High Frequency Branch Research Articles

Towards higher frequencies in a compact prebunched waveguide THz-FEL

Free-electron-lasers fill a critical gap in the space of THz-sources as they can reach high average and peak powers with spectral tunability. Using a waveguide in a THz FEL significantly increases the coupling between the relativistic electrons and electromagnetic field enabling large amounts of radiation to be generated in a single passage of electrons through the undulator. In addition to transversely confining the radiation, the dispersive properties of the waveguide critically affect the velocity and slippage of the radiation pulse which determine the central frequency and bandwidth of the generated radiation. In this paper, we characterize the spectral properties of a compact waveguide THz FEL including simultaneous lasing at two different frequencies and demonstrating tuning of the radiation wavelength in the high frequency branch by varying the beam energy and ensuring that the electrons injected into the undulator are prebunched on the scale of the resonant radiation wavelength.

Face super resolution with a high frequency highway

AbstractFace shape priors such as landmarks, heatmaps, and parsing maps are widely used to improve face super resolution (SR). It is observed that face priors provide locations of high‐frequency details in key facial areas such as the eyes and mouth. However, existing methods fail to effectively exploit the high‐frequency information by using the priors as either constraints or inputs. This paper proposes a novel high frequency highway () framework to better utilize prior information for face SR, which dynamically decomposes the final SR face into a coarse SR face and a high frequency (HF) face. The coarse SR face is reconstructed from a low‐resolution face via a texture branch, using only pixel‐wise reconstruction loss. Meanwhile, the HF face is directly generated from face priors via an HF branch that employs the proposed inception–hourglass model. As a result, allows the face priors to have a direct impact on the SR face by adding the outputs of both branches as the final result and provides an extra face editing function. Extensive experiments show that significantly outperforms state‐of‐the‐art face SR methods, is general for different texture branch models and face priors, and is robust to dataset mismatch and pose variations.

On the energetic particle-induced geodesic acoustic modes with finite-orbit-width effects

This study presents an analytical investigation of energetic particle-induced geodesic acoustic modes (EGAMs) within a gyro-kinetic model, incorporating finite-orbit-width (FOW) effects up to the second order. The inclusion of second-order FOW effects introduces two distinct types of energetic particle-wave resonances, occurring at ω=ωth and ω=2ωth , respectively, where ωth denotes the transit frequency of energetic particles (EPs). It is found that two unstable EGAM branches coexist: a low frequency branch (LFB) characterized by 0<ωLFB<ωt,maxh , and a high frequency branch (HFB) marked by ωt,maxh<ωHFB<2ωt,maxh . The instability of LFB primarily arises from the resonance ω=ωth , mainly introduced by first-order FOW effects. As a result, the instability of LFB always exists regardless of the presence or absence of second-order FOW effects, and is barely modified by these effects. In contrast, the instability of HFB is exclusively attributed to the resonance ω=2ωth induced by second-order FOW effects. Consequently, the HFB exhibits instability in the presence of these effects.

Frequency-guidance Collaborative Triple-branch Network for single image dehazing

Image dehazing is a crucial pre-processing step in many high-level vision applications, e.g., crowd surveillance, automatic driving, and remote sensing. Recently, learning-based methods have achieved promising performance by designing various convolutional neural networks (CNNs). However, most existing CNNs are based on a single feature branch and cannot fully utilize the low-frequency and high-frequency features of haze, which inevitably affects the quality of the restored image. To this end, we develop an effective Frequency-guidance Collaborative Triple-branch Network (FCT-Net) for image dehazing. It casts dehazing as a dual-frequency component restoration task and deals with pertinent features based on a triple-branch-like architecture. Specifically, our FCT-Net can be decomposed into four components: low-frequency branch (LFB), high-frequency branch (HFB), progressive fusion branch (PFB), and image reconstruction tail (IRT). The LFB aims to mine more contextual information with dual-path feature modulation block (DFMB), while the HFB aims to recover the missing structural detail via mixed attention residual block (MARB). PFB is a fusion branch built between LFB and HFB, where different feature streams (low- and high-frequency information) can be integrated together through continuous selective progressive fusion (SPF) modules. Finally, our IRT exploits all these derived features to generate the clear haze-free image. Furthermore, we utilize a multi-strategy loss to optimize our FCT-Net from the perspectives of pixel domain and feature domain, which can achieve better image recovery quality. Extensive experiments on different benchmark datasets show that our FCT-Net is able to get comparable results with other state-of-the-art methods.

