Resonance Layer Research Articles

Heating characteristic of electron Bernstein wave using high-field side X-mode injection in QUEST

Abstract A comparative investigation was conducted to assess the impact of high-field side (HFS) extraordinary (X) mode (HFS X) injection and low-field side (LFS) ordinary (O) mode (LFS O) injection of 8.2 GHz radio frequency (RF) power in the Q-shu University experimental steady-state spherical tokamak. In the case of the HFS X injection, the ratio of electron plasma frequency and RF frequency f pe / f RF was 1.3, indicating an overdense plasma, whereas in the LFS O injection, this ratio was 0.9. Distinctive features emerged in the electron temperature and density profiles between the two injection scenarios. In the HFS X injection, the profiles peaked between the fundamental electron cyclotron resonance (ECR) layer and the upper hybrid resonance layer. Conversely, in the LFS O injection, the peaks were situated near the 1st ECR layer. This implies effective excitation of the electron Bernstein wave (EBW) in the case of the HFS X injection, with the wave power being absorbed through the doppler-shifted ECR of the EBW. Density and temperature oscillations were observed only in the start-up phase of the HFS X injection, which may correlate with the presence of the left-hand cutoff of the X-mode. These findings will be contributed to understand the distinctive characteristics associated with plasma heating through EBW excitation in the HFS X injection.

Effect of parallel flow on resonant layer responses in high beta plasmas

Effect of parallel flow on resonant layer responses in high beta plasmas

Modeling of ion cyclotron resonance frequency heating of proton-boron plasmas in EHL-2 spherical tokamak

Ion cyclotron resonance heating (ICRH) stands out as a widely utilized and cost-effective auxiliary method for plasma heating, bearing significant importance in achieving high-performance discharges in p-11B plasmas. In light of the specific context of p-11B plasma in the EHL-2 device, we conducted a comprehensive scan of the fundamental physical parameters of the antenna using the full-wave simulation program TORIC. Our preliminary result indicated that for p-11B plasma, optimal ion heating parameters include a frequency of 40 MHz, with a high toroidal mode number like to heat the majority H ions. In addition, we discussed the impact of concentration of minority ion species on ion cyclotron resonance heating when 11B serves as the heavy minority species. The significant difference in charge-to-mass ratio between boron and hydrogen ions results in a considerable distance between the hybrid resonance layer and the tow inverted cyclotron resonance layer, necessitating a quite low boron ion concentration to achieve effective minority heating. We also considered another method of direct heating of hydrogen ions in the presence of boron ion minority. It is found that at appropriate boron ion concentrations ( ), the position of the hybrid resonance layer approaches that of the hydrogen ion cyclotron resonance layer, thereby altering the polarization at this position and significantly enhancing hydrogen ion fundamental absorption.

Electron cyclotron current start-up using a retarding electric field in the QUEST spherical tokamak

Abstract The plasma current start-up experiment is conducted through electron cyclotron (EC) heating in the QUEST spherical tokamak. During the EC heating, the application of a toroidal electric field in the opposite direction to the plasma current effectively inhibits the growth of energetic electrons. Observations show rapid increases in plasma current and hard x-ray count immediately following the cancellation of the retarding electric field. When a compact tokamak configuration maintains equilibrium on the high field side, along with the retarding field, it leads to effective bulk electron heating. This heating achieved an electron temperature of T e ≈ 1 keV at electron density n e > 1.0 × 1018 m−3. Ray tracing of the EC wave verifies that more power absorption into plasma through a single-pass occurs around the second resonance layer with higher values of electron density and temperature.

