Large Beam Research Articles

Generation of Airy beam arrays in real and K spaces based on a dielectric metasurface.

Airy beams are widely used in various optical devices and optical experiments owing to their unique characteristics such as self-acceleration, self-recovery, and non-diffraction. Here we designed and demonstrated a metasurface capable of encoding two phase distributions independently in dual circular polarization channels. We experimentally observed the generated Airy beam arrays loaded on the metasurface in the real and K spaces. Compared with the traditional method, such method provides a more efficient solution to generate large capacity Airy beam arrays with switchable working modes in the broadband spectrum. The results may pave the way for the integration and miniaturization of micro-nano devices and provide a platform for information processing, particle manipulation, space-time optical wave packets, and Airy lasers.

A large beam high efficiency radio frequency neutron spin flipper.

A design for a radio frequency (RF) neutron spin flipper obtained from magneto-static and neutron spin transport simulations is presented. The RF flipper constructed from this design provides a flipping probability of 0.999 or better for a beam size 6cm wide and 15cm high and a wavelength band between 0.4 and 0.6nm. Three permanent magnet guide field sections with air gaps provide a linear field gradient along the beam propagation direction over a large cross-sectional area. An RF oscillator based on coupling the resonant coil of a Hartley oscillator to the excitation coil was developed, which provides a higher current and, thereby, a larger RF amplitude, as compared to a conventional RF power amplifier. Two opaque He3 neutron spin filters were employed to measure the flipping probability of the flipper with very high precision. A spatially uniform flipping probability of 0.9995(2) or higher was measured over the large cross-sectional area neutron guide. This RF neutron spin flipper will be employed in a polychromatic beam reflectometer at the National Institute of Standards and Technology Center for Neutron Research. This design can be applied to other polarized neutron instruments or applications requiring a very high continuous flipping probability of the neutron spin for a large cross-sectional area beam.

Strain demand of elastic pipes subjected to permanent ground displacements using the finite difference method

Long-distance pipelines are one of the primary means of oil and gas transportation. During the construction process, long-distance pipelines are inevitably buried across geohazard zones, which potentially generate permanent ground displacements. These ground displacements can potentially induce excessive strains in the pipe posing a great threat to the pipe’s safety and integrity. In this study, a new numerical methodology for the response analysis of pipes subjected to ground displacements is proposed based on the finite difference method. Simulating the pipeline as a large deformation Euler Bernoulli beam, the finite difference method is used to solve the two interacting nonlinear differential equations of equilibrium in the longitudinal and lateral directions considering the nonlinear pipe-soil interaction induced by the ground displacement. Implemented using the nonlinear equation solver of FindRoot by Mathematica for solving nonlinear equations, the longitudinal strain along the pipeline can be subsequently derived, and the tensile and compressive strain demands can be therefore determined for engineering reference. Finally, the applicability of this method is validated based on two hypothetical study cases involving symmetric and non-symmetric soil resistance on the lateral direction of the pipe. Comparing the results with the finite element analysis solver Abaqus, we demonstrate that this present methodology has excellent predictive capabilities. Our study is carried out for elastic response calculation, but the proposed method shows a great promise for further development involving material nonlinearity, which is appropriate for the preliminary safety evaluation for the design of new pipelines or for risk prescreening of existing pipelines.

Double-focusing geometry for phase correction in neutron resonance spin-echo spectroscopy

Neutron spin phase correction is an essential technique for achieving a high energy resolution in neutron spin-echo experiments. We propose a double-focusing geometry in which two identical ellipsoidal mirrors are arranged in series. This arrangement enables us to correct the path length differences for large divergent beams using a complete ellipsoidal focusing mirror. In this study, we evaluated the effectiveness of this geometry both analytically and numerically. A Fourier time of 1 μs is achievable using this geometry.

Accuracy and reproducibility of effective atomic number and electron density measurements from sequential dual energy CT.