Numerical investigation of the effects of symmetric and eccentric earthquake-induced pounding on accelerations and response spectra for two-storey structures

This paper examines the effect of symmetric and eccentric earthquake-induced pounding between adjacent two-storey structures on acceleration times histories and response spectra. The numerical investigation benefits from experimental data issued from an extensive shake table campaign involving two-storey steel–concrete composite structures and slab-to-slab contacts. It is shown that the high frequency branch of the floor response spectra is only related to the maximum impulse acceleration spike during time, characterized by its impulse value, contact duration and time-history shape. In order to numerically reproduce such characteristics, three-dimensional detailed models are set up by modeling the slab with shell elements or hexahedral elements. Previously developed explicit time integrator, the CD-Lagrange scheme, based on non-smooth contact dynamics approach, is employed for ensuring the contact conditions expressed in terms of velocity and impulse, without requiring any contact parameters such as contact stiffness, local damping and restitution coefficient. The eccentric pounding between slabs is simulated by introducing a small defect in the parallelism of the colliding slabs, as measured in experience. The results of eccentric and symmetric pounding, assuming elastic behavior for the steel and concrete materials, are compared with the experimental data in terms of acceleration time-histories and pseudo-acceleration response spectra. It is concluded that symmetric pounding predicts an acceleration spike with a more abrupt increase to the peak and a shorter contact duration in comparison to experimental data, leading to an overestimation of the high frequency branch of the response spectra. On the contrary, eccentric pounding achieves to reproduce the experimental acceleration time histories as well as response spectra up to 400 Hz. Finally, the robustness of the proposed numerical approach is underlined by changing the time step size of the CD-Lagrange scheme as well as by prospecting different relevant choices concerning the Rayleigh damping.

A forgotten participant in pore deblocking of zeolites: dicarbonate in NaMeA zeolites, Me = Na, K, Rb, Cs.

The dynamics of carbon dioxide, carbonate anion (CO32-), and dicarbonate anion (C2O52-) in NaKA zeolite is studied at the DFT GGA level using ab initio molecular dynamics (AIMD). We show the easy formation of C2O52- dicarbonate from the reaction between CO32- and CO2 at high CO2 loading and their equilibrium at low CO2 loading. We have found that the dicarbonate anion can contact up to six cations (Me+ and Na+, Me = Na, K, Rb, Cs), which could reduce the separation properties of NaMeA zeolites relative to CO2 mixtures. The K+ interaction with dicarbonate C2O52- species pushes the cation from 8R site in full analogy with the carbonate's deblocking studied earlier. The easy C2O52- formation in NaMeA is confirmed by modeling reaction of C2O52- formation at the DFT GGA (PBE-D3) and hybrid levels (B3LYP, HISS, HSE06) with cNEB. The calculated intensities for high and low frequency branches of valence vibrations in C2O52- are compared with calculated ones for Me2C2O5 molecules and known IR spectroscopic data in the NaMeA zeolites. This new mechanism of deblocking could be important for a wide family of narrow pore zeolites (CHA, RHO, KFI, etc.) at room temperature where the carbonates are observed in the IR spectra. The possibility of tricarbonate formation is discussed.

Coarse-to-Fine Feedback Guidance Based Stereo Image Quality Assessment Considering Dominant Eye Fusion

