Beam Envelope Research Articles

Optical-thermal modeling and geographic analysis of dusty radiative cooling surfaces

Radiative cooling for energy conservation and harvesting has gained considerable attention worldwide in recent years. The optical and thermal characteristics of radiative cooling surface at outdoors is susceptible to dust soiling. Currently, limited research has been devoted to this problem. This work presents a model utilizing the beam envelope method and effective medium theory to investigate the optical characteristics of radiative cooling surfaces impacted by dust soiling. Subsequently, an optical-thermal coupling model is developed to investigate the spectral emissivity, average absorption/emission rates, and net heat gain of radiative cooling surfaces affected by dust soiling. The effect of dust soiling on the absorption of solar radiation is evident, with a substantial effect observed, but its impact on emissivity in the atmospheric window bands is comparatively minor. Meanwhile, it could be noticed that the mechanisms by which various material parameters of dust layers impact optical characteristics are different. In addition, the integration of the optical-thermal coupling model with EnergyPlus meteorological data and MERRA-2 facilitates an exploration of the geographical distribution of radiative cooling power variations induced by dust soiling across different regions in China. It is concluded that regions in China with a higher potential for radiative cooling often characterized by elevated dust deposition rates and low precipitation, rendering them more susceptible to the adverse effects of dust soiling. Effective cleaning significantly enhances both the optical and thermal characteristics of radiative cooling surfaces. The findings of this study offer a theoretical foundation for the enduring application of radiative cooling technology.

Experimental demonstration of radius-varying active plasma lensing for laser-plasma-accelerated proton beams with increased collection angle

Active plasma lensing, a compact method for intensifying the focus of charged particle beams by providing a magnetic field gradient of kT/m, has emerged as a sought-after technology in laser plasma accelerator applications. However, the utilization of active plasma lenses faces significant hurdles when dealing with laser-driven proton pulses, characterized by their broad bandwidth and high divergence. To address this challenge, we developed a novel active plasma lens with a variable radius, specifically designed to optimize lens geometry in accordance with the beam envelope, and performed the first measurement of its focusing ability. The experimental findings reveal that, compared to conventional cylindrical active plasma lenses, our radius-varying lens exhibits a 2.0-fold improvement in single-energy transmission efficiency, while maintaining comparable achromatic ability. This breakthrough is anticipated to significantly contribute to the miniaturization of laser proton accelerators. Published by the American Physical Society 2024

Machine learning surrogate for charged particle beam dynamics with space charge based on a recurrent neural network with aleatoric uncertainty

In this work, we develop a machine learning (ML) model with aleatoric uncertainty for the low energy beam transport (LEBT) region of the LANSCE linear accelerator in which we model the transport of a space-charge-dominated 750 keV proton beam through a lattice of 22 quadrupole magnets. Our ML model is developed based on data generated by a Kapchinsky–Vladimirsky (KV) envelope model of beam transport. We show that a recurrent neural network can be used as a dynamical surrogate model for fast prediction of the LEBT beam envelope. Furthermore, we endow the model with the prediction of aleatoric uncertainty and compare three different approaches. We demonstrate that the ML-based uncertainty quantification models are well calibrated and produce good estimates of the regions where the model is less certain about its predictions. This ML framework is a necessary step in the development of a real-time virtual diagnostic tool with uncertainty quantification that can be integrated into more complex downstream tasks (e.g., adaptive control or learning flexible control policies via reinforcement learning) for improved efficiency in beam operations. In future work, we plan to expand on this preliminary study by considering more realistic envelope models that include longitudinal momentum spread and dispersive effects in bending magnets, as well as particle tracking codes with 3D space charge (such as and ). Published by the American Physical Society 2024

Installation and integrated testing of magnets for the ESS Linac

The European Spallation Source (ESS) linear accelerator is designed to accelerate a 62.5 mA, 2.86 ms, 14 Hz proton beam up to 2 GeV at the entrance of a rotating tungsten (W) target. Normal Conducting Linac (NCL) and Superconducting Linac (SCL) parts of the accelerator contain over 200 quadrupole, dipole and corrector magnets of different types and parameters for beam envelope and trajectory control. All magnets have been provided to ESS by in-kind collaborators and research institutes across Europe. Following the delivery of these components and their respective power converters to ESS, this proceeding presents the status of the installation together with the methodology and first obtained results from system local and integrated testing phase.

