Showing papers on "Free electron model published in 1987"

Fundamentals of solid state physics

Abstract: A Survey of Solid State Physics. Crystal Lattices. The Structure of Solids. Elastic Scattering of Waves. Bonding. Atomic Vibrations. Electron States. Thermodynamics of Photons and Electrons. Electrical and Thermal Conduction. Dielectric and Optical Properties. Magnetic Properties. Free Electrons and Magnetism. Superconductivity. Physics of Semiconductor Devices. Appendices.

Plasma wave wigglers for free-electron lasers

Abstract: We explore the possibility of using a relativistic plasma density wave as a wiggler for producing free-electron laser radiation. Such a wiggler is a purely electric wiggler with frequency ω p (plasma frequency) and wavenumber k p . If an electron beam is injected parallel to the plasma wave wavefront, it is wiggled transversely with an apparent wiggler wavelength \lambda_{w} = 2\pi c/\omega_{p} . Using plasma densities in the 1017(cm-3) range, λ w of order 100 μm may be obtained, thereby permitting generation of short wavelength radiation with modest energy beams. The effective wiggler strength a_{w} = eA/mc^{2} \sim 0.5 can be extremely large. We discuss the excitation methods for such wigglers and examine the constraints imposed by the plasma medium on FEL gain in this scheme.

Electrons and Holes in Semiconductors

Abstract: A semiconductor can generally be treated in a similar way to a free electron solid. The energies corresponding to the permitted bands are first calculated. The levels in each band are then filled with electrons until the supply has been exhausted. As previously defined, the Fermi level is the top-most filled level at zero Kelvin. As in the Free Electron model, it is only those electrons in the vicinity of the Fermi level which are significant in the conduction process. Therefore, our treatment can be restricted to a study of the properties of the highest filled and lowest empty energy bands, which are more commonly known as the valence band and the conduction band.

Theory of time‐dependent polarization about an ion in an isotropic liquid

Abstract: We have investigated the theory of polarization of a dipolar solvent by a newly created ion or free electron. As a reference system we have used the solvent together with the ‘‘uncharged ion,’’ and by means of statistical mechanics, we have examined the linear response of the system to the imposition of the ionic charge. The response depends upon the response at all k values, the low k behavior corresponding to continuum (universal) macroscopic dielectric effects, and the high k behavior corresponding to molecular (specific) phenomena which may have to be treated by quantum mechanics. We have focused on the quasicontinuum limits for which the low k behavior is assumed, but the sizes of ions and dipoles, and the velocities of ions are incorporated. We also look at some general properties of high k behavior; for example, because the polarization energy for small ions is independent of ion size, it is usually thought that solventbergs form about the ions, but, at least in part, this phenomenon can be explain...

Free-electron lasers with electromagnetic standing wave wigglers

Abstract: A detailed analysis of the electromagnetic standing wave wiggler for free-electron lasers (FEL's) is conducted for both circular and linear wiggler polarizations, following a single-particle approach. After determination of the unperturbed electron orbits in the wiggler field, the single-particle spontaneous emission spectrum and subsequently the gain in the low gain Compton regime (using the Einstein coefficient method) are explicitly calculated. This analysis results in a clear understanding of the resonance conditions and the coupling strength associated with each resonance of this type of FEL. In particular, a striking feature obtained from this investigation is that the electromagnetic standing wave wiggler FEL, under certain circumstances, exhibits a rich harmonic content. This harmonic content is caused by the presence of both the forward and backward wave components of the standing wave wiggler field. In addition, the nonlinear self-consistent equations for this type of FEL are also presented, permitting further investigation of it by the theoretical techniques and numerical codes developed for conventional FEL's.