This study assesses the accuracy of effective atomic number (Zeff ) and electron density measurements acquired from dual energy CT and characterizes the response to clinically relevant variables representative of challenges in patient imaging, including: phantom size, material position within the phantom, variation over time, off-center positioning, and large cone beam angle. The Gammex Multi-Energy CT head and body phantoms were used to measure Zeff and electron density from 35 rod inserts that mimic tissues and varying concentrations of iodine and calcium. Scans were performed on a Canon Aquilion ONE Genesis CT scanner over a period of 6months using default dual energy protocols appropriate for each phantom size. Theoretical Zeff and electron density values were calculated using data provided by the phantom manufacturer and compared to the measurements. Sources of variance were separated and quantified to identify the influences of random photon statistics, ROI placement, and variation over time. A subset of measurements were repeated with the phantom shifted in the vertical and horizontal directions, and over all slices in the volumetric scan. All measurements showed strong correlation (r>0.98) with their corresponding theoretical values; however, the system did demonstrate a bias of -0.58 atomic units in the body phantom and 0.28 atomic units in the head phantom for Zeff measurements. The mean absolute percent error (MAPE) was 6.3% for the body phantom and 3.2% for the head phantom. Electron density measurements of the body and head phantoms gave MAPE values of 4.6% and 1.0%, respectively. Zeff and electron density measurements significantly varied within the solid water background, showing a positional dependence within the phantom that dominated the total standard deviation in measurements. Zeff values dropped by 0.2 atomic units when the phantom was off-center; electron density measurements were less affected by phantom position. Along the z-axis, the accuracy drops off markedly at more than 50-60mm from the central slice. The Canon dual energy system offers an accurate way of measuring the Zeff and electron density of clinically relevant materials. Accuracy could be improved further by calibration to remove bias, careful attention to centering within the FOV, and avoiding measurements at the edges of the cone beam.

Fatigue Performance of Double-Layered Asphalt Concrete Beams Reinforced with New Type of Geocomposites.

The reinforcement of asphalt layers with geosynthetics has been used for several decades, but proper evaluation of the influence of these materials on pavement fatigue life is still a challenging task. The presented study investigates a novel approach to the reinforcement of asphalt layers using a new type of geogrid composite, in which square or hexagonal polypropylene stiff monolithic paving grid with integral junctions is bonded to polypropylene non-woven paving fabric. The laboratory fatigue tests were performed on large asphalt concrete beams reinforced with the new type of geocomposite. Unreinforced samples were used as reference. Test results were analysed in several aspects, including the standardised approach based on stiffness reduction, but also using energy dissipation. The effect of reinforcement on pavement fatigue life was also estimated. Based on the obtained final results of fatigue life calculations, it can be concluded that the evaluated geogrid composites have an evident positive effect on pavement performance and have a significant potential to extend the overall pavement life, especially in the case of hexagonal grid.

Observing flat wavefront formation with diffusive transport in microwave graphene with topological insulator protected edges

The Dirac singularity in 2D graphene structures theoretically allows pseudo-diffusive transport of photons or phonons, under which condition the energy propagates omnidirectionally, while a constant phase velocity is automatically guaranteed, enabling the formation of a uniform wavefront with a large beam aperture. Although similar phase-reconstruction effects were discussed for phononic graphene, none reported observing or taking advantage of the similar effect in microwave/photonic graphene. The challenge lies in the bulk edges where evanescent Bloch states emerge and interfere with the desired propagating Bloch state. We find that by adopting valley-constrained photonic topological insulator (PTI) protected borders and PTI waveguide, the proper modes are preserved, and we are able to experimentally observe the uniform wavefront generated in an all-dielectric microwave graphene bulk within a few wavelengths. This practically provides a possibility of developing low-profile devices with high directivity such as PTI horns or lenses.

On the geometrically exact formulations of finite deformable isogeometric beams

We present a set of advanced analytical formulations that facilitates the accurate analysis and efficient implementation of finite deformable thin Kirchhoff–Love beams. This paper enhances the prevailing differential geometry based large deformation beam models by producing geometrically exact formulations for initial curvatures, non-zero force tangents and external stiffness matrix contributions of spatial beams. Though it is not analytically merged in existing beam models, initial curvatures of beams have a significant influence on the integration of forces over beam cross-sections. We reveal this influence through the systematic deduction of the Jacobian in volume integrals of beam forces. Also, this paper demonstrates the applicability of follower loads on beams with necessary adjustments to the global Hessian matrix. We adopt the isogeometric analysis formalism in beam body discretisation and algorithmic implementation of the presented formulations.

