New acceleration mechanism of electrons by an electromagnetic wave in a weakly magnetized plasma

Acceleration of electrons by an electromagnetic wave has been observed in a weakly magnetized (ω_ce/ω₀ ? 10^?2) inhomogeneous plasma. This acceleration is interpreted as a V_p × B₀ acceleration, which is a new concept for heating or accelerating electrons very efficiently to high energy.     

Effects of sidelobes of focused flat-topped laser beams on vacuum electron acceleration

Using three-dimensional test particle simulations, we investigated electrons accelerated by a focused flat-top laser beam at different intensities and flatness levels of the beam profile before focusing in vacuum. The results show that the presence of sidelobes around the main focal spot of the focused flat-top laser beam influences the optimum (as far as electron acceleration is concerned) initial momentum (and incident angle) of electrons for acceleration. The difference of initial conditions between laser beams with and without sidelobes becomes evident when the laser field is strong enough (a₀>10, corresponding to intensities I>1×10²⁰ W/cm² for the laser wavelength λ=1 μm, where a₀ is a dimensionless parameter measuring laser intensity). The difference becomes more pronounced at increasing a₀. Because of the presence of sidelobes, there exist three typical CAS (capture and acceleration scenario) channels when a₀≥30 (corresponding to I>1×10²¹ W/cm² for λ=1 μm). The energy spread of the outgoing electrons is also discussed in detail. PACS 41.75.Jv; 42.60.Jf; 42.25.Fx     

Mechanism of electron beam poled SHG in 0.95GeS2·0.05In2S3 chalcogenide glasses

Utilizing the Maker fringe method, SHG was observed in the 0.95GeS₂·0.05In₂S₃ chalcogenide glass irradiated by the electron beam and the intensity of SH increases with the enhancement of beam current from 15 to 25 nA. According to Raman spectra of the as-prepared and the irradiated one, no distinct micro-structural transformation was found. In this work, the built-in charge model was founded to interpret the poling mechanism of electron beam irradiation, the emission of the secondary electrons and Auger electrons results in the formation of positive region and the absorbed electrons form negative region. The positive region was situated near the poling surface, and the negative region was much deeper than the positive region. Between the two opposite charged regions, a strong space-charge electrostatic field, E_dc, is created, which leads to the nonzero χ⁽²⁾ in the 0.95GeS₂·0.05In₂S₃ glass. The emission of backscattered electrons does no contribution to the formation of E_dc.     

The quantum field theory of laser acceleration

We determine the scattering rate and the energy loss of electrons due to a laser photon beam. From the energy loss formula we determine the force accelerating an electron by the laser photon beam and the corresponding relativistic dynamical equation describing its motion. Numerically, we calculate the velocity of electron after an acceleration time Δt = 0.1 s.     

Interaction of an electron bunch with a laser pulse in vacuum

The output properties of electrons accelerated by the vacuum laser acceleration scheme CAS (capture and acceleration scenario) are addressed. The transport process of the electron bunch, the fraction of the CAS electrons of the incident electrons, the correlation of electron energy with position and scattering angle, the energy spectrum and angular distributions as well as the emittance of the outgoing electrons are studied at a laser intensity of a₀=10. In addition, the effects of the laser intensity, beam width, and pulse duration on the properties of the output electrons are also examined. Physical explanations of those output characteristics are presented based on the mechanism behind the CAS scheme. The feasibility of CAS to become a realistic laser accelerator scheme is explored. PACS 41.75.Jv; 42.60.Jf; 41.85.Ja     

Estimation of the acceleration ability for electrons in SiO2 and the tunneling effect

Silicon dioxide (SiO₂) plays an important role in layered optimization scheme and solid-state cathodoluminescence (SSCL). Initially, it was believed that the SiO₂ layer would (i) generate extra interface states contributing to a number of primary electrons available for exciting the luminescent centers, and/or (ii) act as acceleration layer resulted in gaining high energy for those electrons that would tunnel into the luminescent layer to excite luminescent centers. Based on the brightness vs. voltage (B-V) measurements, we deem that the latter case, i.e. acceleration and tunneling, is the dominant mechanism. A detailed discussion in terms of electrons acceleration and tunneling as the main contributions to the enhancement of brightness is presented.     

The Dynamics of Electron Orbits during the Acceleration of Electrons in a Betatron

This paper presents a system of equations that describe the motion of charged particles in the electromagnetic field of a betatron. This system of equation was successfully used to study the behavior of the electron orbits and to determine the principal parameters of the electron beam in the electromagnetic field of a betatron during the electron acceleration and deceleration. The results of this study may find application in developing systems designed to accelerate electron beams. It has been shown that in the course of acceleration there is no damping of the betatron oscillations by the law B _z ^–1/2 and, correspondingly, no decrease in beam cross section. In contrast to the existing belief, the initial departure of the kinetic energy (momentum) of the injected electrons from the energy (momentum) of the electrons following the equilibrium orbit is not preserved in the course of acceleration. In the betatron chamber, the electron beam, when accelerated, does not constrict to form a ring but occupies a broad zone, whose dimensions are determined by the initial double amplitudes of the vertical and horizontal oscillations. Despite the large double amplitude of the oscillations of the beam particles, the average energy of the electrons differs from the energy of the electrons following the equilibrium orbit only slightly, and the departure of the average energy from the energy of the equilibrium electrons varies proportionally to the (varying) field of the betatron.     

