Low-frequency Plasma Oscillations Research Articles

Fine-tunable ε′-negative response derived from low-frequency plasma oscillation in graphene/polyaniline metacomposites

Remarkable ε′-negative responses at radio-frequency endow metacomposites with revolutionary applications in electromagnetic (EM) fields. However, uncovering its regulation mechanism and subsequently achieving fine-tunable ε′-negative response remains challenging. Here, we meticulously designed graphene/polyaniline (GR/PANI) metacomposites above percolation, which directly enable tunable negative permittivity at an order of magnitude of −103. By rationally adjusting the three-dimensional GR network, we were able to form collective low-frequency plasma oscillation of free electrons in composites, which intrinsically contributes to the tunable ε′-negative response. Moreover, an ε′-near-zero (ENZ) response was surprisingly observed at ∼920 MHz due to the ineffectiveness of the plasmonic state at high frequencies. In addition, we carried out equivalent circuit analysis to demonstrate the phase relationship and electrical characteristics of these metacomposites. This work could contribute to a new category of polymer-matrix composites and enable exotic properties beyond their natural and bulk counterparts.

Highly tunable ε'-negative and ε'-near-zero response at extremely low frequency region of CaCu3Ti4O12/graphitized-MWCNT metacomposites

CaCu3Ti4O12/graphitized-multiwall carbon nanotube (CCTO/g-MWCNT) nanocomposites were designed according to classical percolation theory. It is found that the ε'-negative response correlates with free carrier density and percolative g-MWCNT networks. The ε'-near-zero response was achieved at ∼802 kHz, which could be explained by the electrical dipole resonance of interconnected g-MWCNT clusters. Once the percolative g-MWCNT networks are formed in nanocomposites, Drude-type ε'-negative response is obtained at kHz region due to the low-frequency plasma oscillation. This work provides a design paradigm of epsilon-negative property by ceramic nanocomposites which will facilitate their practical application of epsilon-negative materials on dielectric ceramics, metamaterials, and electromagnetic shielding.

Graphitized-MWCNT/CaCu3Ti4O12 metacomposites for tunable ε′-negative and ε′-near-zero response with enhanced electromagnetic shielding

Metacomposites with extraordinary electromagnetic (EM) parameters recently offered a brand-new strategy for EM shielding, but still suffered from the unclear paradigm and regulation mechanism for ε′-negative and ε′-near-zero response. Herein, we presented an effective paradigm of binary composites consisting of CaCu3Ti4O12 (CCTO) and graphitized-multiwall carbon nanotube (g-MWCNT) which was designed by percolation theory. The ε′ switched from positive to negative with increasing frequency was obtained in g-MWCNT/CCTO composites, which could be explained by electrical dipole resonance of interconnected g-MWCNT clusters. Once the percolated g-MWCNT network is formed, Drude-type ε′-negative response would be obtained due to the low-frequency plasma oscillation. Epsilon-near-zero effect (at ∼385 MHz and ∼135 MHz) were tuned by adjusting g-MWCNT content, which respectively corresponds to the loss peaks. The EM medium of CCTO/g-MWCNT composites with ε′-negative or ε′-near-zero response showed great EM shielding performance at specific frequency region. The g-MWCNT/CCTO metacomposites provide a classical paradigm of ceramic matrix composites for ε′-negative and ε′-near-zero response which will guide the practical application of metacomposites in dielectric ceramics and metamaterials, and promote the next steps in EM shielding field.

Rotating spoke frequency in E × B penning source with two-ion-species plasma

In E B sources with different degrees of magnetization between electrons and ions, collisionless Simon–Hoh instability is prone to be induced, which may result in long-wavelength and low-frequency plasma oscillations along the azimuthal direction known as the rotating spoke. Recent studies have shown that the inertial response of unmagnetized ions is a key factor that affects the rotating spoke frequency, and in-depth studies on the correlation between the ion mass and spoke frequency have been conducted for various types of E B sources (e.g. Hall thruster, magnetron and Penning sources). In the case of the E B Penning source where the generation of desired ion species is maximized through molecular gas discharge, the intrinsic characteristics of the instability dependent on the ion mass inevitably require consideration of the coexistence of various ion species. In this work, we experimentally studied the spoke in the E B Penning source with two-ion-species plasma. By adjusting the mixture ratio of argon and krypton gases, we investigated the changes in instability frequency with electrical diagnostics. Two frequency peaks were clearly observed, with the peaks of lower and higher frequencies corresponding to mode numbers 2 and 3, respectively. The frequencies of each peak shifted to higher values when the argon content in the gas mixture was increased. Finally, the theoretical scaling of the spoke suggests that the changes in the frequencies are proportional to the inverse square root of the effective ion mass.

