Dipole Loops Research Articles

High-Q magnetic toroidal dipole resonance in all-dielectric metasurfaces

High quality (Q) factor toroidal dipole (TD) resonances have played an increasingly important role in enhancing light–matter interactions. Interestingly, TDs share a similar far-field distribution as the conventional electric/magnetic dipoles but have distinct near-field profiles from them. While most reported works focused on the electric TD, magnetic TDs (MTDs), particularly high-Q MTD, have not been fully explored yet. Here, we successfully realized a high-Q MTD by effectively harnessing the ultrahigh Q-factor guided mode resonances supported in an all-dielectric metasurface, that is, changing the interspacing between silicon nanobar dimers. Other salient properties include the stable resonance wavelength but a precisely tailored Q-factor by interspacing distance. A multipole decomposition analysis indicates that this mode is dominated by the MTD, where the electric fields are mainly confined within the dielectric nanostructures, while the induced magnetic dipole loops are connected head-to-tail. Finally, we experimentally demonstrated such high-Q MTD resonance by fabricating a series of silicon metasurfaces and measuring their transmission spectra. The MTD resonance is characterized by a sharp Fano resonance in the transmission spectrum. The maximum measured Q-factor is up to 5079. Our results provide useful guidance for realizing high-Q MTD and may find exciting applications in boosting light–matter interactions.

Design and evaluation of a wideband reflectarray antenna using cross dipole with double‐ring elements

This article reports the design and analysis of a wideband, single layer, and dual-polarized microstrip reflectarray for Ku-band applications. The array element is constructed using a crossed dipole enclosed by two cross dipole loops. The element is optimized to exhibit a linear phase response and wide reflection phase range. The reflection characteristic of the unit cell is analyzed using an infinite array approach and the results are presented. A 324-element Ku-band microstrip reflectarray antenna of size 20 × 20 cm2 is simulated, fabricated, and experimentally evaluated. The 2.9-dB gain-bandwidth and aperture efficiency of the reported reflectarray are 45.3% and 35%, respectively. The reflectarray operates in the frequency range of 10.6 to 16.8 GHz.

A low‐cost variable polarization reflectarray based on controllable wave‐path difference

AbstractA variable polarization reflectarray antenna is investigated in this communication. It consists of the dual‐layer three parallel dipole loops (TPDLs) to independently control the reflected spatial compensative phase and the phase difference of two orthogonal LP components. In this case, the proposed variable polarization reflectarray can transform a linear polarization (LP) mode into an arbitrary polarization mode by simply rotating the feeder and adjusting the wave‐path difference between the dual‐layer TPDL. A detailed operating mechanism of the variable polarization is introduced in this work. As an example, a 13 × 13‐element offset reflectarray is designed, fabricated, and measured. Four different polarization states of right‐hand circularly polarized (RHCP), 45°LP, left‐hand CP (LHCP), and 135°LP are measured and validated with a specific incident LP wave. The measured gains of 22 dB and 1.5‐dB gain bandwidths of 11% are achieved for all the states. This new reflectarray antenna is especially useful for low‐cost variable polarization applications.

Investigating Equations Used to Design a Very Small Normal-Mode Helical Antenna in Free Space

A normal-mode helical antenna (NMHA) has been applied in some small devices such as tire pressure monitoring systems (TPMS) and radio frequency identification (RFID) tags. Previously, electrical characteristics of NMHA were obtained through electromagnetic simulations. In practical design of NMHA, equational expressions for the main electrical characteristics are more convenient. Electrical performances of NMHA can be expressed by a combination of a short dipole and small loops. Applicability of equations for a short dipole and a small loop to very small normal-mode helical antennas such as antennas around 1/100 wavelengths was not clear. In this paper, accuracies of equations for input resistances, antenna efficiency, and axial ratios are verified by comparisons with electromagnetic simulation results by FEKO software at 402 MHz. In addition, the structure of the antenna equal to 0.021 λ is fabricated, and measurements are performed to confirm the design accuracy.

Bowing out of a dislocation from wall of persistent slip bands (PSB)

Abstract Edge dislocation segments bowing out of dislocation walls of persistent slip bands (PSBs) during cycling are experimentally well documented. Instead of dealing with the problem in its complexity a simplified computer simulation of this process is presented. The bowing out segment interacts with the wall of the PSB. The walls are represented by the elastic field of a cluster of edge dipolar loops. Mutual interactions between the loops in the cluster and an interaction between the loop cluster and the bowing out dislocation segments are incorporated in the simulation procedure. The dislocation segments deposited at the walls are trapped in the elastic potential valleys produced by the dipole loops. Our aim is to determine the local shear stress needed to bow out fully the edge dislocation segment from the PSB wall. The obtained result is in reasonable agreement with the local shear stress at the wall deduced from measurements.

