Transition radiation from an extended bunch of charged particles

The spatial distribution of the transition radiation generated by an extended system of charges is studied. The charges sequentially cross the interface at equispaced points. Also, the transition from a spherical bunch with the uniform charge distribution is considered. The radiation patterns produced by the point charge and by the bunch of the charges are shown to differ significantly at certain sizes of the bunch expressed in terms of the wavelength. Charge distributions such that the transition radiation exhibits the properties typical of the Doppler effect or of Vavilov-Cherenkov radiation are found to be possible.     

The mechanism of Vavilov-Cherenkov radiation

The mechanism of generation of Vavilov-Cherenkov radiation is discussed in this article. The developers of the theory of the Vavilov-Cherenkov effect, I.E. Tamm and I.M. Frank, attributed this effect to their discovery of a new mechanism of radiation when a charged particle moves uniformly and rectilinearly in the medium. As such a mechanism presupposes the violation of the laws of conservation of energy and momentum, they proposed the abolition of these laws to account for the Vavilov-Cherenkov radiation mechanism. This idea has received a considerably wide acceptance in the creation of other theories, for example, transition radiation theory. In this paper, the radiation mechanism for the charge constant motion is demonstrated to be incorrect, because it contradicts not only the laws of conservation of energy and momentum, but also the very definitions of uniform and rectilinear motion (Newton's First Law). A consistent explanation of the Vavilov-Cherenkov radiation microscopic mechanism that does not contradict the basic laws is proposed. It is shown that the radiation arises from the interaction of the moving charge with bound charges that are spaced fairly far away from its trajectory. The Vavilov-Cherenkov radiation mechanism bears a slowing down character, but it differs fundamentally from bremsstrahlung, primarily because the Vavilov-Cherenkov radiation onset results from a two-stage process. First, the moving particle polarizes the medium; then, the already polarized atoms radiate coherently, provided that the particle velocity exceeds the phase speed of light in the medium. If the particle velocity is less than the phase speed of light in the medium, the polarized atoms return energy to the outgoing particle. In this case, radiation is not observed. Special attention is given to the relatively constant particle velocity as the condition of the coherent composition of waves. However, its motion cannot be designated as a uniform and rectilinear one in the sense of its definition by Newton's First Law, and it also contradicts the laws of conservation of energy and momentum.     

Radiation of charged particles interacting with interface of two different transparent media

It is shown that the Vavilov-Cherenkov radiation has an essential contribution in experiments on optical transition radiation. The properties of the Vavilov-Cherenkov radiation, such as direction, threshold condition, and polarization, are considered. Peculiarities appearing at sliding incidence of electrons at the target in experiments on optical transition radiation are discussed.     

Angular distribution of Cherenkov radiation from relativistic heavy ions taking into account deceleration in the radiator

Numerical methods are used to study the dependence of the structure and the width of the angular distribution of Vavilov-Cherenkov radiation with a fixed wavelength in the vicinity of the Cherenkov cone on the radiator parameters (thickness and refractive index), as well as on the parameters of the relativistic heavy ion beam (charge and initial energy). The deceleration of relativistic heavy ions in the radiator, which decreases the velocity of ions, modifies the condition of structural interference of the waves emitted from various segments of the trajectory; as a result, a complex distribution of Vavilov-Cherenkov radiation appears. The main quantity is the stopping power of a thin layer of the radiator (average loss of the ion energy), which is calculated by the Bethe-Bloch formula and using the SRIM code package. A simple formula is obtained to estimate the angular distribution width of Cherenkov radiation (with a fixed wavelength) from relativistic heavy ions taking into account the deceleration in the radiator. The measurement of this width can provide direct information on the charge of the ion that passes through the radiator, which extends the potentialities of Cherenkov detectors. The isotopic effect (dependence of the angular distribution of Vavilov-Cherenkov radiation on the ion mass) is also considered.     

Vavilov-Cherenkov effect in a chiral waveguide

The Vavilov-Cherenkov radiation appearing in a circular regular ideal waveguide with a chiral medium is considered. The features of the emission spectrum excited by a charge moving along the waveguide axis are investigated. Expressions for the energy loss of a moving charge are obtained. Zh. Tekh. Fiz. 69, 69–71 (March 1999)     

Field Structure of the Cherenkov Radiation in a Waveguide

We study the field structure of the Vavilov-Cherenkov radiation in planar and cylindrical waveguides filled with a continuous dielectric medium or a uniaxial crystal. It is shown that the Vavilov-Cherenkov radiation has the form of wave packets propagating with the same phase and group velocities. Radiation from a system of charged-particle bunches is considered.     

