Peak Beam Research Articles

Simultaneous imaging of luminescence and prompt x-rays during irradiation with spot-scanning proton beams at clinical dose level

Prompt x-ray imaging is a promising method for observing the beam shape from outside a subject. However, its distribution is different from dose distribution, and thus a comparison with the dose is required. Meanwhile, luminescence imaging of water is a possible method for imaging the dose distribution. Consequently, we performed simultaneous imaging of luminescence and prompt x-rays during irradiation by proton beams to compare the distributions between these two different imaging methods. Optical imaging of water was conducted with spot-scanning proton beams at clinical dose level during irradiation to a fluorescein (FS) water phantom set in a black box. Prompt x-ray imaging was also conducted simultaneously from outside the black box using a developed x-ray camera during proton beam irradiation to the phantom. We measured images of the luminescence of FS water and prompt x-rays for various types of proton beams, including pencil beams, spread-out Bragg peak (SOBP) beams, and clinically used therapy beams. After the imaging, ranges were estimated from FS water and prompt x-rays and compared with those calculated with a treatment planning system (TPS). We could measure the prompt x-ray and FS water images simultaneously for all types of proton beams. The ranges estimated from the FS water and those calculated with the TPS closely matched, within a difference of several mm. Similar range difference was found between the results estimated from prompt x-ray images and those calculated with the TPS. We confirmed that the simultaneous imaging of luminescence and prompt x-rays were possible during irradiation with spot-scanning proton beams at a clinical dose level. This method can be applied to range estimation as well as comparison with the dose for prompt x-ray imaging or other imaging methods used in therapy with various types of proton beams at a clinical dose level.

Terahertz-driven positron acceleration assisted by ultra-intense lasers.

Generation and acceleration of energetic positrons based on laser plasma have attracted intense attention due to their potential applications in medical physics, high energy physics, astrophysics and nuclear physics. However, such compact positron sources face a series of challenges including the beam dispersion, dephasing and unstability. Here, we propose a scheme that couples the all-optical generation of electron-positron pairs and rapid acceleration of copious positrons in the terahertz (THz) field. In the scheme, nanocoulomb-scale electrons are first captured in the wakefield and accelerated to 2.5 GeV. Then these energetic electrons emit strong THz radiation when they go through an aluminum foil. Subsequently, abundant γ photons and positrons are generated during the collision of GeV electron beam and the scattering laser. Due to the strong longitudinal acceleration field and the transvers confining field of the emitted THz wave, the positrons can be efficiently accelerated to 800 MeV, with the peak beam brilliance of 2.26 × 1012s-1mm-2mrad-2eV-1. This can arouse potential research interests from PW-class laser facilities together with a GeV electron beamline.

Propagation properties of the Airy vortex beam in the linear potential

In this paper, the propagation properties of the Airy vortex beam (AVB) in the transverse linear potential are investigated analytically and numerically. The analytical expression describing the propagation of the AVB in the linear potential is derived. And it should be noted that the paths of the beam and the optical vortex (OV) are functions of the linear potential. Specifically, by setting a suitable linear potential, it becomes feasible to manipulate the beam and the OV along a predesigned trajectory without altering the peak intensity of the beam or compromising its inherent non-diffracting and self-healing properties. In addition, the off-axis position and the topological charge of the OV are found to have an effect on the position and the magnitude of the beam peak intensity in the linear potential.

Optimization of proton beam quality by obliquely incident laser pre-pulse

An investigation has been made to enhance the proton acceleration through the use of an additional obliquely incident lower intense pre-pulse. We observed by means of two-dimensional particle-in-cell simulation that the surface plasma waves can be generated on a flat target using a low-intensity pre-pulse, which is beneficial for enhancing the laser self-focusing and consequently boosting the proton acceleration. We noticed that the proton acceleration is driven by a hybrid acceleration process. The obtained proton beam peak energy and the maximum energy are found to increase by about 205% and 68%, respectively, for the case with pre-pulse in comparison to the cases where the pre-pulse is absent. Moreover, significant degradation of the proton beam energy spread and angular divergence are also seen.