Considering that the human brain always follows a coarse-to-fine (low-to-high spatial frequency) visual processing and fusion mechanism, we propose a coarse-to-fine feedback guidance based stereo image quality assessment (SIQA) network which considers a coarse-to-fine feedback guidance and adaptive dominant eye mechanism. The proposed network consists of two main sub-network streams, each of which has three branches to extract low, middle and high spatial frequency information in parallel. To better realize the guidance of the high-level features in the low spatial frequency branch to the low-level features in the high spatial frequency branch, an information feedback guidance module (IFGM) is proposed, which realizes a top-down guidance mechanism in each sub-network stream. Simultaneously, according to the theory of ocular dominance in human visual system (HVS), we design an adaptive bi-directional parallax-based binocular fusion module (BPBFM), which synthesizes two types of fusion feature by taking the left and right view features as dominant eye input. Furthermore, in order to obtain the better perceptual quality of stereo images, we design a weighted fusion strategy to weigh the quality scores from the two types of fusion features obtained by using an ensemble model with two multi-layer perceptrons (MLPs). The experimental results on four public stereo image datasets show that the proposed method is superior to the mainstream metrics and achieves an excellent performance.

Runaway electron dynamics in Experimental Advanced Superconducting Tokamak helium plasmas

The generation of runaway electrons (REs) is observed during the low-density helium ohmic plasma discharge in the Experimental Advanced Superconducting Tokamak (EAST). The growth rate of hard x-ray (HXR) is inversely proportional to the line-average density. Besides, the RE generation in helium plasma is higher than that in deuterium plasma at the same density, which is obtained by comparing the growth rate of HXR with the same discharge conditions. The potential reason is the higher electron temperature of helium plasma in the same current and electron density plateau. Furthermore, two Alfvén eigenmodes driven by REs have been observed. The frequency evolution of the mode is not fully satisfied with the Alfvén scaling and when extension of the Alfvén frequency is towards 0, the high frequency branch is ∼50 kHz. The different spatial position of the two modes and the evolution of the helium concentration could be used to understand deviation between theoretical and experimental observation.

Vibration Measurements of the Gerbil Eardrum Under Quasi-static Pressure Sweeps.

Tympanometry provides an objective measurement of the status of the middle ear. During tympanometry, the ear-canal pressure is varied, while the response of the ear to sound pressure is measured. The effects of the pressure on the mechanics of the middle ear are not well understood. This study is a continuation of our previous work in which the vibration response of the gerbil eardrum was measured in vivo under quasi-static pressure steps. In this study, we delivered a continuous pressure sweep to the middle ear and measured the vibration response at four locations for six gerbils. Vibrations were recorded using a single-point laser Doppler vibrometer and glass-coated reflective beads (diameter ~ 40µm) at the umbo and on the mid-manubrium, posterior pars tensa and anterior pars tensa.The vibration magnitudes were similar to those in the previous step-wise pressurization experiments. Most gerbils showed repeatability within less than 10dB for consecutive cycles. As described in the previous study, as the frequency was increased at ambient pressure, the vibration magnitude on the manubrium increased slightly to a broad peak (referred to as R1) and then decreased until a small peak appeared (referred to as R2), followed by multiple peaks and troughs as the magnitude decreased further. The low-frequency vibration magnitude (at 1kHz) decreased monotonically as the pressure became more negative except for a dip (about 500Pa wide) that occurred between - 700 and - 1800Pa. The lowest overall magnitude was recorded in the dip at mid-manubrium. The vibration magnitudes also decreased as the middle-ear pressure was made more positive and were larger than those at negative pressures. R1 was only visible at negative and small positive middle-ear pressures, while R2 was visible for both positive and negative pressures. R2 split into multiple branches after the middle-ear pressure became slightly positive. No magnitude dip was visible for positive middle-ear pressures.The low-frequency vibration magnitudes at negative middle-ear pressures on the pars tensa were higher than those on the manubrium. R1 was not visible for large negative middle-ear pressures on the pars tensa. R2 appeared as a multi-peak feature on the pars tensa as well, and a higher-frequency branch on the posterior pars tensa appeared as a trough on the anterior pars tensa. The magnitude dip was not present on the pars tensa. The largest overall magnitude was recorded at the R2 peak on the posterior pars tensa.The results of this study expand on the findings of the step-wise pressurization experiments and provide further insight into the evolution of the vibration response of the eardrum under quasi-static pressures.