Superluminal motion (review)

Prior to the development of Special Relativity, no restrictions were imposed on the velocity of the motion of particles and material bodies, as well as on energy transfer and signal propagation. At the end of the 19th century and the beginning of the 20th century, it was shown that a charge that moves at a velocity faster than the speed of light in an optical medium, in particular, in vacuum, gives rise to impact radiation, which later was termed the Vavilov-Cherenkov radiation. Shortly after the development of Special Relativity, some researchers considered the possibility of superluminal motion. In 1923, the Soviet physicist L.Ya. Strum suggested the existence of tachyons, which, however, have not been discovered yet. Superluminal motions can occur only for images, e.g., for so-called ??light spots,?? which were considered in 1972 by V.L. Ginzburg and B.M. Bolotovskii. These spots can move with a superluminal phase velocity but are incapable of transferring energy and information. Nevertheless, these light spots may induce quite real generation of microwave radiation in closed waveguides and create the Vavilov-Cherenkov radiation in vacuum. In this work, we consider various paradoxes, illusions, and artifacts associated with superluminal motion.     

Influence of an ambipolar field and target boundary blurring on transition radiation from fast electrons of laser plasma

It is shown that the blurring of the rear side of a thin laser target leads to a decrease in the intensity of higher harmonics in the spectrum of coherent transition radiation and that the scale of the boundary inhomogeneity can be estimated from the amplitude ratio of harmonics. Deceleration of the electron flow in an ambipolar electric field at the rear boundary of a target leads to a decrease in the intensity of lower harmonics in the spectrum of coherent transition radiation, and the strength of the ambipolar field can be estimated from the amplitude ratio of harmonics. A change in the permittivity of a dielectric laser target with frequency can lead to an increase in the intensity of some harmonics in the spectrum due to the Vavilov-Cherenkov mechanism.     

Observation of Vavilov-Cherenkov radiation and EAS charged particles at large zenith angles

Extensive air showers (EASes) at zenith angles of 70–80 degrees with Vavilov-Cherenkov radiation having two and three maxima were registered at the Tian Shan Mountain Station of the Lebedev Physical Institute. In each such event, the subsequent maxima came with a time delay of 100 or more nanoseconds. Extensive air showers at a zenith angle of 70° with charged particles and Vavilov-Cherenkov radiation were also registered.     

Vavilov-Cherenkov radiation from a model cascade developing near the lunar surface

The model problem of Vavilov-Cherenkov radiation emergence from lunar regolith into vacuum under LORD experimental conditions is considered. The boundary problem on radiation emergence into vacuum is solved numerically in the given field approximation (Kirchhoff approximation) from the cascade near the boundary (near-field region of the radiation source). The results are of great importance to interpret future experimental data.     

Relativistic generalization of the Landau criterion as a new foundation of the Vavilov-Cherenkov radiation theory

The relativistic generalization of the Landau criterion is obtained for the determination of threshold conversion of medium elementary Bose-condensed excitation into Cherenkov's photon unbremsstrahlung radiation. In contraposition to classic Vavilov-Cherenkov radiation (VCR) theory, the new VCR theory admits the conditions for effective and direct VCR realization even for high-frequency transverse electromagnetic waves in isotropic plasma.     

Scattering of unformed Vavilov-Cherenkov radiation by the rough lunar surface during its emergence into vacuum

The model boundary problem of scattering of unformed Vavilov-Cherenkov radiation by the rough lunar surface during its emergence into vacuum is considered in the Born approximation. The case of the horizontally directed cascade is considered. The point approximation for the shower disk, but taking into account its evolutions along the track, is used. The lunar surface is defined to be sinusoidal or wavy with a wavelength varying along one of the axes, and also is simulated by various spectra of characteristic roughness sizes: Gaussian, exponential, and uniform spectra.     