A perfect absorber based on a VO2-tunable Fabry–Perot cavity: an analysis of periodic oscillation absorption characteristics

Terahertz metamaterial absorbers (TMAs) have garnered significant attention as vital electromagnetic wave-absorbing devices. In this study, we designed a terahertz metamaterial absorber (TMA) utilizing an asymmetric Fabry–Perot nanocavity comprising vanadium dioxide (VO2), gold (Au), and polyimide. The TMA exhibits five perfect absorption peaks within 0.1 THz to 10 THz, with an absorption rate exceeding 97%, peaking at 99%. The absorption rate oscillates periodically between 0 and 1, and its oscillating absorption peak can be determined by f m ( fm=(2m+1)c0/(4tε2 ), while tunability of the absorption rate between 15% and 97% is achievable by adjusting the conductivity of (VO2) (2×102∼2×105S/m). The physical mechanism of the absorption peak was analyzed by simulation and compared with theoretical analysis. The results show that the absorption peak of the absorber’s absorptivity can be insensitive to polarization and has a wide absorption angle of 80% up to 40◦ or more. Importantly, the thickness of the absorber (VO2) layer can be calculated from the desired absorption frequency and dielectric constant of the interlayer medium, d=ε0/μ0/σ , thus reducing the need for redesigning different resonant layer patterns. This work provides a new perspective on terahertz absorber design.

Optimization of ECR assisted pre-ionization in GLAST-III via Multiphysics simulation

Abstract Electron cyclotron resonance (ECR) is a common pre-ionization technique to assist ohmic startup in tokamaks. In this research, ECR assisted pre-ionization in GLAST-III tokamak is comprehensively investigated via Multiphysics 3D COMSOL simulations. Optimal conditions for ECR microwave absorption at fundamental mode are determined to achieve reliable plasma startup. The effect of filled gas pressure on the ECR plasma parameters such as plasma density and plasma temperature is thoroughly investigated in the range of 0.08–1 Pa. The results reveal that the ECR absorption of the incident microwaves is significantly enhanced at low enough filled argon pressure. The behavior of absorbed microwave power is significantly changed for different filled gas pressures. In case of high gas pressure, the microwave power is deposited locally in front of the waveguide, whereas in case of low pressure (0.08 Pa), the microwave power is uniformly deposited along the resonance layer. Moreover, the experimental investigation extends to nearly identical operating conditions, confirming the ECR absorption. The experimental data aligns with the simulation results, collectively affirming the efficacy of optimized pre-ionization in the low gas pressure scenario (0.08 Pa) within the GLAST-III tokamak.

Thermotunable mid-infrared metamaterial absorption material based on combined hollow cylindrical VO2 structure

We have designed a metamaterial absorption material based on the VO2 phase transition effect. The aim is to realize thermally tunable broadband absorption in the mid-infrared band. The metamaterial absorption material is a three-layer structure. From top to bottom, the combined hollow cylindrical VO2 resonant layer, the SiO2 dielectric layer and the Ti substrate. In this work, we perform numerical simulations using the three-dimensional time-domain finite element difference method. The simulation shows that the absorption material's average absorption intensity is adjustable by temperature between 0.09 and 0.95. At a temperature of 342 K, the absorption material has an absorption bandwidth of 17.3 μm (6.43 μm to 23.73 μm) with an absorption rate above 90%. And the average absorption in this band range is 94.95%. And this band covers the wavelength of the atmospheric transparency window (8 μm∼14 μm), which suggests that our absorption material has great application prospects. In this work, we first analyze the electromagnetic field at the absorption material's resonant wavelength and explain the absorption material's absorption mechanism. Then, we investigated the absorption effect of the absorption material under the VO2 structure with different temperatures. And we explain temperature's effect on the absorption material's absorption effect by analyzing the electric field's change on the absorption material's surface at different temperatures for the same wavelength. The effect of the absorption material structural parameters on the absorption material absorption performance to determine the optimal structural parameters was then investigated. Finally, we investigate the absorption curves of the absorption material under different columnar VO2 layers and demonstrate the innovative and unique nature of the designed absorption material. The absorption material we designed has a simple structure and is easy to configure. And the absorption material has high average absorption rate and a wide absorption bandwidth. We therefore believe that the metamaterial absorption material has great application prospects in optoelectronic detection, infrared imaging and infrared stealth.