Measurement and Analysis of the Intensity-dependent Effects on the CSNS Medium Energy Beam Transport line

A power upgrade project (CSNS-II) has been approved in 2021 to increase the beam power to 500 kW for the China Spallation Neutron Source (CSNS), for which the Linac energy will reach 300 MeV and the beam intensity is expected to be 50 mA. In this study, the beam measurement results given by the wire scanners at various current intensities, and the numerical modeling and fitting methods to obtain the evolution of the beam envelope and emittance along the CSNS medium energy beam transfer (MEBT) are given and discussed. Finally, new matching results have been given to control the emittance blowup.

Propagation characteristics of astigmatic elliptic vortex beams in nonlocal media with anisotropic nonlinearity

The propagation characteristics of the astigmatic elliptic vortex beam (AEVB) in nonlocal media with isotropic and anisotropic nonlinearities are analytically and numerically investigated. Three different kinds of dynamics are discussed: AEVB in nonlinear isotropic media, elliptic vortex beam in nonlinear anisotropic media, and astigmatic circular vortex beam (ACVB) in nonlinear anisotropic media. The isotropic nonlinearity cannot support the formation of elliptic vortex solitons for any input power, and the initial phase-front curvature (IPC) will induce the pulsating behavior of the AEVB. For anisotropic nonlinearity, when the nonlinear anisotropic parameter (NAP) of the medium and the initial ratio of the beam width satisfy γx/γy = (w0y/w0x)2, a quasi-stable elliptic vortex soliton can be obtained, in which the vortex angular velocity and vortex rotation angle evolve periodically. Circular vortex soliton cannot exist in nonlinear anisotropic media, and the rotation of the ACVB can be controlled by adjusting the NAP and the IPC. In both isotropic and anisotropic nonlinear media, the rotation direction of the vortex core is opposite to that of the beam envelope.

Design and simulation of a radius-varying active plasma lens for focusing laser-accelerated protons

With plasma as the medium, laser driven accelerator can generate high energy pulsed particles with initially micron scale source size and broad energy spread. Efficient transport of the beam with above characteristics is a challenging task. Active plasma lenses (APLs) provide strong, tunable, axisymmetric focusing with weak chromatic dependencies, making them highly beneficial for compact laser accelerators. However, due to the large divergence angle of the laser-accelerated proton beams, a majority of the protons cannot be transmitted through the APLs, resulting in losses. In this study, we propose a radius-variable APL (RV-APL) that optimizes the capillary’s geometry based on the beam envelope. The collection angle of RV-APL under the design parameters in the article can be increased by about 80% compared to a cylindrical APL (C-APL) with a constant radius, achieving the transmission capacity of traditional magnets. The magnetic field distribution of the RV-APL was predicted using a two-dimensional axisymmetric plasma discharge model, and the focused beam’s chromaticity and emittance properties were theoretically analyzed through particle tracking simulation.

Instability and halo for double-periodic focusing structure

This paper presents a comprehensive investigation into the beam transport issue in the double-period focusing structure of low energy proton/ion cryogenic superconducting linear accelerators (CSLA). The structure, consisting of a large period corresponding to a cryogenic refrigeration module and a small period of focusing solenoid, is modeled with solenoids and drifts to study the impact of higher-order resonances on beam halo in transverse phase spaces without considering the acceleration. Numerical simulation reveals that the double-period structure can cause beam envelope instability even when the small periodic zero-current phase advance (σ0f) is less than 90°, making it more susceptible than the fully periodic structure. The study identifies that 2:1 resonance remains a major factor contributing to the formation of beam halo, particularly in the double-period structure. Furthermore, through exploration of different matching modes, the paper elucidates that not only 2:1 resonance but also higher-order resonances can lead to beam halo formation for space charge dominated beam. The oscillation frequency spectrum of the matched envelope is obtained using fast Fourier transform. It is recommended that particle oscillation frequencies avoid matching the identical fundamental frequency and its multiples of the matched envelope. Overall, the findings of this study can serve as valuable references for the design and optimization of CSLA.

