Resonance Layer Research Articles

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.

A tunable terahertz broadband absorber based on patterned vanadium dioxide

A patterning vanadium dioxide (VO2) tunable broadband terahertz absorber is designed, which consists of a resonant layer of VO2 and a metallic reflector layer separated by a dielectric layer of Topas. VO2 is a phase-changing material that behaves as a metallic phase at high temperatures and an insulating phase at low temperatures. Different from conventional metal-dielectric-metal three-layer structure, the innovation of this paper is that the resonant layer adopts vanadium dioxide, which is a phase change material, not only does it increase the operating bandwidth but it also allows active tuning of the absorption amplitude by varying the temperature and hence the vanadium dioxide conductivity. By optimizing the width of the vanadium dioxide resonant layer square ring and the thickness of the dielectric layer, broadband absorption is achieved in the frequency range of 0.72–1.92 THz. Numerical simulation results show that VO2 with low conductivity then behaves as an insulating phase, and its peak absorption in the corresponding broadband absorption band is only 3%, so it can be used for the reflector. When VO2 has a high conductivity, its absorption is greater than 90% in the absorption bandwidth up to 1.2 THz. In addition, the proposed absorber is polarization insensitive under vertical incidence conditions because of the symmetry of the structure. The absorber can also maintain good absorption performance by changing the angle of incidence and polarization state. In the terahertz wave sector, the tunable terahertz absorber developed in this article has several potential applications, including detectors, sensors, stealth technologies, etc.

Quasi-optical design and analysis of a remote steering launcher for CFETR ECRH system

In order to optimize the operational safety and reliability of the upper launcher for the CFETR ECRH system, a design of the launcher for NTM control based on the remote steering concept is currently being carried out for comparison with the front steering equivalent. This paper presents the layout design and analysis of the quasi-optical system in the remote steering launcher. A 3D visual quasi-optical design tool has been developed for the quasi-optical system, which can parameterize modeling, perform general astigmatic beam calculation and show the accurate beam propagation path in the upper port. Three identical sets of quasi-optical modules are arranged in the launcher, and each one consists of two fixed double-curvature focusing mirrors, which focus and reflect the steering beams (−12°–12°) from two square corrugated waveguides. The beam characteristics at the resonance layer are described, and the average beam radius is < 100 mm. The peak head loads on the surfaces of the two fixed mirrors are 1.63 MW/m2 and 1.52 MW/m2. The position and size of the beam channel in the blanket are obtained, and the opening apertures on the launcher-facing and plasma-facing sides of the blanket module are 0.54 m2 and 0.4 m2, respectively.

X-band frequency selective surface with low loss and angular stability

A bandpass frequency selective surface (FSS) with low loss and angular stability based on three metal layers is proposed in this paper. The top and bottom resonant layers are coupled through a metal ring layer to build the FSS structure. The equivalent circuit model is derived to explain the operation principle of the proposed FSS. In the design, two transmission zeros (TZs) are generated at both sides of the passband contributing to the high selectivity performance. Furthermore, current distributions are given to analyze the generation mechanism of the TZs. The performance of the proposed FSS is insensitive to the polarization and angle of the incident wave. To verify the performance, a prototype of the FSS is fabricated, whose measured results agree with the simulated ones. The FSS shows low-profile of about 0.07λ (λ is the free space wavelength at the center frequency).

Stacked Chip-Based Terahertz Metamaterials and Their Application

A terahertz (THz) metamaterial design mechanism based on a stacked chip is proposed. Unlike the traditional sandwich-type metamaterial design mechanism based on the “resonant layer–dielectric layer–ground layer” structure, it adopts a stacked design of upper and lower metamaterial chips to achieve a new structure based on the “dielectric layer–resonant layer–air layer–ground layer” structure. This could break through the thickness limitations and construct an ultra-thin metamaterial upper chip. To verify the effectiveness of this method, we applied it to the field of THz perfect absorbers. We designed, simulated, and prepared a terahertz stacked chip-based perfect absorber with an upper-chip thickness less than 1/800 of the wavelength. Then, a reflective spectroscopy system based on a vector network analyzer is built to test the absorption performance. The measured results show that it has an absorptivity of 98.4% at 0.222 THz, which is in good agreement with simulations.