Fast Beam Training With True-Time-Delay Arrays in Wideband Millimeter-Wave Systems

The best beam steering directions are estimated through beam training, which is one of the most important and challenging tasks in millimeter-wave and sub-terahertz communications. Novel array architectures and signal processing techniques are required to avoid prohibitive beam training overhead associated with large antenna arrays and narrow beams. In this work, we leverage recent developments in true-time-delay (TTD) arrays with large delay-bandwidth products to accelerate beam training using frequency-dependent probing beams. We propose and study two TTD architecture candidates, including analog and hybrid analog-digital arrays, that can facilitate beam training with only one wideband pilot. We also propose a suitable algorithm that requires a single pilot to achieve high-accuracy estimation of angle of arrival. The proposed array architectures are compared in terms of beam training requirements and performance, robustness to practical hardware impairments, and power consumption. The findings suggest that the analog and hybrid TTD arrays achieve a sub-degree beam alignment precision with 66% and 25% lower power consumption than a fully digital array, respectively. Our results yield important design trade-offs among the basic system parameters, power consumption, and accuracy of angle of arrival estimation in fast TTD beam training.

Investigation of High-Depostition-Rate Additive Manufacturing of Ti-6Al-4V via Laser Material Deposition

Ti-6Al-4V is the most prominent titanium alloy widely used e.g. for aerospace applications. Conventionally, many Ti-6Al-4V aerospace components are produced by a multi-stage hot forging process followed by subsequent machining which often generates a high amount of scrap. Additive manufacturing (AM), such as powder-based laser material deposition (p-LMD), enables parts to be made with geometric freedom and near-net-shape, but so far lacks high deposition rates. The present study proposes high-deposition-rate laser material deposition manufacturing using a large laser beam diameter and increased scanning speed to achieve deposition rates up to 5 kg/h. As Ti-6Al-4V is prone to oxygen pick-up, the process was performed in an inert atmosphere. We determined suitable process windows for tracks without fusion defects and low porosity and investigated microstructure and hardness.

Small footprint phase locking system for a large tiled aperture laser array.

A phase sensing system fitted to the control of coherent laser beam array of large cross section is experimented. It is based on the use of a fiber bundle that collects a weak part of the synthetic wavefront, that scales it down (1/40) and that reshapes it in a more compact arrangement (2D to 1D array). Then, the reconfigured beam array can be analyzed by a small footprint system making the large laser beam array easier to phase-lock. The discrete laser array wavefront transmitted by the meter long fiber bundle was stabilized thanks to a multiple arm servo loop. Laser array phase locking was further ensured by random scattering through a diffuser, associated to an alternating projection algorithm. Six fiber laser beams constituting a 110 mm diameter synthetic aperture, were phase-locked with λ/16 accuracy.

Monte Carlo Comparison of Proton and Helium-ion Minibeam Generation Techniques

Proton minibeam radiation therapy (pMBRT) is a novel therapeutic strategy that combines the normal tissue sparing of submillimetric, spatially fractionated beams with the improved dose deposition of protons. In contrast to conventional approaches which work with comparatively large beam diameters (5 mm to several centimetres) producing laterally homogeneous fields, pMBRT uses submillimetric minibeams to create a distinct spatial modulation of the dose featuring alternating regions of high dose (peaks) and low dose (valleys). This spatial fractionation can increase the tolerance of normal tissue and may allow a safe dose escalation in the tumour. Important quantities in this context are the valley dose as well as the peak-to-valley dose ratio (PVDR). Creating submillimetric proton beams for clinical applications is a challenging task that until now has been realized with mechanical collimators (metal blocks with thin slits or holes). However, this method is inherently inefficient, inflexible and creates undesirable secondary neutrons. We therefore recently proposed a method for obtaining clinical minibeams using only magnetic focusing. In this study, we performed Monte Carlo simulations in order to compare minibeams generated using the new method of magnetic focusing with two techniques involving mechanical collimators (collimator and broad beam irradiation, collimator and pencil beam scanning). The dose deposition in water was simulated and dosimetric aspects [beam broadening, depth-dose profiles, PVDR and Bragg-peak-to-entrance dose ratio (BEDR)] as well as irradiation efficiencies were evaluated. Apart from protons, we also considered helium ions which, due to their reduced lateral scattering and sharper Bragg peak, may present a promising alternative for minibeam radiation therapy. Magnetically focused minibeams exhibited a 20–60 times higher PVDR than mechanically collimated minibeams and yielded an increase in irradiation efficiency of up to two orders of magnitude. Compared to proton minibeams, helium ion minibeams were found to broaden at a slower rate and yield an even higher PVDR (at the same minibeam spacing) as well as a more favourable BEDR. Moreover, the simulations showed that methods developed for proton minibeams are suitable for the generation of helium ion minibeams.