THE ROLE OF ACTIVE REGION TOPOLOGY IN EXCITATION, TRAPPING, AND DAMPING OF CORONAL LOOP OSCILLATIONS

We investigate the role of magnetic field topology in dense coronal loop oscillation by the means of three-dimensional magnetohydrodynamic numerical simulations of two models of idealized active regions (ARs). The first AR model is initialized as a straight cylinder surrounded by the field lines of the same length and orientation. The second model consists of a potential dipole magnetic configuration and contains a loop with a higher density than its surroundings. Dipole field lines have position-dependent length and orientation in contrary to straight ones. We study different ways of excitation of transverse loop oscillations by an external pulse and a nearly eigenmode excitation implemented inside the loop. We find that perturbation acting directly on a single loop excites oscillations both in cylindrical and dipole loops. However, the leakage of the wave energy is larger in a curved loop compared to a straight loop. External excitation of the whole AR is efficient in the excitation of oscillation in the straight field configuration, but results in less efficient excitation in the case of dipole field. We show that excitation of collective motion of straight field lines having the same wave periods and planes of the oscillations requires much less energy than excitation of dipole field lines having position-dependent orientation and wave periods and being excited individually, not having a collective mode of oscillation. We conclude that coherent motion of straight field lines is one of the factors that decrease the energy leakage from an oscillating loop, while individual motions of dipole field lines require more energy from the source to produce the loop oscillations, and also lead to higher damping rate compared to the straight field case. We discuss Transition Region and Coronal Explorer (TRACE) observations of coronal loop oscillations in view of our theoretical findings. We show several examples of time signatures of transversal loop oscillations observed by TRACE that agree with numerical simulations of externally excited oscillations.

Simulation of dynamical interaction between dislocations and dipolar loops

The article describes a model of interaction dynamics between a dislocation and dipolar dislocation loops. The interaction is essential for dipolar dislocation structure formation in early stages of a hardening process. For the description of the dislocation curve a direct parametric approach is employed whereas the loops are treated as rigid objects. The model equations are solved approximately by means of the finite-volume method. Physically interesting phenomena can be captured by the model provided the simulation covers long time periods. The strong interaction between the dislocation and the loops causes growing nonuniformity of distribution of discrete nodes along the dislocation curve. This effect is balanced by two proposed types of tangential redistribution of the discrete nodes. The redistribution is tested in simulations of loop clustering.

A parametric simulation method for discrete dislocation dynamics

A new computer simulation method employed in discrete dislocation dynamics is presented. The article summarizes results of an application of the method to elementary interactions among glide dislocations and dipolar dislocation loops. The glide dislocations are represented by parametrically described curves moving in glide planes whereas the dipolar loops are treated as rigid objects. All mutual force interactions are considered in the models. As a consequence, the computational complexity rapidly increases with the number of objects considered. This difficulty is treated by advanced computational techniques such as suitable accurate numerical methods and parallel implementation of the algorithms. Therefore the method is able to simulate particular phenomena of dislocation dynamics which occur in crystalline solids deformed by single slip: generation of glide dislocations from the Frank-Read source, interaction of glide dislocations with obstacles, their encounters in channels of the bands, sweeping of dipolar loops by glide dislocations and a loop clustering.

Statistical foundation of continuum dislocation plasticity

A continuum model of mesoscale plasticity intended to cover dislocation pattering and size effects is proposed. The statistical approach to dislocation dynamics put forward by Groma et al. [Acta Mater. 51, 1271 (2003)] is applied to curved glide dislocations and dipolar dislocation loops appearing in a crystal deformed by single slip. Due to the very different size and mobile properties of the glide dislocations and the loops, the two entities are differently treated. The statistics of Groma et al. is directly applicable to the loops, which behave as rigid objects of restricted mobility. The glide dislocations are divided into groups, which in the averaging process lead to so-called single-valued dislocation fields. The averaged continuous distribution of the glide dislocations is treated as a superposition of these fields. The system of equations of the continuum model consists of (i) equations for the flow of glide dislocations of each single-valued field and the balance equation for the density of dipolar loops (these equations are derived from the discrete equations of motion and contain constitutive functionals representing short range interactions), (ii) the conservation of Burgers vector in the single-valued fields, (iii) the equations of continuum crystal plasticity, and (iv) the Orowan law for the shear strain rate caused by the glide dislocations. The possibilities for specification and evaluation of the constitutive functionals, namely, the short range correlations among dislocations (both the glide dislocations and the loops), are indicated.