A Geant4 based simulation platform of the HollandPTC R&D proton beamline for radiobiological studies

A Geant4 based simulation platform of the Holland Proton Therapy Centre (HollandPTC, Netherlands) R&D beamline (G4HPTC-R&D) was developed to enable the planning, optimisation and advanced dosimetry for radiobiological studies. It implemented a six parameter non-symmetrical Gaussian pencil beam surrogate model to simulate the R&D beamline in both a pencil beam and passively scattered field configuration. Three different experimental proton datasets (70 MeV, 150 MeV, and 240 MeV) of the pencil beam envelope evolution in free air and depth-dose profiles in water were used to develop a set of individual parameter surrogate functions to enable the modelling of the non-symmetrical Gaussian pencil beam properties with only the ProBeam isochronous cyclotron mean extraction proton energy as input. This refined beam model was then benchmarked with respect to three independent experimental datasets of the R&D beamline operating in both a pencil beam configuration at 120 and 200 MeV, and passively scattered field configuration at 150 MeV. It was shown that the G4HPTC-R&D simulation platform can reproduce the pencil beam envelope evolution in free air and depth-dose profiles to within an accuracy on the order of ±5% for all tested energies, and that it was able to reproduce the 150 MeV passively scattered field to the specifications need for clinical and radiobiological applications.

Photonic Möbius topological insulator from projective symmetry in multiorbital waveguides.

The gauge fields dramatically alter the algebraic structure of spatial symmetries and make them projectively represented, giving rise to novel topological phases. Here, we propose a photonic Möbius topological insulator enabled by projective translation symmetry in multiorbital waveguide arrays, where the artificial π gauge flux is aroused by the inter-orbital coupling between the first (s) and third (d) order modes. In the presence of π flux, the two translation symmetries of rectangular lattices anti-commute with each other. By tuning the spatial spacing between two waveguides to break the translation symmetry, a topological insulator is created with two Möbius twisted edge bands appearing in the bandgap and featuring 4π periodicity. Importantly, the Möbius twists are accompanied by discrete diffraction in beam propagation, which exhibit directional transport by tuning the initial phase of the beam envelope according to the eigenvalues of translation operators. This work manifests the significance of gauge fields in topology and provides an efficient approach to steering the direction of beam transmission.

Finite element analysis of beam optics, high voltage ceramic seal and focusing magnet for CEPC C-band 80 MW klystron

For CEPC LINAC, the first 80 MW class klystron at C-band (5720 MHz) has been proposed to accelerate a high-current beam at an acceleration gradient of 45 MV/m. This paper discusses the design and simulation of beam optics, beam envelope, drift tube, and solenoid of the klystron. Space charge beam current of 390 A at an acceleration potential of 410 kV is obtained with average cathode loading of less than 6.0 A/cm2. The maximum surface electric field at the beam optics and high voltage ceramic seal have been evaluated less than 22 kV/mm and 4.50 kV/mm, respectively. The electron beam with an average beam radius of 5.40 mm has successfully been transported to the interaction cavity with beam ripple rate around 4.0% with low scalloping factor and a fill factor of 0.670. 2-D, DGUN & POISSON simulations results are validated by the 3-D CST simulation and found in agreement.

Predicting the transverse emittance of space charge dominated beams using the phase advance scan technique and a fully connected neural network

The transverse emittance of a charged particle beam is an important figure of merit for many accelerator applications, such as ultra-fast electron diffraction, free electron lasers and the operation of new compact accelerator concepts in general. One of the easiest to implement methods to determine the transverse emittance is the phase advance scan method using a focusing element and a screen. This method has been shown to work well in the thermal regime. In the space charge dominated laminar flow regime, however, the scheme becomes difficult to apply, because of the lack of a closed description of the beam envelope including space charge effects. Furthermore, certain mathematical, as well as beamline design criteria must be met in order to ensure accurate results. In this work we show that it is possible to analyze phase advance scan data using a fully connected neural network (FCNN), even in setups, which do not meet these criteria. In a simulation study, we evaluate the perfomance of the FCNN by comparing it to a traditional fit routine, based on the beam envelope equation. Subsequently, we use a pre-trained FCNN to evaluate measured phase advance scan data, which ultimately yields much better agreement with numerical simulations. To tackle the confirmation bias problem, we employ additional mask-based emittance measurement techniques.

