Effect of sheath potential on electromagnetic radiation emitted from the rear surface of a metallic foil target

In ultra-intense laser--matter interactions, intense electric fields formed at the rear surface of a foil target may have strong influences on the motion of energetic electrons, and thereby affect the electromagnetic emissions from the rear surface, usually ascribed to transition radiation. Due to the electric fields, transition radiation occurs twice and bremsstrahlung radiation also happens because the electrons will cross the rear surface twice and have large accelerations. In the optic region, transition radiation is dominant. The radiation spectrum depends on the electric field only when the electrons are monochromatic, and becomes independent of the electric field when the electrons have a broadband momentum distribution. Therefore, in an actual experiment, the electric field at the rear surface of a foil could not be studied just with the measurement of optic emissions. In the terahertz region, both bremsstrahlung and transition radiations should be taken into account, and the radiation power could be enhanced in comparison with that without the inclusion of bremsstrahlung radiation. The frequency at which the maximum terahertz radiation appears depends on the electric field.     

Transition radiation from relativistic electrons crossing a perfectly conducting conical surface: Calculation and experiment

The angular distributions of the transition radiation intensity when a charged particle passes through the vertex of a perfectly conducting conical surface have been calculated. The radiation generated both when the particle exits the conductor and when it falls on the conductor has been considered. The angular distributions of the intensity of the transition radiation generated by a bunch of relativistic electrons have been measured in the millimeter wavelength range. A microtron with a particle energy of 7.4 MeV was the source of electrons. The influence of the particle injection direction and the conical-surface opening angle on the angular distribution of the radiation intensity has been studied. The measurements have shown that the distribution of the radiation generated by a charge when it enters the horn differs significantly in pattern from the distribution when it exits the horn.     

Transition radiation of fast electrons as a broadband vacuum-ultraviolet radiation source

The characteristics of a possible radiation source in the vacuum-ultraviolet and soft X-ray range based on the transition radiation of electrons with an energy of about 100 keV are calculated. The possibilities of enhancing the radiation yield in the geometry of the glancing interaction of electrons with a target are analyzed. The possibility of creating a source with a yield on the order of 10^?5 photon/(eV sr) per electron is demonstrated.     

On the eikonal approximation in the theory of transition radiation

The transition radiation of relativistic electrons in nonuniform media is considered. Based on the equivalent photon method and the eikonal approximation in wave mechanics, a method for describing this process is proposed. For the case in which the permittivity depends on several coordinates, equations for the spectral-angular density of transition radiation are obtained. The main results obtained in the Born and eikonal approximations of the theory of transition radiation are compared. The equations obtained are used to analyze the transition radiation process for a fiberlike target.     

X-ray transition radiation of electrons with energy of 300 to 900 MeV in periodic multifoil radiators

Results of experiments conducted at the Tomsk synchrotron to study resonant X-ray transition radiation generated by relativistic electrons in periodic multifoil radiators are reviewed. Both the internal synchrotron beam and the external secondary electron beam from the pair magnetic γ-spectrometer with energies ranging from 300 to 900 MeV were used in the experiments. The radiators consisted of many thin amorphous foils of various materials. The generation of X-ray radiation in a compound radiator consisting of a multifoil radiator and a crystal is also studied. In this case, the resonant X-ray transition radiation generated in the multifoil radiator is diffracted in the crystal and emitted at Bragg angles, together with the parametric X-ray radiation generated in the crystal. Spectral and angular properties of the resonant X-ray transition radiation and diffracted resonant X-ray transition radiation are investigated. The ratio between the contributions from the diffracted resonant X-ray transition radiation and other types of radiation to the total coherent X-ray radiation flux generated by electrons in periodic structures and crystals is estimated.     

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.     

Experimental study of coherent transition radiation from relativistic electrons in a dihedral angle

Angular intensity distributions for transition radiation excited by a beam of relativistic electrons in the emitter in the form of a dihedral angle are measured in the millimeter range. The angle is formed by the intersection of two conducting planes. The source of radiation is a microtron with an electron energy of 7.4 MeV. We analyze the effect of the magnitude of the dihedral angle of the emitter, the position of the electron transition point on the surface of the angle, and the direction of motion of electrons on the angular distribution of radiation intensity. It is shown that the spectral and angular distributions of radiation intensity in the dihedral angle substantially differ from analogous distributions for a particle intersecting a planar conducting surface. The possibility of using radiation to measure the energy, spatial position, and direction of motion of charges is considered.     