Atomic force microscopy of the intense slip localization causing fatigue crack initiation in polycrystalline brass

For wavy slip materials like copper a recent nanotheory suggests that destabilization (splitting) of edge dipolar loops (EDLs) triggers formation of intense slip lines (ISLs), which nucleate persistent slip bands (PSBs) in a thermally activated process of cyclic saturation. The nanotheory also has implications for fatigue of planar slip materials like brass, which do not display saturation. Also has implications for polycrystalline brass (Cu–30% Zn) was cyclically strained at room temperature and constant plastic strain amplitude into the secondary hardening stage associated with nucleation of intense slip bands (ISBs). After unloading from compression and electropolishing to remove surface fatigue damage the specimen was loaded to its stress amplitude in tension. Slip lines formed during the tensile half-cycle were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) and the observations were repeated after an additional compressive half-cycle.

On the sweeping mechanism of dipolar dislocation loops under fatigue conditions

Dislocation dynamics simulations of the interaction between prismatic dipolar loops and a glide screw dislocation are presented. As a result of stress gradients generated by the glide dislocation, dipolar loops execute drift motion along their glide cylinders. Simulations of the combined motion of two dipolar loops in the stress field of a glide dislocation clearly show that inter-loop forces are essential in determining their spatial trajectories. Under cyclic stress conditions, it is shown that same-type dipolar loops drift monotonically in the same direction, and that their spatial clustering is largely determined by strong inter-dipolar binding forces. The investigated sweeping mechanism is discussed in terms of the prerequisites for the formation of persistent slip bands under fatigue loading conditions.

Mutual impedance of air-cored coils above a conducting plate

The change in mutual impedance ΔZ12 due to eddy-current induction is considered for an arbitrary pair of air-cored coils located above a conducting plate. A general expression is derived for ΔZ12 and applied to the specific case of cylindrical coils with rectangular cross-section. A series of closed-form expressions for ΔZ12 is presented for the geometries that cover all of the possible coil configurations for which the orientation of the individual coils is either normal or tangential to the plate. Expressions are also obtained for ΔZ12 in the dipole limit and the results are compared with earlier work for dipole loops. The validity of the theory is tested against experimental measurements for representative high-symmetry coil systems above an aluminium-alloy plate. The theoretical calculations and experimental results are in excellent agreement over the experimental frequency range 100 Hz–50 kHz. The significance of the results for eddy-current non-destructive inspection using driver–pickup coils is discussed.

Conformal invariant saturation

We show that, in onium–onium scattering at (very) high energy, a transition to saturation happens due to quantum fluctuations of QCD dipoles. This transition starts when the order α 2 correction of the dipole loop is compensated by its faster energy evolution, leading to a negative interference with the tree level amplitude. After a derivation of the one-loop dipole contribution using conformal invariance of the elastic 4-gluon amplitude in high energy QCD, we obtain an exact expression of the saturation line in the plane ( Y, L) where Y is the total rapidity and L, the logarithm of the onium scale ratio. It shows universal features implying the Balitskyi–Fadin–Kuraev–Lipatov (BFKL) evolution kernel and the square of the QCD triple Pomeron vertex. For large L, only the higher BFKL Eigenvalue contributes, leading to a saturation depending on leading log perturbative QCD characteristics. For initial onium scales of same order, however, it involves an unlimited summation over all conformal BFKL Eigenstates. In all cases, conformal invariance is preserved for the saturation mechanism based on dipole loops.

The annihilation by glide of prismatic loops in walls

Two mechanisms are conjectured for the non-diffusional refinement of prismatic dislocation loops. These mechanisms offer an alternative to the widely accepted concept of spontaneous disintegration below some critical dipole height. In one, a loop array is refined by reaction with a mobile dislocation having the same Burgers vector. Loop shrinkage then depends on the geometry of the array and on the position of the plane of incidence. In principle, any loop array can be refined via this mechanism provided that all the loops have the same sign. An alternative mechanism may take place in dense walls formed of randomly arranged dipolar loops of both signs. As the wall is densified by impacting dislocations that push pre-existing loops on their glide prism against each other, refinement occurs conservatively when two loops of opposite sign come into contact.

