Angular Spread Research Articles

Design of a compact proton beam energy modulator for imaging

Proton beam therapy is the current state of the art for irradiation of cancerous tumors near vital organs. Proton imaging is a new technology that images tumors with the same particle interaction as proton therapy, promising more accurate treatment preparation compared to traditional computerized tomography scans. A specific proton imaging technique in development at Massachusetts General Hospital requires an energy modulator that can fit inside the gantry nozzle. This work presents the design of a compact proton beam energy modulator required for such imaging applications. A combination of steel and lead wedges was used to create a balanced modulation wheel, with an axis of rotation perpendicular to the incident beam. The mechanical design also allows interchangeable disks and precise position control. The modulator design was verified by testing on a proton beam line. The final device achieved a wide range of energy modulation (21 cm water-equivalent thickness) while maintaining a constant exiting beam angular spread meeting device requirements. This compact design facilitates proton imaging to be practically incorporated into future gantry systems, which will advance proton treatment for tumors near vital organs.

Frequency-Hopped Signal Source Identification in Frequency-Selective Channels

Blind source separation of frequency-hopped signals is a challenging problem for cases involving multipath exhibiting temporal and angular spread. A novel solution is proposed that exploits polarization-frequency correlation to separate signals, enabling association of hopped signal in these adverse conditions. Performance depends upon signal-to-noise ratios and channel-induced signal feature differences. The method has the advantage of not requiring model order estimation of multipath. The efficacy of the approach is demonstrated in experiments and numerical analyses.

High-quality high-order harmonic generation through preplasma truncation.

By introducing preplasma truncation to cases with an initial preplasma scale length larger than 0.2λ, the efficiency of high-order harmonics generated from relativistic laser-solid interactions can be enhanced by more than one order of magnitude and the angular spread can be confined into near-diffraction-limited divergence. Numerical simulations show that density truncation results in more compact oscillation of the surface electron sheet and the curvature of the reflection surface for the driving laser is greatly reduced. This leads to an overall improvement in the harmonic beam quality. More importantly, density truncation makes the harmonic generation weakly dependent on the preplasma scale length, which provides a way to relax the extremely high requirement on the temporal contrast of the driving laser pulse. A feasible scheme to realize the required preplasma truncation is also proposed and demonstrated by numerical simulations.

Learning the Structured Sparsity: 3-D Massive MIMO Channel Estimation and Adaptive Spatial Interpolation

This paper addresses the channel estimation problem for three-dimensional (3-D) massive multiple-input multiple-output (MIMO) systems, where the base station (BS) is equipped with a two-dimensional uniform planar array (UPA) to serve a number of user equipments (UEs). To implement with low hardware complexity, the number of available radio-frequency (RF) chains at BS is constrained to be much smaller than the number of antennas. The theoretical analysis of sparse property for 3-D massive MIMO channel reveals that there exists two kinds of sparse structures in beam-domain channel vector, namely the common sparsity structure among sub-arrays of UPA and block sparsity structure per sub-array. Based on this property, a novel structured sparse Bayesian learning framework is proposed to estimate the channel between BS and UE reliably. Moreover, an adaptive spatial interpolation scheme is proposed to further reduce the number of required RF chains at BS while maintaining the estimation performance. The simulation results show that the proposed scheme provides stable estimation performance for a variety of scenarios with different numbers of RF chains, transmit signal-to-noise ratios, Rician factors, and angular spreads, and outperforms the reference schemes significantly.

Controlling polarization of THz radiation in pair plasma

Polarization-tunable terahertz (THz) radiation in pair plasma has been generated under the combined configuration of a helical wiggler and solenoidal magnetic fields. The control over the electromagnetic vector of the emitted field (i.e. polarization) furnishes the knob of another degree of freedom which is very useful in numerous applications. With the optimization of wiggler period and frequency, the total angular spread of the field has been reduced which provides a narrow-band THz spectrum. Additionally, the solenoidal field not only prevents the escape of particles during laser-matter interaction but also enhances the magnitude of emitted field. Along with the external magnetic field, collisions among the particles and ripples in plasma density have also been considered to fine-tune the radiation.

Verification of dry storage cask loading using monoenergetic photon sources

Safeguarding the contents of spent nuclear fuel dry storage casks requires the verification of cask contents, especially if continuity of knowledge has been lost. The high attenuation of the cask’s thick shielding and stored fuel assemblies makes verification difficult using conventional photon radiography. Thomson-scattering quasi-monoenergetic photon sources offer narrow angular and energy spread for increased penetration. This paper demonstrates the advantages of such a source by examining effects of source properties on transmission through thick, highly attenuating objects. This information is then applied to calculations and simulations of transmission for a variety of photon sources and cask configurations. The performance of the source is compared to traditional bremsstrahlung sources and other advanced methods, in the context of typical nuclear fuel dry storage casks. It is shown that the use of a Thomson scattering based quasi-monoenergetic photon source enables determination of fuel assembly occupancies faster and more accurately than other methods.