Large deflection stability of axially functionally graded tapered cantilever column

This article studies the large deflection stability of axially functionally graded (AFG) cantilever column for analyzing post-buckling behavior. Consideration is given to a nonlinearly tapered column with a regular polygon cross section, whose volume is constant. The governing differential equations of the problem are derived based on the large deflection beam theory. To calculate the elastica and buckling load of the AFG column, the differential equations are solved with the direct integration method together with the nonlinear solution method. The computed results are compared with those reported in literature and obtained from finite element method. Numerical examples are provided to ascertain the effects of the geometric and material parameters on the elastica and buckling load. The elastica shape is shown to depend on applied load and column properties, and the optimal shape such that the AFG column with fixed volume enables to bear the maximum buckling load is discussed.

A coherent harmonic generation method for producing femtosecond coherent radiation in a laser plasma accelerator based light source.

The electron beam generated in laser plasma accelerators (LPAs) has two main initial weaknesses - a large beam divergence (up to a few milliradians) and a few percent level energy spread. They reduce the beam brightness and worsen the coherence of the LPA-based light source. To achieve fully coherent radiation, several methods have been proposed for generating strong microbunching on LPA beams. In these methods, a seed laser is used to induce an angular modulation into the electron beam, and the angular modulation is converted into a strong density modulation through a beamline with nonzero longitudinal position and transverse angle coupling. In this paper, an alternative method to generate microbunching into the LPA beam by using a seed laser that induces an energy modulation and transverse-longitudinal coupling beamlines that convert the energy modulation into strong density modulation is proposed. Compared with the angular modulation methods, the proposed method can use more than one order of magnitude lower seed laser power to achieve similar radiation performance. Simulations show that with the proposed method a coherent pulse of a few microjoules pulse energy and femtosecond duration can be generated with a typical LPA beam.

Computational weld-mechanics assessment of welding distortions in a large beam structure

Unwanted distortions are typically observed in components after the welding process. Physical trial tests and extra post-treatments are being widely utilized in industries to minimize and correct the out of tolerance distortions. These methods are time-consuming and costly. There has been growing interest in digital tools which have great potential to minimize the physical test loops and corrections. In this study welding distortions analysis has been carried out on a large beam structure experimentally and numerically using computational welding mechanics (CWM) techniques such as the inherent strain (local–global) method and the shrinkage method, together with the lumping approach. The estimated distortions from the shrinkage together with lumping approaches were in good agreement with the experimental measurements and the computational time affordable. The inherent strain (local–global) method captured the trend of distortion with an underestimation of distortions. The accuracy of the estimated residuals stresses from the inherent strain (local–global) approach is higher than the one from shrinkage together with lumping approaches. Moreover, the effects of various welding process parameters (i.e. welding sequence, fixture, and weld pool size) on welding distortions were investigated. It is found that following the proper welding sequence could minimize the welding distortion of the beam structure. Increasing the constraints of fixtures can prevent welding distortion effectively and reducing weld pool size results in less welding distortions of the beam structure.

Generation of a large orbital angular momentum beam via an optical fiber winding around a curved path and its application* *Project supported by the National Natural Science Foundation of China (Grant No. 11504074) and the State Key Laboratory of Quantum Optics and Quantum Optics Devices, Shanxi University (Grant No. KF202004).

Many papers have been published on the study of orbital angular momentum (OAM) of the laser modes based on the Laguerre–Gaussian (LG) beam and helical phase plate or rotating cylindrical lens, and the principal ray of the LG beam remains straight line. These ways are difficult to get a large OAM modes. In this paper, we propose a novel method to prepare a large OAM modes when the light propagates through a optical fiber winding around a curved path, and establish a theoretical framework based on the principal ray path changing. Firstly, we investigate three kinds of winding structure. Secondly, based on the analytical solutions and numerical calculations, we can find that the spiral body can achieve a large OAM temporal pulse. At the same time, based on the zero OAM diffraction diagram, we can obtain an improvement of resolving power beyond Rayleigh’s criterion. Finally, applying a large OAM diffraction pattern to realize 12-bitencodes, we can obtain a high-security optical information transfer system.