Superlight source of radiation with a waveguide used for bunching an electron beam

A scheme of a generator of electromagnetic-wave radiation is proposed in which a radiating region moves along a radiator with a velocity greater than the velocity of light in vacuum. The superlight motion of the generating region leads to the situation in which the resulting radiation has the properties of Vavilov-Cherenkov radiation. The electron beam of a superlight source is formed while the particles travel across a waveguide along which an electromagnetic wave propagates. The construction of the generator makes it possible to vary the velocity of the radiating region, the radiation pattern, and the radiation beamwidth. Calculations are performed that allow one to evaluate the parameters of the generator and the characteristics of radiation.     

Excitation of a wake field by a relativistic electron bunch in a semi-infinite dielectric waveguide

A wake field excited by a relativistic electron bunch in a semi-infinite metal waveguide filled with a dielectric consists of the Vavilov-Cherenkov radiation, the “quenching”-wave field, and transient radiation, which interfere with each other. An exact analytic expression for the transient component of the field of a thin relativistic annular bunch is derived for the first time. The evolution of the space distribution of a field excited by a finite-size electron bunch is numerically calculated. The excitation of the wake field by a periodic train of electron bunches in a finite-length waveguide is studied.     

Emission accompanying superluminal motion of light

Conclusion We hope that we have succeeded in demonstrating the great scientific interest that attaches to superluminal motions of charges and to investigation of the radiation accompanying these motions. An extensive literature on superluminal motion is on hand even now. It deals, however, mainly with radiation in rectilinear motion, i.e., phenomena connected with the Vavilov-Cherenkov effect, the anomalous Doppler effect, ect. In the present note, on the other hand, principal attention is paid to the influence of the breakup of radiation of a single real source on the emission of several images visible by an immobile observer, which can take place for both uniform and nonuniform motion of the charges. From this viewpoint, new light is cast also on the anomalous Doppler effect. Breakup of one source into several images has been observed in all the considered examples.There is no doubt that the effect of several images can be observed in experiments, and possibly even used in practical electronics. From the viewpoint of observing the effect, most attention should be paid, in our opinion, to observation of the Vavilov-Cherenkov effect in a magnetic field (i.e., on a circular trajectory of a charge) in a medium. We note in this connection that to observe a doubling of a radiation source or vanishing of a pair of images there is no need to observe the entire circular trajectory—it suffices to observe only that part containing the points at which doubling of the source or vanishing of a pair of images takes place.Theoretical Division, Theoretical and Mathematical Physics, Lebedev Physics Institute. Translated from Preprint No. 194, Lebedev Physics Institute of the Academy of Sciences of the USSR, Moscow, 1989.     

The parametric Doppler effect upon oblique incidence of probe radiation

Transformation of frequency and intensity of probe radiation obliquely incident on a moving inhomogeneity induced by an intense laser radiation in a nonlinear medium is analyzed. The dependences of frequency and intensity of radiation reflected from the inhomogeneity have been found for a variant of frequency dispersion corresponding to collision-free plasma. These dependences possess singularities under conditions corresponding to the Vavilov-Cherenkov effect. The initially weak noise components of probe radiation increase in the reflected radiation up to a considerable value.     

Radiation from uniformly moving sources (Vavilov-Cherenkov effect,transition radiation,and some other phenomena)

The radiation produced by uniformly moving sources (the Vavilov-Cherenkov effect, the transition radiation, and some other phenomena) is discussed. This area of physical research originated in the Lebedev Physical Institute of the Russian Academy of Sciences and now represents an integral part of modern physics.     

THz Coherent Vavilov-Cherenkov Radiation in a Special Three-Mirror Cavity

In the modern science and technology a compact and having enough output power terahertz radiation source working in room temperature have earned great attention. This paper is devoted to utilize electron bunches stimulate Vavilov-Cherenkov Radiation (VCR) in a special three-mirror quasi-optical cavity to generate coherent THz waves. This novel three-mirror quasi-optical resonant cavity has the coaxial field pattern which enables field establish in this cavity effectively. The analytical theory of the radiation exited by a train of electron bunches in the special kind of three-mirror cavity has been carried out and the coherent VCR has been achieved by the computer simulation. All those shows that this method can be used to establish useful THz radiation source by the normal electron gun and the commonly used microwave devices.     

The SPHERE experiment: Baikal 2010

The SPHERE-2 detector was lifted above the snow-covered surface of Lake Baikal by a captive balloon several times in 2010. The Vavilov-Cherenkov radiation of extensive air showers was measured. Preliminary results from processing the data of the SPHERE-2 experiment at various altitudes of observation are presented.