Real-time state estimation of low-frequency plasma oscillations in Hall effect thrusters

An extended Kalman filter (EKF) is developed to estimate unobserved states and parameters in plasma dynamical systems. Physical constraints are satisfied by adapting the process and measurement noise covariances to account for consistency between the estimates and the physical processes. First, the EKF is tested using the Lorenz system to demonstrate the robustness of the EKF with sparse measurement data. Then, the capabilities of the EKF are applied to investigate discharge current oscillations in a Hall effect thruster. It is demonstrated that the dynamics of the electron temperature can be estimated using the discharge current fluctuation as the measurement data. The propagation of the uncertainties of such estimates is also quantified.

Communication—Dielectric Dispersion of Chromium Carbide/Copper Calcium Titanate Metacomposites: Epsilon-Negative, Epsilon-Near-Zero, and Inductive Character

Chromium carbide/copper calcium titanate (Cr3C2/CCTO) metacomposites were fabricated, wherein epsilon-negative and epsilon-near-zero (ENZ) properties were simultaneously obtained. When Cr3C2 content exceeds percolation threshold, the formation of Cr3C2 networks triggers a well-pronounced epsilon-negative behavior. The epsilon-negative property originates from low-frequency plasma oscillation which mainly correlates with concentration of free electrons in Cr3C2 networks. Epsilon-negative is remarkably achieved at absolutely weak value (∼−102) at 30 wt% content of Cr3C2, and negative permittivity was tuned to ∼−103 by increasing Cr3C2 content. Transition of permittivity from positive to negative was accompanied by electrical character changing from capacitive to inductive.

Tailorable epsilon-negative and epsilon-near-zero behavior of TiC/CCTO metacomposites: Low-frequency plasma oscillation

Epsilon-negative ([Formula: see text] < 0) and epsilon-near-zero (ENZ) property was demonstrated in titanium carbide/copper calcium titanate (TiC/CCTO) metacomposites. Benefiting from the moderate concentration of free electrons in TiC filler and its adjustable three-dimensional (3D) networks, weakly negative permittivity ([Formula: see text]200 at 20 MHz) was achieved. Not only that, tailoring the negative permittivity of metacomposites from [Formula: see text]200 to [Formula: see text]2060, [Formula: see text]4200, [Formula: see text]14000 and [Formula: see text]70000 at 20 MHz was realized by simply increasing TiC content. Besides, Drude model was used to explain the radio-frequency (RF) negative permittivity and quantified the collective plasma oscillation in TiC networks.

Low-frequency plasma oscillations in transient conditions of an accelerator with an anode layer

The results are the research of oscillations discharge voltage and ion current of an accelerator with an anode layer (AAL), working without a cathode-compensator. The ion beam flows into the region, occupied by a volume charge and created by its own particles of the beam. Oscillogram received the oscillations of the plasma discharges (AAL) from this operating mode. This mode called the transient mode. Based on the analysis of the obtained oscillograms, an explanation of the reason for the development of oscillations. The effect of power supply voltage ripple is established (AAL) on the development of oscillations discharge voltage and current of the ion beam in the transient (unstable) mode of the single-stage ion accelerator with an anode layer (AAL), working without a cathode-compensator.

