Electron Beam Channel Research Articles

Trivelpiece–Gould modes and low-frequency electron–ion instability of non-neutral plasma

The frequency spectra of the Trivelpiece–Gould modes of a waveguide partially filled with non-neutral plasma are determined numerically by solving the dispersion equation. The modes having azimuthal number $m = 1$ are considered. The results are presented for the entire acceptable range of electron densities, magnetic field strengths, for different values of the charge neutralization coefficient. The Cherenkov resonance condition of an ion with a diocotron mode having a finite value of the longitudinal wave vector was studied. The characteristics of resonant low-frequency electron–ion instability caused by relative azimuth motion of electrons and ions in crossed fields and by the anisotropy of the distribution function of ions are discussed. Ions are created by ionization of residual gas in the plasma volume. Due to the anisotropy, instability occurs not only in the vicinity of the resonance, but also outside it. For typical values of plasma parameters in experiments, estimations of the frequency growth rate are given. A conclusion is drawn that this instability can be the cause of the low-frequency oscillations observed in linear devices with non-neutral plasma produced in an electron beam channel.

A novel all-metal metamaterial for constructing relativistic slow wave structure

In recent vacuum electronic devices, metamaterials have shown an obvious advantage of miniaturization. Due to increasing miniaturization demand, research of metamaterials in high-power microwave (HPM) field is now also a hotspot. For better applications of metamaterials, there are still two issues to be solved, the low space limit current of the structure and the low uniformity of the working electric field. To solve these problems, we construct a novel all-metal metamaterial structure by applying a 45° rotational arrangement in space and a four-support rod structure. This new metamaterial structure has a great uniformity of the axis electric field, and its electric field fluctuation is reduced from 8.6% to 2.8%. Based on this metamaterial structure, we proposed a novel relativistic slow wave structure, and its working mode has a negative dispersion characteristic. Using the method of expanding the electron beam channel, the space limit current of the relativistic slow wave structure is greatly improved to 12.3 kA, and the particle simulation method is used to prove the increase in the space limit current. In addition, it is found that this slow wave structure has higher coupling impedance, which provides a better working environment for an annular relativistic electron beam. Therefore, this metamaterial structure has great potential in the HPM field.

Cold Characteristics of Overmoded Ridge- and Groove-Loaded Folded Waveguides for High-Power Traveling Wave Tubes

The TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sub> high-order mode operation characteristics of the ridge-loaded (RL) and groove-loaded (GL) folded waveguide slow wave structures (FWG SWSs) with two electron beam channels are studied for high-power traveling wave tubes (TWTs) at the terahertz (THz) frequency regime. Operating in a higher order mode means that the SWSs can have larger structure dimensions to reduce the fabrication difficulty at the THz frequency band and use higher current to increase the output power. For FWG SWSs operating in the high-order mode, it is difficult to suppress backward wave oscillation (BWO) of fundamental mode. In this article, we proposed the RL and GL FWG SWSs to suppress BWO and studied the dispersion, coupling impedance, and transmission loss of them. The results show that the RL structure can substantially increase the start oscillation length of the interaction circuit and avoid using many circuit sections for spurious mode depression which ease the assembly process of the tube. The GL structure can broaden the bandwidth with high-order mode operation but with the ease of BWO of fundamental mode. The straight RL and GL waveguides are used to explain the above results, and the effect of electron beam channel position on coupling impedance is also studied.

Determination of atomic positions and polar direction in half-Heusler material Sb1-xSnxTi1-y-zHfyZrzCo using electron channeling