Coil Error Analysis of a Curved Canted-Cosine-Theta Magnet Applied to a Superconducting Gantry

A lightweight superconducting (SC) gantry with large momentum acceptance is under research at Huazhong University of Science and Technology (HUST). The curved alternating gradient Canted-Cosine-Theta (AG-CCT) magnets are the essential components of this SC gantry. This study presents the findings on coil error analysis of the AG-CCT magnets. Based on multi-line model using Biot-Savart law, a coil model built with coil winding error is developed, and it is adopted to evaluate the influence on the proton beam. The tolerance of coil winding is determined by the study on different error amplitude in the beam envelope. Additionally, the compensation method is also discussed.

Coupling Design of Combined Function Bending Magnet and High Current Electron Beam for 100 kW Irradiation Accelerator

RF system is one of the key components of a 2 GeV, 6 MW high power fixed field alternating gradient (FFAG) accelerator being designed at China Institute of Atomic Energy (CIAE). In order to verify the design principle and the engineering feasibility, a scaled-down RF cavity is under construction, which can also be used as the main accelerating cavity for a 100 kW electron irradiation accelerator. By the deflections of several 180 degrees bending magnets, the electrons emitted from the electron gun can pass the cavity for multiple times. Due to the limited length of the cavity, the deflection radius should be as small as possible to ensure more acceleration times and consequently higher beam power, which will make the fringe field effect of the bending magnet be a problem. In addition, the bending magnet should provide beam focusing in both the radial and axial directions of the beam and keep the beam envelope stable during the whole acceleration process. Based on the above reasons, the parameters of the combined function bending magnet, such as the pole face rotations (Liu, 1994), magnetic field gradient and shielding distance, have a great influence on the beam dynamics behavior in the accelerator, which bring great difficulty to the design of the combined function bending magnet. In this paper, the coupling design of the bending magnet with focusing function and the high intensity electron beam for this 100 kW irradiation accelerator will be illustrated in detail.

Theoretical Analysis and Simulated Verification of Circular Beam Electron Optical System for Terahertz Vacuum Electron Devices

The micro-structure miniaturization of vacuum electron devices (VEDs) in terahertz band requires the high precision description of formation and transportation for the ultrahigh density electron beams with the novelty theoretical analysis and simulation. In this article, the 3-D spatial potential of circular electron beam formation is presented by adopting differential geometry and tensor analysis theory. The specific numerical solution of the circular electron beam potential can be obtained by establishing a moving coordinate frame, carrying out numerical calculation, series expansion with adopting of Riemann method. In order to verify the complex theory, a 0.34 THz vacuum electron tube using thin circular electron beam gun and corresponding optical system are designed and analyzed. The beam voltage and current are 23.8 kV and 0.28 A respectively, with beam channel radius of 0.16 mm and magnetic field of 5250 Gs. Using 3-D simulation of CST, the ultrahigh density electron beam can transport 30 mm with good stability. The spatial potential of numerical calculation with the obtained theoretical functions and the simulation model are compared to verify our theoretical description. Both the results are with good consistent which preliminary proves the effectiveness of the theoretical function. Furthermore, we combine the theoretical function potential with the electron beam envelope under the same coordinate frame, the detailed characteristics of ultrahigh density beam formation with the potential function can be directly understood easily, which will give the deep physical mechanics vision for the development of high power and high-efficiency terahertz tube in the future.

Rotating optical vortex clusters in competing cubic-quintic media

We put forward a rich variety of optical vortex structures nested in a localized beam envelope supported by cubic-quintic media confined in a rotating harmonic trap. The globally linked vortex cluster comprises an even number of vortices with topological charges equaling 1 and $\ensuremath{-}1$ alternately. In the nonrotating frame, single-charged vortices reside evenly on a ring. Yet, the system rotation induces the Coriolis force, which in turn leads to a strong asymmetry of the vortex cluster. With the increase of rotation frequency, vortex clusters with a different number of vortices transform into rotating nonlinear states with different symmetries. Meanwhile, the beam envelope is deformed obviously. Nonrotating vortex clusters are stable provided that their power exceeds a certain critical value. Unstable rotating states are very robust and survive over thousands of diffraction lengths.