Acceptor levels in indium selenide. An investigation by means of the Hall effect, deep-level-transient spectroscopy and photoluminescence

Abstract: Acceptor levels related to I, II, IV, and V group impurities in indium selenide are studied by means of the Hall effect, deep-level-transient spectroscopy (DLTS) and photoluminescence. Activation energies for hole concentrations in the range from 200 to 300 meV have been measured. A reversible change of sign of the Hall voltage has been observed below 215 K. This behaviour can be explained through a model in which acceptor levels are assumed to be shallow and interlayer planar precipitates of ionized shallow donors create potential wells that behave as deep donors and in which a low concentration of bidimensional free electrons can exist. This model also explains the capacitance-voltage characteristics of both ITO/p-InSe and Au/p-InSe barriers. DLTS results are coherent with this model: hole traps in high concentration located about 570 meV above the valence band are detected. Photoluminescence also confirms the shallow character of acceptor levels. A broad band whose intensity is related to p conductivity appears in the PL spectra of low resistivity p-InSe. The shape and temperature dependence of this band can be explained through self-activated photoluminescence in a complex center in which the ground acceptor level must be at about 50 meV above the valence band.

Density dependence of the atomic transition probabilities in hot, dense plasmas.

Abstract: The atomic properties and transition probabilities of highly ionized aluminum in hot, dense plasmas were studied. In particular, we present computational results of the variations with density of the following atomic parameters: atomic potential and screening factor (due to both bound and free electrons), free-electron distribution, atomic wave functions, binding energies, line shifts, and, finally, transition probabilities and oscillator strengths. The calculations were carried out using the ion-sphere model (ISM) which treats the bound and free electrons in the atom self-consistently in a central potential. This potential is produced by the combination of the nuclear Coulomb field together with contributions by the bound- and free-electron charge distributions. The results indicate an increasing effect of the plasma on the atomic properties with increasing plasma density. Particularly, the free-electron screening reduces the atomic potential, pushes the atomic wave functions away from the nucleus, and reduces the binding energy of the bound electrons. The transition probability also decreases monotonically with density up to the ionization limit of the upper state beyond which it drops to zero. The computational results are compared to those expected from a homogeneous free-electron spatial distribution.

Optical gain, phase shift, and profile in free-electron lasers.

Abstract: The gain, phase shift, wave-front curvature, and radius of the radiation envelope in a free-electron-laser amplifier are obtained in the small-signal regime. The electron beam is assumed to have a Gaussian density distribution in the transverse direction. Numerical calculations indicate that the radius and curvature of the radiation beam entering a wiggler asymptote to unique spatially constant values after a finite transition region. However, in the asymptotic region the wave fronts are divergent. Analytical expressions for the gain, phase shift, curvature, and spot size are derived. It is shown analytically that small perturbations of the optical waist and curvature about the matched value are spatially damped out, indicating the stability of the matched envelope. When the electron-beam envelope is modulated in space, the optical spot size oscillates with an almost identical wavelength but is delayed in phase. In the case of small-amplitude long-wavelength betatron modulation of the electron-beam envelope, generation of optical sidebands in wave-number space is examined and the effect on the dispersion characteristics of the primary wave is found to be negligible for typical experimental parameters.

Theory of the Hollow Cathode Glow Discharge

Abstract: A theory of the hollow cathode glow discharge based on the two-temperature electron model is developed. The following two interdependent mechanisms are consistently taken into account: the creation of ions and excited atoms due to electron collisions in the discharge and the emission of secondary electrons on the cathode surface due to ions and ultraviolet photons. Trapped electrons in the hollow are also considered. Numerical calculations are made for helium gas in a cylindrical hollow cathode whose inner diameter and length are 10 and 32 mm, respectively, on the basis of a onedimensional steady state model. Voltage-current relations are obtained at gas pressures of 0.13, 0.27 and 0.40 kPa. Population densities of 64 excited atomic levels are determined as a function of the radial distance r as well as the densities of the ions and electrons.

Second harmonic electron cyclotron breakdown in stellarators

Abstract: In linear wave-particle interaction models, the coupling between cold electrons and microwaves with frequency equal to twice the electron gyrofrequency is so weak that the ionization of a significant number of neutral hydrogen atoms would seem impossible in practical applications. However, the non-linear interaction of a cold electron with the wave is very large if the electron becomes trapped near resonance in a shallow, static magnetic well. A model has been developed to describe the breakdown of a neutral gas when these non-linear interactions are considered, and it is in reasonable agreement with the limited amount of available experimental data. For gas pressures that are too large, electron-neutral collisions inhibit the non-linear interaction and prevent breakdown. For gas pressures that are too low, the growth rate of the free electron population is limited because electrons capable of causing ionization are lost before suffering a collision with a neutral. Quantitative growth rate predictions are presented for stellarators, and formulae for rough estimates are given.

