电子在激光驻波场中运动产生的太赫兹及X射线辐射研究

采用单电子模型和经典辐射理论分别对低能和高能电子在线偏振激光驻波场中的运动和辐射过程进行了研究. 结果表明: 垂直于激光电场方向入射的低速电子在激光驻波场中随着光强的增大, 逐渐从一维近周期运动演变为二维折叠运动, 并产生强的微米量级波长的太赫兹辐射; 高能电子垂直或者平行于激光电场方向入射到激光驻波场中, 都会产生波长在几个纳米的高频辐射; 低能电子与激光驻波场作用中, 激光强度影响着电子的运动形式、辐射频率以及辐射强度; 高能电子入射时, 激光强度影响了电子高频辐射的强度, 电子初始能量影响着辐射的频率; 电子能量越高, 产生的辐射频率越大. 研究表明可以由激光加速电子的方式得到不同能量的电子束, 并利用电子束在激光驻波场的辐射使之成为太赫兹和X射线波段的小型辐射源. 研究结果可以为实验研究和利用激光驻波场中的电子辐射提供依据.     

Distribution and evolution of electrons in a cluster plasma created by a laser pulse

We analyze the properties and the character of the evolution of an electron subsystem of a large cluster (with a number of atoms n～10⁴?10⁶) interacting with a short laser pulse of high intensity (10¹⁷?10¹⁹ W/cm²). As a result of ionization in a strong laser field, cluster atoms are converted into multicharged ions, part of the electrons being formed leaves the cluster, and the other electrons move in a self-consistent field of the charged cluster and the laser wave. It is shown that electron-electron collisions are inessential both during the cluster irradiation by the laser pulse and in the course of cluster expansion; the electron distribution in the cluster therefore does not transform into the Maxwell distribution even during cluster expansion. During cluster expansion, the Coulomb field of a cluster charge acts on cluster ions more strongly than the pressure resulting from electron-ion collisions. In addition, bound electrons remain inside the cluster in the course of its expansion, and cluster expansion therefore does not lead to additional cluster ionization.     

Spatial-temporal measurements of magnetic fields in laser-produced plasmas

The results of an investigation of the electromagnetic wave polarization, probing high-temperature laser plasma, as well as spatial-temporal structure of the magnetic fields, electron density, current density, and electron drift velocity are presented. To create the plasma, plane massive Al targets were irradiated with the second harmonic of a phoenix Nd laser at intensities up to 5·10¹⁴ W/cm². It was shown that the magnetooptical Faraday effect is the main mechanism responsible for the changing polarization of the probing wave. Magnetic fields up to 0.4 MG with electron densities ∼10²⁰ cm⁻³ were measured. Analysis of the magnetic field spatial distribution showed that the current density achieved the value ∼90 MA/cm² on the laser axis. The radial structure of the magnetic field testified to the availability of the reversed current in the laser plasma. The spatial and temporal resolutions in these experiments were equaled to ∼5 μsec and ∼50 psec, respectively. Translated from Preprint No. 35 of the Lebedev Physics Institute, Moscow, 1993.     

Superradiation from non-ideal plasmas in electric field

An effect of external static electric field on emission of radiation from non-ideal plasmas of erosion focus has been experimentally observed. An order-of-magnitude increase in radiation intensity for spectral interval hv = 40 ? 350 eV with electric field increasing up to 10³V/cm has been found for plasmas with $\text{ξ}\text{Z}\text{?1}$, where ξ is the number of electrons in Debye sphere, Z = 2 ? 3 is the stage of ionization. The energy emitted has been several times higher than the black body energy for the same spectral interval at maximum electric field achieved. The effect vanishes at $\text{ξ}\text{Z}\text{?10}$.     

