Showing papers on "Free electron model published in 1980"

Theory of free-electron lasers

Abstract: Abstract : A general analysis is presented of free electron lasers in which a static periodic magnetic pump field is scattered from a relativistic electron beam. The steady state formulation of the problem is fully relativistic and contains beam thermal effects. Growth rates associated with the radiation field, efficiencies, and saturated field amplitudes are derived for various modes of operation. Effects of space charge on the scattering process are included and shown to play a dominant role in certain situations. Scaling laws for the growth rates and efficiencies at a fixed radiation frequency as a function of the magnetic pump amplitude are obtained. The shear in beam axial velocity due to self fields is discussed and various methods of reducing it are suggested. Finally, a detailed illustration of a far infrared (lambda = 2mu m) two-stage free electron laser using a 3 MeV electron beam and a 2 cm wavelength magnetic pump field is presented.

Properties of a fluctuating double layer in a magnetised plasma column

Abstract: Wave phenomena at a strong electric double layer are investigated experimentally in a magnetised plasma column where the electron gyro frequency is smaller than the plasma frequency. Fluctuations with frequencies below the ion plasma frequency and high frequency fluctuations, peaked close to the electron plasma frequency, are found to be dominating. The low and high frequency fields are shown to have characteristic spatial distributions with the maximum fields occurring in localised regions at the double layer. Spatial correlation measurements show that the high frequency field is associated with axially propagating waves. The phase velocity is nearly constant over a wide range of frequencies and 10 to 20% smaller than the velocity of the electron beam that is formed in the layer. The double layer is characterised by a steep potential gradient in a central region with a thickness of about 20 Debye lengths. This region is surrounded by 'presheaths' where free electrons and ions are accelerated towards the layer. The role of the ionisation in the anode plasma is also discussed.

Non-radiative surface plasma-polariton modes of inhomogeneous metal circular cylinders

Abstract: The theory is given of the non-radiative surface plasmon-polariton modes of metal cylinders, modelled by a spatially dispersive free electron gas within a Bloch hydrodynamic formulation. Dispersion equations for homogeneous and inhomogeneous cylinders are derived for nth cylindrical harmonic, and the dispersion properties are investigated as a function of n, cylindrical radius and surface structure. The position, and splitting with n, of the surface modes is found to be considerably different from those based on a model that neglects spatial dispersion. This is particularly true for very small cylinder radii.

Electrically pumped, relativistic, free electron wave generators

Abstract: Stimulated scattering induced by the longitudinal electric field of a pump wave is studied theoretically for the case of dense relativistic electron beams traveling in cylindrical metal waveguides. Two processes are examined. In one, the pump wave decays parametrically into a slow and a fast space‐charge wave. In the other, it decays into a slow space‐charge wave and a TM wave of the guide. The frequency characteristics and stimulated growth rates are given for each process, as a function of beam diameter, velocity, and density.

Isochronous Storage-Ring Laser: A Possible Solution to the Electron Heating Problem in Recirculating Free-Electron Lasers

Abstract: It is possible to design a storage-ring free-electron laser so that the electrons remain trapped in the optical potential wells formed by the laser fields. Such a design eliminates the heating of the electrons caused by the interaction and could yield both high power output and efficiency.

Some features of inverted-V events as seen from simulated double layers

Abstract: Results from a numerical simulation of a double layer show some features similar to those of inverted-V events. The strong heating of thermal and precipitating electrons is observed along with extremely low frequency fluctuations found during inverted-V events. It is suggested that after the acceleration of auroral electrons by the double layer, the precipitating free electrons are heated by the nonlinear effects of the electron beam plasma instability. Fluctuations and pulsations of auroral electron fluxes during auroral events are caused by a relaxation type of oscillation. The finite extent of one dimensional plasma is simulated by solving the Vlasov and Poisson equations as an initial and boundary value problem.

