference effects in the coherent X-ray radiation of relativistic electrons in two- and three-layer targets

A dynamic theory of the coherent X-ray radiation of relativistic electrons in “amorphous medium–single-crystal” and “amorphous medium–free-space–single-crystal” targets is developed. Expressions for the spectral-angular distributions of diffracted transition radiation (DTR), parametric X-ray radiation, and a term describing their interference are obtained. The spectral-angular density of the DTR is represented as a sum of terms describing the diffracted transition radiation from various boundaries and a term describing the interference of these components. Interference effects in the spectral-angular and angular densities of radiation are studied.     

Momentum, radiation pressure, and other second-order quantities in ideal gas (liquid) in some boundary-value problems

Problems related to such properties of sound waves as momentum, radiation pressure, and sound energy density and flux are investigated on the basis of the solutions of particular problems in the first-and second-order approximations using the Eulerian representation. Specifically, it is shown that a disturbance propagating in a continuous medium may have a nonzero momentum when the average density of the medium in the volume occupied by the wave coincides with the density of the undisturbed medium. In this case, the momentum and the related mass transfer and radiation pressure are caused by variations in the wave profile (envelope). Andreev’s expression for the energy density that differs from the commonly used one is verified, and some other paradoxical consequences of the theory of sound are considered. The correctness of using the quantities averaged over time and space is discussed.     

Speckle diagnostics of multiple scattering media with the use of frequency-modulated laser radiation

A method for probing randomly inhomogeneous multiple scattering media with the use of frequency-modulated laser radiation is considered. The method is based on analysis of the dependence of the blinking index of time-averaged speckles formed upon scattering of the probing radiation in a medium on the frequency modulation depth of the probing radiation. In the case of a binary frequency modulation, the blinking index of the detected speckle-modulated radiation is determined by the cosine Fourier transform of the probability density of the optical path-length difference of partial components of the scattered field in the probed medium. The values of the probability density of the optical path-length difference reconstructed with the use of the proposed method from the measured blinking index of speckles for model scattering media (fluoroplastic layer and layer of TiO₂ particles on a glass substrate) are in a good agreement with the results of statistical simulation of the probing radiation transfer in multiple scattering media.     

Jets in QCD media: From color coherence to decoherence

We investigate soft gluon radiation off a quark-antiquark antenna in both color singlet and octet configurations traversing a dense medium. We demonstrate that, in both cases, multiple scatterings lead to a gradual decoherence of the antenna radiation as a function of the medium density. In particular, in the limit of a completely opaque medium, total decoherence is obtained, i.e., the quark and the antiquark radiate as independent emitters in vacuum, thus losing memory of their origin.     

Nonlinear transmittance of ZnSe:Fe<Superscript>2+</Superscript> crystal at a wavelength of 2.92 μm

ZnSe crystals doped with Fe²⁺ ions are produced with the diffusion method under the conditions for thermodynamic equilibrium of solid ZnSe, solid Fe, and vapors (S_ZnSe-S_Fe-V). The transmittance of the samples is varied from 7 to 50% (in the absence of the antireflection coating) for a wavelength of about 3 μm. It is demonstrated that the transmittance of the ZnSe:Fe²⁺ crystals increases with an increase in the energy density of the high-power laser radiation with a wavelength of 2.92 μm. An equation that describes the propagation of the resonant radiation in a medium with saturable absorption at an arbitrary ratio of the radiation pulse duration to the relaxation time of the medium is introduced for the analysis of the experimental dependence of the transmittance on the energy density.     

Helmholtz integral in the acoustics of an inhomogeneous medium

Integral relations that generalize the Helmholtz integral for an inhomogeneous medium with arbitrary gradients of its density and the sound velocity in it are obtained. Expressions that determine the Helmholtz integral for problems related to the diffraction and radiation of sound in an inhomogeneous medium are derived. It is shown that, in the case of an inhomogeneous medium, an additional factor depending on the density distribution in the medium appears in the integrand.     

Mass change of dielectric media induced by propagation of electromagnetic waves

It has been experimentally confirmed that an electromagnetic wave carries momentum and thus exerts forces on dielectrics. Combining wave-particle duality with the theory of relativity, it was shown in the present work that the electromagnetic wave also increases the mass of the dielectric medium through which the incident wave propagates. The change in the mass density of the medium, or the mass density of the electromagnetic wave in the medium, was found to be proportional to the energy density of the incident wave. At an absolute temperature T, the mass density of the black body radiation was shown to be proportional to T⁴. “Weighing” the light wave in a glass fiber provides a novel technique to study the particle nature of electromagnetic waves in dielectric media.     