Absorption of electromagnetic energy by slow electrons under scattering from Coulomb centers

Simple analytical expressions are obtained for the rate of the inverse stimulated bremsstrahlung absorption under electron scattering from a Coulomb center with charge Z in the presence of the electromagnetic field. The initial and final values of electron energy are assumed to be small compared to the Rydberg energy Z ² (atomic units are used throughout). Single-photon processes of absorption and induced radiation of photon by electron are treated. It is assumed that the electromagnetic field frequency ω is rather low, so that the condition Zω/p ³ ? 1, where p is the electron momentum, and the condition ?ω ? p ² are valid. However, this frequency is assumed to be fairly high compared to the electron-Coulomb center collision frequency: ω ? v _nei. The dependences of the rates of photon absorption and induced radiation on the angle θ between the direction of incident electron and the electromagnetic field polarization vector (assumed to be linearly polarized) are obtained. It is demonstrated that, for any angles θ, the rate of photon absorption is higher than the rate of induced radiation and, therefore, the Marcuse effect for slow electrons (electromagnetic field amplification) is absent. It is further demonstrated that a slow electron on the average absorbs double ponderomotive energy per collision with an ion (Coulomb center) in Maxwellian plasma. This agrees both with the known results calculation for fast electrons and with the known results of the calculation based on the classical Boltzmann kinetic equation for plasma.     

Elektronenbeschleunigung in helikalen Feldern

An electron accelerator with a betatronlike air core accelerating field and with helical stabilizing fields is described. It consists of a glass torus having a medium radius of 20 cm which is surrounded by three coils generating the following magnetic fields: a) a betatronlike accelerating field, b) a toroidal field (B _z-field) and c) a helical quadrupole field. Electrons are injected into the torus by a spark discharge at a gas pressure of less than 10^?5 mm Hg. In aB _z-field of 18000 gauss electrons have been accelerated up to 6 MeV, as measured with a 1,5 l bubble chamber, and up to 10 MeV, as measured with scintillation counters. In the energy range of 0,5 to 1,6 MeV, depending on experimental conditions, electron currents between 100 and 1000 amps have been observed, independently by Rogowsky-belt measurements and by determining the bremsstrahlung intensity. Considering the superposition of various drift velocities stability conditions are derived for the motion of the electrons.     

Synchrotron radiation from relativistic electrons in intense magnetic fields

The synchrotron radiation spectrum is calculated for relativistic electrons in the case where no restriction is placed on the strength of the magnetic field. It is shown that in intense fields H? H₀ = m²c³/e₀? =4.41 · 10¹³ G a major contribution to the total radiation intensity comes from transitions to the ground state and also to weakly excited levels. In particular, the contribution from transitions to the ground state (final electron energy E' =mc²) for electrons of initial energy E = 10 MeV in a field H = 2H₀ is 14% of the contribution from transitions to highly excited states (E'?mc²).     

Overheated plasma at the surface of a target with a periodic structure induced by femtosecond laser radiation

A periodic structure is induced at the surface of a metal target exposed to a series of p-polarized 200-femtosecond laser pulses with intensity close to the melting threshold of the target material. The period of the structure is determined by the interference between the incident pump wave and the surface electromagnetic wave. Exposure of the obtained structure to the same laser pulse, but with an intensity of ～10¹⁶ W/cm², provides resonant excitation of the surface electromagnetic waves at the plasma-vacuum interface. This leads to an increase in the X-ray output and the temperature of plasma hot electrons.     

Analyzing the transition rates of the ionization of atoms by strong fields of a CO<Subscript>2</Subscript> laser including nonzero initial momenta

Here, the method of including nonzero initial momenta for ejected electrons in strong infrared laser fields is further developed [8]. It has been shown that, apart from being natural, including the nonzero initial momenta enables one to go into a deeper analysis of the process of tunnel ionization of atoms in strong laser fields (intensity up to 10¹⁶ W/cm²). This is due to looking closely at Fig. 2, which indicates that all electrons that could be ejected, under the circumstances, are ejected at a field intensity ～10¹³ W/cm², and that the effect of ionization after that is strongly diminished, which can be seen from the slope of the plates on Figs. 2 and 4. This also explains the saturation effect for fields up to 10¹⁶ W/cm² [1, 4, 5, 7], and probably this saturation goes on until the fields raising relativistic effects ～10¹⁸ W/cm² [7]. Opposite to what was believed earlier [7], the atomic field intensities could be increased to values over 10¹⁷ W/cm² only when more than 10 electrons are ejected from the atom, it is shown that the properly calculated ionization of 9 electrons increases the atomic field intensity to ～10¹⁸ W/cm².     