Variation of electron-induced x-ray emission with the crystal direction of an incoming electron beam (channeling) is used to study the crystal site preference of different elements in a promising thermoelectric half-Heusler material. This material has Sb, Co and Ti as the main constituting elements, with lower contents of Hf, Zr and Sn. It is confirmed experimentally, in accordance with previous assumptions, that Sb and Sn occupy one of the octahedral sites, 4a (0, 0, 0), Ti, Zr and Hf the other octahedral site, 4b (½, ½, ½), and Co atoms occupy the tetrahedral site, 4c (¼, ¼, ¼) in the cubic spacegroup F4¯3m with lattice constant a = 0.598 ± 0.001 nm. Furthermore, a strong asymmetry along the polar directions is observed, which is as it should be in a half-Heusler material, when the Co atoms have a strong preference for one of the two sets of tetrahedral sites. Such asymmetries can not be observed by kinematical diffraction according to Friedel’s law, thus demonstrating that the observation of element characteristic signals under channeling conditions is a unique technique for determining the sense of polar directions and distinguishing crystal symmetry alternatives in small crystal grains and nanocrystals.

Simplifying Electron Beam Channeling in Scanning Transmission Electron Microscopy (STEM).

Sub-angstrom scanning transmission electron microscopy (STEM) allows quantitative column-by-column analysis of crystalline specimens via annular dark-field images. The intensity of electrons scattered from a particular location in an atomic column depends on the intensity of the electron probe at that location. Electron beam channeling causes oscillations in the STEM probe intensity during specimen propagation, which leads to differences in the beam intensity incident at different depths. Understanding the parameters that control this complex behavior is critical for interpreting experimental STEM results. In this work, theoretical analysis of the STEM probe intensity reveals that intensity oscillations during specimen propagation are regulated by changes in the beam's angular distribution. Three distinct regimes of channeling behavior are observed: the high-atomic-number (Z) regime, in which atomic scattering leads to significant angular redistribution of the beam; the low-Z regime, in which the probe's initial angular distribution controls intensity oscillations; and the intermediate-Z regime, in which the behavior is mixed. These contrasting regimes are shown to exist for a wide range of probe parameters. These results provide a new understanding of the occurrence and consequences of channeling phenomena and conditions under which their influence is strengthened or weakened by characteristics of the electron probe and sample.

Simplifying Electron Beam Channeling in STEM

Simplifying Electron Beam Channeling in STEM

Space charge effect simulation at electrons channeling in laser fields

In this work we present simulation results for electron beam channeling in ponderomotive potential of laser fields, calculated with a newly created code for electron beam dynamics taking into account space charge effect. It is shown that the use of laser field allows the electron beam to be shaped including focusing and collimation.

Effects of Boundary Current on Electromagnetic Dispersion Characteristics for a Relativistic Electron Beam**supported by the National ITER Project Foundation of China (Nos. 2013GB106001 and 2013GB106003)

With betatron oscillation characteristics of the electron beam and ion channel effect taken into account, dispersion characteristics of electrostatic modes and TM modes for a relativistic electron beam guided by ion channel are studied. Dispersion relations are derived and solved numerically to investigate the dependence of the dispersion characteristics for electrostatic modes and TM modes on the betatron oscillation frequency and the ratio of the relativistic electron beam radius to the waveguide radius. The effects of the boundary current on the dispersion characteristic of the TM modes and the interaction between the betatron modes and TM modes are analyzed. When considering the boundary current, for a strong ion channel, a new low-frequency branch of the TM modes arises and the interaction frequency between the betatron modes and the TM01 modes is increased with the same parameters.

Coherent terahertz radiation generated from a square-shaped free-electron beam passing through multiple stacked layers with sub-wavelength holes

This paper presents terahertz (THz) coherent radiation from the interaction between a square-shaped electron beam and guiding wave mode in a multiple stacked layers with sub-wavelength holes structure. The simulated results show that such a guiding wave mode enhances the field intensity in the electron beam channel so that the interaction is more efficient than the electron beam-surface wave interaction in traditional grating and bi-grating structures. This electron-beam-driven THz source provides a way with much simpler fabrication processes and a larger square size electron beam rather than sheet beam. Furthermore, such a source can be integrated to construct a 2D source-array structure, which demonstrates a good opportunity to generate high power THz radiation.