Design and analysis of terahertz multifunctional tunable metasurfaces based on single-layer square split ring

Aiming at the problems of single function, limited application range and complex structure of most metasurfaces, we propose a terahertz multifunctional tunable metasurface based on VO2-metal hybrid structure. The metasurface consists of three layers, from top to bottom, VO2-metal resonance layer, Topas dielectric layer and bottom gold film reflective layer. When circularly polarized light is incident, the metasurface can achieve broadband circular dichroism (CD) function, and the CD intensity generated at 3.19–6.24 THz is greater than 0.8, and the relative bandwidth is as high as 64.69%. When linearly polarized light is incident, the dual-band reversible linear dichroism (LD) function, broadband linear-to-circular polarized conversion function, and high-efficiency linear cross-polarization conversion function can be achieved by adjusting the polarization state of the incident light. Moreover, by adjusting the conductivity of VO2 and the state of the incident light, the adjustment of each functional intensity can be achieved. Finally, we believe that the proposed multifunctional metasurface has significant reference value in the terahertz range for constructing chiral photodetectors, CD metamirrors, intelligent switches, and polarization digital imaging systems.

Kinetic study of the bifurcation of resonant magnetic perturbations for edge localized mode suppression in ASDEX Upgrade

The correlation between the bifurcation of resonant magnetic perturbations (RMPs) to the unshielded state and edge localized mode (ELM) suppression in ASDEX Upgrade is studied using a kinetic plasma response model numerically and analytically. For the numerical studies, the linear kinetic Maxwell solver KiLCA for cylindrical geometry and the quasilinear transport code QL-Balance are used in combination with the ideal MHD solver GPEC to account for realistic tokamak geometry. Based on this modelling, a numerical local bifurcation criterion is introduced which estimates the effect of RMP-induced temperature plateau formation in the resonant layer. Its analytical form is derived in constant-psi approximation. The kinetic model reproduces the known gyrocenter resonance, Er=0 , and the electron fluid resonance. In contrast to MHD theory, the latter is located at the zero of the perpendicular electron fluid velocity computed only with half of the electron temperature gradient. The application of the criterion to experimental data shows a correlation between bifurcation and the ELM suppression phase. Moreover, an electron density limit is found resembling the one observed in experiments.

Numerical simulation of ultrashort-pulse reflectometry (USPR) on EAST

The microwave reflectometer is a popular non-intrusive plasma density diagnostic instrument on tokamaks that provides centimeter and millisecond level resolution. The ultrashort-pulse reflectometer (USPR) achieves plasma density measurement by emitting a chirped wave containing a broadband signal and measuring the time of flight from different frequency components. A USPR system is currently being built on EAST (Experimental Advanced Superconducting Tokamak) to meet the needs of diagnostic of the pedestal density evolution, such as high-frequency small edge-localized modes. In order to predict the density reconstruction of the EAST USPR system, this work presents a numerical simulation study of the beam propagation of the chirped wave of extraordinary waves (X-mode) in the plasma based on Python. The electron density profile has been successfully reconstructed by the reflection signal interpretation. The small gap between the right-hand cut-off layer and the electron cyclotron resonance layer, due to the low plasma density on the plasma edge, causes unexpected leakage from the transmitting microwave beam to the pedestal and the core region. This kind of ‘tunneling’ effect will cause the reflected signal to have energy loss in the low-frequency band. The study also discusses the influence of the poloidal magnetic field on the reflected signal. The spatial variation of the poloidal magnetic field will lead to the conversion between extraordinary (X) waves and ordinary (O) waves, which leads to energy loss in the reflected signals. The simulation results show that the ‘tunneling’ effect and the O-X mode conversion effect have little effect on the EAST USPR system. Therefore, the currently designed transmit power meets the working requirements.