Static structure, collective dynamics and transport coefficients in the liquid Li-Pb alloy. An ab initio molecular dynamics study

Several static and dynamic properties of the liquid Li-Pb alloy at diverse compositions, have been calculated by means of ab initio molecular dynamics simulation study. This alloy has attracted much attention because of the finding of fast sound at the Li0.80Pb0.20 composition and also the technological interest of the the eutectic composition, Li0.17Pb0.83, as a component of the blanket in fusion reactors.Results are reported for total static structure factors, which are compared with the available experimental data. An additional analysis of the structure allows the quantification of the heterocoordinating tendencies in this alloy, which at the Li0.80Pb0.20 composition are largest and lead to a Pb-centered polyhedral structure, where, however, Li4Pb units are not present.Regarding the collective dynamics, the calculated partial dynamic structure factors exhibit side peaks indicative of propagating density fluctuations, including density fluctuation modes with phase velocity greater than the hydrodynamic sound velocity. Also, the longitudinal and transverse dispersion relations have been calculated and its different branches analysed. We find all the high frequency branches to behave as optic-like modes, contrary to other interpretations in terms of an acoustic-like fast sound mode.Some transport coefficients such as self- and inter-diffusion coefficients, shear viscosities and adiabatic sound velocities, have also been calculated. Finally, the obtained results for the electronic density of states clearly indicate the metallic character of the liquid LixPb1-x alloy.

Application of high frequency biasing and its effect in STOR-M tokamak

A pulsed oscillating power amplifier has been developed to apply high frequency biasing voltage to an electrode at the edge of the STOR-M tokamak plasma. The power amplifier can deliver a peak-to-peak oscillating voltage ±60 V and current 30 A within the frequency range 1 kHz–50 kHz. The electrode is located in the equatorial plane at radius ρ = 0.88. The frequency of the applied voltage has been varied between discharges. It is observed that the plasma density and soft x-ray intensity from the plasma core region usually increase at lower frequency regime 1 kHz–5 kHz as well as relatively higher frequency regime 20 kHz–25 kHz but seldom increase in between them. Increment of τ\\!E has been observed 40% and 20% for the frequency regimes of 1 kHz–5 kHz and 20 kHz–25 kHz, respectively, and τ\\!p increment is 25% for both frequency regimes. Transport simulation has been carried out using the ASTRA simulation code for STOR-M tokamak parameters to understand the physical process behind experimental observations at the higher frequency branch. The model is based on geodesic acoustic mode(GAM) excitement at resonance frequency associated with Ware-pinch due to the oscillating electric field produced by biasing voltage, which can suppress anomalous transport. Simulation results reproduce the experimental trends quite well in terms of the density, particle confinement, as well as energy confinement time evolution. All the results indicate that high frequency biasing is capable of improving confinement efficiently.

‘Magneto-elastic’ waves in an anisotropic magnetised plasma

The linear waves that propagate in a two fluid magnetised plasma allowing for a non-gyrotropic perturbed ion pressure tensor are investigated. For perpendicular propagation and perturbed fluid velocity a low frequency (magnetosonic) and a high frequency (ion Bernstein) branch are identified and discussed. For both branches a comparison is made with the results of a truncated Vlasov treatment. For the low frequency branch we show that a consistent expansion procedure allows us to recover the correct expression of the finite Larmor radius corrections to the magnetosonic dispersion relation.

Electromagnetic dispersion characteristics of a high energy electron beam guided with an ion channel

Taking self-fields into consideration, dispersion properties of two types of electromagnetic modes for a high energy electron beam guided with an ion channel are investigated by using the linear perturbation theory. The dependences of the dispersion frequencies of electromagnetic waves on the electron beam radius, betatron frequency and boundary current are revealed. It is found that the electron beam radius and betatron frequency have different influences on the electromagnetic waves dispersion behavior by compared with the previous works. As the boundary current is taken into account, the TM modes will have two branches and a low-frequency branch emerged as the new branch in strong ion channel case. This new branch has similar dispersion behavior to the betatron modes. For TE modes, there are two branches and they have different dispersion behaviors in strong ion channel case. However, in weak ion channel case, the dispersion behaviors for both of the low frequency and high frequency branches are similar.