A beam dynamics study for efficient extraction from the VECC K500 superconducting cyclotron

Beam extraction from the K500 superconducting cyclotron is very challenging due to its long extraction path of about 330° with very high magnetic field gradient and low acceptance. The initial attempts of beam extraction showed that the pilot beams could successfully clear only a few magnetic channels. To investigate the causes of the beam loss, magnetic field measurements in different zones of the cyclotron were repeated. The total field has been generated by superposition of the measured field and the numerically computed field of the extraction elements. Based on these data, extensive beam dynamics simulation has been carried and beam distribution at the entry of the extraction path obtained by accelerated orbit calculations. The energy spread due to multi-turn deflector entry has been included in the simulation. The beam envelope along the extraction path as well as the beam transmission efficiency has been determined for different positions of the extraction elements. The radial positions of the extraction elements have been optimized parametrically to compute the maximum transmission efficiency. The present studies have enabled us to successfully extract the pilot beam (N4+) from the cyclotron with beam current of around 20 enA.

Numerical Simulation of the Self-Imaging at Different Cascaded Optical Fiber Specifications

Cascaded optical fiber single mode-no core-single mode fiber (SNS) attracted attention for being the base of various photonic devices. These devices are used in optical communication, fiber sensors, and fiber laser technology. The effect of variable NCF specifications, length, diameter, external refractive index (ERI), propagating wavelength on the self-imaging position, and the multimode interference (MMI) is studied. The study aims to simulate and analyze cascaded optical fiber by using the finite element beam envelope method (BEM). To the best of our knowledge, this is the first report that studied the self-imaging in cascaded optical fiber longitudinally by using BEM. The NCF length is important in determining the coupled out intensity and peak transmission wavelength. The field in the cascaded fiber is simulated for single and multi-wavelengths to evaluate the maximum transmission and study the structure's tunability. A tunable filter is simulated, where varying the length of the NCF about 0.6 mm produces a wavelength shift of about 40 nm. The BEM is effective in studying the field propagation in large guiding photonic devices

Stable transport of relativistic electron beams in plasmas

The long-distance stable transport of relativistic electron beams (REBs) in plasmas is studied by full three-dimensional particle-in-cell simulations. Theoretical analysis shows that the beam transport is mainly influenced by three transverse instabilities, where the excitation of self-modulation instability, and the suppression of the filamentation instability and the hosing instability are important to realize the beam stable transport. By modulating the transport parameters such as the electron density ratio, the relativistic Lorentz factor, the beam envelopes and the density profiles, the relativistic bunches having a smooth density profile and a length of several plasma wave periods can suppress the beam-plasma instabilities and propagate in plasmas for long distances with small energy losses. The results provide a reference for the research of long-distance and stable transport of REBs, and would be helpful for new particle beam diagnosis technology and space active experiments.

Ion-focused propagation of a relativistic electron beam in the self-generated plasma in atmosphere

It is known that ion-focused regime (IFR) can effectively suppress expansion of a relativistic electron beam (REB). Using the particle-in-cell Monte Carlo collision (PIC-MCC) method, we numerically investigate the propagation of an REB in neutral gas. The results demonstrate that the beam body is charge neutralization and a stable IFR can be established. As a result, the beam transverse dimensions and longitudinal velocities keep close to the initial parameters. We also calculate the charge and current neutralization factors of the REB. Combined with envelope equations, we obtain the variations of beam envelopes, which agree well with the PIC simulations. However, both the energy loss and instabilities of the REB may lead to a low transport efficiency during long-range propagation. It is proved that decreasing the initial pulse length of the REB can avoid the influence of electron avalanche. Using parts of REB pulses to build a long-distance IFR in advance can improve the beam quality of subsequent pulses. Further, a long-distance IFR may contribute to the implementation of long-range propagation of the REB in space environment.