Simultaneous epsilon-negative and mu-negative property of Ni/CaCu3Ti4O12 metacomposites at radio-frequency region

Metacomposites with simultaneous epsilon-negative and mu-negative property have drawn tremendous attentions due to their promising applications in electromagnetic field. Herein, Ni/CaCu3Ti4O12 (Ni/CCTO) metacomposites were prepared by low-temperature pressureless sintering method. It was observed that Ni chains were gradually interconnecting to each other with increasing Ni content, and a percolation behavior was observed. Epsilon-negative property was obtained above percolation threshold attributing to the synergistic effect of induced electric dipole and low-frequency plasma oscillation in Ni networks, which was explained by the superposition of Drude and Lorentz model. Mu-negative property was observed above ∼34 MHz and ∼120 MHz for 60 wt% and 70 wt% Ni/CCTO composites respectively, which may originate from the combined effects of diamagnetic response, domain wall motion and gyromagnetic spin rotation. Hence, simultaneous epsilon-negative and mu-negative property at radio-frequency region was realized in Ni/CCTO metacomposites. Besides, the composites presented hopping conduction behavior, and then changed into metal-like conduction behavior with increasing Ni content. The impedance response of Ni/CCTO composites clarified the correlation between epsilon-negative and inductive character. This work could broaden the scope of metacomposites and pave its way to potential applications of electromagnetic shielding, novel antenna, and high-k capacitors.

Tunable radio-frequency negative permittivity of Carbon/CaCu3Ti4O12 metacomposites

Abstract In this paper, metacomposites of carbon nanotubes/CaCu3Ti4O12 (CNTs/CCTO), carbon blacks/CaCu3Ti4O12 (CBs/CCTO), and CNTs-CBs/CCTO were fabricated by low-temperature sintering process. The CNTs-CBs building blocks were fabricated by one-step electrostatic self-assembly method. In carbon/CCTO metacomposites, magnitude of negative permittivity ranged from 103 to 100 while its band-width ranged from 300 MHz to 1 GHz. An electrical percolation phenomenon was verified in ac conductivity spectra, suggesting the transition of conductive mechanism. Negative permittivity was ascribed to low-frequency plasma oscillation in carbon networks, corresponding to inductive character. The pendulum-like simple harmonic motion (SHM) model was applied to clarify the negative permittivity behavior. Accumulating of carbon clusters or networks in composites resulted in colossal dielectric loss (10−1

Epsilon-negative behavior of BaTiO3/Ag metacomposites prepared by an in situ synthesis

Epsilon-negative behavior in metamaterials has attracted increasing attention recently. In this work, excessively percolating BaTiO3/Ag composites were synthesized by an in-situ preparation method to achieve negative permittivity. The epsilon-negative behavior is explained by low-frequency plasma oscillation of free electrons, which can be well fitted by the Drude model. Moreover, the negative permittivity of composites can be tuned by adjusting the content of randomly distributed Ag particles. The ac conductivity of excessively percolating BaTiO3/Ag composites shows a typical metal-like behavior. Furthermore, the investigation of reactance reveals that the inductive character derived from the conductive network is the main contributor to negative permittivity, as confirmed by the equivalent circuit analysis. This work provides a theoretical recipe for designing negative permittivity, supplements the family of metamaterials, and would further facilitate the practical applications of epsilon-negative materials in electromagnetic devices.

Wavelet analysis of low frequency plasma oscillations in the magnetosheath of Mars

Wavelet analysis was employed to identify the major frequencies present in the plasma oscillations in the Martian magnetosheath. We have selected magnetosheath crossings and applied the Morlet wavelet transform to the electron density and temperature data, obtained from the Analyzer of Space Plasmas and Energetic Atoms experiment (ASPERA-3), onboard the Mars Express (MEX). In a study of 9667 magnetosheath crossings observed from 2005 to 2016, we have found 22,912 frequencies with high wave power between 5 and 60 mHz for the electron density data, For about 2/3 of the high wave power periods (63.3%), the most energetic frequencies were identified in the range 5–20 mHz. The analysis of the electron temperature data led to similar results, with 61.9% of the highest power frequencies in the 5–20 mHz range. No significant influence of the solar cycle on the frequencies of ULF waves has been observed in our analysis. The frequencies obtained here are near the O+ gyrofrequency. The main fact that controls the ion gyroradius is the interplanetary magnetic field that is compressed around the planet.