Ion-beam generated ion acoustic solitons in beam plasma system with non-isothermal electrons

Abstract: Reductive perturbation method has been employed to obtain a modified KdV equation and to show the excitation of ion acoustic solitons in an ion beam-plasma system consisting of bulk of cold ions, beam ions and non-isothermal and resonant electrons (both trapped and free electrons). The effect of ion beam velocity, beam ion density and plasma ion to beam ion mass ration on the amplitude and width of ion acoustic solitons has been investigated. The beam amplification rate of solitons amplitude is found to depend upon the ion beam velocity. There exists a critical ion beam velocity below which the ion beam is unable to generate solitons and this threshold ion beam velocity is an increasing function of the ion beam concentration and the mass ratio of background plasma ions and beam ions.

Single-electron analysis of the space-charge effect in free-electron lasers

Abstract: An exact treatment of the space-charge effect in the single-electron analysis of a free-electron laser is presented to calculate its small-signal gain. With the inclusion of the repulsive force between electrons, it is found that the trajectory of an electron can be solved from a generalized equation which includes a space-charge term. The results show the gain is saturated with decreasing growth rate due to high electron density. The radiation frequency is found to increase with the electron density and approach the value at plasma resonance. The condition ωpL/c = π clearly defines the boundary between the noninteracting and the collective regime of an electron beam, where ωp is the plasma frequency, L is the device length, and c is the light velocity in vacuum.

Electrical and optical properties of chalcogenide amorphous semiconductors modified with Ni

Abstract: The electrical and optical properties of the chalcogenide semiconductor (Se 32 Te 32 As 4 Ge 32 ) 100− x Ni xitx have been studied. As the Ni concentration is increased the electrical dc conductivity is drastically increased and variable range hopping conduction becomes dominant even above room temperature. The optical energy gap decreases with the Ni concentration from 1.18–0.95 eV. Ni-atoms in the chalcogenide semiconductor donate free electrons which occupy the gap state. This occupation causes the shift of the Fermi level toward the conduction band. It is an effect of this shift that the thermal activation energy is decreased. The decrease in optical energy gap is independent of the shift of the Fermi level and is ascribable to the appearance of the additional level located at 0.95 eV above the top of the valence band. This level originates from the 3d-level of the Ni-atom.

Optically induced band structure of free electrons in an external plane wave field

Abstract: The motion of a non-relativistic free charged particle in a classical electromagnetic plane wave field is investigated. By introducing the ansatz of a modulated plane wave for the wavefunction, the problem is reduced to the solution of the general Mathieu equation for a circularly polarised field and to the solution of the Hill equation for a linearly polarised field. The corresponding eigenvalue equations show that physical values of the energy are separated into bands. The relationship with earlier methods is also discussed.

Effect of a dc electric field on the trapping dynamics of a cold electron beam

Abstract: The self‐consistent modification of the trapping dynamics of a low density cold electron beam due to the external application of a dc electric field is considered. For dc fields smaller than the peak amplitude of the saturated beam‐plasma instability, the beam energy remains clamped while the wave amplitude grows secularly. Large traveling potential wells appear and create strongly focused charge clumps. By considering the role of wave dissipation, an exact dynamic Bernstein–Greene–Kruskal equilibrium is found analytically. It consists of a singular charge clump which propagates through the medium at constant velocity even though a dc field is present. The numerical study of the basic equations shows that the system evolves asymptotically to this singular state. A variety of experimentally relevant properties associated with the trapping dynamics is investigated, including the stability of the dynamic nonlinear equilibrium to sideband growth.

Dissociative recombination in an ambient electron gas

Abstract: It is found that the ambient electron gas may increase the rate of dissociative recombination appreciably even when the number density of the bound Rydberg electrons is considerably less than that of the free electrons.

The Pressure-Volume Relation and Cohesive Properties in Transition Metal Alloys

Abstract: A method to treat the pressure-volume relation in alloys on the basis of the virial theorem is proposed, and then the lattice parameter, the bulk modulus and the formation energy of the disordered 4d transition metal alloys are calculated. In this calculation, the s-d hybridization is neglected and the d band is treated in the single band off-diagonal C.P.A, the s band in the free electron like rigid band model. Assuming the charge neutrality condition at each site, it is found that the calculated lattice parameters, bulk moduli and formation energies of Nb–Zr alloy and Pd-base 4d transition metal alloys agree with the observed trends. Particularly, it is found that the negative deviation from the Vegard law is mainly due to the d part pressure. The origin of this attractive pressure lies in the gain of the Pd-site bond energy due to alloying.

