Transition radiation at the boundary of a chiral isotropic medium.

This study analyzes the radiation produced by a point charge intersecting the interface between a vacuum and a chiral isotropic medium. We deduce analytical expressions for the Fourier components of an electromagnetic field in both vacuum and medium for arbitrary charge velocity. The main focus is on investigating the far field in a vacuum. The distinguishing feature of the interface with a chiral isotropic medium is that the field in the vacuum area contains both copolarization (coinciding with the polarization of the self-field of a charge) and cross-polarization (orthogonal to the polarization of the self-field). Using a saddle-point approach, we obtain asymptotic representations for the field components in the far-field zone for typical frequency ranges of the Condon model of the chiral medium. We note that a so-called lateral wave is generated in a vacuum for certain parameters. The main contribution to the radiation at large distances is presented by two (co- and cross-) spherical waves of transition radiation. These waves are coherent and result in a total spherical wave with elliptical polarization, with the polarization coefficient being determined by the chirality of the medium. We present typical radiation patterns and ellipses of polarization.

Radiation of charged-particle bunches passing perpendicularly by the edge of a semi-infinite planar wire structure.

The radiation of a charged-particle bunch moving perpendicularly to a semi-infinite plane grid composed of thin parallel wires is analyzed using the method of averaged boundary conditions (the period of the grid is assumed to be much less than the wavelengths under investigation). We perform an analysis of the volume radiation and surface waves generated by a bunch of finite length. It is shown that the patterns of the volume radiation fundamentally differ from those that arise in the case of an infinite grid. The properties of the surface waves are similar to the properties of Cherenkov radiation in a three-dimensional wire metamaterial. These waves propagate along the wires at the speed of light in a vacuum and do not diminish with distance (if absorption is negligible). The structure of the surface waves allows for the determination of the size and form of the particle bunches.

Dielectric concentrator for Cherenkov radiation.

We report on a dielectric target that concentrates Cherenkov radiation into a small spatial area. In contrast to traditional devices, this target can focus almost all of the radiation without using additional lenses or mirrors. We consider the case where radiation is produced by a point charge moving along the axis of a cylindrical channel inside an axially symmetrical target. The specific form of the target is determined using the laws of ray optics. The field is calculated using an aperture integration method that can determine the field near the focus. Typical field plots and the spatial distribution of the field outside the target are presented. We demonstrate that at terahertz frequencies, this concentrator can increase the field magnitude by up to at least 2 orders of magnitude relative to that on the surface of the target.

Radiation of charges moving along the boundary of a wire metamaterial.

The electromagnetic fields of charges moving along the boundary of a "wire metamaterial" perpendicularly to the wires are investigated. The metamaterial under consideration represents a volume-periodic structure of thin parallel wires located in a square lattice. This structure is described by an effective permittivity tensor and exhibits both spatial and frequency dispersion. It is shown that the charge generates nondivergent radiation, as in the case of an infinite metamaterial. However, unlike the infinite-metamaterial case, the radiation concentrates near a certain plane (not line) behind the charge, and it is asymmetric with respect to this plane. An algorithm for calculating the wave fields of finite-length bunches is developed, and some typical numerical results are given. They demonstrate that the structure under consideration can be applied to determine the size and form of such bunches. The stopping and deflection forces acting on the charge are also calculated.

Electromagnetic field of a charge moving in a chiral isotropic medium.

We analyze the electromagnetic field generated by a point charge moving with a constant velocity in an isotropic chiral medium. We work in the frame of the Condon dispersion model for the weak chirality and ultrarelativistic motion of the charge. We show that the field of a moving charge contains two low-frequency wave processes with right- and left-hand circular polarizations and a high-frequency wave process with a right-hand polarization. The low-frequency wave field exists at an arbitrary charge velocity and oscillates at a frequency of the order of the resonant frequency of the medium. This effect is of most importance near the charge trajectory. The high-frequency wave field arises at an ultrahigh velocity and is essential near the plane of charge dislocation for a sufficiently large offset from the trajectory. This wave field oscillates at a frequency that is considerably greater (up to several orders) than the resonant frequency of the medium. Intriguingly, both of these phenomena exist in the domain in front of the charge, thus producing the low- and high-frequency wave forerunners correspondingly.

Approximate method for calculating the radiation from a moving charge in the presence of a complex object.

An approximate method for calculating the radiation from a moving charge in the presence of a dielectric object is developed. The method is composed of two steps. The first step is calculation of the field in the medium without considering the external boundaries of the object, and the second step is an approximate (ray-optical) calculation of the wave propagation outside the object. As a test problem, we consider the case of a charge crossing a dielectric plate. Computations of the field are performed using exact and approximate methods. It is shown that the results agree well. Additionally, we apply the method under consideration to the case of a cone-shaped object with a vacuum channel. The radiation energy spectral density as a function of the location of the observation point and the problem's parameters is given. In particular, the convergent radiation effect is described.

Electromagnetic field of a charge moving in a cold magnetized plasma.