Electrodynamics and thermodynamics of a paramagnetic system in a metallic resonator

The well-known problem of the phase transition in a two-level system coupled with the radiation field of a resonator is considered subject to the conduction electrons of the resonator walls. It is shown that the conduction electrons are important for consideration of the high-frequency properties of the system to cut off the frequency spectrum of the radiation field at the plasma frequency. In the vicinity of the phase transition the conduction electrons lead to the damping of the soft mode.     

Experimental study of coherent transition radiation from relativistic electron bunches entering a dihedral angle

The angular distributions of the intensity of transition radiation from a bunch of relativistic electrons entering a dihedral angle between two conducting planes have been measured in a millimeter wavelength range. A microtron with a particle energy of 7.4 MeV is used as a source of electrons. The effect of the particle injection direction and the magnitude of the dihedral angle on the angular distribution of the radiation intensity has been analyzed. The measurements show that the character of the distribution of radiation from a charge entering the dihedral angle significantly differs from that for a charge escaping the angle. A comparatively small change in the magnitude of the dihedral angle can lead to qualitative changes in the angular distribution of radiation from a charge entering the dihedral angle.     

Reabsorption of resonant transition radiation

We study reabsorption of resonant transition radiation (RTR) of fast particles in a magnetoactive plasma with random inhomogeneities of electron density. The RTR growth rates are obtained for isotropic and anisotropic distribution functions of fast electrons. The coefficient of radio wave absorption by thermal electrons near the plasma frequency is obtained. It is shown that the RTR instability can be realized in the case of sufficiently strong anisotropy. The properties of such maser transition radiation allow us to distinguish it from other coherent mechanisms of radiation when interpreting laboratory and astrophysical observation results. St. Petersburg State Technical University, A. F. Ioffe Physical and Technical Institute, Russian Academy of Sciences, St. Petersburg, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 8, pp. 941–951, August, 1997.     

Coherent X-ray radiation generated by a beam of relativistic electrons in a single crystal under conditions of multiple scattering

The dynamic theory of the coherent X-ray radiation of a divergent beam of relativistic electrons generated in a single-crystal wafer under conditions of the multiple scattering of incident particles is developed. Radiation cross sections are averaged over the divergent beam of rectilinear trajectories of electrons. Expressions describing the spectral-angular characteristics of parametric X-ray radiation and diffracted transition radiation under conditions of multiple scattering are obtained. Conditions under which the contribution of diffracted brehmsstrahlung can be disregarded are shown, and the spectral-angular characteristics of parametric X-ray radiation and diffracted transition radiation are calculated numerically for such conditions.     

Coherent emission of transition radiation in periodic radiators

Measurements have been made of X-ray transition radiation spectra, which were produced by relativistic electrons traversing periodic stacks of mylar and lithium foils. The dependence of the spectral yield on the stack parameters and on the momentum of the electrons establishes the influence of long-range coherence in the emission of transition radiation.     

径向三腔渡越时间振荡器数值模拟

 基于渡越辐射机理结合径向结构的低阻抗特性,提出了一种新型的低阻抗高功率微波器件——径向三腔渡越时间振荡器,它由3个等间距的边耦合同轴腔组成,径向运动电子束与谐振腔中的角向均匀模式场相互作用。采用PIC粒子模拟程序进行了模拟研究。在电子束能量450 keV、束流60 kA且无外加引导磁场的条件下,当结构参数网长为4.8 cm,腔间距为1.4 cm,电子发射面为0.8 cm,内径为8 cm时,获得了平均功率7.4 GW,频率4.1 GHz的微波输出,效率达27.4%,阻抗7.5 Ω。通过粒子模拟给出了束波互作用效率随电子束电压、电流以及谐振腔间距的变化曲线,电子束电压对输出微波频率的影响曲线以及不同谐振频率与最佳谐振腔间距的对应曲线,这些曲线表明该器件具有渡越时间效应的基本特征,属于渡越辐射器件,且具有对电子束质量要求不高的特点。     

Coherent X-ray diffraction radiation produced by microbunched beams passing close to the edge of a slab

With use of formulas for single-particle X-ray transition and diffraction radiation (XTR and XDR), we obtain expressions for spectral-angular distributions and total numbers of emitted quanta in coherent X-ray transition radiation (CXTR2) and coherent X-ray diffraction radiation (CXDR) arising when a microbunched beam of electrons, respectively, intersects the interfaces of a slab or flies at small distance from the edge of the slab. Comparison of obtained results with those known for CXTR from a single interface is performed. It is shown on the basis of numerical calculations that experimental study of CXDR is at present possible on the Linac Coherent Light Source (LCLS, SLAC).     