Dynamics of Electron Beam Channeling in Single Atomic Column and in Crystals

Dynamics of Electron Beam Channeling in Single Atomic Column and in Crystals

Electron Beam Channeling in Single Atomic Column

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

140 GHz菱形微带曲折线慢波结构行波管的模拟计算

A rhombus-shaped microstrip meander-line slow-wave structure is proposed for use in a low voltage, wide bandwidth, moderate power and high efficiency millimeter-wave traveling-wave tube. The structure is evolved from the original V-shaped microstrip meander-line slow-wave structure. Compared with the conventional slow-wave structure, it is a kind of planar structure, whose fabrication can be easily realized by utilizing the technology of 2-D micro-fabrication. Whats more, the structure can realize beam-wave interaction with sheet electron beam, and no additional electron beam channel is needed. The electromagnetic characteristics and the interaction between the sheet electron beam and slow wave in the structure are obtained by utilizing the electromagnetic simulation software HFSS and the particle-in-cell code in CST Particle Studio, respectively. Our calculations indicate that, at a beam voltage of 7 kV and a beam current of 90 mA, the traveling wave tube based on the structure is capable of delivering several tens of watts output power with an interaction efficiency of 14.3％ and a transient 3 dB bandwidth of 18 GHz(ranging from 132 GHz to 150 GHz)

Research on the V-shape folded rectangular groove slow-wave structure

A novel slow-wave structure called V-shape folded rectangular groove waveguide is proposed. This structure evolves from a conventional rectangular groove waveguide bending the groove with V-shape along its longitudinal direction, and the gap between metal plates forms a sheet electron beam channel naturally. Compared with the traditional U-shape structure, it can increase the interaction area without changing good high-frequency properties, which can adopt the sheet electron beam with a larger area to acquire more output power. In this paper, the high-frequency properties of this structure are analyzed, the interaction circuit for the V-band TWT is designed and the PIC simulation is performed to predict the operating characteristics. From our calculations, this tube can produce average saturation output power over 1000 Watts in a frequency range from 58 GHz to 64 GHz when the cathode voltage and beam current are set to be 12.8 kV and 600 mA respectively. The corresponding saturation gain and electron efficiency can reach over 33 dB and 13.2% respectively.

Surface Channeling in Aberration-Corrected Scanning Transmission Electron Microscopy of Nanostructures

The aberration-corrected scanning transmission electron microscope can provide information on nanostructures with sub-Angström image resolution. The relatively intuitive interpretation of high-angle annular dark-field (HAADF) imaging technique makes it a popular tool to image a variety of samples and finds broad applications to characterizing nanostructures, especially when combined with electron energy-loss spectroscopy and X-ray energy-dispersive spectroscopy techniques. To quantitatively interpret HAADF images, however, requires full understanding of the various types of signals that contribute to the HAADF image contrast. We have observed significant intensity enhancement in HAADF images, and large expansion of lattice spacings, of surface atoms of atomically flat ZnO surfaces. The surface-resonance channeling effect, one of the electron-beam channeling phenomena in crystalline nanostructures, was invoked to explain the observed image intensity enhancement. A better understanding of the effect of electron beam channeling along surfaces or interfaces on HAADF image contrast may have implications for quantifying HAADF images and may provide new routes to utilize the channeling phenomenon to study surface structures with sub-Angström spatial resolution.

Interface structure and polarity of GaN/ZnO heterostructure

Gallium nitride (GaN) films were grown with and without lattice-matched (In,Ga)N buffer layers on the c(+) and c(-) faces of ZnO single-crystal substrates using molecular beam epitaxy, and their interface structures, including the relationship between the crystallinity of the GaN film and the polarity of the ZnO substrate, were investigated. Growth at a high temperature (e.g., 800°C) was made possible by using an (In,Ga)N buffer layer, which improved the crystallinity of the GaN films compared with that of GaN films grown directly on a ZnO substrate. Ion scattering, i.e., coaxial impact-collision ion scattering spectroscopy, and electron beam channeling, i.e., convergent beam electron diffraction, profiles revealed that the GaN films grown on c(+)- and c(-)-ZnO substrates with an (In,Ga)N buffer layer had a (0001) surface (i.e., c(+) polarity). That is, polarity inversion occurs when GaN film is grown with an (In,Ga)N buffer layer on a c(-)-ZnO substrate.