Influence of target curvature on the characteristics of particle beams generated by laser ion acceleration with microstructured enhanced targets at ultra high intensity

Following the advances in laser science that have been performed recently, the push to improve the quality of the laser generated particle beams has seen a lot of interest in the past decade. The work presented in this paper aims to study the role of target curvature using 2D PIC simulations to ascertain its influence on the angular spread and peak energy for the accelerated proton beam, and the characteristics of the gamma photons produced for potential usage as secondary sources. For the scope of this work we parametrically probed different curvature radii of a microstructured enhanced target impacted by a laser pulse with predetermined parameters. This paper continues past studies (Tatomirescu et al 2017 AIP Conf. Proc. 1796 020013; Tatomirescu et al 2017 AIP Conf. Proc. 1916 030002) and concentrates on determining the target curvature effect when using conical structures for pulse focusing. The results show promising enhanced focus of the accelerated particles and an enhancement of the peak energy of the accelerated particles.

Resonant slow extraction with constant optics for improved separatrix control at the extraction septum

Losses and component activation are limiting performance factors for slow extraction with high-power applications, and new techniques of loss-reduction, such as bent crystals, require a stable and narrow separatrix angular spread. Conventional tune-sweep slow extraction results in an optics change and an accompanying separatrix rotation through the spill. This can be compensated by a dynamic closed-orbit bump, but requires a high level of complexity for setting up and monitoring. For the Super Proton Synchrotron (SPS), a simpler and powerful new extraction technique has been developed and deployed, providing a mechanism to fix the machine optics and hence separatrix completely through the spill. The technique with the name constant optics slow extraction (COSE) relies on high chromaticity and scaling all machine settings with beam rigidity following the momentum distribution of the beam. In this paper we describe the new COSE concept and its successful operational deployment in the SPS during the 2018 run.

DOA Estimation of Two-Dimensional Coherently Distributed Sources Based on Spatial Smoothing of Uniform Rectangular Arrays

Aiming at the direction-of-arrival (DOA) estimation of two-dimensional (2D) coherently distributed (CD) sources which are coherent with each other, we explore the propagator method based on spatial smoothing of a uniform rectangular array (URA). The rotational invariance relationships with respect to the nominal azimuth and nominal elevation are obtained under the small angular spreads assumption. A propagator operator is constructed through spatial smoothing of sample covariance matrices firstly. Then, combination of propagator and identical matrix is divided according to rotational operators, and the nominal angles can be obtained through eigendecomposition lastly. Realizing angle matching automatically, the proposed method can estimate multiple DOAs of 2D coherent CD sources without spectral peak searching and prior knowledge of deterministic angular signal distribution function. Simulations are conducted to verify the effectiveness of the proposed method.

Very-High-Frequency Single-Input–Multiple-Output Acoustic Communication in Shallow Water

Very-high-frequency (VHF) underwater acoustic communication is a relatively unexplored terrain, but its importance is expected to increase with the growing demands for larger bandwidths and higher data rates. Knowledge of VHF propagation channels is a prerequisite to achieve high data throughput. In this paper, we present time-variant angle-resolved impulse responses for a set of 250-kHz shallow-water channels. The measurements were performed at high resolution using 64 hydrophones arranged in a line array. The measured channels are characterized by delay spreads of several milliseconds, Doppler spreads of tens of hertz, and angular spreads up to 30 °. The angular spread is not separable from the delay-Doppler spread, and the impulse response depends heavily on the direction of arrival. Different beamforming and multichannel equalizer strategies are compared for a single-input-multiple-output configuration. A method combining subarray beamforming and multichannel equalization delivers in both noise- and interference-limited scenarios, transferring data at 78.125 kb/s over ranges up to 770 m.

A Novel Random Angular Sampling Method for Spatial and Temporal Channel Emulation

In this letter, a novel angular sampling method–random offset sampling (ROS) is proposed for the simulation of ray-based physical reference channel models. This new method employs an asymmetrical random angular offset to the equal power sampling method, which makes the simulated channel match the spatial–temporal characteristics of the reference channel model more exactly. It is shown that this new method can make better utilization of the symmetrical property of power angular spectrum distributions with very limited sampling sinusoids, especially performing pretty well in case of small angular spread. With small modifications, ROS can be directly embedded in the state-of-the-art standardized channel simulation models.