The Optimization of a Segmented Cladding Fiber via the Response Surface Methodology Approach for a Large Mode Area

In this work, we have proposed and optimized a segmented cladding fiber (SCF) with a large mode area (LMA) consisting of a uniform core and a double cladding. The outer cladding of the SCF consisted of a periodic alternation of high- and low-refractive-index segments, while the inner cladding consisted of a resonant layer of rods surrounded by high-refractive-index rings. The three geometrical parameters chosen as design variables were the (a) doped rod refractive index differences, (b) refractive index differences of rings, and (c) ring thickness. Using the Box–Behnken approach, we selected thirteen different design cases and modeled them numerically using the finite element method (FEM). In order to optimize the features of the proposed fiber, such as the effective mode area (EMA), we applied response surface methodology (RSM). The EMA of the optimal SCF was significantly improved and markedly enlarged to about 706 µm2 at a wavelength of 1.550 µm. Different properties of the optimized double-cladding octo-wing SCF (DC-OW-SCF), such as confinement losses in the core mode and the first higher-order mode, were studied. The DC-OW-SCF offers the advantages of feasibility in fabrication using the powder-in-tube (PIT) method and provides the possibility of utilization in compact amplifier devices and high-power fiber lasers.

Optimization of the operational domain for ICRH scenarios in WEST from statistical analysis

In the WEST tokamak, the Ion Cyclotron Resonance Heating (ICRH) system plays a substantial role in increasing the plasma temperature. However, its efficiency can be lowered by two main phenomena: ripple-induced fast ion losses and technical limits on the antenna current. In this work, a new technique using IR thermography was employed to measure the flux of particles coming from the trapping of fast ions in the toroidal magnetic field ripple. These measurements provided the opportunity to fit parametric scaling laws in order to predict the ion flux intensity and the total power loss for experiments with a plasma current of kA and a major radius of the cyclotron resonance layer of m depending on the heating power and the line-averaged electron density. Furthermore, another semi-empirical parametric scaling was developed to evaluate the coupling resistance depending on controllable parameters such as the line averaged electron density and the radial outer gap between the separatrix and the ICRH antenna. These laws were used to define an operational domain from the database of previous experiments made during campaign C4. The settled operational domain suggests that high power ( MW) and high electron density ( m−3) discharges are suitable for optimized steady-state high confinement scenarios in WEST using ICRH.

Circuit design for doubling the toroidal magnetic field on the QUEST spherical tokamak

Various scenarios of electron cyclotron heating can be explored in a spherical tokamak with a high toroidal field. On the QUEST spherical tokamak, doubling the toroidal field results in the emergence of the fundamental resonance layer for 28 GHz electron cyclotron waves. The second-harmonic resonance layer shifts toward the plasma central area. To increase the coil current from 50 to 100 kA, a current source is designed with 125 modules in parallel. The module power supply comprises a bank of lithium-ion capacitors and a DC–DC buck converter. In the bench test of a single module, the current was found to increase linearly and maintained the flat-top as programmed.

Second-harmonic electron-cyclotron resonance heating: A possible impact from mode coupling near the resonance.

The short-wavelength paraxial asymptotic technique, known as Gaussian beam tracing, is extended to the case of two linearly coupled modes in plasmas with resonant dissipation. The system of amplitude evolution equations is obtained. Apart from purely academic interest, this is exactly what happens near the second-harmonic electron-cyclotron resonance if the microwave beam propagates almost perpendicularly to the magnetic field. Because of non-Hermitian mode coupling, the strongly absorbed extraordinary mode may partly transform into the weakly absorbed ordinary mode near the resonant absorption layer. If this effect is significant, it could impair the well-localized power deposition profile. The analysis of parameter dependencies gives insight into what physical factors affect the power exchange between the coupled modes. The calculations show a rather small impact of non-Hermitian mode coupling on the overall heating quality in toroidal magnetic confinement devices at electron temperatures above 200 eV.

Characterisation of electron cyclotron wall conditioning plasma in ASDEX Upgrade

Electron cyclotron wall condition (ECWC) discharges are characterised in ASDEX Upgrade with full tungsten plasma facing components and X2 polarised waves launched from the equatorial ports, relevant to ECWC conditions in ITER Pre-Fusion Power Operation phase 1. The characterisation of the deuterium plasmas is based on experimental inputs such as electron density measurements, in-vessel pressure measurements, poloidal field maps obtained from the measured coil currents, as well as advanced tomographic methods on camera images filtered at the hydrogen Balmer lines. TOMATOR-1D simulations and collective Thomson scattering radiometer spectra complement the findings. The cold, high density and partially ionized toroidal plasmas show significant levels of stray radiation. The measured radiation includes waves at half of the gyrotron frequency suggesting the occurrence of parametric decay instabilities at 2nd harmonic upper hybrid resonance that locates at the low field side of the resonance. A displacement of the plasma emission along the resonance layer is observed at higher discharge power in discharges with a vertical poloidal magnetic field only. By optimizing the poloidal field pattern, along with the location of the electron cyclotron heating (ECH) resonance, the strongest surface interaction regions for the charged particles can be controlled. Directing plasma flux to inner wall surfaces, and same for the inner divertor apron, is found less effective in ASDEX Upgrade due to magnetic mirror effects and outward convective flows. Modeling however predicts the presence of an intense and uniform flux of low energy atoms produced at the ECH absorption layer that may be effective for conditioning the high field side surfaces after use of the disruption mitigation system.

