Thin Electron Beam Research Articles

The Small Signal Analysis of a Centered Dielectric-Rod Loaded, Arbitrarily-Shaped Helical Groove Traveling-Wave-Tube

Properties of traveling wave-beam interaction in a centered dielectric-rod loaded, arbitrarily-shaped helical groove slow-wave structure (SWS) are investigated for a thin annular electron beam. The “hot” dispersion equation is obtained by means of the self-consistent field theory, and the small signal analysis is carried out including the effects of the dielectric-rod parameters and the groove shapes. The numerical results show that the bandwidth of the helical groove TWT is expanded by loading dielectric-rod, however, the small-signal gain is reduced; and when the groove shape changes from the swallow-tail shape to the triangle shape, the working frequency increases , while the peak gain decreases.

Electromagnetic wave radiation by an electron beam spiraling in a magnetized plasma column

The paper studies the electromagnetic wave radiation by a density modulated and thin electron beam of finite length injected obliquely with respect to the constant external magnetic field into a cylindrical plasma column embedded in a homogeneous medium (plasma, dielectric, or free space) and aligned along the magnetic field lines. The time-averaged power radiated at the modulation frequency is determined as a function of the beam, the plasma column, and the medium parameters. Particular attention is devoted to the case when the beam modulation frequency belongs to the whistler frequency band. The paper shows what significant differences exist between the physical features of the emissions when the beam radiates in a plasma column embedded in a homogeneous medium or in a uniform and unbounded magnetized plasma. Based on the results of numerical calculations, the time-averaged power radiated by pulsed and modulated beams has been estimated for typical laboratory plasma experiments. In particular, it is shown that a beam propagating in a plasma column can efficiently enhance its wave emission due to Cherenkov and normal cyclotron excitation of guided whistler modes.

Excitation of THz symmetric TM-modes in a cylindrical metallic waveguide with an axial magnetized degenerate plasma rod by an electron beam

The dispersion relation of symmetric electromagnetic TM-modes in a cylindrical metallic waveguide with a magnetized degenerate plasma column protected by an annular dielectric layer are obtained. The permissible frequency regions for slow E-modes (TM) are presented. In addition, the time growth rate for excitation of these waves by a thin annular relativistic electron beam (TAREB) is studied. The dependency of time growth rate on the strength of the external magnetic field, hole plasma frequency and accelerating voltage are investigated.

THE THEORETICAL INVESTIGATION OF THz ELECTROMAGNETIC WAVES IN A ROD DEGENERATE PLASMA-WAVEGUIDE

By using the Fourier components of dielectric tensor elements of cold collisionless degenerate plasmas, the dispersion relation of electromagnetic waves in a cylindrical metallic waveguide with a degenerate plasma column protected by an annular dielectric layer are obtained. The permissible frequency regions for slow and fast waves in E-mode (TM) are presented. Furthermore, the graph of cutoff frequency versus the radius of plasma column for the fast waves is investigated. In addition, the time growth rate for excitation of symmetric slow E-modes (TM) by a thin annular relativistic electron beam (TAREB) is studied. The graph of time growth rate respect to radius of dielectric layer and accelerating voltage are presented.

Slow cyclotron instability due to surface modulation of an annular beam

The Cherenkov instability used in slow-wave devices has been well studied in the literature. However, in previous analyses, the beam motion is restricted to the longitudinal direction assuming an infinitely strong magnetic field. For the finite strength magnetic field, the transverse beam perturbation cannot be ignored and leads to the slow cyclotron instability. Recently, a new version of self-consistent field theory considering three-dimensional perturbation has been developed based on a solid beam, in which the effect of the transverse perturbation appears as a surface charge at a fixed boundary. In the case of a thin annular beam, the boundary is modulated and is essentially different from the solid beam case. We propose a self-consistent field theory considering the moving modified boundary surface. The slow cyclotron instability due to the modulation of an infinitesimally thin annular electron beam is presented.

Palladium–silver thin film for hydrogen sensing

In this paper, a palladium–silver (Pd–Ag) thin film was fabricated and studied for hydrogen gas sensing. A zigzag-shaped microstructure of Pd–Ag was coated on a porous alumina substrate through a combination of microfabrication techniques including photolithography, thin film sputtering and electron beam vacuum evaporation. The atomic ratio of the two metals is controlled at Pd:Ag = 3:2. To evaluate the H 2 sensitivity of the fabricated device, the electrical resistance of the film was measured toward hydrogen concentration change. Experimental results show that the sensing element fabricated this way demonstrates high sensitivity, fast response and excellent reversibility to hydrogen gas. The sensing performance of the mixed metal film is much better than that of the palladium film. The present sensor is promising for room temperature hydrogen leakage detection and related applications.