Diamagnetic and Paramagnetic Effects in Free-Electron Lasers

Abstract: The effect of high-current relativistic electron beams (REB's) on the undulator field amplitude in free-electron lasers (FEL's) is investigated. Two mechanisms of excitation of periodic magnetostatic self-fields by REB are considered: 1) a static mechanism that is realized at stationary motion of REB in the undulator field; and 2) a dynamic mechanism that is realized at signal wave amplification. The static mechanism in the absence of an axial magnetic field leads to a decrease of the total undulator field amplitude (a diamagnetic effect). The dynamic mechanism for low-density beams leads to an increase of the total undulator field amplitude (a paramagnetic effect), with a subsequent increase of electron efficiency. For high-density beams, the effect of the phase shift of the total undulator field is most essential, due to which the growth of the signal wave amplitude is limited by nonlinear mismatch of synchronism.

Photoemission from nearly free electron metals

Abstract: The properties of the angle resolved photoemission spectra from nearly free electron (NFE) metals have been investigated for a wide range of electron densities, from K (1.4 × 1022 electrons/cm3) through Be (24.7 × 1022 electrons/cm3). These data are used in conjunction with theoretical calculations to illustrate the importance of many body effects in the excitation spectra. Characteristic features of the emission from the bulk and surface of NFE metals will be described as well as the possibility for interference between the emission from the bulk and surface.

Optical Properties of Polycrystalline YBa2Cu3O7-δ

Abstract: Reflectivity of polycrystalline YBa2Cu3O7-δ has been measured in the energy ranges from 2.5 to 0.05 eV and from 27 to 3.7 meV at room temperature. A Drude edge due to the free electron has been observed at 1.4 eV and the carrier concentration is estimated as 3.2×1021 cm-3. The observed low reflectivity suggests the two dimensional character of the free electron. Below Tc a structure has been observed around 17.7 meV (143 cm-1), from which the energy gap 2Δ is tentatively estimated as 15.5 meV (125 cm-1) at 15 K. Alternatively, the structure may be due to a transverse collective excitation.

Renormalized-free-atom model and Compton profile of hcp cobalt

Abstract: Compton profile of cobalt has been calculated employing the renormalized-free-atom model for fcc as well as hcp phases choosing several 3d-4s configurations. The results have been compared with recent gamma-ray measurements on polycrystalline Co. Best agreement between theory and experiment is found for 3d74s2 configuration. Comparison with free electron model has also been made for this case.

Three dimensional modelling and numerical analysis of super-radiant harmonic emission in an FEL (optical klystron)

Abstract: A full 3-D analysis of super-radiant (bunched electron) free electron harmonic radiation is presented. A generalized form of the FEL pendulum equation was derived and numerically solved. Both spectral and phasor formulation were developed to treat the radiation in the time domain. In space the radiation field is expanded in terms of either a set of free space discrete modes or plane waves. The numerical solutions reveal some new distinctly 3-D effects to which we provide a physical explanation.

Quasi One-Dimensional Conductors: The Far Infrared Problem

Abstract: In the metallic state the quasi one-dimensional conductors are characterized by very high values of dc conductivity equaling in some cases pure copper at room temperature. Thus one might expect their optical properties to follow the Drude free electron theory where the conductivity remains equal to the dc value up to a frequency of the order of the relaxation time. Instead the quasi one-dimensional conductors show a region of depressed optical conductivity in the far infrared. The dc conductivity1 at low temperature of a material such as (TMTSF)2ClO4 is of the order of 5·105 (Ω·cm)-1 whereas the optical conductivity2 at 50 cm-1 is only a few hundred (Ω·cm)-1. The change from high to low conductivity occurs in the experi mentally difficult microwave region and very few experiments have been done at these limiting frequencies. The phenomenon of suppressed far infrared conductivity is almost universal in the quasi one-dimensional materials2–7. It was first observed in TTF-TCNQ but subsequently also reported in (TMTSF)2X family of compounds in their conducting state. This discrepancy has been generally explained in terms of a narrow collective mode with a very large effective oscillator mass centered at zero frequency. Such a model implies that the electrical current at low temperature is carried by the collective mode.