Ponderomotive acceleration of electrons by an ultrashort laser pulse

By using the corrected solutions for an ultrashort laser pulse, we study the laser-driven electron violent acceleration in vacuum. Our simulations demonstrate that an ultrashort laser pulse with an intensity a₀≡eE₀/m_eωc=3 can accelerate electrons to an energy more than 0.5 GeV. The scaling laws for the net energy gain in different pulse length and laser radius at focus are also studied. Its acceleration mechanism is found to be ponderomotive acceleration.     

Hot electron photoluminescence in GaAs crystals

The spectrum and the linear polarization of photoluminescence of hot electrons in GaAs crystals were investigated. Oscillations in the hot photoluminescence (HPL) spectrum due to the subsequent emission of LO-phonons were observed. The study of HPL depolarization in an external magnetic field yielded the scattering time due to the emission of a LO-phonon by a hot electron in the Γ-valley (τ_?0 = 1 × 10^?13 sec) as well as the Γ?L intervalley scattering time. The radiative recombination of hot electrons created in the central Γ-valley via the subsidiary L-valley was observed. The distribution function of hot electrons in a wide energy range was evaluated from the spectra.     

Field properties and vacuum electron acceleration in a laser beam of high-order Laguerre–Gaussian mode

The electric field intensity distribution and the phase velocity distribution of high-order Laguerre–Gaussian (LGρ?) mode laser beams are analyzed. Using three-dimensional test particle simulation, the numerical results of electrons accelerated by LG00, LG40 and LG41 mode laser beams are presented. Compared with the LG00 mode (the fundamental mode) laser beam, low-energy injection electrons can be more favorably accelerated in a high-order LG mode laser beam. Contrary to anticipation, a high-order LG mode laser beam with intense axial electric field distribution is inferior to the LG00 mode in capture acceleration for electrons with high injection energy.     

Collisionless self-consistent trapping of electrons into a nonstationary potential well: Dynamics of trapped electrons

The subject of investigation is the early stage of self-consistent trapping of electrons into a potential well that forms during the development of aperiodic Pierce instability. An analytical estimate for threshold gap δ_th = d _th/λ_D (λ_D is the Debye beam length) above which the trapping begins is derived. The nonlinear dynamics and distribution function of trapped electrons are studied in detail using a numerical method ((E, K) code). It is found that the trapped particles produce a localized steep-edge bunch, which “dangles” around between the electrodes, causing potential oscillations. Trapped electrons render the well shallower. Some of the particles in the bunch are shown to periodically escape to the electrodes. As a result, the potential oscillation amplitude fades away and the mean depth of the well increases.     

Nonlinear oscillatory waveguide propagation in a plasma

The presence of an intense Gaussian laser beam gives rise to a ponderomotive force on electrons in a collisionless plasma, leading to a redistribution of electron density along the wave-front and consequently to an intensity dependent dielectric constant which saturates with increasing intensity. The intensity dependent dielectric constant is responsible for beam propagation in an oscillatory waveguide. It is seen that (i) a beam of radiusr ₀ less thanr _0min (?c/ω _p) cannot be focused in the plasma regardless of its power, (ii) minimum dimension of oscillatory waveguide increases with increasing power of the beam. Similar results are also obtained for collisional plasma where nonlinearity arises due to nonuniform heating and consequent redistribution of carriers.     

Heavy ion beam polarization produced by the multi-tilted-foil interaction

Nuclear polarization of the 7/2^? ground-state of⁵¹V was produced via the Multi-Tilted-Foil (MTF) interaction with a V beam. The induced polarization was determined by measuring the left-right asymmetry of Coulomb excited⁵¹V nuclei and, for a⁵¹V beam at E=100 MeV, was measured to beP _I =0.012(2). The nuclear polarization was also induced atE=50 MeV and, after further acceleration, determined at E=195 MeV to be P_I=0.010(1). These experiments demonstrate the feasibility of polarizing a great variety of heavy-ion beams at arbitrary velocities with subsequent acceleration and passage through magnetic beam-optics elements. Such polarization, albeit small, can be utilized for the determination of electromagnetic moments of exotic beams and separated reaction products.     

Acceleration of electrons by a Bessel-Gaussian beam in vacuum

The acceleration of electrons by using a Bessel-Gaussian (BG) beam in vacuum is studied. It is shown that the axial electric field of a linearly or circularly polarized BG beam of order n = 1 can be used to accelerate electrons. The general features of the acceleration of electrons by using a linearly or circularly polarized BG beam, such as the transversal and axial electric-field components, phase velocity, slippage distance, accelerating potential, and energy gain etc., are analyzed.     