Selection rules for the orbital angular momentum of optically produced THz radiation.

In this work, we theoretically study the transduction of orbital angular momentum (OAM) l for infrared pump lasers into the THz domain. In the case of optical rectification, the transduction of OAM occurs only through a spin-orbit interaction, with the selection rule on the OAM l=0 valid for any kind of polarization of the pump, which means that there is no transfer of OAM along the propagation axis. In difference frequency generation, the selection rule for the difference Δl between the OAM of the pump fields with linear or circular polarization is l=Δl, whereas l ranges from Δl-2 to Δl+2 in cases of both radial and azimuthal polarization. Moreover, for THz generation in the latter case, the high diffraction obtained with tightly focused pumps yields l tending to Δl±2, while l tends to zero in the opposite case of large pump beams.

Design and Performance of a Miniaturized, Low-Energy, Large Beam Spot Electron Flood Gun

Charge accumulation often occurs in electron optics equipment and interferes with their operation. The trouble can be handled by using the electron flood gun. However, there are still some scenarios where neutralization is not as desired. To achieve a better charge neutralization effect and to facilitate work in confined spaces, a small size, low-energy electron flood gun providing a large area and uniform electron beam has been required. This article employs Munro’s Electron Beam Software (MEBS) to simulate the effect of the structure parameters on the performance of the beam. Based on the simulation results, the electron flood gun is processed and assembled. To verify the performance of the electron flood gun, this paper proposes a new “pinhole scanning method”. By using the method, we have achieved in-situ measurements of beam current and beam spot. The experimental results generally match the simulation results, which demonstrates that the electron flood gun has good performance and is likely to have many applications.

Limiting radiation exposure during prostatic arteries embolization: influence of patient characteristics, anatomical conditions, and technical factors.

To assess the influence of patient characteristics, anatomical conditions, and technical factors on radiation exposure during prostatic arteries embolization (PAE) performed for benign prostatic hyperplasia. Patient characteristics (age, body mass index (BMI)), anatomical conditions (number of prostatic arteries, anastomosis), and technical factors (use of cone beam computed tomography (CBCT), large display monitor (LDM), and magnification) were recorded as well as total air kerma (AK), dose area product (DAP), fluoroscopy time (FT), and number of acquisitions (NAcq). Associations between potential dose-influencing factors and AK using univariate analysis and a multiple linear regression model were assessed. Forty-one consecutive men (68 ± 8 years, min-max: 40-76) were included. LDM and CBCT decreased the use of small field of view with 13.9 and 3.8% respectively, both p < 0.001. The use of a LDM significantly reduced AK (1006.6 ± 471.7 vs. 1412 ± 754.6 mGy, p = 0.02), DAP (119.4 ± 64.4 vs. 167.9 ± 99.2, p = 0.04), FT (40.4 ± 11.5 vs. 53.6 ± 25.5 min, p = 0.01), and NAcq (16.3 ± 6.3 vs. 18.2 ± 7, p = 0.04). In multivariate analysis, AK reduction was associated with lower patient BMI (β = 0.359, p = 0.002), shorter FT (β = 0.664, p < 0.001) and CBCT use (β = - 0.223, p = 0.03), and decreased NAcq (β = 0.229, p = 0.04). LDM and CBCT are important technical dose-related factors to help reduce radiation exposure during PAE, and should be considered in standard practice. • The use of large display monitor (LDM) and cone beam computed tomography (CBCT) both decreased the need for magnification during prostatic arteries embolization (PAE). • The use of LDM reduces radiation exposure during PAE. • Total air kerma is associated with patient's body mass index, fluoroscopy time, CBCT, and the number of acquisitions.

When invariable cross sections change: The Electron-Ion Collider case

In everyday research, it is tacitly assumed that the scattering cross-sections have fixed values for the given particle species, centre-of-mass energy, and particle polarizations. However, this assumption has been called into question after several observations of suppression of high-energy bremsstrahlung. This process will play a major role in experiments at the future Electron-Ion Collider, and we show here how variations of the bremsstrahlung cross-section can be profoundly studied there using the lateral beam displacements. In particular, we predict very strong increase of the observed cross-sections for large beam separations. We also discuss the relation of these elusive effects to other quantum phenomena occurring over macroscopic distances. In this context, spectacular and possibly useful properties of the coherent bremsstrahlung at the EIC are also evaluated.