Improved Colored State Reflectivity In Lithiated WO 3 Films

Abstract: Improved colored state reflectivity in in the Drude model there are 3 important lithiated WO3 films parameters; Ep (eV), the plasma energy =(h/2n)\l [ne] q2/fm*e0 ], (h=Planck's constant,R.B.Goldner, G.Seward, K.Wong. G.Berera, Cne3 = free electron density, q = electronic T.Haas and P.Norton charge, m* = electron effective mass, eQ =free space dielectric constant); Er (eV), theElectro-Optics Technology Center, Tufts loss parameter (also known as the damping orUniversity, Medford, MA 02155 scattering parameter); and e^ = the bound"'"Honeywell Electro-Optics Center, Lexington, electron contribution to the dielectricMA 02173 response function. Ideally, to obtainreflectivities > 1, for photon energies belowABSTRACT E , one needs Er ^ 0. Also, for Er » 0 theresulting spectral dependence for thePolycrystalline W03 films have been reflectivity is very "step-like," and the prepared which exhibit high near infrared optical absorption > O.6 Furthermore, a low reflectance (-69% at 2.5ym wavelength) when Er leads to a considerably reduced switching lithiated. This is a significant increase in energy requirement for smart windows. This is near infrared reflectance over what has been because the switching energy is proportional previously reported for lithiated WO3. The to the square of transferred charge and films also exhibit a decrease in the Drude the transferred charge required to achieve scattering parameter and a reversible a desired reflectivity decreases with Ep and structure change to higher symmetry consequently film thickness, [e.g^, for Er A (monoclinic to cubic) with increased .05eV and Ep a. 5eV, films of 80%.] Thus, identifying the significance for smart window glass will be source(s) of free electron scattering and discussed. reducing the scattering is important for thedesign of smart windows. By comparing ourreflectivity results for Lix WO3 films with the reflectivity of single crystals of1. INTRODUCTION Na^WO^, we concluded that there was anadditional major source of electronThere are a number of important scattering in our films besides ionized applications for which it is highly desirable impurity scattering, which most likely to have spectrally selective reflection results from the insertion of H"1" or Li"*") . On modulation, or tuning. One such application the basis of a spectroscopic ellipsometry is that of smart window glass, which could study, we showed that the major additional be used for significantly increasing the source of electron scattering was energy efficiency of buildings and vehicles, scattering associated with extended defects, especially by controlling solar gain.1 which acted like monopole line charges.tt Reflection modulation is desirable for at Furthermore, we attributed these extended least 3 reasons: 1) to avoid excess strain defects crystallographic shear planes and consequent degradation associated with primarily arising from oxygen deficiency. We heat-producing optical absorption; 2) to also ruled out phonon scattering as dominant avoid a decrease in thermal efficiency by obtaining no measurable change in the near associated with the radiative emission infrared reflectivity as we lowered the arising from the heat-producing optical temperature to close to that of liquid absorption; and 3) spectrally-selective nitrogen (~ 80°K). In this paper we wish to modulation (filtering) can be more readily report having achieved in polycrystalline achieved than with bound electron absorption films of WO3 a lithiated state reflectivity modulation. Consequently, one of our efforts at \ = 2.5 ym of approximately 69%, a has been directed towards developing an significant improvement over previous electrochromic layer for which a high degree results, and we wish to discuss this as well of reflection modulation can be achieved. We as related results, have concentrated much of that effort on attempting to model and optimize thereflection modulation in polycrystalline 2. SAMPLE PREPARATION films of lithium-insertable WO3. We havedemonstrated that the Drude quasi-free Thin films were prepared by rf sputter electron model applies to polycrystalline deposition in an oxygen atmosphere. An MRC films of HXWO3 and LixWO3. ~4 Previously we rf sputter deposition system (Model SM 8500) reported attaining a significant increase in with a 5 inch diameter WO3 target (purchased reflection modulation for polycrystalline from Cerac, Inc.) was used. The substrates films of H^WOo over what had been reported by were either bare or ITO-coated microscope Schirmer et.al.5 In particular we obtained glass slides and were heated to temperatures protonated (or "colored") state reflectivity in excess of 400°C. Lithiation was done in at X= 2.5ym (X = wavelength) of approximately either of two ways: a) for bare glass 60% as compared to approximately 35%, substrates, the RCA technique (previously obtained by Schirmer et.al. described)10 was used - i.e., a metal