Hall mobility of hot electrons in CdS

Photo-Hall effect of hot electrons in a pure single crystal of CdS was observed, for the first time. The measurement was carried out for electric fields up to 4,300 V/cm in magnetic fields up to 40 kOe at 4.2 K. The saturation of the drift velocity V_d of electrons due to optical phonon emission was observed. The saturated value of V_d is found to be about 1.7 × 10⁷ cm/sec.     

A New Scheme for High‐Intensity Laser‐Driven Electron Acceleration in a Plasma

We propose a new approach to high‐intensity relativistic laser‐driven electron acceleration in a plasma. Here, we demonstrate that a plasma wave generated by a stimulated forward‐scattering of an incident laser pulse can be in the longest acceleration phase with injected relativistic beam electrons. This is why the plasma wave has the maximum amplification coefficient which is determined by the acceleration time and the breakdown (overturn) electric field in which the acceleration of the injected beam electrons occurs. We must note that for the longest acceleration phase the relativity of the injected beam electrons plays a crucial role in our scheme. We estimate qualitatively the acceleration parameters of relativistic electrons in the field of a plasma wave generated at the stimulated forward‐scattering of a high‐intensity laser pulse in a plasma. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electroluminescence and its excitation mechanism of SiOx films deposited by electron-beam evaporation

Blue electroluminescence from SiO_x films deposited by electron beam evaporation was observed. This blue emission blueshifted from 450 to 410 nm with increasing applied voltage. The dependences of blue emission on applied voltage, frequency and conduction current were studied. Our experimental data support that blue emission from SiO_x films is the result of both recombination of charge carriers injected from opposite electrodes and impact excitation of hot electrons, the recombination of carriers injected is dominant in low and medium electric fields but hot electron impact excitation is dominant under high electric fields.     

Collisionally pumped hybrid soft X-ray laser in Ne-like sulphur

We describe an experiment demonstrating XUV amplification following collisional excitation in a capillary discharge plasma irradiated by a picosecond IR laser pulse. Guiding and temporally resolved transmission of the pump laser beam are also demonstrated and analysed. The short pump laser pulse heated rapidly the electrons producing amplification in the 3p¹S₀–3s¹P₁ transition of Ne-like sulphur at 60.84 nm. The estimated gain–length product was equal to 6.8, while the beam divergence reached 2.5 mrad for 30 mm capillary. This new, hybridly pumped collisional soft X-ray laser with the transient gain offers a new way towards efficient table-top XUV sources.     

A new semiconductor superlattice with tunable electronic properties and simultaneously with mobility enhancement of electrons and holes

A new artificial semiconductor superlattice with tunable electronic properties and simultaneously with significant mobility enhancement of both 2-dimensional electrons and 2-dimensional holes has been prepared by molecular beam epitaxy. The structure consists of a periodic sequence ofn-Al _x Ga_1?x As/i-GaAs/n-Al _x Ga_1?x As/p-Al _x Ga_1?x As/ i-Ga.As/p-Al _x Ga_1?x As stacks with undoped Al _x Ga_1?x As spacers between the intentionally doped Al _x Ga_1?x As and the nominally undopedi-GaAs layers. In this newheterojunction doping-superlattice we have for the first time achieved a spatial separation of electrons and holes by half a superlattice period as well as simultaneously a spatial separation of both types of free carriers from their parent ionized impurities. These unique properties are demonstrated by the strongly increased tunability of bipolar conductivity with bias. In addition, the observed temperature dependence of Hall mobilities provides direct evidence for a strong mobility enhancement of both electrons and holes in the spatially separated 2-dimensional accumulation channels formed in the lower band gap material.     

Growth of fullerene on Ag and hydrogen-passivated Si substrates: Effect of electron beam exposure on growth modes

We have used Auger electron spectroscopy (AES) to investigate the effect of electron beam exposure on growth modes of fullerene (C₆₀) on substrates like Ag and hydrogen-passivated Si(1 1 1). The electron beam comprises of 3.4 keV electrons, which are used in the AES study. To investigate the effect, Auger signal (AS) vs. deposition time (t) measurements were conducted in a sequential mode, i.e., alternating deposition of C₆₀ and analysis using the electron beam. Duration of AES data collection after each deposition was the duration of exposure to electron beam in this experiment. For the growth study of C₆₀ on Ag, three AS-t plots were recorded for three different durations of exposure to electron beam. Changes in the AS-t plot, depending on the duration of exposure to the electron beam, reflect the electron beam-induced damage. Electron beam-induced damages of C₆₀ produce carbon materials of different densities and consequently transmission coefficient (α) of Auger electron through this material changes. In order to fit the AES (AS vs. t) data a model has been used which simultaneously provides the growth mode and the transmission coefficient. Observation of an increasing transmission coefficient with the increasing duration of exposure to the electron beam from α=0.34 to 0.60 indicates the change of the nature of the carbon material due to the partial damage of C₆₀.