Simulation of Smith-Purcell terahertz radiation using a particle-in-cell code

A simulation of the generation of Smith-Purcell (SP) radiation at terahertz frequencies has been performed using the two-dimensional particle-in-cell code MAGIC. The simulation supposes that a thin (but infinitely wide) monoenergetic electron beam passes over a diffraction grating. We simulate two configurations, one similar to the Dartmouth SP free-electron laser, with a low-energy continuous beam (we use an axial magnetic field to constrain the electrons to essentially one-dimensional motion). The other is similar to the recent MIT experiment that uses a prebunched 15 MeV beam.

High-Frequency Wave Excitation by the Modulated Electron Beam From Spacecraft Injected Into the Ionosphere Plasma

Waves excitation in the ionosphere by the modulated electron beams was observed in some active plasma experiments in space. In the active beam-plasma experiments electromagnetic wave can be excited by injecting modulated electron beam into the ionosphere, by various mechanisms according to different experimental conditions. The frequency of this sort of radioemission is determined by the beam modulation; it can lie in the whistler band or in the high frequency band (higher than electron plasma frequency). This article gives a theoretical analysis as well as quantitative calculation of the radioemission of semi-restricted thin modulated electron beam injected from the spacecraft into the ionosphere plasma along the geomagnetic field. The result shows that when the modulated electron beams are injected along the magnetic field line, they can produce high-frequency electromagnetic radiations in the ionosphere plasma. This electromagnetic radiation is mainly observed in the plane that is perpendicular to the velocity direction of the injected electron beam. Estimations show that the radiated power is high enough to be detected.

Modification of Silicon Oxide by High Energy Electron Beam

During interaction between thin film SiO2 and electron beam with high power density, amorphous silicon dioxide modifies. Silicon nanoclusters are formed in radiated area. Result of this interaction is formation of Si/SiO2 nanocomposite. We studied modified SiO2 by TEM, microdiffraction and cathodoluminescence.

Electromagnetic Wave Excitation by a Modulated Electron Beam Injected from Spacecraft into the Ionosphere

Electromagnetic wave excitation can be excited by injecting a modulated electron beam into ionosphere, with varied mechanisms under different experimental conditions. This article presents calculation of the radioemission of a semi-restricted thin modulated electron beam injected from the spacecraft into the ionosphere plasma along the geomagnetic field. The result shows that the radiation pattern for the whistlers' frequency range has a sharp peak that corresponds to the Cerenkov resonance condition. In addition, the radiated power increases with the length of relaxation characteristic. This shows that the intensity of electromagnetic wave can be strengthened by electron beam modulation.

Time growth rate of symmetric TM mode of a rod dielectric Cerenkov plasma maser

The dispersion relation of symmetrical TM waves (usual operating mode of a traveling wave tube) propagating in a Cerenkov maser including a thin annular relativistic electron beam (TAREB), a strongly magnetized plasma column, and a dielectric rod is investigated. The effects of relative permittivity constant of dielectric rod, accelerating voltage, radii of the rod, TAREB, and plasma column as well, on the frequency spectra and growth-rate coefficient are presented. The conditions under which the growth rates are maximum are studied as well.

Spatial growth rate and field profiles of symmetric mode in a rod dielectric Čerenkov maser with a magnetized plasma column

By numerically solving the exact dispersion equation, the dispersion relation of symmetrical TM waves propagating in a Čerenkov maser including a thin annular relativistic electron beam (TAREB), a strongly magnetized plasma column, and a dielectric rod is investigated. The effects of accelerating voltage, radii of TAREB and plasma column as well, on the frequency spectra and spatial growth-rate coefficient are presented. The axial electric field profiles during the wave amplification and the conditions under which the spatial growth rates are maximum are presented.

The virtual cathode velocity during electron beam transport in a drift tube

A solution corresponding to a thin annular electron beam in a homogeneous drift tube featuring a uniformly moving virtual cathode (VC) has been obtained. The dependence of the VC velocity on the injected and transmitted currents is determined and it is established that this velocity has a limiting value. Theoretical results agree with the results of numerical calculations performed by means of the electromagnetic PIC code KARAT.

Space charge limited flow of a thin electron beam confined by a strong magnetic field

An approximate analytic theory is developed and implemented numerically for calculating the space charge limited current and electric field of a thin cylindrical beam or current sheet between two wide parallel electrodes. The flow is confined by a sufficiently strong magnetic field. Assuming that the potential and current density are almost homogeneous in the direction transversal to the flow, the beam current and its profile are computed by a variational method. The average current density scales as the reciprocal of the beam width when the latter becomes very small. The total cylindrical beam current thus decreases proportionly to its diameter while the total current of a sheet becomes almost independent of the width in this regime.