A unified section-based restoring force model for reinforced concrete coupling beams based on the XGBoost model: Establishment, validation and application

To enhance the seismic performance of reinforced concrete (RC) coupling beams, improved reinforcement details such as diagonal layout and high-performance concrete including fiber reinforced concrete (FRC) as well as engineered cementitious composites (ECC) are widely investigated and applied. Despite the numerous experimental studies on various reinforced concrete coupling beams, there is still a lack of unified, efficient and accurate numerical model applicable to all kinds of concrete and reinforcement layouts, leading to difficulty in numerical study and proper design of coupling beams and coupled shear walls. In this study, a database containing 180 reinforced concrete coupling beam specimens is first set up. Then the key mechanical indexes of test curves including the peak load, peak beam rotation, yielding beam rotation and softening beam rotation are predicted using the trained XGBoost models. The feature importance and partial dependence of different independent variables are analyzed. Based on the predicted results, the skeleton curves of the section-based restoring force model can be obtained. Besides, a new hysteretic rule for the restoring force model which can account for the pinching effect, strength and stiffness degradation is proposed. The proposed model is then implemented in ABAQUS 2017 software, of which the accuracy and applicability are validated by twelve coupling beam specimens with different concrete materials and reinforcement layouts. Finally, a coupled shear wall specimen is simulated to quantitatively and thoroughly evaluate the effects of key indexes of the coupling beams on the seismic performance of the coupled shear wall structures.

Prompt X-ray imaging during irradiation with spread-out Bragg peak (SOBP) beams of carbon ions

Prompt secondary electron bremsstrahlung X-ray (prompt X-ray) imaging using a low-energy X-ray camera is a promising method for observing a beam shape from outside the subject. However, such imaging has so far been conducted only for pencil beams without a multi-leaf collimator (MLC). The use of spread-out Bragg peak (SOBP) with an MLC may increase the scattered prompt gamma photons and decrease the contrast of the images of prompt X-rays. Consequently, we performed prompt X-ray imaging of SOBP beams formed with an MLC. This imaging was carried out in list mode during irradiation of SOBP beams to a water phantom. An X-ray camera with a 1.5-mm diameter as well as 4-mm-diameter pinhole collimators was used for the imaging. List mode data were sorted to obtain the SOBP beam images as well as energy spectra and time count rate curves. Due to the high background counts from the scattered prompt gamma photons penetrating the tungsten shield of the X-ray camera, the SOBP beam shapes were difficult to observe with a 1.5-mm-diameter pinhole collimator. With the 4-mm-diameter pinhole collimators, images of SOBP beam shapes at clinical dose levels could be obtained with the X-ray camera. The use of a 4-mm-diameter pinhole collimator attached to the X-ray camera is effective for prompt X-ray imaging with high sensitivity and low background counts. This approach makes it possible to image SOBP beams with an MLC when the counts are low and the background levels are high.

Astigmatic electron beam propagation

An astigmatic electron beam with an ellipticity ratio of approximately 2:1 was examined using the Enhanced Modified Faraday Cup (EMFC) diagnostic to measure its properties and show how the power density distribution varies near the beam waist. Results show that the beam has two power density peaks, one on either side of the beam crossover point, resulting from different focal distances of the major and minor axes of the elliptical beam shape. Quantification of the beam properties was used to establish a depth-of-field where the beam diameter and peak power densities are relatively constant. For the 1 kW beam studied here, the depth-of-field measured approximately ±5 mm from the beam crossover point, illustrating how electron beam diagnostics can be used to identify process control limits for repeatable and reliable welds under non-ideal electron beam power density distribution conditions.

Microwave generation from betatron oscillation of slow electron beam in DC cross-fields configuration

A high-efficiency mechanism of microwave (MW) generation from slow electron beam (e-beam) is investigated in details. Space-inhomogeneous density profile of the e-beam enables its current vortex profile ∇ × j easy to be modified significantly by external fields. Suitable DC cross-fields configuration can modify time behavior of the ∇ × j of an e-beam from monotonically-varying to oscillatory. Under available not-too-severe values of beam peak density and beam drifting velocity, such a time-oscillatory current vortex can be tuned to have a narrow-spectrum around a desired central frequency within a broad range from MHz to Thz. The mechanism is applicable to slow non-relativistic e-beam and hence does not need bulky cooling component and high-voltage component. This favors the whole vacuum electron devices (VEDs) to be compact enough without at the cost of lower output power.