Analysis of transmitarray antenna elements using polarizability tensors

This paper presents an analytical approach to analyze transmitarray (TA) antenna elements using generalized boundary conditions (GBCs) and polarizability tensors. The proposed method analyzes multi-layer TA elements, which are composed of scatterers and dielectric substrates. The method combines an equivalent circuit model and network theory to evaluate total transmission matrix of multi-layer elements. Initially, each layer of scatterers is modeled by its collective polarizability tensor, which is calculated from individual polarizabilities of scatterers and their interactions. Next, analytical sheet admittance of layers is derived from their collective polarizabilities. Finally, total transmission matrix of multi-layer elements is calculated, taking into account the effect of resonant layers and dielectric spacers.This method determines the contribution of different items in the scattering properties of the elements separately, including the angle and polarization of the incident wave, the geometry and material of scatterers, and the array period. To validate the proposed model, several examples of multi-layer elements are analyzed, and the analytical results are compared with full-wave simulation results. A good agreement between them is achieved.The proposed approach is an efficient tool for the precise and fast design of transmitarray antennas, particularly where the effect of the incident angle needs to be considered.

Examination of High-Density Plasma Heating on the GAMMA 10/PDX Central Cell with 3D Wave Analysis Code

In GAMMA 10/PDX, ion heating is achieved using slow wave in the ion cyclotron frequency absorbed near the resonance layer. However, in high-density plasma, the slow wave is less likely to be excited due to the shielding effect. To overcome this challenge, researchers have investigated difference frequency (DF) waves excited by the nonlinear interaction of fast waves have been investigated. The fast waves can propagate to the core plasma even in high-density plasma making DF-waves an efficient option for ion heating. This study has suggested that a DF-wave with left-handed polarization, which is excited as a slow wave, is effective for ion heating in high-density plasma. The characteristics of the DF-wave were analyzed using a three-dimensional (3D) wave analysis code, TASK/WF3D, to investigate the ion resonance absorption of DF-wave heating. The results showed that when the antenna is positioned inside the core plasma to simulate DF-wave heating, ion resonance absorption is higher in high-density plasma than in low-density plasma. These results indicate that DF-wave heating is effective in high-density plasmas.

Multi-functional terahertz metamaterials based on nano-imprinting.

This paper reports a multi-functional terahertz (THz) metamaterial based on a nano-imprinting method. The metamaterial is composed of four layers: 4 L resonant layer, dielectric layer, frequency selective layer, and dielectric layer. The 4 L resonant structure can achieve broadband absorption, while the frequency selective layer can achieve transmission of specific band. The nano-imprinting method combines electroplating of nickel mold and printing of silver nano-particle ink. Using this method, the multilayer metamaterial structures can be fabricated on ultrathin flexible substrates to achieve visible light transparency. For verification, a THz metamaterial with broadband absorption in low frequency and efficient transmission in high frequency is designed and printed. The sample's thickness is about 200 µm and area is 65 × 65 mm2. Moreover, a fiber-based multi-mode terahertz time-domain spectroscopy system was built to test its transmission and reflection spectra. The results are consistent with the expectations.