Electromagnetic Waves Dispersion and Interaction of an Annular Beam-Ion Channel System in Plasma Waveguide

A linear theory for the electromagnetic properties and interactions of an annular beam-ion channel system in plasma waveguide is presented. The dispersion relations for two families of propagating modes, including the electrostatic and transverse magnetic modes, are derived. The dependencies of the dispersion behavior and interaction for different wave modes on the thickness of the annular beam and betatron oscillation frequency are studied in detail by numerical calculations. The results show that the inner and outer radii of the beam have different influences on propagation properties of the electrostatic and electromagnetic modes with different betatron oscillation parameters. In the weak ion channel situation, the two types of electrostatic waves, that is, space charge and betatron modes, have no interaction with the transverse magnetic modes. However, in the strong ion channel situation, the transverse magnetic modes will have two branches and a low frequency mode emerged as the new branch. In this case, compared with the solid beam case, the betatron modes not only can interact with the high frequency branch at small wavenumber but also can interact with the low frequency branch at large wavenumber.

Effects of passing energetic particles on geodesic acoustic mode

Effects of passing energetic particles on geodesic acoustic modes (GAMs) are investigated using the hybrid kinetic-fluid model. The local dispersion relation of GAM is derived by adopting the equilibrium distribution function for slowing-down energetic ions with a single pitch angle. The dependence of the distribution function on the poloidal angle is first taken into account and shows to play a crucial role in determining the instability criterion as well as the frequency of GAM, although the poloidal asymmetry is of order O(ϵ). A high frequency branch of GAM resonantly excited is always stable, and a low frequency branch could be unstable. The case of zero pitch angle is specifically discussed. This case is always responsible for stable modes when disregarding the poloidal asymmetry, but can be unstable when the poloidal asymmetry is considered.

Parametric excitation of axisymmetric toroidal electrostatic mode by drift wave turbulences

The axisymmetric toroidal electrostatic mode discussed in this paper refers collectively to the nearly ideal electrostatic fluid mode with zero toroidal mode number in magnetically confined toroidal plasmas like tokamak, including geodesic acoustic mode, sound waves and the so-called nearly zero-frequency zonal flow. Use is made of cold ion fluid model in the toroidal coordinate system with a circular cross section to develop the theory of parametric excitation for the three above mentioned modes systematically to the first order of inverse large aspect ratio, which ends up with the four following observations: (1) The density zonal flow is only associated with the excitation of the first harmonic cosine sound wave and is independent of the potential zonal flow. (2) The geodesic acoustic mode is the high frequency branch of the dispersion in the form of coupling between the first harmonic sine sound wave and the nearly zero-frequency zonal flow due to geodesic curvature, while the low frequency branch of the same dispersion is identified to be the ‘toroidally modified nearly zero-frequency zonal flow’. (3) Only a weak coupling exists between the second harmonic sine sound wave and the nearly zero-frequency zonal flow. (4) All cosine sound waves and sine sound waves beyond the second harmonic are decoupled to the nearly zero-frequency zonal flow. A Gaussian type of drift wave energy spectrum with only a few parameters is introduced for calculation. Emphasis is laid on the effects resulting from the finite radial spectrum width such as double Landau-singularity, which reveal a significant modification to the δ -spectrum, thus resulting in serious restriction to the parametric excitation of geodesic acoustic mode and nearly zero-frequency zonal flow. Also discussed is the possibility of excitation of density zonal flow in the high q region. Numerical results are presented graphically and discussed in the reasonable physical regime. It is indicated that the geodesic acoustic mode and the nearly zero-frequency zonal flow cannot be parametrically excited at the same radii, and that if the geodesic acoustic mode is parametrically excited, the density zonal flow is expectedly to be observed.

Er doped ZnO nanoplates: Synthesis, optical and dielectric properties

We report on synthesis of rare earth Er doped ZnO nanoplates via low cost wet precipitation method. The effect of varying Er doping concentration in the range from 3, 5 to 7 wt% on structural, optical and electrical properties of ZnO nanoplates has been successfully investigated. X-ray diffraction (XRD) studies show hexagonal wurtzite phase structure of prepared Er doped ZnO nanoplates. Scanning electron microscopy (SEM) reveals the growth of nanoplates like structures of the prepared samples. Further, the presence of Er ions doping in ZnO matrix has been confirmed by EDS measurement. A sharp and strong peak at around 435 cm−1 was observed in Raman spectra of nanoplates which correspond to the high frequency branch of the E2 mode of ZnO. The frequency dependence of dielectric constant (εr), dielectric loss (tan δ) and AC conductivity of ZnO nanoplates of different Er doping concentration were measured. The dielectric properties of pure and Er doped ZnO nanoplates follow a behavior based on Maxwell–Weigner model. The AC conductivity was found to decrease with Er doping.