The sweeping of a dipolar loop by a glide dislocation in a PSB channel

The sweeping of small dipolar loops by a gliding dislocation is thought to be the main mechanism responsible for dislocation pattern formation during cyclic deformation. The present work extends a previous investigation where the individual sweeping event was calculated within the framework of isotropic linear elasticity for an infinite straight gliding dislocation and a rigid dipolar prismatic loop, Here, the motions of a flexible glide dislocation segment and a rigid loop of finite dimension are considered under stress in the channel of an idealized wall structure, The shape of the dislocation segment as well as the behavior of the dipolar loop is obtained from the numerical solution of the equations of motion and balance of forces for the two defects.

Self-organization model of localization of cyclic strain into PSBs and formation of dislocation wall structure

Development of dislocation structures in cubic metal crystals cyclically deformed in single slip is treated as a self-organization process. The proposed model consists of the crystal plasticity equations, the equation of motion of glide dislocations, and the balance equation for stored dislocations (dipolar loops). The model incorporates hardening by the loops, sweeping of the loops by glide dislocations, drift of the loops in the stress gradients, the mutual loop interaction, their generation and annihilation. By using analytical methods it is briefly shown that in the early stage of deformation the vein or extended wall structure is formed. In the saturation state when all newly formed dislocations are annihilated strain becomes localized. In the lamellae of concentrated slip (PSBs) the dislocation structure may be but need not to be rearranged. The criterion for rearrangement is specified. The transformation of veins to walls is a typical rearrangement in PSBs.

Annihilation of interstitial atoms to dislocations in neutron-irradiated Cu and Ni at high temperature

When Cu and Ni were irradiated at 300°C by neutrons in a reactor, interstitial clusters decorate dislocations at a low fluence such as 5×10 17 n/ cm 2 . Computer simulation suggests that this is due to the accumulation of interstitial atoms in the dislocation strain field and also the difficulty of absorption of interstitials to extended dislocation which existed prior to irradiation. Interstitial clusters accumulated along dislocations coagulated to form dislocation loops in Cu and dipole loops which were along 〈1 1 2〉 direction with a 〈1 1 0〉 Burgers vector in Ni. These newly grown loops and dipoles can absorb interstitial clusters due to the unextended nature of these dislocations, and cause disappearance of decoration of interstitial clusters. Dislocations grew to a bow-out shape and was pinned at voids.

MIC - a self deploying magnetically inflated cable system for large scale space structures

A new approach, termed MIC (Magnetically Inflated Cable) that enables large, lightweight very strong and rigid space structures is described. MIC would be launched as a compact package of coiled superconducting (SC) cables. After reaching orbit, the cables would be cryogenically cooled and electrically energized by a small power source. The resultant repulsion magnetic forces between the DC currents in the SC cables automatically cause the coiled launch package to self deploy into the final large space structure. The SC cables are held in place by a distributed network of high tensile strength tethers (e.g., Spectra material), creating a very stiff, rigid truss structure that strongly resists bending and torsional, etc. movements, without the need for gravity gradient stabilization. A linear quadrupole (LQ) MIC configuration is described that is suitable for large solar power satellites, space stations, space hotels, propellant tanks, manned Mars spacecraft, etc. The LQ has 2 long SC dipole loops, of horizontal width W, length L, and opposite magnetic polarity, which are vertically separated by distance W, producing a long truss structure of square cross-section (width W) with the 4 SC cables at the corners of the square. The SC currents are opposite in adjacent cables, yielding an outwardly directed net radial force on each cable. The ends of each SC loop experience outwards longitudinal forces. The magnetic forces are very strong, even for modest supercurrents. For example, a 4 meter square truss with I = 250 kiloamp has an outwards radial force of 220 kg per meter of cable. and 5250 kg outwards longitudinal force at the ends of each SC loop. The network of restraining tensile lines can support lightweight structures, including solar panels, propellant tankage, habitat modules, power transmission lines, etc. The design of a 1 kilometer long, 4 meter square cross section MIC truss for solar power satellites is described. The MIC launch package fits within the length/weight constraints of the shuttle bay, and includes all of the helium coolant lines, thermal insulation, and refrigeration equipment required.

Elastic model for the sweeping of dipolar loops

Abstract The sweeping of small dipolar loops by gliding dislocations is thought to be one of the mechanisms leading to dislocation pattern formation during cyclic deformation. The individual sweeping event was calculated within the framework of isotropic linear elasticity for an infinite straight gliding segment and a rigid dipolar prismatic loop. With respect to previous estimates, the present one includes the finite size of the loop and the character of the gliding dislocation. The conditions for sweeping were quantitatively estimated as a function of the geometrical factors and the local effective and friction forces. The sweeping mechanism is found to be effective for distances between the slip planes of the two defects smaller than 9 times the height of the dipolar loop.

Elastic model for the sweeping of dipolar loops

Elastic model for the sweeping of dipolar loops