Efficient Phase-Only Dual- and Multi-Beam Pattern Synthesis With Accurate Beam Direction and Power Control Employing Partitioned Iterative FFT

In this communication, a novel phase-only method based on a partitioned iterative Fourier transform (PIFT) is presented to efficiently synthesize dual- and multibeam patterns for uniformly spaced linear and planar arrays. To achieve accurate mainlobe peak directions, the PIFT integrates a partitioned beam calibration strategy into the IFT to successively calibrate the beams by partitioning the multibeam pattern into single-beam patterns and calibrating them individually. Besides, the peak powers of the multiple beams are precisely controlled through proper mainlobe corrections. Several numerical examples of synthesizing dual- and multibeam patterns with accurately controlled peak directions and beam powers as well as reduced sidelobe levels (SLLs) are presented to demonstrate the effectiveness and efficiency of the proposed method. Synthesized results are validated with full-wave simulation. Comparisons with other phase-only methods are also presented to show the superiority of the proposed method.

Controllable self-rotating array beam with an arc-shaped accelerating trajectory.

In this study, a modified interfering vortex phase mask (MIVPM) is proposed to generate a new type of self-rotating beam. The MIVPM is based on a conventional and stretched vortex phase for generating a self-rotating beam that rotates continuously with increasing propagation distances. A combined phase mask can produce multi-rotating array beams with controllable sub-region number. The combination method of this phase was analyzed in detail. This study proves that this self-rotating array beam has an effectively enhanced central lobe and reduced side lobe owing to adding a vortex phase mask compared with a conventional self-rotating beam. Furthermore, the propagation dynamics of this beam can be modulated by varying the topological charge and constant a. With an increase in the topological charge, the area crossed by the peak beam intensity along the propagation axis increases. Meanwhile, the novel self-rotating beam is used for optical manipulation under phase gradient force. The proposed self-rotating array beam has potential applications in optical manipulation and spatial localization.

Fast and flexible control of beam longitudinal phase space for high-repetition-rate x-ray free-electron lasers.

The wide-ranging requirements for the photon properties from multiple beamlines in superconducting based free-electron lasers (FELs) demand more challenging beam manipulation techniques. Shot-by-shot control of electron beam bunch length and peak current at high repetition rate up to megahertz is highly desired. In this paper, we present a comprehensive study of a method based on a 2-m-long normal conducting radio-frequency cavity to achieve fast and flexible control of beam compression and realize the full potential of the facility, including theoretical analysis, beam dynamics simulations, and conceptual cavity design for the Linac Coherent Light Source II and its high-energy upgrade. We illustrate the physical mechanism of the chirping cavity on the control of the final beam compression and propose methods to lower the requirements for the cavity parameters. The application of this method will allow tailored photon properties of individual beamlines to optimize their performance and drastically improve the multiplexing capabilities of a high-repetition rate FEL facility.

Vibration induced by active nematics

Active elements in active nematics can impose forces on immersed bodies and move them accordingly. We numerically investigate the vibrational motion of a cantilever beam placed in active nematics. The continuous energy transfer from vortices to the beam results in beam oscillation, whose direction and amplitude depend on the vortex strength, size and position. Referring to the kinetic-energy spectrum, we indicate that both the large- and small-scale vortices are the primary mechanism for the energy transfer between the fluid and beam, leading to the beam oscillatory motion, with the contribution from the large-scale vortices being higher. We investigate the effect of fluid properties such as activity, viscosity and elastic constant on the oscillation frequency. We show that the intensification of the activity increases peak frequency, and there is a linear correlation between the peak frequency and activity. We further demonstrate the reciprocal relationship between viscosity and peak frequency. Subsequently, we relate the increase and decrease in the peak frequency to the energy injection/dissipation by activity/viscosity. Moreover, we reveal the negligibly small dependency of beam peak frequency on the elastic constant and discuss free energy's role in accounting for this behaviour. The findings clearly demonstrate that active fluids can impose an oscillatory motion on flexible bodies, which might be used as a novel method for measuring the critical properties of active nematics.

Impact of secondary particles on the magnetic field generated by a proton pencil beam: a finite-element analysis based on Geant4-DNA simulations.