Electronic and vibrational properties of vanadium-carbide nanowires

We have made an effort to understand the properties of transition metal carbide nanowires (NWs) and studied vanadium-carbide (VC) nanowires as a specific case. Different structures have been considered and their electronic and vibrational properties studied employing density functional theory. The effect of dimensionality is very well brought forth by these NWs, narrow/thinner structures have clear preference for magnetic state with sizeable magnetic moment at the V sites. As the thickness/width increases, the margin decreases and the magnetic moment disappears altogether for structures like square and rectangular NWs. The cohesive energy per atom increases with the increase in lateral dimensions of the NW, and it is about 88% of the bulk value for the rectangular NW, while it is only 50% for the linear chain. All the wires are conducting in nature, with the linear and zigzag wires having half-metallic character. Our calculations show that the V atoms decide the electronic and magnetic properties in these while compressibility, a mechanical property, is governed by the C atoms. The electron localization function beautifully illustrates the closeness of thicker/wider NWs to the bulk. It also reveals that electrons are highly localized around C atoms; however, the amount of charge transferred depends strongly on the structure of wire. The optical properties unfurl the impact of different spatial expanse in the cross section of NW in a nice way, e.g., ε2xx &amp;gt; ε2yy (ε2 is imaginary part of dielectric function) for all those with a larger expanse along X compared to Y and vice-versa. Thicker nanowires seem to be more suitable for optical applications. Site-resolved phonon density of states shows that presence of C atoms is responsible for high frequency branches. The heat capacity variation for various structures closely follows the magnitude of respective phonon density of states.

Dispersion relations of externally and thermally excited dust lattice modes in 2D complex plasma crystals

The dispersion relations of the externally and thermally (naturally) excited dust lattice modes (both longitudinal and transverse) in two-dimensional Debye-Yukawa complex plasma crystals are investigated. The dispersion relations are calculated numerically by taking the neutral gas damping effects into account and the numerical results are in agreement with the experimental data given by Nunomura et al. [Phys. Rev. E 65, 066402 (2002)]. It is found that for the mode excited by an external disturbance with a real frequency, the dispersion properties are changed at a critical frequency near where the group velocity of the mode goes to zero. Therefore, the high frequency branch with negative dispersion cannot be reached. In contrast, for the thermally excited mode, the dispersion curve can extend all the way to the negative dispersion region, while a “cut-off” wave number exists at the long wavelength end of the dispersion in the transverse mode.

Ferromagnetic resonance phenomenon of magnetic thin film under a perpendicular field

Regarding ferromagnetic resonance frequency as a function of the perpendicular external magnetic field, the ferromagnetic resonance phenomenon is investigated. The results show that for the case where the external magnetic field is parallel to the its plane and the magnetic thin film has a perpendicular uniaxial magnetiocrystalline anisotropy, magnetic resonance frequency is divided into two categories, i.e., low frequency branch and high frequency branch, their specific relation depends on the magnetic anisotropy of the magnetic film; when external magnetic field is perpendicular to the systemic plane, the magnetic resonance frequency displays only a branch with the change of external magnetic field. In general, the magnetic resonance frequency decreases with the increase of the external magnetic field nonlinearly and monotonically. However, when the ratio between the cubic magnetocrystalline anisotropic field Hk1 and uniaxial magnetocrystalline anisotropic field Ha is about 2/3 Hk1/Ha &lt; 1, the magnetic resonance frequency increases with the increase of external magnetic field, which is in good agreement with the experimental results. The results show that through magnetic resonance spectrum, the vertical magnetic anisotropy in magnetic thin film can be distinguished.