Polarization-insensitive terahertz ultra-wideband absorber with an actively tunable bandwidth

This study presents a novel, to our knowledge, polarization-insensitive terahertz absorber with an actively adjustable bandwidth. The absorber consists of a vertically stacked three-layer structure with target-shaped resonant layers. Its absorption bandwidth is dynamically controlled, exhibiting ultra-broadband characteristics in the metallic phase. The absorber achieves over 90% absorption from 1.8 to 31.7 THz, with a relative bandwidth of 178%. In the insulating phase, the device transforms into a narrowband absorber, showing a precise absorption peak within the 0 to 20 THz frequency range. Notably, this absorber demonstrates polarization-insensitive and wide-angle absorption properties, making it suitable for different application scenarios. Compared to traditional broadband absorbers, the absorber proposed in this study features an ultra-wide absorption bandwidth and switchable operating modes, enhancing its flexibility and adaptability. The excellent performance of the absorber indicates its suitability for various applications, such as sixth-generation (6G) wireless communication, security detection, imaging, military stealth technology, and energy harvesting.

Enhanced plasma ion heating by lasers in inhomogeneous external magnetic field

Particle–In–Cell simulations have been carried out to study the propagation of a laser pulse inside a plasma medium threaded with an external magnetic field. The external magnetic field is chosen to have space variations, which ensure that the laser pulse penetrates inside the plasma slab following the pass band dispersion relation of the X-mode. The spatial variation of the magnetic field is chosen such that the LH resonance is encountered within the bulk plasma medium. Enhanced deposition of laser energy to ions at the resonance layer has been demonstrated. This scheme would thus be useful to heat ions at any desired bulk region of the plasma.

Polarization and Incident Angle Independent Multifunctional and Multiband Tunable THz Metasurface Based on VO2.

Aiming at the limitations of single-functionality, limited-applicability, and complex designs prevalent in current metasurfaces, we propose a terahertz multifunctional and multiband tunable metasurface utilizing a VO2-metal hybrid structure. This metasurface structure comprises a top VO2-metal resonance layer, a middle polyimide dielectric layer, and a gold film reflective layer at the bottom. This metasurface exhibits multifunctionality, operating independently of polarization and incident angle. The varying conductivity states of the VO2 layers, enabling the metasurface to achieve various terahertz functionalities, including single-band absorption, broadband THz absorption, and multiband perfect polarization conversion for linear (LP) and circularly polarized (CP) incident waves. Finally, we believe that the functional adaptability of the proposed metasurface expands the repertoire of options available for future terahertz device designs.

Active control of Alfvén eigenmodes by external magnetic perturbations with different spatial spectra

Alfvén eigenmodes have been suppressed and excited in tokamak plasmas by (just) modifying the poloidal spectra of externally applied static magnetic perturbations. This effect is observed experimentally when toroidal spectra of n = 2, n = 4 as well as a mixed spectrum of n = 2 and n = 4 is applied. Under the n = 2 magnetic perturbations, the modes are excited or suppressed by modifying the coil phasing between the upper and the lower set of coils. Regardless of the absolute rotation, an even parity for the n = 4 perturbation is observed to reduce the amplitude of the Alfvénic instabilities, while an odd parity amplifies it. To combine the stabilizing (and destabilizing) effect of n = 2 and n = 4, a mixed spectrum is applied, finding similar reduction (and amplification) trends. However, the impact on the mode amplitude is more subtle, due to the reduced coil current required for a mixed spectrum. The signal level on the fast-ion loss detector is sensitive to the applied poloidal spectrum, which is consistent with Hamiltonian full-orbit modelling of an edge resonant transport layer activated by the 3D perturbative fields. An internal redistribution of the fast-ion population is induced, modifying the phase-space gradients driving the Alfvénic instabilities, and ultimately determining their existence. The calculated edge resonant layers for both n = 2 and n = 4 toroidal spectra are consistent with the observed suppressed and excited phases. Moreover, hybrid kinetic-magnetohydrodynamic (MHD) simulations reveal that this edge resonant transport layer overlaps in phase-space with the population responsible for the fast-ion drive. The results presented here may help to control fast-ion driven Alfvénic instabilities in future burning plasmas with a significant fusion born alpha particle population.