To investigate the static magnetic field generated by a proton pencil beam as a candidate for range verification by means of Monte Carlo simulations, thereby improving upon existing analytical calculations. We focus on the impact of statistical current fluctuations and secondary protons and electrons. We considered a pulsed beam (10 s pulse duration) during the duty cycle with a peak beam current of 0.2 A and an initial energy of 100MeV. We ran Geant4-DNA Monte Carlo simulations of a proton pencil beam in water and extracted independent particle phase spaces. We calculated longitudinal and radial current density of protons and electrons, serving as an input for a magnetic field estimation based on a finite element analysis in a cylindrical geometry. We made sure to allow for non-solenoidal current densities as is the case of a stopping proton beam. The rising proton charge density toward the range is not perturbed by energy straggling and only lowered through nuclear reactions by up to 15%, leading to an approximately constant longitudinal current. Their relative low density however (at most 0.37 protons/mm3 for the 0.2 A current and a beam cross-sectionof 2.5mm), gives rise to considerable current density fluctuations. The radial proton current resulting from lateral scattering and being two orders of magnitude weaker than the longitudinal current is subject to even stronger fluctuations. Secondary electrons with energies above 10eV, that far outnumber the primary protons, reduce the primary proton current by only 10% due to their largely isotropic flow. A small fraction of electrons (<1%), undergoing head-on collisions, constitutes the relevant electron current. In the far-field, both contributions to the magnetic field strength (longitudinal and lateral) are independent of the beam spot size. We also find that the nuclear reaction-related losses cause a shift of 1.3mm to the magnetic field profile relative to the actual range, which is further enlarged to 2.4mm by the electron current (at a distance of mm away from the central beam axis). For mm, the shift increases linearly. While the current density variations cause significant magnetic field uncertainty close to the central beam axis with a relative standard deviation (RSD) close to 100%, they average out at a distance of 10cm, where the RSD of the total magnetic field drops below 2%. With the small influence of the secondary electrons together with the low RSD, our analysis encourages an experimental detection of the magnetic field through sensitive instrumentation, such as optical magnetometry orSQUIDs.

Machine learning for predicting mechanical behavior of concrete beams with 3D printed TPMS

Bioinspired structures are remarkable porous structures with great strength-to-weight ratios. Hence, they have been applied in various fields including biomedical, transportation, and aerospace materials, etc. Recent studies have shown the significant impact of the plastic 3D printed triply periodic minimal surfaces (TPMS) structure on the cement beam including increasing the peak load, reducing the deflection, and improving the ductility. In this study, a machine learning (ML) surrogate model has been conducted to predict the beam behavior under static bending load. At first, various combinations of plastic volume fractions and numbers of core layers have been adopted to reinforce the constituent beam. The finite element method (FEM) was implemented to investigate the influences of these reinforcement strategies. Next, the above data were employed to create the ML model. A three-process assessment was proposed to achieve the most suitable model for the present problem, these processes were the model hyperparameter tuning, the performance assessment, and the handling overfitting with deep learning (DL) techniques. Consequently, both beam peak loads and maximum deflections were proportional to the volume fraction. The increment in TPMS layers could lead to the enhancement in both traits but with a nonlinear relationship. Furthermore, each trait may be a ceiling value that could not be exceeded with a specific volume fraction despite any number of layers. This conclusion was indicated by the surrogate model predictions. The final model in this study could deal with noisy data from FEM and with the support of a new early stopping condition, excellent performance could be found on both train and test data. The maximum deviations of 2.5% and 3.5% for peak loads and maximum midpoint displacements, respectively, have verified the robustness of the present surrogate model.

Shaped beam from multiple beams using planar phased arrays

ABSTRACT This paper proposes a synthesis and optimization procedure to generate user-specific shaped and multiple beams in any arbitrary 3D space. The synthesis method generates independently controllable multiple beams over different azimuth and elevation angles with variable peak powers. These multiple beams can form any user-defined shaped pattern with an optimum number of beams, their power, and placement in the angles using the proposed direct synthesis method. Different evolutionary algorithms are employed to optimize the beam peak parameters to mitigate the ripple, side lobe level, and beam spread. The synthesis method is validated on a fully functional integrated 5G mmWave phased array for various shaped beam patterns. Measured sector, isoflex beams between , , and dual beams with unequal powers are presented and compared with the shaped beam synthesis method.

High-power diode lasers with in-situ-structured lateral current blocking for improved threshold, efficiency and brightness