Transition radiation of sound under uniform motion of momentum and mass sources in inhomogeneous media

Theoretical investigations of transition radiation of sound under the uniform motion of small momentum and mass (heat) sources in a gas with chaotic inhomogeneities of density and temperature are briefly discussed. We analyze two basic methods for calculating the source losses by the radiation of acoustic waves. The first method is based on the calculation of the energy flux in the Born approximation over small perturbations of the parameters of the medium, and allows us, in particular, to clarify the problem of the angular distribution of the radiation. The second method is based on the use of the effective permittivity tensor, which links together the spatiotemporal Fourier transforms of the average momentum of a unit volume of the medium and the average (over the ensemble) velocity of the wave perturbations. The dependence of the radiation reaction force on the velocity of the motion of source is analyzed for the case of small-scale and large-scale inhomogeneities of the medium.Radiophysical Research Institute, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, Nos. 1–2, pp. 44–55, January–February, 1995.     

The Milne problem in a continuous stochastic planar medium with Gaussian statistics

The Milne problem of radiative transfer in a planar medium, with isotropic scattering is considered. The medium is assumed to be continuous stochastic medium, with fluctuations described as Gaussian field. Pomraning-Eddington method is used to obtain an explicit form for the radiation energy density in the deterministic case. It depends on two random variables, namely the optical space variable and the optical thickness of the medium. The Gaussian joint probability density function of these two random variables is defined and used to find the ensemble-averaged energy density and the linear extrapolation distance. It is shown that the statistical nature of the medium leads to two quite different solutions of the Milne problem. Numerical results are implemented for the sake of clarification.     

Electromagnetic radiation and instability of superthermal particles in a magnetoactive plasma with strong small-scale irregularities

The electromagnetic radiation of superthermal electrons in a magnetoactive plasma with strong small-scale irregularities is calculated. The emissive power of such a medium is found. The flux density of electromagnetic radiation I_f is determined for an optically thin medium. The values of I_f are estimated for different ionospheric conditions. The induced radiation of a directed electron beam of charged particles in a magnetoactive plasma with such irregularities is studied. The gain (absorption) factor is derived. The possibility of electromagnetic instability in different ionospheric conditions is analyzed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 3, pp. 286–300, March, 1996.     

Simultaneous solution of Kompaneets equation and radiative transfer equation in the photon energy range 1-125 keV

Radiative transfer equation in plane parallel geometry and Kompaneets equation is solved simultaneously to obtain theoretical spectrum of 1-125 keV photon energy range. Diffuse radiation field are calculated using time-independent radiative transfer equation in plane parallel geometry, which is developed using discrete space theory (DST) of radiative transfer in a homogeneous medium for different optical depths. We assumed free-free emission and absorption and emission due to electron gas to be operating in the medium. The three terms n, n² and (∂n/∂x_k) where n is photon phase density and x_k=(hν/kT_e), in Kompaneets equation and those due to free-free emission are utilized to calculate the change in the photon phase density in a hot electron gas. Two types of incident radiation are considered: (1) isotropic radiation with the modified black body radiation I^MB[1] and (2) anisotropic radiation which is angle dependent. The emergent radiation at τ=0 and reflected radiation τ=τ_max are calculated by using the diffuse radiation from the medium. The emergent and reflected radiation contain the free-free emission and emission from the hot electron gas. Kompaneets equation gives the changes in photon phase densities in different types of media. Although the initial spectrum is angle dependent, the Kompaneets equation gives a spectrum which is angle independent after several Compton scattering times.     

On the Spectral Distribution of Equilibrium Radiation and Zero-Point Fluctuations in the Coulomb System

The consideration of equilibrium radiation in plasma-like media shows that the spectral energy distribution of such radiation differs from that of Planck equilibrium radiation. Based on the previously derived relation for the spectral energy density of equilibrium radiation in the system of charged particles, accounting for finite damping in a medium with spatial dispersion, the limiting case of infinitesimal damping dispersion is considered. It was shown that zero-point vacuum fluctuations being a component of the total spectral energy distribution in the medium should be renormalized when using certain models for the transverse plasma permittivity. In this case, renormalized zero-point vacuum fluctuations become dependent on plasma parameters. The possibility of the manifestation of this effect is discussed.     