Photonic bound states in the continuum governed by heating.

A photonic crystal microcavity with the liquid crystal resonant layer tunable by heating has been implemented. The multiple vanishing resonant lines corresponding to optical bound states in the continuum are observed. The abrupt change in the resonant linewidth near the vanishing point can be used for temperature sensing.

Broadband terahertz absorber based on patterned slotted vanadium dioxide

This study aims to explore broadband terahertz absorbers based on metamaterials by introducing a design featuring patterned slotted vanadium dioxide. This absorber structure includes a vanadium dioxide resonator layer, an intermediate Topas dielectric layer, and an underlying metal reflector layer. To enhance the absorption bandwidth, the vanadium dioxide resonant layer is strategically patterned to optimize impedance matching. Simulation outcomes reveal that, under positive incidence, the absorption rate exceeds 90 % within the frequency range of 1.5–4.2 THz. The absorber demonstrates a substantial absorption bandwidth of 2.7 THz, centered at 2.85 THz, corresponding to a relative bandwidth of 94.7 %. Additionally, through modulation of the conductivity of vanadium dioxide, the absorption peak can be finely tuned, spanning from approximately 2.5 %–99 %. In addition, it's angularly symmetrical and exhibits absorption properties for wide-angle incidence. The tunable terahertz absorber designed in this paper simultaneously achieves high absorption over a wide frequency range, is dynamically tunable, and has a simple structure. From the application point of view, it can provide a way to realize detection, and imaging in the terahertz range.

Polarization and incident angle independent multifunctional tunable terahertz metasurface based on graphene

Motivated by the imperative demand for design integration and miniaturization within the terahertz (THz) spectrum, this study presents an innovative solution to the challenges associated with singular functionality, limited application scope, and intricate structures prevalent in conventional metasurfaces. The proposed multifunctional tunable metasurface leverages a hybridized grapheme–metal structure, addressing critical limitations in existing designs. Comprising three distinct layers, namely a graphene–gold resonance layer, a Topas dielectric layer, and a bottom gold film reflective layer, this terahertz metasurface exhibits multifunctionality that is both polarization and incident-angle independent. The metasurface demonstrates a broadband circular dichroism (CD) function when subjected to incident circularly polarized waves. In contrast, under linear incidence, the proposed design achieves functionalities encompassing linear dichroism (LD) and polarization conversion. Remarkably, graphene's chemical potential and the incident light’s state can be manipulated to tune each functional aspect's intensity finely. The proposed tunable multifaceted metasurface showcases significant referential importance within the terahertz spectrum, mainly contributing to advancing CD metamirrors, chiral photodetectors, polarization digital imaging systems, and intelligent switches.

Numerical study of minority ion heating scenarios in a spherical tokamak plasma

In this study, D(H) minority ion cyclotron resonance heating (ICRH) scenarios in Nan Chang spherical tokamak (NCST) were simulated using the full-wave code TORIC. NCST is a low-aspect-ratio (R/a = 1.67) spherical tokamak, with its core plasma parameters characterized by a magnetic field intensity of 0.36 T and a density of 1018 m−3. Our simulation results demonstrate that the ion cyclotron wave can penetrate the core plasma of the NCST more effectively with a lower toroidal mode number, indicating that resonant ions can absorb the wave energy efficiently. Furthermore, it is found that as the minority ion H concentration is increased, a noticeable decline in the left-handed electric field adjacent to the ion cyclotron resonance layer is observed. Optimal heating efficiency is attained when maintaining a minority ion H concentration within the range 5%–10%. The minority ion velocity distribution was simulated to estimate the tail temperature of minority-ICRH, which is expected to exceed 10 keV. The difference in the power efficiency with different plasma compositions [Ar(H) and D(H)] was also simulated. When the H-ion cyclotron resonance layer is located at the core plasma, the power-absorption fraction of H in Ar(H) plasma surpasses that of D and H combined in D(H) plasma under identical conditions. These simulations provide a crucial foundation and theoretical reference not only for NCST but also for other spherical tokamaks conducting ICRH experiments.

