A. R. Mkrtchyan

ON THE INFLUENCE OF ACOUSTIC WAVES ON COHERENT BREMSSTRAHLUNG IN CRYSTALS

We investigate the coherent bremsstrahlung by relativistic electrons in a single crystal excited by hypersonic vibrations. The formula for the corresponding differential cross-section is derived in the case of a sinusoidal wave. The conditions are specified under which the influence of the hypersound is essential. The case is considered in detail when the electron enters into the crystal at small angles with respect to a crystallographic axis. It is shown that in dependence of the parameters, the presence of hypersonic waves can either enhance or reduce the bremsstrahlung cross-section.

ON THE THEORY OF COHERENT PAIR PRODUCTION IN CRYSTALS IN THE PRESENCE OF ACOUSTIC WAVES

The influence of hypersonic waves excited in a single crystal is investigated on the process of electron–positron pair creation by high-energy photons. The coherent part of the corresponding differential cross-section is derived as a function of the amplitude and wave number of the hypersound. The values of the parameters are specified for which the latter affects remarkably on the pair creation cross-section. It is shown that under certain conditions the presence of hypersonic waves can result in enhancement of the process cross-section.

Quantum-mechanical approach to planar positron channeling in a longitudinal hypersonic field

The wave function of a planar-channeled positron with energy MeV under the resonant influence of hypersound is calculated. It is found that the longitudinal hypersound changes the spectral distribution of the channeling radiation significantly. With decreasing wavelength λs of the hypersound, starting from λs=λE, where λE is the characteristic length related to one transverse oscillation of the channeled positron, a new type of radiative transition occurs. It is characterized by the stimulated enhancement of transverse energy of the positron at channeling (inverse radiative transition). When λs approaches λE/2 the inverse transitions show a resonant behaviour.

Radiation from a charged particle-in-flight from a laminated medium to vacuum

The radiation from a charged particle-in-flight from a semi-infinite laminated medium to vacuum and back,- from vacuum to the laminated medium, has been investigated. Expressions for the spectral-angular distribution of radiation energy in vacuum (at large distances from the boundary of laminated medium) were obtained for both the cases with no limitations on the amplitude and variation profile of the laminated medium permittivity. The results of appropriate numerical calculations are presented and possible applications of the obtained results are discussed.

Resonant influence of a longitudinal hypersonic field on the radiation from channeled electrons

Abstract The wave function of a planar/axially channeled electron with energy 10 MeV ⩽E≪1 GeV under the influence of a longitudinal hypersonic wave excited in a single crystal is calculated. Conditions for the resonant influence of the hypersonic wave on the quantum state of the channeled electron are deduced. Expressions for the wave function that are applicable in the case of resonance are obtained. Angular and spectral distributions of the radiation intensity from the planar/axially channeled electron are also calculated. The possibility of significant amplification of channeling radiation by a hypersonic wave is substantiated. It is found that the hypersound can excite inverse radiative transitions through which the transversal energy of the channeled electron is increased. These transitions have a resonant nature and can lead to a considerable intensification of the electron channeling radiation. In the case of axial channeling, the resonance radiation is sustained also by direct radiative transitions of the electron.

Resonant influence of hypersound on the radiation of an axially channeled electron

Abstract The spectral intensity of the radiation emitted by an axially channeled electron in a single crystal excited by a longitudinal hypersonic wave propagating along the channeling direction has been calculated for the energy range 10MeV ≤ E ≤ 100 MeV. It has been shown that under the influence of acoustic vibrations excited in the single crystal a resonant intensification of the electron channeling radiation, a variation of its spectral distribution as well as inverse radiative transitions are possible.

Self-amplified Cherenkov radiation from a relativistic electron in a waveguide partially filled with a laminated material

The radiation from a relativistic electron uniformly moving along the axis of cylindrical waveguide filled with laminated material of finite length is investigated. Expressions for the spectral distribution of radiation passing throw the transverse section of waveguide at large distances from the laminated material are derived with no limitations on the amplitude and variation profile of the layered medium permittivity and permeability. Numerical results for layered material consisting of dielectric plates alternated with vacuum gaps are given. It is shown that at a special choice of problem parameters, Cherenkov radiation generated by the relativistic electron inside the plates is self-amplified. The visual explanation of this effect is given and a possible application is discussed.

Radiation from an electron bunch flying over a surface wave

Abstract Radiation generated by the passage of a monoenergetic electron bunch above the surface wave excited in plane interface between homogeneous media with different dielectric constants is investigated. For the surface wave of general profile the radiation intensity is expressed via the radiated power from a single charge and bunch form factor. Various types of transverse and longitudinal distributions of electrons in the bunch have been considered including Gaussian, asymmetrical Gaussian, two Gaussian and rectangular distribution with asymmetrical exponential tails. Conditions are specified under which the coherent radiation essentially exceeds the incoherent part.

Quantum mechanical approach to planar electron channeling in a hypersonic field (I)

Abstract The wave function of an electron of energy 10 MeV < E ≤ 100 MeV planar channeled in a longitudinal hypersonic superlattice has been found. Conditions for the resonant influence of the acoustic waves on the quantum states of the electron were deduced. The expression obtained for the wave function is applicable at resonance.

Resonant influence of hypersound on the quantum states of an axially channeled electron

Abstract The wave function of an electron of energy 10 MeV ≤ E ≤ 100 MeV axially channeled in a longitudinal hypersonic superlattice has been obtained within the framework of a quantum mechanical approach. Conditions for the resonant influence of the hypersonic wave excited in the single crystal on the quantum states of the channeled electron have been deduced.