The present paper addresses the electromagnetic field generated by a point charge or a small charged particle bunch moving with constant velocity in a cold magnetized plasma, along the external magnetic field. Attention is focused on the case of ultrarelativistic motion. The field surrounding the point charge is investigated both analytically and numerically. In the analytical study, we obtain rigorous decomposition of the field into quasistatic and wave components. Beating behavior in the far-field zone and harmonic behavior in the vicinity of the charge trajectory are found using suitable approximate approaches. The transverse component of the electric field exhibits a strong (inversely proportional) singularity on the charge trajectory, while the longitudinal components of both the electric and magnetic fields exhibit a weaker (logarithmic) singularity. An efficient numerical approach is developed to calculate the field for arbitrary parameters. An efficient algorithm for calculating the fields of small bunches with different forms is also presented, using a thin charged disk and a charged cylinder as representative examples.

Nondivergent Cherenkov radiation in a wire metamaterial.

The electromagnetic radiation of a charge moving in an infinite 3D structure made of parallel wires is considered. The periods of the structure are assumed to be small; therefore, it can be described by an effective permittivity tensor. The charge velocity is perpendicular to the wires. Analytical and numerical investigations are performed, and some unusual properties of the radiation are noted. It is shown that the radiation propagates along the wires and concentrates near certain rays behind the charge. The wave field does not vary with distance from the charge along these rays (if energy loss in the medium is negligible). The prospects for the use of the structure under consideration for diagnostics of bunches are noted.

Electromagnetic field of a charge traveling into an anisotropic medium.

We analyze the electromagnetic field generated by a point charge intersecting the interface between vacuum and a nonmagnetic anisotropic medium with a plasma-type dispersion of the dielectric permittivity tensor. After penetrating the medium, the charge moves along its main axis. The total field is presented as a sum of a self-field (i.e., a charge field in a corresponding unbounded medium) and a scattered field associated with the boundary influence. We show that the self-field in the considered anisotropic medium is divided into a quasistatic field and a wave field (the so-called "plasma trace" is absent in the case under consideration). Under certain conditions, the Vavilov-Cherenkov radiation generated in the medium is reversed (i.e., the energy flux density vector forms an obtuse angle with the direction of the charge motion). Accordingly, so-called reversed Cherenkov-transition radiation (RCTR) can be generated. We analytically and numerically investigate both the scattered field and the total one, and we show that RCTR exists in the vacuum region if the charge velocity exceeds a certain threshold value associated with total internal reflection. Computations of the Fourier harmonics of the field as well as the total field itself demonstrate that RCTR in vacuum can be a dominant effect. Some properties of RCTR can be useful for diagnostics of particle bunches and determination of medium characteristics.

Electromagnetic field of a charge intersecting a cold plasma boundary in a waveguide.

We analyze the electromagnetic field of a charge crossing a boundary between a vacuum and cold plasma in a waveguide. We obtain exact expressions for the field components and the spectral density of the transition radiation. With the steepest descent technique, we investigate the field components. We show that the electromagnetic field has a different structure in a vacuum than in cold plasma. We also develop an algorithm for the computation of the field based on a certain transformation of the integration path. The behavior of the field depending on distance and time and the spectral density depending on frequency are explored for different charge velocities. Some important physical effects are noted. A considerable increase and concentration of the field near the wave front in the plasma is observed for the case of ultrarelativistic particles. In the plasma, the mode envelopes and spectral density show zero points when the charge velocity is within certain limits.

Reversed Cherenkov-transition radiation by a charge crossing a left-handed medium boundary.

We analyze the radiation from a charged particle crossing the boundary between an ordinary medium and a "left-handed" metamaterial. We obtain exact and approximate expressions for the field components and develop algorithms for their computation. The spatial radiation in this system can be separated into three distinct components, corresponding to ordinary transition radiation having a relatively large magnitude, Cherenkov radiation, and reversed Cherenkov-transition radiation (RCTR). The last one is explained by reflection and refraction of reversed Cherenkov radiation at the interface. Conditions for generating of RCTR are obtained. We note properties of this radiation that have potential applications in the detection of charged particles and accelerator beams and for the characterization of metamaterial macroscopic parameters (epsilon, mu).

Vavilov-Cherenkov radiation in passive and active media with complex resonant dispersion.

Some of the problems with the theory of moving charge radiation in media with frequency dispersion are analyzed. First, some general properties of the integrals for field components are described. The results are applied to the cases of passive and active media. In one instance, the field of a charge moving in passive media with an arbitrary number of resonances is considered. Components of the field have been presented as a sum of the "quasi-Coulomb" field, the wave field, and the "plasma trace." In another example, the case of an active medium with two resonant frequencies is considered. It has been demonstrated that radiation is amplified even with a purely real refractive index if the following conditions are fulfilled: the "lower" resonance is active, the "upper" one is passive, and the charge movement velocity lies within a certain range. Efficient algorithms for the computation of fields in the cases of passive and active media have been developed.