We present high-power GaAs-based broad-area diode lasers with a novel variant of the enhanced self-aligned lateral structure ‘eSAS’, having a strongly reduced lasing threshold and improved peak conversion efficiency and beam quality in comparison to their standard gain-guided counterparts. To realize this new variant (eSAS-V2), a two-step epitaxial growth process involving in situ etching is used to integrate current-blocking layers, optimized for tunnel current suppression, within the p-Al0.8GaAs cladding layer of an extreme-triple-asymmetric epitaxial structure with a thin p-side waveguide. The blocking layers are thus in close proximity to the active zone, resulting in strong suppression of current spreading and lateral carrier accumulation. eSAS-V2 devices with 4 mm resonator length and varying stripe widths are characterized and compared to previous eSAS variant (eSAS-V1) as well as gain-guided reference devices, all having the same dimensions and epitaxial structure. Measurement results show that the new eSAS-V2 variant eliminates an estimated 89% of lateral current spreading, resulting in a strong threshold current reduction of 29% at 90 μm stripe width, while slope and series resistance are broadly unchanged. The novel eSAS-V2 devices also maintain high conversion efficiency up to high continuous-wave optical power, with an exemplary 90 μm device having 51.5% at 20 W. Near-field width is significantly narrowed in both eSAS variants, but eSAS-V2 exhibits a wider far-field angle, consistent with the presence of index guiding. Nonetheless, eSAS-V2 achieves higher beam quality and lateral brightness than gain-guided reference devices, but the index guiding in this realization prevents it from surpassing eSAS-V1. Overall, the different performance benefits of the eSAS approach are clearly demonstrated.

Three‐dimensional shaped and contour pattern synthesis with multiple beam approach

AbstractA multiple‐beam synthesis method is proposed to generate three‐dimensional (3D) shaped and contour patterns from a planar phased array antenna. Three‐dimensional radiation patterns are synthesized primarily with optimum beam positions and variable mainbeam power levels of multiple beams, followed by calculating the resultant element weights. Contour patterns are synthesized from multiple beam peaks by mapping the individual beam power levels to their corresponding 3 dB beamwidth coverage areas at each designated location on the contour. The far‐field contour shape affected by the influence of the element pattern, mutual coupling, and scan loss is compensated by the peak power of the scanned beam patterns. The proposed synthesis supports dynamically changeable contour shapes. Flat‐topped, stepped 3D patterns and arbitrary contour shapes with shaped null zones inside the contour are simulated and implemented on a 16 × 16 element 5G mmWave planar phased array. Measured radiation patterns are compared with simulations and presented.

Scattering Mitigation Frequency Selective Surface for Unidirectional Blockage Compensation

This communication describes a concept of scattering mitigation frequency-selective surface (FSS) for unidirectional blockage compensation using a single substrate FSS without modification of the obstacle. The devised scattering mitigation FSS generates the complementary electric field to that of the blockage, enabling the blockage to become electromagnetically transparent to the antenna. Using superposition and linearity, the spatial impedance distribution of the FSS is calculated. The devised FSS consists of indented patch elements consisting of four symmetric via structures to exhibit a wide tunable transmission phase range and less than 1 dB insertion loss at the center frequency of 30.0 GHz for proof-of-concept. The proposed scattering mitigation FSS is verified in transverse magnetic (TMz)- and transverse electric (TEz)-polarized cylinder scattering scenarios. The measurement confirms the improvements of the main beam peak gain and restoration of far-field radiation patterns. The proposed investigation is expected to provide concise and practical scattering reduction solutions for blockage scenario such as vehicular communication in urban areas.

Knock sensing and imaging from structural bending waves

This paper presents a novel knock imaging technique for human-to-things interfaces. It allows any object to use its own surface as an imaging sensor. The methodology is to sense the structural bending waves caused by the knock and reconstruct the intensity image through the beamforming of the wave signals. To verify this, we prepare a thin piezoelectric sensor array on a square plate. The performances are evaluated by measuring the beam peak, beamwidth, and sidelobe level in the reconstructed image. Finally, a real finger knock test is demonstrated, showing an image reconstruction that the position error is only 3 mm.

Transmitarray Antenna for Conical Beam Scanning

Conventional transmitarray antennas are designed to produce a directional pattern with the beam peak in a single direction when the feed is placed at a distinct location. A real-life application requiring conical beam scanning necessitates the development of antennas with patterns that sweep out a cone in space through successive excitation of feeds placed at multiple locations along the body of the revolution ring focus. In this communication, we propose transmitarrays as an antenna solution that can achieve the conical beam sweep. Compared to the previously reported transmitarrays, the development of an off-axis-fed transmitarray for conical beam scanning requires significant advancements regarding the synthesis and optimization of the off-axis-fed transmitarray. A novel synthesis algorithm is presented based on the optimization of the feed location and a modified transmitarray phase compensation. To validate the approach, an 18 cm transmitarray with a thickness of 0.8 cm operating at 13.4 GHz is designed, simulated, prototyped, and measured. Predictions obtained by full-wave simulations agree well with the measured results. The conical beam scanning capability of the transmitarray is demonstrated through measurements taken with the feed placed at five locations within the ring focus.