Emission of a spatially modulated resonant medium excited with superluminal velocity

Specific characteristics of the radiation of a resonant medium excited by an ultrashort light pulse propagating through the medium with a superluminal velocity are considered. The medium is assumed to consist of identical linear harmonic oscillators with a spatial density periodically modulated along the direction of propagation of the superluminal excitation. The field of radiation of the resonant medium under these conditions is calculated. It is shown that, under the superluminal excitation, the radiation spectrum of the medium shows, along with the fundamental frequency of the oscillators, new frequencies that depend on the spatial frequency of the distribution of oscillators and on the angle of observation. Possible application of the effect is discussed.     

Simulation of interaction of radiation with nanoparticles

Enhancement of microwave radiation at wavelength λ ∼ 10 cm in a cavity is simulated using the system of constituent equations derived earlier. It is shown that a radiation energy density of W ∼ 1000 J/m³ can be attained. Pumping of the medium containing conducting nanoparticles is carried out with a stationary electric field. The required mass concentration of nanoparticles and pumping field are estimated. A method of obtaining active medium using a statitionary electric field for enhancement of microwave radiation in a wavelength range of λ ∼ 10 cm is proposed. In this method, extended conducting nanoparticles should be sputtered.     

Finite element analysis of concurrent radiation and conduction in participating media

A method is proposed to calculate temperature, conductive and radiative heat flux distributions in a participating medium. The method is based on the simultaneous solution of two non-linear and mutually conjugated equations describing distribution of both temperature and the so-called radiation function in the medium. In the case of isotropic scattering, the latter quantity, is proportional to the local energy density of radiation. The solution of the coupled non-linear equations is based on the finite element spatial discretization combined with the iterative technique.     

Phototransformations of substituted <Emphasis Type="Italic">p</Emphasis>-terphenyl upon excitation by a XeCl laser

Phototransformations of 1,8-dicarboxyisobutyl-substituted p-terphenyl in ethanol are studied upon excitation by a XeCl laser at a power density of up to 180 MW/cm² for different types of radiation and different excitation geometries. The absorption and emission spectra are studied before and after irradiation. The resources of the lasing medium, quantum yields of phototransformations, and relative quantum yields of photoproducts are determined. The transmission of the solutions at the excitation wavelength and the shape of radiation pulses are measured as functions of the pump power density.     

Ionization yield of two-step photoionization process in an optically thick atomic medium of barium

The kinetics of a two-step photoionization process in optically thick atomic medium of barium (Ba) is studied using the rate equation approach. In the first step, Ba atoms get resonantly excited by laser radiation from their ground state to an intermediate excited state and subsequently are ionized in the second step by another laser radiation. The absorption of exciting radiation is taken into account along its propagation direction (optically thick). However, the medium is assumed to be optically thin for the ionizing radiation. A numerical simulation is done to estimate the ionization yield for time-varying Gaussian shaped laser pulses. The required energy density of the laser pulse to saturate the excitation transition throughout the thick medium is calculated. The effect of optical delay between the laser beams on the ionization yield is simulated. The calculated ionization yield from the simulation is compared with the measured values.     

X光辐射在泡沫介质中超声速传输研究进展

X光辐射输运过程通过光子的发射与吸收使能量在介质内进行再分配,而光子的辐射和吸收过程对从介质内出射的能谱产生十分显著的影响。辐射输运由积分-微分方程来描述,得到它的解极其困难,这是因为此方程依赖于局域和非局域的条件。当外加的非局域的X光辐射场作用在介质上一个强的X光辐射能流时,介质的局域温度和密度将影响X光的吸收和发射。本文将对辐射输运的方程和基本理论进行阐述,并对辐射超声速传输的实验结果进行评述。首先,介绍辐射在介质中的传输的理论基础以及简化分析模型;其次,对辐射在介质中扩散超声速输运进行解析分析,我们首次导出辐射的超声速传输条件下的辐射能流与物质能流之比与马赫数和光学厚度的定量关系;最后,介绍国外的主要实验结果,同时也给出我们近期的研究结果。我们的实验结果表明,不同能区的光子因辐射不透明度不同使得在介质中的传播时间不同,并且实验测出光学厚度。