Electromagnetic Properties of Fe50Co50/Cu Granular Composite Materials Containing Flaky Particles

Electromagnetic properties of Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> /Cu hybrid granular composite materials containing flaky Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> particles and flaky Cu ones have been studied in the RF to microwave frequency range to realize the double negative (DNG) material. The electrical percolation along the dispersed Cu particle chains takes place at the critical Cu particle volume fraction φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c_Cu</sub> = 0.07; the σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> value of composites with φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c_Cu</sub> and above was more than 10-1 S/cm. The negative permittivity caused by the low-frequency plasma oscillation of conduction electron in percolated Cu particle chains was observed above φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cu</sub> = 0.08; the hybrid composites with φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cu</sub> 0.08 are in the low-frequency plasmonic state. Meanwhile, the negative permeability due to the magnetic resonances was obtained above φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cu</sub> = 0.10. Therefore, in the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> /Cu flaky particle composites with φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cu</sub> 0.10, the DNG characteristics were realized by the low-frequency plasma oscillation and magnetic resonances in the RF to microwave frequency range. In the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> /Cu flaky particle composites, the DNG spectrum was observed at relatively low Cu particle content and the DNG frequency band becomes broad compared to the Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> /Cu hybrid composites containing spherical or arborized shape particles.

Tunable and weakly negative permittivity at radio frequency range based on titanium nitride/polyethylene terephthalate composites

Metacomposites have been induced widespread concern in the realization of negative permittivity. In this paper, composites with titanium nitride (TiN) particles homogeneously dispersed in polyethylene terephthalate (PET) resin were prepared by high energy ball-milling and adhesive hot-pressing method. The influences of TiN networks in composites on the electric and dielectric properties were investigated in detail. With the formation of conductive TiN networks, the conductance mechanism changed from hopping conduction to metal-like conduction. The tunable and weakly negative permittivity in the radio frequency range was obtained in TiN/PET composites by adjusting the frequency range and volume fraction (v) of TiN in composites. Negative permittivity behavior raises from the low-frequency plasma oscillation of free electrons in TiN networks, which could be analyzed by the Drude model. The impedance of TiN/PET composites were investigated by the equivalent circuit analysis, demonstrating the capacitive or inductive of the composites. This paper shows an effective way toward the tunable and weakly negative permittivity, which will promote practical applications of metacomposites in electromagnetic shielding and impedance matching fields.

Numerical study of low-frequency discharge oscillations in a 5 kW Hall thruster

A two-dimensional particle-in-cell plasma model is built in the R–Z plane to investigate the low-frequency plasma oscillations in the discharge channel of a 5 kW LHT-140 Hall thruster. In addition to the elastic, excitation, and ionization collisions between neutral atoms and electrons, the Coulomb collisions between electrons and electrons and between electrons and ions are analyzed. The sheath characteristic distortion is also corrected. Simulation results indicate the capability of the built model to reproduce the low-frequency oscillation with high accuracy. The oscillations of the discharge current and ion density produced by the model are consistent with the existing conclusions. The model predicts a frequency that is consistent with that calculated by the zero-dimensional theoretical model.

Functional nano-units prepared by electrostatic self-assembly for three-dimension carbon networks hosted in CaCu3Ti4O12 ceramics towards radio-frequency negative permittivity

Carbonaceous materials served as functional units have trigged tremendous fundamental and practical interest in preparing electronic composites especially in meta-composites. In this paper, carbon functional nano-units were built by electrostatic self-assembly of carbon nanotubes (CNTs) and carbon black (CB). Three-dimension carbon networks hosted in CaCu3Ti4O12 (CCTO) matrix were constructed by randomizing CNTs/CB nano-units. Negative permittivity was observed when increasing CNTs/CB nano-units contents exceeded the percolation threshold. The negative permittivity behavior considered as deriving from the low frequency plasma oscillation of delocalized electrons in carbon networks was analyzed by Drude model and equivalent circuit model. Besides, the electrical properties influenced by percolating phenomenon including ac conductivity, dielectric loss, and impedance were investigated. This work presents a novel meta-composites with negative permittivity built by CNTs/CB nano-units filling in low-temperature sintering CCTO matrix which extends the fields of meta-composites and will greatly facilitate its practical applications.