Linear plasma device GyM for plasma-material interaction studies

GyM is a linear plasma device operating at Istituto per la Scienza e Tecnologia dei Plasmi, Consiglio Nazionale delle Ricerche, Milan, with the original aim of studying basic plasma physics, such as turbulent processes. Since 2014, GyM experimental program has been mainly focused on the issue of plasma-material interaction (PMI) for magnetic confinement nuclear fusion applications. GyM consists of a stainless steel vacuum chamber (radius and length of 0.125 m and 2.11 m), a pumping system, a gas injection system, 10 magnetic field coils and two magnetron sources at 2.45 GHz, capable of delivering a total microwave power up to 4.5 kW. Highly reproducible steady-state plasmas of different gas species, at a maximum working pressure of ∼10−1 Pa, can be obtained by electron cyclotron resonance heating in the resonance layer at 87.5 mT. Plasmas of GyM have electron and ion temperature ≤15 eV and ∼0.1 eV, respectively. The electron density is in the range of 1015–1017 m−3 and the ion flux is ≤5 × 1020 ions⋅m−2s−1. Main plasma diagnostics of GyM comprise Langmuir probes, an optical emission spectrometer, a mass spectrometer and a fast camera system equipped with an image intensifier unit. For the purpose of investigating the topic of PMI, GyM is provided with two sample exposure systems. Both are biasable at a negative bias voltage down to −400 V, to tune the energy of the impinging ions. One of them is also equipped with a heating lamp and can reach and sustain a temperature of 990 K for several hours, thus allowing to study the role of sample temperature during the plasma-material interaction. This contribution presents the layout of GyM, the diagnostics, the sample exposure systems and the typical plasma parameters. A brief overview of the main PMI activities carried out so far and a description of future machine upgrades are also given.

Kinetic theory of parametric decay instabilities near the upper hybrid resonance in plasmas

Parametric decay instabilities (PDIs) near the upper hybrid resonance layer are studied with a 1D framework. In a uniform plasma, the kinetic nonlinear dispersion relation of PDI is numerically calculated for parameters corresponding to electron cyclotron heating experiments at the ASDEX-U tokamak, in which O-mode radiation was converted to X-mode radiation by reflection from the high-field sidewall. The forward scattering processes driven by X-mode and linearly converted electron Bernstein waves (EBWs) are investigated and found to lead to a primary PDI where the pump waves decay into lower hybrid waves and sideband EBWs. A frequency shift of 930 MHz is obtained for the sideband EBWs in the primary PDIs. Subsequently, the sideband EBWs can decay into a low-frequency ion Bernstein quasi-mode (IBQM) and a secondary EBW, where the dominant forward scattering channel is the first-order IBQM with a frequency close to twice the ion cyclotron frequency. The decay channels obtained by numerical calculation can explain the characteristics of the signal observed in ASDEX-U experiments. The threshold of the pump electric field strength required to excite the primary PDI in the presence of plasma inhomogeneity is also estimated.

Active Control of Alfvén Eigenmodes by Externally Applied 3D Magnetic Perturbations.

The suppression and excitation of Alfvén eigenmodes have been experimentally obtained, for the first time, by means of externally applied 3D perturbative fields with different spatial spectra in a tokamak plasma. The applied perturbation causes an internal fast-ion redistribution that modifies the phase-space gradients responsible for driving the modes, determining, ultimately their existence. Hybrid kinetic-magnetohydrodynamic simulations reveal an edge resonant transport layer activated by the 3D perturbative field as the responsible mechanism for the fast-ion redistribution. 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.

Optimization of X-mode electron cyclotron current drive in high-electron-temperature plasma in the EAST tokamak

A discharge with electron temperature up to 14 keV has been achieved in EAST. Analysis of the electron cyclotron current drive (ECCD) efficiency at high electron temperature under EAST parameters is presented using C3PO/LUKE code. Simulation results show that the ECCD efficiency of X-mode increases with central electron temperature up to 10 keV and then starts to decrease above 10 keV, at a specific magnetic field and toroidal angle. The efficiency degradation is due to the presence of the third harmonic extraordinary (X3) downshifted absorption at the low field side (LFS); even the cold resonance of X3 mode is located outside the plasma. As the electron temperature increases from 5 to 20 keV, the X3 absorption increases from 0.9% to 96.4%. The trapping electron effect at the LFS produces a reverse Ohkawa current. The competition between the Fisch–Boozer current drive and the Ohkawa current drive results in a decrease in ECCD efficiency. ECCD efficiency optimization is achieved through two methods. One is to increase the toroidal angle, leading to X2 mode predominating again over X3 mode and the electron resonance domain of X2 mode moving far from the trapped/passing boundary. The second one is to increase the magnetic field to move away the X3 resonance layer from the plasma, hence less EC power absorbed by X3 mode.