Parity violation in pion electroproduction and photoproduction with a proton target

Parity violation in pion electroproduction and photoproduction on a proton target is investigated as a possible means of elucidating the structure of the electronic and non-leptonic neutral weak currents. We calculate both parity-violating effects caused directly by Z-boson exchange, which contributes only to electron scattering, and those due to parity-violating electromagnetic and nuclear interactions. The asymmetry in electroproduction is of order 20^?5, whereas that in photoproduction is ～10^?7. The sensitivity to the parameter sin²θ_w is investigated; it is a maximum at backward electron scattering angles. In addition to the Born terms, contributions from the Δ(1232) resonance and the light vector mesons [?(776) and ω(782)] are included in the model. Fits to the observed π⁰ and π⁺ electroproduction and photoproduction cross sections indicate that the model accounts for the production data in the medium energy region.     

Motion and acceleration of electrons in high-intensity laser standing waves

The motion and the energy of electrons driven by the ponderomotive force in linearly polarized high-intensity laser standing wave fields are considered. The results show that there exists a threshold laser intensity, above which the motion of electrons incident parallel to the electric field of the laser standing waves undergoes a transition from regulation to chaos. We propose that the huge energy exchange between the electrons and the strong laser standing waves is triggered by inelastic scattering, which is related to the chaos patterns. It is shown that an electron's energy gain of tens of MeV can be realized for a laser intensity of 10²⁰ W/cm².     

A new mechanism of electromagnetic electron radiation in a medium (spin light)

Based on the method of exact solutions to the quantum equations for the wave functions of particles in external fields and media within the framework of the standard interaction model, the modified Dirac equation for the electron is derived that allows its interaction with the medium to be considered. An exact solution to the equation and energy spectrum of the electron states are determined. In the context of this approach, a new type of electromagnetic radiation-spin light of electron in a neutron medium-is predicted and studied. General expressions for the probability of the process in unit time and for the radiation intensity are derived, and a dependence of the radiation intensity on the electron energy and density of the medium is analyzed. The limiting cases of the process and polarization properties of radiation are investigated. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 66–73, June, 2007.     

Amplification of electromagnetic field in the course of the nonrelativistic electron scattering by ion in the presence of the field of the medium-intensity elliptically polarized light wave

The amplification factor of the electromagnetic field is theoretically studied for the scattering of nonrelativistic electrons by ions in the presence of the field of the elliptically polarized electromagnetic wave. A simple analytical formula for the gain is derived for the medium-intensity range. The formula supplements and extends the domain of applicability of the known Marcuse formula for the linear polarization in the presence of a weak field. It is demonstrated that the maximum gain is reached when the initial electron velocities belong to the polarization plane of the electromagnetic wave. In the range of optical frequencies, the amplification factor of the laser radiation can be significant for relatively high powers of electron beams.     

Generation of electron nanobunches and short-wavelength radiation upon reflection of a relativistic-intensity laser pulse from a finite-size target

We have proposed an efficient scheme of generation of short dense electron bunches during the interaction at large angles of incidence of a laser pulse with a thin transversally semibounded laser target. Streams of bunches can be used to simultaneously and independently generate pulsed X-ray radiation as fast electrons hit secondary targets. Dependences of bunch parameters (the number of particles in the bunch and the bunch energy and thickness) on the angle of incidence and laser intensity have been obtained. It has been shown that, upon reflection from the target, the relativistic-intensity laser wave is efficiently converted (the energy-conversion factor reaches ～20%) into a sequence of electromagnetic tens-of-nanometer-long atto pulses, which follow one after another in the period of the initial laser wave. We have investigated how the parameters of the atto pulse depend on the angle of incidence and the laser intensity. We have shown that atto pulses are generated most efficiently at large angles of incidence (≥50°) of the laser pulse on the target.     