Intense wideband terahertz amplification using phase shifted periodic electron-plasmon coupling

We report efficient wideband amplification of terahertz frequency electromagnetic waves from sequential electron-plasmon coupling along a half-period staggered double grating structure. Numerical eigenmode calculations show that the fundamental plasmonic band has a strong symmetric TE-mode, with a significant longitudinal electric field component, that is confined to the electron beam channel. A particle-in-cell simulation analysis with a single frequency excitation, combined with a short pulse broad spectrum drive, confirms that this terahertz plasmonic circuit produces three or four orders of magnitude power amplification with up to 30% bandwidth and 3%–5.5% saturated energy conversion efficiency.

Simulation analysis of rectangular dielectric-loaded traveling wave amplifiers for THz sources

Nonlinear simulation results for a 220-GHz rectangular dielectric-loaded traveling-wave amplifier are presented. Simulations are used to check a linear theory that is developed by phenomenological introduction of an effective dielectric parameter for electron beam channel, and it is found that the rf power gains from Pierce three-wave theory and particle simulations are in reasonable agreement. It is shown that the rf power gain during initial beam-wave interaction is positive; the falling on the initial rf power profile, which has been thought to be the rf power transferred to the beam for bunching buildup (negative gain effect), is probably resulting from numerical errors. Beam-wave interaction mechanism is analyzed by examining the evolution of beam bunching centers. Influences of various parameters on amplifier performance are examined, and transverse space-charge effect is analyzed. A symmetric excitation scheme for rf couplers is proposed, and rf field jumps on the common intersection line of vacuum, dielectric, and metal wall, which were found in rf simulations, are explained theoretically.

Diagnosis and dynamics in a simple low energy medium current electron beam channel

We present a simple experimental setup and an associated method enabling both the non-destructive diagnosis and the calculation of the beam evolution in a low energy medium current electron beam channel, where the space-charge and emittance effects are comparable. The diagnosis makes use of an axially symmetric magnetic lens while a second lens is added to increase the flexibility in the beam processing. The paper emphasizes the three steps involved in the method: the evaluation of the lenses' magnetic field by numerical simulation, the beam diagnosis, and the computation of the beam envelope. The calculation of the magnetic field is based on the finite element method. Subsequently, the beam parameters at the electron source exit – emittance and cross-over radius and position – are found with the modified three gradient method. Finally, the beam dynamics are modeled with the K–V equation adapted for the particular case of axial symmetry. The results obtained in this paper can be used to optimize technological processes, such as welding, hardening, cladding, and surface alloying.

Analysis for the double staggered ladder circuit

A new type of mm-wave slow wave system, double staggered ladder circuit is analyzed in this paper, by means of “average cavity field method” and three-dimensional finite element method. The influence of the electron beam channel on the dispersion characteristics is discussed. An experimental research is made, the experimental results agree well with the theoretical one.

Radiation guiding in channeling beam x-ray laser by Bragg reflection coupling.

The effects of radiation guiding on an electron-beam channeling x-ray laser are investigated. An essential feature of this laser is distributed feedback induced by Bragg reflections. Using the Maxwell-Bloch scheme including the transverse Laplacian it is shown that reflection guiding and absorption guiding can avoid the diffraction of the amplified radiation out of the amplification medium for a beam radius in the range of 100--1000 A\r{}. This opens the possibility of considering a channeling x-ray laser with high current density ${(10}^{5}$ A/${\mathrm{cm}}^{2}$), low emittance, but with very small total current (10 \ensuremath{\mu}A), similar to the beam used in scanning electron microscopy. On the other hand, gain guiding can avoid radiation diffraction in such small beam systems, but only with a further increase in the requirements on the beam current density. It is noted that planar superlattice crystals with a periodic structure of the order of 100 A\r{} transverse to the beam propagation can be useful for radiation guiding.