Direct growth of mm-size twisted bilayer graphene by plasma-enhanced chemical vapor deposition

Plasma enhanced chemical vapor deposition (PECVD) techniques have been shown to be an efficient method to achieve single-step synthesis of high-quality monolayer graphene (MLG) without the need of active heating. Here we report PECVD-growth of single-crystalline hexagonal bilayer graphene (BLG) flakes and mm-size BLG films with the interlayer twist angle controlled by the growth parameters. The twist angle has been determined by three experimental approaches, including direct measurement of the relative orientation of crystalline edges between two stacked monolayers by scanning electron microscopy, analysis of the twist angle-dependent Raman spectral characteristics, and measurement of the Moiré period with scanning tunneling microscopy. In mm-sized twisted BLG (tBLG) films, the average twist angle can be controlled from 0° to approximately 20°, and the angular spread for a given growth condition can be limited to < 7°. Different work functions between MLG and BLG have been verified by the Kelvin probe force microscopy and ultraviolet photoelectron spectroscopy. Electrical measurements of back-gated field-effect-transistor devices based on small-angle tBLG samples revealed high-quality electric characteristics at 300 K and insulating temperature dependence down to 100 K. This controlled PECVD-growth of tBLG thus provides an efficient approach to investigate the effect of varying Moiré potentials on tBLG.

Vehicular Channel in Urban Environments at 23 GHz for Flexible Access Common Spectrum Application

With the development of the vehicular network, new radio technologies have been in the spotlight for maximizing the utilization of the limited radio spectrum resource while accommodating the increasing amount of services and applications in the wireless mobile networks. New spectrum policies based on dynamic spectrum access technology such as flexible access common spectrum (FACS) have been adopted by the Korea Communications Commission (KCC). 23 GHz bands have been allocated to FACS bands by the KCC, which is expected extensively for vehicular communications. The comprehensive knowledge on the radio channel is essential to effectively support the design, simulation, and development of such radio technologies. In this paper, the characteristics of 23 GHz vehicle-to-infrastructure (V2I) channels are simulated and extracted for the urban environment in Seoul. The path loss, shadow factor, Ricean K-factor, root-mean-square (RMS) delay spread, and angular spreads are characterized from the calibrated ray-tracing simulation results, and it can help researchers have a better understanding of the propagation channel for designing vehicular radio technologies and a communication system in a similar environment.

Channel Characteristics of High-Speed Railway Station Based on Ray-Tracing Simulation at 5G mmWave Band

In order to satisfy the increasing demand for the higher transmission capacity of “smart station”, millimeter wave (mmWave) technology is expected to play a significant role in the high data rate communication system. Based on the ray-tracing simulation technology, this paper would study wireless channel characteristics of the three-dimensional (3D) model of high-speed railway station at the mmWave band. Key parameters such as path loss exponent, shadow fading factor, delay spread, Rician K-factor, angular spread, power angle spectrum, and spatial correlation are extracted and investigated. These channel characteristics are of value for the selection of antenna arrays and even the design of future 5G communication networks in the railway environment.

Analytical Framework for Full-Dimensional Massive MIMO With Ray-Based Channels

The performance of baseband beamforming in multi-user multiple-input multiple-output (MU-MIMO) systems has been extensively studied for simplified statistical channel models where no angular parameters are taken into account. In contrast, there is little performance analysis with ray-based models, which are more physically motivated, feature prominently in standardization and have been experimentally validated. Thus, unlike previous studies, we present a mathematical framework to analyze the performance of zero forcing (ZF) and minimum mean-squared error (MMSE) combining. Using a central result for averaging in the angular domain, we derive tight approximations for the uplink signal-to-noise ratio and signal-to-interference-and-noise ratio (SINR) for ZF and MMSE processing, respectively, and the resulting spectral efficiencies. The remarkably simple expressions offer the following insights into the effects of the propagation environment. We demonstrate an improvement in performance when moving from vertical uniform rectangular array (URA) to horizontal URA to uniform linear array (ULA) antenna configurations. There is also a corresponding increase in the robustness of the performance to propagation scenarios. We demonstrate that under specific conditions increasing the angular spread can decrease the SINR for a ULA - an unexpected behavior which we link to the effects of end-fire radiation. Furthermore, our results allow us to investigate the impact of different array configurations and system parameters on the rate of convergence to favorable propagation conditions. Finally, we evaluate the spatial correlation properties intrinsically present in ray-based models, and compare them to the commonly used simple exponential model which yields equal, fixed correlation characteristics for each user.

