arXiv subjects

Lekdar A. Gevorgian

Publications and source records attributed to Lekdar A. Gevorgian.

5 recordsLinked to original sources

arXiv2025

Positron Bunch Radiation in the System of Tightly Packed Nanotubes

Radiation emitted by a bunch of positrons channeled in nanotubes at zero emission angle is studied taking into account medium polarization. The formation of radiation is characterized by an energy threshold that depends on the oscillation amplitude of each positron. When the bunch energy reaches the maximum value of the threshold energy, radiation is produced by all positrons in the bunch. The nanotube potential barrier is described using a harmonic model. The spectral line shape of the radiation from the positron bunch, the fundamental radiation frequency, and the number of emitted photons are determined. It is shown that a system of tightly packed carbon nanotubes can generate an intense, quasi-monochromatic, and directed beam of circularly polarized soft X-ray photons with an energy of about $3$~\SI{}{\kilo\electronvolt} (wavelength $4.1$~\SI{}{\angstrom}).

physics.acc-ph↗

arXiv2025

FEL Gain Enhancement in an Optical Klystron

A system comprising two identical helical or planar undulators separated by a gap -- viz., an optical klystron (OK) -- is investigated. A formula for the frequency distribution of spontaneous radiation at zero angle is derived. It is shown that the spontaneous radiation line shape gradually narrows with increasing distance (up to an optimal value) between the undulators due to the constructive interference of the radiation fields formed in each of them, while the number of radiated photons decreases. The free-electron laser (FEL) gain coefficient also gradually increases, since it is proportional to the derivative of the spontaneous radiation line shape. Using the undulator parameters of the SASE XFEL and the bunch parameters of the LCLS, the total gain coefficient in the X-ray range is on the order of 2.5 and, for a different electron energy, reaches approximately 90 in the water window frequency range.

physics.acc-ph↗

arXiv2025

Partially Coherent X-Ray Oscilex Radiation from a FEL-Modulated Positron Bunch during Its Planar Channeling in a Crystalline Undulator

The radiation emitted at zero angle by a microbunched positron bunch undergoing planar channeling in a crystalline undulator (CU) is studied. The bunch energy is assumed to be far above the threshold for radiation generation in the dispersive CU medium. Besides the usual ``hard'' undulator radiation produced by channeling oscillations (channeling undulator radiation) and by the CU bending (crystalline undulator radiation), a ``soft'' medium-polarization component also appears at zero angle due to the oscillations that excite atomic electrons. We refer to this soft component as Oscilex (oscillationally-excited) radiation. Since the two types of oscillations have different frequencies, they yield two distinct frequency components of both undulator and Oscilex radiation. The Oscilex frequencies are set by the plasma frequency and the characteristic oscillation frequency and are, to high accuracy, independent of the positron energy. The CU period is chosen so that the radiation wavelength is not shorter than the microbunch length, ensuring coherent emission from microbunches and partially coherent Oscilex emission from the full bunch. Analytical expressions are obtained for the spectral line shapes and the number of photons of spontaneous Oscilex radiation. For partially coherent emission, Gaussian distributions are used for both the bunch and microbunches. Gain factors for the two Oscilex components, including longitudinal form-factors, and the total number of partially coherent photons are derived. A positron bunch with LCLS parameters, modulated by SASE XFEL, channeling between (1 1 0) planes of a periodically bent diamond crystal is analyzed. The number of spontaneously emitted Oscilex photons exceeds the number of positrons by $1\text{÷}2$ orders of magnitude, and the gain factors reach $10^3 \text{÷} 10^4$.

physics.acc-ph↗

arXiv2024

Line shape of soft photon radiation generated at zero angle in an undulator with a dispersive medium

The problem of undulator radiation from a bunch of charged particles, taking into account a medium polarization, is considered. In a dispersive medium, at a zero angle, in addition to hard photons, soft photons are also generated. If the wavelength of the soft photons is greater than or equal to the longitudinal size of the microbunches formed during the FEL process, the microbunches radiate coherently. Consequently, the radiation of the bunch will be partially coherent. As a result, intense, quasi-monochromatic, and directed X-ray photon beams are produced, which can have wide practical applications.

physics.acc-ph↗

arXiv2024

Dielectric permittivity of electron plasma with laser beat waves

Within the framework of the kinetic method, the process of interaction of a rarefied electron plasma with laser beat waves is investigated taking into account the initial spread in the velocities of plasma particles. Using the expression for the spectral distribution function of plasma particles in the presence of laser beat waves, an anisotropic tensor of the plasma dielectric permittivity is obtained. In this case, an expression for the nonequilibrium current density induced by the perturbed field and the constitutive equation connecting the conductivity tensor with the dielectric permittivity are determined. Assuming that in equilibrium the particle distribution function has the form of the Maxwellian distribution function, the dielectric tensor is diagonalized. Analytical expressions are obtained for the longitudinal and transverse components of the dielectric permittivity.

physics.plasm-ph↗