Mobility oscillations of electrons on thin He films

Abstract: Among the various probes for studying surfaces, free electrons probably play the most important role nowadays. More and more soph~sticated techniques have been developed to investigate the structure and the excitation of surfaces by low energy electron diffraction, electron energy loss spectroscopy, etc. These methods usually require luItra high vacuum conditions and are therefore suitable only for studying solid surfaces. For liqu~ds the vapor pressure in general is so high and hence the mean free path of the electrons so short that these techniques are not applicable. Here we discuss a method that although quite different from the conventional electron spectroscopies, also makes use of free (or rather quasl-free) electrons to obtain information about substrate surfaces via the interaction of the electrons with these substrates. This method relies on materials with a negative electron affinity and is therefore, in particular, suited for the quantum liquids and solids formed by helium hydrogen; it can be applied even under conditions where the mean free path of the electrons is relative1 y short. In the work presented here we have used the electrons to study thin liquid films of He covering solid substrates, mainly

Density and deceleration limits in tapered free electron lasers.

Abstract: The competition between the bunching force of the ponderomotive potential and the repulsive force of the self electrostatic potential in a tapered free electron laser limits the beam density, deceleration rate due to tapering, and the local electric field spatial exponentiation rate. The limit on the spatial exponentiation rate is below that predicted by linear theory at the transition from the Compton to Raman regimes. Furthermore, this limit is independent of the form of the particle energy distribution function.

Three-dimensional theory of free-electron lasers in the collective regime.

Abstract: We present an analysis of a free-electron laser within a three-dimensional model, in which the finite transverse dimensions of the electron beam are taken into account together with the confinement of the electromagnetic modes by a guiding structure. The analysis is based on the expansion of the beam's self-potential into an infinite set of modes that interact simultaneously with the wiggler and signal fields. Explicit gain-dispersion equations are developed for several cases including a waveguide tube completely filled by an electron beam of uniform density, and a uniform-density electron beam partially filling a waveguide (or in free space). This is carried out in both the magnetized beam limit and the general case which includes surface currents. The results bear significant effect on free-electron-laser gain operating parameters in the collective regimes and on the threshold of absolute instability oscillation.

BRIGHTNESS OF AN INTENSE ELECTRON BEAM GENERATED BY A PULSE-LASER IRRADIATED PHOTOCATHODEa

Abstract: Linac-driven free electron lasers, new powerful synchrotron radiation sources, and advanced highpower microwave devices all require very bright emitters of electrons to operate efficiently. By irradiating a Cs3Sb photocathode with a frequency-doubled Nd:

Two-dimensional electron fluid computer calculations of nanosecond pulse, low pressure microwave air breakdown in a rectangular waveguide and their verification by experimental measurement

Abstract: To assess the feasibility of the propagation of intense microwaves through the atmosphere, we must understand their interaction with the atmospheric air molecules. When an intense electromagnetic pulse passes through a gaseous medium such as the atmosphere, a number of physical processes can occur. The most important of these is avalanche ionization, which occurs when initially present free electrons are accelerated by the wave electric field and cause ionizing collisions with air molecules. If not rapidly removed from the region, free electrons can then multiply exponentially and reach a density, at which they remove energy from the electromagnetic wave. This energy loss can be strong enough to absorb the trailing part of the pulse and thus severely limit the energy the wave can transmit. Electrons may be lost by the processes of attachment, recombination with air ions, and diffusion out of the wave region. For pulse lengths of 100 nanoseconds and less, the characteristic times for attachment, recombination and diffusion are too long to be important and therefore may be neglected. Except for a case of detector diode saturation, the 2-D numerical electron-fluid model for air breadkdown of the TE10 mode in the WR-284 waveguide agrees well with transmitted powermore » measurements for pressures from 3.5 to 300 Torr. The peak values are within 20% and the integrated envelope energies are within 13%. The modeled and measured values do not agree as well at 0.8 Torr, where the peak values are within 39% and the transmitted envelope energies are within 49%.« less