V2V channel characterization and modeling for underground parking garages

As an important part of the intelligent transportation system (ITS), vehicle-to-vehicle (V2V) communication will improve the safety and efficiency of the transportation system by realizing the information transmission between vehicles. The underground parking garage is a typical scenario in V2V communication, but as an indoor environment, it is different from the conventional outdoor road scenarios significantly. Therefore, the purpose of this paper is to analyze and model the V2V channel characteristics of an underground garage. In this paper, a novel channel measurement carried out in the underground garage is first introduced. Considering that there are a lot of obstacles in an underground garage, the non-line-of-sight (NLoS) condition is taken into account during the measurement. Then, channel characteristic parameters, including large-scale fading, delay spread, and K-factor, are analyzed based on the measured data. In addition, we also carry out channel angle of arrival analysis, show the distribution of azimuth of arrival (AOA) and elevation of arrival (EOA) of multipath components (MPC), and conduct a statistical analysis of the angular spread. In the process of analysis and modeling, we focus on the differences between line-of-sight (LoS) and NLoS conditions. The analysis and conclusions presented in this paper will enrich the understanding of V2V channel and benefit the design of V2V communications.

Spatial and temporal resolution in entangled ghost imaging

ABSTRACTWe show that, when the integration time of the single photon detectors is longer than the correlation time of the biphoton, the attainable spatial resolution in ghost imaging with entangled signal-idler pairs generated in type II spontaneous parametric down conversion is limited by the angular spread of single-frequency-signal-idler pairs. If, however, the detector integration time is shorter than the biphoton correlation time, the transverse k-vectors of different spectral components combine coherently in the image, improving the spatial resolution.

Simulation of a radiobiology facility for the Centre for the Clinical Application of Particles

The Centre for the Clinical Application of Particles’ Laser-hybrid Accelerator for Radiobiological Applications (LhARA) facility is being studied and requires simulation of novel accelerator components (such as the Gabor lens capture system), detector simulation and simulation of the ion beam interaction with cells. The first stage of LhARA will provide protons up to 15 MeV for in vitro studies. The second stage of LhARA will use a fixed-field accelerator to increase the energy of the particles to allow in vivo studies with protons and in vitro studies with heavier ions.BDSIM, a Geant4 based accelerator simulation tool, has been used to perform particle tracking simulations to verify the beam optics design done by BeamOptics and these show good agreement. Design parameters were defined based on an EPOCH simulation of the laser source and a series of mono-energetic input beams were generated from this by BDSIM. The tracking results show the large angular spread of the input beam (0.2 rad) can be transported with a transmission of almost 100% whilst keeping divergence at the end station very low (<0.1 mrad). The legacy of LhARA will be the demonstration of technologies that could drive a step-change in the provision of proton and light ion therapy (i.e. a laser source coupled to a Gabor lens capture and a fixed-field accelerator), and a system capable of delivering a comprehensive set of experimental data that can be used to enhance the clinical application of proton and light ion therapy.

Hierarchical Twinning Induced Texture Weakening in Lean Magnesium Alloys

Rolled and annealed Mg-1wt. %Zn-1wt. %Gd-0.6wt. %Zr (ZEK110) alloys were subjected to room temperature in-plane compression along the rolling direction, followed by isochronal annealing treatments for 1 h. Despite a starting orientation favoring c-axis extension, the as-deformed microstructure revealed a hierarchical network of twins with profuse quantities of second and third generation twinning appearing within primary tension and compression twins. Complex twin-twin and twin–particle interactions were accompanied by the activation of both basal and non-basal dislocation slip in the neighborhood. While the population density of twins nucleated was significantly high, twin growth was severely retarded due to the presence of secondary phases. In terms of the overall distribution of grain orientations, the as-deformed texture displayed a rather weak basal component with a large portion of the basal poles aligned towards the longitudinal direction with an angular spread of ± 30°. Recrystallization commenced within twins, at twin-twin intersections at lower annealing temperatures and occurred additionally near secondary phases at higher annealing temperatures, giving rise to diverse orientations of both basal and off-basal character. Subsequent growth led to favorable coarsening of the off-basal orientations resulting in an overall texture weakening. The findings provide critical insights with respect to engineering high-strength, high-ductility lean magnesium alloys, comprising hierarchical microstructures that are not only associated with favorable crystallographic textures but additionally display a multi-scale strengthening behavior.

Estimation for Two-Dimensional Incoherently Distributed Source in Double L-Shape Arrays

Distributed sources can be regarded as an assembly of point sources within a spatial distribution. In this paper, we explore the estimation of the two-dimensional incoherently distributed sources using double L-shape arrays. The rotational invariance properties of the nominal elevation and nominal elevation are firstly obtained by taking first-order Taylor series expansions with regard to the generalized steering vectors of two pairs of parallel subarrays. The rotation operators can be solved based on signal subspace. Then the nominal elevation and nominal elevation can be obtained from parameters matching method. Estimation of direction of arrival can be used in multi-source scenario and needn't peak-finding search. Lastly the angular spreads can be solved through two-dimensional Capon search based on nominal angles. The simulation experiments show that the proposed method has good performance on the estimation of two-dimensional incoherently distributed sources. Investigating different experimental conditions, sources with different angular spreads, simulations are conducted to validate better estimation accuracy of the proposed method.