Electrostatic free electron laser

Abstract: An electrostatic free electron laser having an electromagnetic waveguide, means to produce a periodic longitudinal electrostatic field along the waveguide and an electron beam which passes through the waveguide. Radiative energy is generated by the laser as a result of the kinetic energy lost by the electron beam as it is decelerated during its interaction with the electromagnetic wave and the electrostatic field within the waveguide. In use as an oscillator, the electrostatic free electron laser includes a resonator either in addition to or instead of the waveguide.

Light impurities in a uniform electron gas

Abstract: The electronic properties of H and He embedded in a uniform electron gas are studied from an atomic point of view abandoning the local density formalism. The effect of the electron gas on the bound states of the proton is analysed: (i) by using a linear dielectric response in which the free electrons are described by orthogonalised plane waves instead of plane wave states; (ii) by including the exchange and correlation effects between the orthogonalised plane waves and bound states. The electronic bound states are then determined by means of a variational procedure. By using many-body techniques, the quasiparticle spectrum is also determined. The authors results are in reasonable agreement with those obtained in the local density formalism.

Free-electron screening of short-range scattering potentials in semiconductors

Abstract: The authors consider the scattering of conduction electrons in a semiconductor by static short-range potentials, e.g., the short-range potential of an impurity or the disorder potential in an alloy. The response of free electrons consists of producing a long-range potential, due to off-diagonal elements of the dielectric matrix. The electron matrix elements of these short- and long-range potentials partially cancel. The matrix element of the total potential is equal to the matrix element of the short-range potential divided by the free-electron dielectric function. Thus, the scattering by short-range potentials is affected ('screened') by free electrons in the same way as the scattering by long-range potentials (e.g., by Coulomb potential). Both single-valley and multi-valley cases are considered.

On the generation of charge carriers by exciton-exciton collisions in organic molecular crystals

Abstract: The rate of generation of a free electron and a trapped hole due to the Coulomb interaction between charge carriers of two Frenkel excitons (one free and one trapped) is derived for molecular crystals by time dependent perturbation theory. Electron-lattice and spin-orbit interactions are neglected. The theory is finally applied to benzophenone single crystals where a transition rate approximately 109 cm-3 s-1 is obtained.

Two-element free-electron lasers.

Abstract: The interaction between the electrons and the radiation in a free-electron laser leads to a shift and a spread of the electron velocity distribution. The electron dynamics of a two-element system are studied in the small signal region. It is found that the efficiency and gain can be increased through introduction of an adjustable drift distance between two identical wigglers.

A three dimensional calculation of electron energy loss in a variable parameter free-electron laser

Abstract: The motion of an electron beam through a longitudinally varying period undulator for a single-pass free-electron laser is studied. It is shown that, under certain conditions, the electrons are trapped in an optical “bucket”, and deliver on the average more energy to the laser field than in constant period undulators of comparable parameters. The limits set by a finite beam emittance and energy spread are studied in detail by three-dimensional computer simulation.

Very narrow quasi elastic scattering in BaTiO3 close to the phase transition

Abstract: An excess electron in a first order phase transition ferroelectric material (eg BaTiO3) produces a local change in the spontaneous polarization: it can result in the creation of a new self-consistent state (phason) The phason states and the photoferroelectric phenomena can exist simultaneously if the initial free electron is the result of an optical excitation The connection between the concept of photofluctuation (introduced for the explanation of the photoinduced Rayleigh scattering and of the photovoltaic effect) and the phason state is discussed The calculations are restricted to the non degenerate case for the electrons; we consider also that the phasons are not quasiparticles

Effects of Fermi Surface Nesting on Magnetic Instability in Magnetic Superconductors

Abstract: Possible magnetic instability in magnetic superconductors is studied by calculating the wave number dependent susceptibility χ( q ) in the presence of superconductivity. Effects of Fermi surface nesting on χ( q ) are examined for three models: Free electron gas model and tightly binding cosine band model of one-dimension, and Liu's model for imperfect nesting. All model calculations reveal that stable magnetic ordering which coexists with superconductivity is antiferromagnetic one. Possibility of an incommensurate spin density wave state is discussed.