Radiative process in strong magnetic fields

Abstract

The behavior of electromagnetic processes in strong magnetic fields is currently of great interest in high-energy astrophysics. Observations of neutron stars indicate that magnetic fields larger than 10¹² Gauss exist in nature. In fields this strong, where electrons behave much as if they were in bound atomic states, familiar processes undergo profound changes and exotic processes become important. Strong magnetic fields affect the physics in several fundamental ways: energies prependicular to the field are quantized, transverse momentum is not conserved and electron/positron spin is important. The relaxation of transverse mometum conservation allows first order processes and their inverses: one-photon pair production and annihilation, synchrotron/cyclotron radiation and absorption, which are kinematically forbidden under field-free conditions. The first two are essentially quantum-mechanical and hence significant only in fields whose strength approaches the critical field, B _cr = 4.414 × 10¹³ Gauss. One-photon pair production is likely to be the dominant source of e ⁺ -e ^? pairs in fields exceeding 10¹² Gauss. While synchrotron radiation and absorption are observable as classical electromagnetic processes in weak fields, they are considerably different in high fields, where the classical synchrotron radiation formulae can violate conservation of energy, and predict too large an emissivity and electron energy loss rate. The second-order processes: two-photon pair production and annihilation and Compton Scattering, are also modified in strong fields. The discreteness of e ⁺ - e^? pair states causes resonant behavior in the cross sections and decreases the second-order rates from their free-space values. These processes play an important role in modelling high energy emission from pulsars and gamma-ray bursts.     

A free-electron laser in a uniform magnetic field

The interaction of free electrons and free electromagnetic radiation, in the presence of a uniform magnetic field, can result in stimulated emission or absorption. We analyze the dynamics of single electrons by solving the classical, relativistic Lorentz force equations of motion in these combined fields. An electron may gain energy from, or lose energy to, the radiation field, depending crucially on the phase and oscillation frequency of the electron's helical motion within the superposed, circularly polarized light wave. To first order in the radiation field strength, electrons in a monoenergetic, uniformly distributed beam become spatially bunched, but there is no net energy change. To second order, however, the beam may experience a gain or loss of energy, corresponding to attenuation or amplification of radiation. We compare the bunching of this laser process to the bunching processes involved in 1) the Stanford free-electron laser and 2) the cyclotron maser, and find significant differences in each case. Our analytic results provide a clear, simple picture of the interaction process, and can be useful in exploring light amplification in astrophysical magnetic fields, the magnetosphere, or in laboratory devices. Supported in part by Army Contract No. DASG 60-77-C-0083 and NASA Grant NSG-7490.     

On <Emphasis Type="Italic">e</Emphasis><Superscript>+</Superscript><Emphasis Type="Italic">e</Emphasis><Superscript>−</Superscript> pair production by colliding electromagnetic pulses

Electron-positron pair production from vacuum in an electromagnetic field created by two counterpropagating focused laser pulses interacting with each other is analyzed. The dependence of the number of produced pairs on the intensity of a laser pulse and the focusing parameter is studied with a realistic three-dimensional model of the electromagnetic field of the focused wave, which is an exact solution of the Maxwell equations. It has been shown that e⁺e^? pair production can be experimentally observed when the intensity of each beam is I～10²⁶ W/cm², which is two orders of magnitude lower than that for a single pulse.     

Laser-induced amplification and visualization of β-ray ionized tracks in dense gases

We describe some experiments on Bremsstrahlung energy transfer from an intense laser light towards electrons sorrounded by a dense gas. The electrons gain eventually enough energy to radiate light and to excite and ionize the surrounding atoms. We have measured the spectra of the light emitted in these conditions and the electron number multiplication factor resulting from successive ionization. In krypton we observe that a static field enhances significantly the rate of energy transfer from light to electrons. At the pressure of 200 bar we obtained controllable electron multiplication factors up to 10⁶. At this level of amplification the ionized track of a ⁹⁰Yβ-ray becomes visible. This latent electronic image development takes place in 10^?8s.     

Electromagnetic excitation of infrasound in a conducting medium

A comparative analysis is made of the mechanisms of interaction between the electromagnetic fields of a global resonator and hydrodynamic and acoustic disturbances in a conducting medium. A universal boundary condition at the interface between air and the conducting medium, which takes into account the motion of the electrolyte, is obtained in an explicit analytical form to calculate the long-wavelength electromagnetic fields. The intensity of the electromagnetic field excited by a vertical hydroacoustic wave is estimated together with the efficiency of excitation of infrasonic oscillations of a conducting medium in the field of a global resonator. Zh. Tekh. Fiz. 68, 80–83 (January 1998)