Multi-Layered Metamaterial Absorber: Electromagnetic and Thermal Characterization

Metamaterials, recognized as advanced artificial materials endowed with distinctive properties, have found diverse applications in everyday life, military endeavors, and scientific research. Starting from monolayer metamaterials, multilayer ones are increasingly researched, especially in the field of electromagnetic wave absorption. In this article, we propose a multilayer metamaterial-absorber (MA) structure comprising two resonant layers crafted with copper and FR-4 dielectric. The presented multilayer MA structure exhibited an absorption greater than 90% in a frequency range from 4.84 to 5.02 GHz, with two maximum absorption peaks at 4.89 and 4.97 GHz. The bandwidth of the multilayer MA surpassed that of the individual single-layer MAs, with extension fractions reaching 360% and 257%, respectively. Through the simulation and calculation, the field distribution and equivalent circuit model elucidated that both individual magnetic resonances and their interplay contribute significantly to the absorption behavior of the multilayer MA. The absorption of the proposed multilayer MA structure was also investigated for the oblique incidence in the transverse electric (TE) and transverse magnetic (TM) modes. In the TE mode, the absorption intensity of two maximum peaks was maintained at over 93% up to an incident angle of 40 degrees and dropped to below 80% at an incident angle of 60 degrees. In the TM mode, the absorption was more stable and not significantly affected by the incident angle, ranging from 0 to 60 degrees. An absorption greater than 97% was observed when the incident angle increased from 0 to 60 degrees in the TM mode. Additionally, the approach in our work was further demonstrated by adding more resonant layers, making 3- and 4-layer structures. The results indicated that the absorption bandwidths of the 3- and 4-layer structures increased by 16% and 33%, respectively, compared to the bilayer structure. Furthermore, we analyzed the thermal distribution within the MA to understand the dissipation of absorbed electromagnetic energy. This research offers valuable insight into the augmented MA through a multilayer structure, presenting the implications for microwave applications like electromagnetic shielding, as well as in the design of MAs for terahertz devices and technologies, including emission and thermal imaging. These findings contribute to the advancement of knowledge in enhancing the absorption capabilities across various frequency ranges, expanding the potential applications of metamaterials.

Using Planar Metamaterials to Design a Bidirectional Switching Functionality Absorber—An Ultra-Wideband Optical Absorber and Multi-Wavelength Resonant Absorber

This study aimed to investigate a bidirectional switching functionality absorber, which exhibited an ultra-wideband characteristic in one direction, while in the other direction it demonstrated the absorption of three different resonant wavelengths (frequencies). The fully layered planar structure of the absorber consisted of Al2O3, Zr, yttria-stabilized zirconia (YSZ), Zr, YSZ, Al, YSZ, and Al. The simulations were conducted using the COMSOL Multiphysics® simulation software (version 6.1) for analyses, and this study introduced three pivotal innovations. Firstly, there had been scarce exploration of YSZ and Zr as the materials for designing absorbers. The uses of YSZ and Zr in this context were a relatively uncharted territory, and our research endeavored to showcase their distinctive performance as absorber materials. Secondly, the development of a planar absorber with multifunctional characteristics was a rarity in the existing literature. This encompassed the integrations of an ultra-wideband optical absorber and the creation of a multi-wavelength resonant absorber featuring three resonant wavelengths. The design of such a multi-wavelength resonant absorber holds promise for diverse applications in optical detection and communication systems, presenting novel possibilities in related fields. Lastly, a notable discovery was demonstrated: a discernible redshift phenomenon in the wavelengths of the three resonant peaks when the thickness of YSZ, serving as the material of resonant absorber layer, was increased.