On possibility of measurement of the electron beam energy using absorption of radiation by electrons in magnetic field

A possibility of precise measurement of the electron beam energy using absorption of radiation by electrons in a homogeneous magnetic field for electrons of high energy in the range up to a few hundred GeV, was considered earlier. In this paper, with the purpose of experimental checking of this method in the range of several tens MeV of electrons energies, a possibility of measurement of absolute energy of the electron beam with a relative accuracy up to 10^?4, is considered. We take into account influence of the laser beam diffraction, of the spread of electrons over energies, and of the length of formation of radiation absorption in the process of electron beam energy measurement. The laser wavelength and the length of the magnet are chosen depending on the length of photon absorption formation. It is found that the kinematical restrictions on the photon absorption process lead to the selection in angles of propagation of photons, which can be absorbed by the beam electrons. It is shown that parameters of the electron beam will noticeably not vary during the measurement of the energy.     

Conditions for generation of X-ray cherenkov radiation during motion of charges in realistic media

The influence of the spatial distribution of electrons, atoms, and nuclei in condensed media on a medium’s susceptibility and on the conditions for the formation of short-wavelength Cherenkov radiation are considered. It is shown that taking into account the inhomogeneous (atomic and electronic) structure of material media in which fast charged particles travel leads to a change in the effective susceptibility and permittivity in the X-ray range, as compared with cases of model homogeneous media with the same average concentration of electrons. The influence of the distribution functions of electrons and nuclei in a target on the conditions for Cherenkov radiation generation and its parameters and on the threshold energy of fast charged particles required to generate such radiation is studied. It is shown that the function of the spatial distribution of electrons and nuclei in a target affects the conditions for generating laser radiation in the X- and λ-ray ranges (on the problem of X- and λ-ray lasers). The obtained results show that using the Fresnel approximation in the X-ray range is insufficiently justified and can lead to significant errors.     

Near QED regime of laser interaction with overdense plasmas

Interaction of laser plulses with intensities up to 10²⁵?W/cm² with overdense plasma targets is investigated via three-dimensional particle-in-cell simulations. At these intensities, radiation of electrons in the laser field becomes important. Electrons transfer a significant fraction of their energy to γ-photons and obtain strong feedbacks due to radiation reaction (RR) force. The RR effect on the distribution of laser energies among three main species: electrons, ions and photons is studied. The RR and electron-positron pair creation are implemented by a QED model. As the laser intensity inreases, the ratio of laser energy coupled to electrons drops while the one for γ-photons reaches up to 35%. Two distinctive plasma density regimes of the high-density carbon target and low-density solid hydrogen target are identified from the laser energy partitions and angular distributions of photons. The power-laws of absorption efficiency versus laser intensity and the transition of photon divergence are revealed. These show enhanced generation of γ-photon beams with improved collimation in the relativistically transparent regime. A new effect of transverse trapping of electrons inside the laser field caused by the RR force is observed: electrons can be unexpectedly confined by the intense laser field when the RR force is comparable to the Lorentz force. Finally, the RR effect and different regions of photon emission in laser-foil interactions are clarified.     

Investigation of resonance diffractive radiation from relativistic electrons in conducting periodic targets

Optical resonance diffractive and transition radiation from 200-MeV electrons in conducting periodic targets with spaced strips are investigated experimentally at the Tomsk synchrotron. The orientation and spectral properties of the radiation are measured.     

Transition radiation,Cerenkov radiation and energy losses of relativistic charged particles traversing thin foils at oblique incidence

Semiclassical relativistic energy losses and the transition radiation are calculated for fast charged particles (e.g. electrons) traversing a thin dielectric foil at oblique incidence. The transition radiation formula is generalized for foils with spatial dispersion. This formula for oblique electron incidence is of particular interest for the observation of Cerenkov radiation, emitted from a dielectric foil. The emission of Cerenkov radiation is discussed for varying electron incidence angle and foil thickness by the aid of numerical computations.     

Optical transition radiation of relativistic electrons in amorphous quartz

Induced optical transition radiation of electrons with the energy of 7.5 MeV on inhomogeneities caused by energy losses of electrons has been investigated experimentally.