Excitation of Microwave by a Thin Annular Relativistic Electron Beam in a Plasma-Filled Dielectric-Loaded Coaxial Cylindrical Waveguide

A new type of slow-wave structure, i.e., plasma-filled dielectric-loaded coaxial cylindrical waveguide with a dielectric ring tightly enclosing the inner conductor, is developed. The collision between electrons and ions in the background plasma is taken into account. The microwave generation based on the Cherenkov effect excited by a thin annular relativistic electron beam in the new slow-wave structure is examined by use of the self-consistent linear field theory, considering the effect from this kind of collision via the collision frequency n/sub e,i/ term. The dispersion equation of the beam-wave interaction with a complex value of angular frequency is derived. The effects of the plasma angular frequency and collision frequency on the dispersion characteristics, i.e., the output frequency and the wave growth rate of the microwave radiation, are calculated and discussed. It is clearly shown that the microwave radiation excited by the beam-wave interaction results from the coupling between the slow TM modes propagated along the slow-wave structure and the negative-energy space-charge wave propagated along the relativistic electron beam.

The electron beam with virtual cathode in a two-section drift tube

An analysis based on the law of conservation of the z component of field and particle momentum in a thin annular magnetized monoenergetic electron beam in a two-section drift tube composed of tube segments with different radii (R1<R2) allowed the critical injected beam current to be determined for which a virtual cathode, formed in the wider tube at the joint section, starts traveling through the narrower tube toward the beam injection region. A region behind the traveling virtual cathode features a “squeezed” single-flux state of the beam (corresponding to the “slow” left branch of the current characteristic, high charge density, but low relativistic factor). When the injected current decreases below a critical transition level (Iin<ITr), the virtual cathode returns to the initial position and restores the double-flux electron beam. This current is smaller than (Ilim1+Ilim2)/2, and, depending on R2, varies from the limiting transport current for the narrower section (Ilim1) up to IF/2, where IF is the Fedosov current for this tube section.

On the current of an annular electron beam with a virtual cathode in a drift tube

It is shown that beyond a virtual cathode the current of a magnetized steady-state thin annular electron beam in a drift tube may range from zero to the limiting current of the tube.

The current of an annular electron beam with virtual cathode in a drift tube

An analysis based on the laws of conservation of energy and the z components of the field and particle momentum shows that a thin magnetized annular electron beam in a homogeneous drift tube behind a virtual cathode under stationary conditions occurs in a “squeezed” state corresponding to a slow left-hand branch of the current characteristic with a relativistic factor in the interval 1≤γ≤Γ1/3. The beam current I 1 behind the virtual cathode in a homogeneous drift tube can vary from zero up to a limiting value I lim, while the injection currents (I 2) and the current of electrons reflected from the virtual cathode (I 3) for every stationary state are single-valued functions of I 1 and fall within the intervals I F/2≤I 2≤I lim and 0≤I 3≤I F/2, respectively, where I F is the Fedosov current.

Interaction of a wave packet with a thin electron beam spiraling in a magnetized plasma

The nonlinear processes governing the resonant interaction of a packet of lower hybrid waves with a radially bounded electron beam spiraling in a magnetized plasma are investigated. In particular, the paper tries to answer the fundamental following questions: What are the causes of the beam bunching and the main agents of the beam self-organization occurring during the nonlinear wave–particle evolution? What is the influence of the wave packet on the stability of the formed bunches? This paper shows that, owing to the presence of wave energy dissipation out of the bounded beam volume, a competition takes place between the beam relaxation and the particle bunching processes, leading to the structuring of the beam over long distances from the injection point. In particular, two main mechanisms govern the behavior of the particles in resonance with the waves: First, the process of particle bunching which gives rise to dynamically stable and long living bunches of particles which keep resonance and strong correlations with several waves of the packet while continuously decelerated (Cherenkov resonance is considered) in the frame moving with the initial parallel beam velocity and, second, a process of particle diffusion in the velocity space concerning particles which do not experience trapping by waves or which leave a bunch through the action of large stochastic oscillations. This diffusion process cannot be well described in the frame of the weak turbulence theory, as it is noticeably perturbed by the presence of a large number of small unstable bunches which appear, merge together and disappear during the system’s evolution.

Microstructural and Electrical Properties of ZrO2 ThinFilms Prepared on silicon on Insulator with Thin Top silicon

Amorphous zirconia thin films were deposited directly onsilicon-on-insulator (SOI) substrates with thin top silicon byultra-high vacuum electron beam evaporation. Spreading resistanceprofile and scanning transmission-electron microscopy (TEM) were usedto detect the interface quality and microstructure, revealing that theinterface between the zirconium oxide films and top silicon was abruptand clear. The films kept to be amorphous up to the rapid thermaltemperature of 700°C for 300 s, but arriving at 700°C anunknown interfacial product appeared, which was probablyZrSixOy. High frequency capacitance-voltage (C-V)characteristics at 1 MHz performed on metal-oxide-SOI structurerevealed that this interfacial product exhibited good electricalproperties of zirconia thin films. When the annealing temperatureincreased from 600°C to 700°C, flat voltage VFBchanged from -2.451 to -1.741 eV, showing the improvement in thequality of the films. The cumulative region capacitance decreased from3.058×10-11F to 3.012×10-11F, indicating increasingequivalent oxide thickness, which is in agreement with the resultof high-resolution cross-sectional TEM.