Experimental investigation of Rayleigh wave propagation in a locally resonant metamaterial layer resting on an elastic half-space

In this experimental investigation, we explore the propagation characteristics of surface Rayleigh waves in a Locally Resonant Metamaterial (LRM) layer positioned on an elastic half-space. The study focuses on characterizing the dispersion and attenuation properties of these waves and validating analytical and numerical models of the LRM. For practical purposes, we utilize a thin-plate sample and construct the LRM layer, featuring multiple rows of sub-wavelength resonators, by machining the resonators at one edge of the plate. Employing a piezoelectric transducer coupled to the plate and a laser vibrometer, we actuate and receive the surface-like waves propagating at the plate edge. Two resonant layer configurations, comprising 3 and 5 rows of resonators, corresponding to heights of ∼0.6λh and λh, where λh represents the reference wavelength of Rayleigh waves, are examined. The experimental observations reveal the hybridization of the fundamental surface mode at the resonant frequency of the embedded resonators, leading to the creation of a low-frequency bandgap. This bandgap, attributed to the local resonance mechanism, exhibits a remarkable attenuation of surface wave amplitudes. To support our experimental findings, we conduct both analytical and numerical studies. These analyses demonstrate the confinement of the lowest-order surface mode within the frequency ranges proximate to the resonators’ resonance. The insights gained from this experimental study contribute to the advancement of strategies for mitigating surface waves through the application of resonant metamaterials and metastructures.

Effects of electron viscosity on resonant layer responses to non-axisymmetric magnetic perturbations

The resonant field penetration to magnetic islands is the central MHD mechanism of non-axisymmetric plasma responses in a tokamak such as disruptive locking or favorable ELM stabilization. The resonant field penetration can be induced by any non-ideal processes as manifested in the delicate balance under the generalized Ohm's law. Here, we show that the viscous effects by electrons are not ignorable in the field penetration unlike previous presumption, even if the electron viscosity is as small as the square root of its mass compared to the ions. It is clear that its effects become only bigger if the electron viscosity becomes anomalously large. The work strictly follows the three-field model in the linear regime targeting the prediction of the onset of the field penetration and successfully extending it with electron viscosity and identifying new regimes. The results also indicate that the error field thresholds become more strongly dependent on plasma density than ones predicted in the linear regimes without the electron viscosity, which is consistent with experimental observations and thus a significant implication.

An all-composite sandwich structure with PMI foam-filled for adjustable vibration suppression and improved mechanical properties

A novel all-composite double-layer sandwich structure with tubular cores (DSST) is designed and fabricated to achieve the both of vibration suppression and enhancement of mechanical properties. The suppression effect of the proposed sandwich structure on the structural vibration is verified numerically and experimentally, and the mechanism of bandgap generation as well the structural wave propagation modes are revealed and analyzed. The anisotropy of the carbon fiber reinforced polymer (CFRP) is utilized to enables the intermediate resonant layer to exist a wide frequency adjustment range of vibration suppression without altering its geometrical parameters. Then, the improvement of structural vibration characteristics (i.e., natural frequencies and mode shapes) by filling the polymethacrylimide (PMI) foam in the DSST is discussed. And PMI foam-filling also leads to improved mechanical properties, out-of-plane compression tests are conducted to reveal the mechanism of mechanical enhancement, and it is found that the interaction effect of the foam filled in DSST in the axial direction enhances the compressive strength and the specific energy absorption (SEA) compared to the one without foam by 35.7 % and 26.2 %, respectively. In addition, the core configuration and the composite material preparation enable the proposed structure to outperform competing ones in terms of load-bearing capacity and bandgap characteristics.