Iron Granular Percolative Composites toward Radio-Frequency Negative Permittivity

The immense potential of metamaterials with negative electromagnetic parameters has propelled recent research and development. Here, the two-phase composites with Fe or insulated PVP-coated Fe powders dispersed in epoxy resin were prepared by blending and hot-molding procedure. Microstructures, ac conductivity, permittivity, and permeability of the resulted composites were investigated at 10 MHz-1 GHz region. Three-dimensional conductive networks were constructed when the Fe content exceeds the percolation threshold, where the conduction mechanism changed from hopping conduction to metal-like conduction. Negative permittivity behavior was attributed to low-frequency plasma oscillation of delocalize electrons explained by Drude model. Besides, the diamagnetism of current loops induced by external magnetic field lead to the negative magnetic susceptibility. After Fe particles were coated by insulated PVP, permittivity was positive and there is no longer obvious diamagnetic phenomenon, indicating that insulating layer changed the conductive path in composites. This work opens up a promising strategy to tune negative permittivity and permeability which will facilitate applications in electromagnetic cloaking, absorbing, attenuation and so on.

The importance of the cathode plume and its interactions with the ion beam in numerical simulations of Hall thrusters

Numerical simulations with a 2-D axisymmetric multi-fluid plasma code illustrate the significance of the near-plume interactions in investigations of the anomalous electron transport in Hall thrusters. In our simulations, the transport of electrons is modeled using an anomalous collision frequency, νanom, yielding νanom ≈ ωce (i.e., the electron cyclotron frequency) in the near-plume region. We first show that restricting the anomalous collision frequency in this region to only within the ion beam, where the current density of ions is large, does not alter the plasma discharge in the Hall thruster as long as the interaction between the beam and the cathode plume is captured properly. These simulations suggest that electron transport occurs largely inside the beam. A second finding is on the significance of accounting for the ion acoustic turbulence (IAT), now known to occur in the vicinity of the cathode exit. We have included in our simulations a model of the IAT-driven anomalous collision frequency based on Sagdeev's model for saturation of the ion-acoustic instability. This implementation has allowed us to achieve excellent agreement with experimental measurements in the near plume of the H6 Hall thruster. Low frequency plasma oscillations similar in both magnitude and frequency to those found in the H6 thruster are recovered in our simulations when the model for the anomalous collision frequency in the cathode plume is included.

Double negative electromagnetic property of granular composite materials in the microwave range

The double negative (DNG) electromagnetic property, i.e. the simultaneous negative permittivity and permeability, of granular composite materials has been studied in the microwave frequency range. The negative permittivity spectrum can be realized by the low frequency plasma oscillation which is generated in the percolated metal particle chain as well as the dielectric resonance of the induced dipole in the isolated metal particle clusters. Meanwhile, the negative permeability spectrum can be obtained by the magnetic resonance of the embedded ferromagnetic particles in the granular composite structure. By combining these negative electromagnetic properties, the DNG characteristics can be produced in the granular composite material. The DNG properties of the Cu/Yttrium Iron Garnet (Cu/YIG) granular composite materials under external magnetic field will be presented; the negative refractive index of the Cu/YIG composite material will also be discussed.

Longitudinally propagating arc wave in the pre‐onset optical aurora

We present the first systematic observational evidence for a traveling periodic structure in the pre‐onset optical aurora – the longitudinally propagating arc wave (LPAW) – associated with flapping oscillations in the magnetotail. The LPAW is characterized by azimuthally moving intensity enhancements inside auroral arcs as seen by THEMIS ground‐based all‐sky imagers. It travels westward in the pre‐midnight auroral sector during the 10–20 minutes preceding auroral breakup with a velocity of 2–10 km/s, time period 40–110 s, and wavelength 250–420 km. Magnetically conjugate measurements by THEMIS satellites show low frequency plasma oscillations consistent with the parameters of the arc wave in the course of current sheet thinning. When mapped into tail, wavelength (4800–9400 km) and velocity (70–190 km/s) of the LPAW are compatible with observations and theoretical predictions for current sheet flapping motions. Our results strongly suggest that LPAW is an auroral footprint of the drift wave mode (kink, sausage, ballooning, etc.) in a stretched magnetotail.