High-energy Electron Accelerators Research Articles

Development of a 90–600 MHz Meter-wave Solar Radio Spectrometer

Radio observation is important for understanding coronal mass ejections (CMEs), coronal shock waves, and high-energy electron acceleration. Here, we developed a new Chashan broadband solar radio spectrometer at a meter wavelength for observing the (super)fine structure of the solar radio burst spectrum. In the signal-receiving unit, we adopt an antenna system consisting of a 12 m large-aperture parabolic reflector and dual-line polarized logarithmic periodic feed source, as well as a high-precision Sun-tracking turntable system, all of which ensure the high-precision acquisition of solar radiation signals. For the digital receiver, we use a high-speed analog-to-digital converter with a sampling rate of 1.25 GSPS to directly sample the signal amplified and filtered by the analog receiver, simplifying the structure of the analog receiver, and design a 16k-point fast Fourier transform algorithm in the field programmable gate array to perform time–frequency transformation on the sampled signals. The default frequency and temporal resolution of the system are 76.294 kHz and 0.839 ms (up to 0.21 ms), respectively. The noise coefficient of the system is less than 1 dB, the dynamic range is more than 60 dB, and the sensitivity is as high as 1 sfu. We have observed a large number of radio bursts, including type I radio storms, hundreds of type III, ∼20 type II, and ∼15 type IV bursts in the past year. These high-quality data are useful in the further study of CMEs and associated particle acceleration and the origins of solar radio bursts.

Electron beam irradiation-induced co-grafting and degradation of cotton cellulose with ionic liquid

We developed a cellulose upgrading method based on electron beam processing technology by using a carbon-carbon double bond-containing ionic liquid as an auxiliary reagent. The irradiation of cellulose was implemented by a 10 MeV high-energy electron accelerator or a 200 keV low-energy electron accelerator. The established method enabled grafting of the ionic liquid onto cellulose while triggering an irradiation degradation of cotton cellulose. By employing irradiation co-grafting degradation with a high-energy electron accelerator in the dose range of 30–40 kGy, highly uniform cotton cellulose with a high degree of polymerization (DP) of 500 and low degree of polymerization deviation (DPD) was obtained. By using a low-energy accelerator in the dose range of 4–6 kGy, cotton cellulose with a high DP of 1000 and low DPD was prepared. This method to upgrade cellulose has advantages of simple operation, short reaction time, and minimal amount of chemical reagents. It opened an avenue to access high quality of cellulose that cannot be attained with other method.

Analysis of frequency chirping on the electron bunch acceleration during Laguerre–Gauss laser interaction

We intend to examine the impact of frequency chirping on the efficient vacuum acceleration of an electron bunch using a chirped Laguerre–Gauss (LG) laser pulse. The study demonstrates that achieving high-energy electron acceleration relies not only on significant pulse asymmetry but also on ensuring phase synchronization in the LG laser interaction. The findings highlight the feasibility of generating outgoing electron bunches with high energy and low divergence through the selection of an optimal chirping value. Furthermore, we establish that chirping the LG laser pulse facilitates the acceleration of electrons to high energy using a lower laser intensity compared to the scenario without chirping.

X-ray fluorescence (XRF) analysis and prediction of induced radioactivity components for high-energy electron accelerator

The objective of this research was to predict the induced activation in high-energy particle accelerator components to determine the isotope inventories and remanent dose rates for considering the replacement materials. The long-lived radionuclides produced by nuclear reactions caused biological health effects to human as well as environmental impacts. The bremsstrahlung photon beam by the high-energy electron accelerator produced the radioactive nuclei from direct interactions or indirect interactions. The study highlighted the determination of chemical compositions and the estimation of induced activation in particle accelerator components. The elemental compositions of materials were investigated by the X-ray fluorescence (XRF) technique. The heavy metal elements were detected with different percentages, i.e., Zr, Zn, Ti, Pb, Ni, Mn, Fe and Cu. To calculate the induced activation in the materials, the FLUKA simulation was performed. The 1.2 GeV high-energy electron beam had been defined to irradiate the materials for this simulation. The daughter nuclei were obtained, and their decay followed. The estimation of the residual dose rate in activated materials after various cooling times from the end of the irradiation was presented.

Controllable electron self-injection in laser wakefield acceleration with asymmetric gas-jet nozzle

AbstractBeam charge control in the staging of laser wakefield acceleration (LWFA) is a crucial technique for developing full-optical jitter-free high-energy electron accelerators. Precise control of total charge in pre-accelerated electron bunches is necessary to achieve practical electron beam characteristics in the final acceleration stage(s). In contrast to the well-known cathode techniques in conventional accelerators, in LWFA the electron injection results from non-linear processes originating from plasma wave breaking. Therefore, the development of charge control requires a deep understanding of the electron self-injection processes and applications of non-trivial tools. The use of asymmetric gas-jet nozzles seems to be a promising way in developing charge control via tuning the target parameters such as plasma density, density slope, and acceleration length. Here, we demonstrate and characterize controllable electron self-injection, owing to a parametric resonance in slantwise density gas jets irradiated by 50 TW femtosecond laser pulses. The measured characteristics of the electron bunches, in which charge and energy distribution depend on the gas density and gas density gradient, agree well with those obtained by multidimensional particle-in-cell simulation and confirm the possibility of charge control.

Determination of Optimum Materials Thickness for Converting E-Beam Energy into Bremsstrahlung X-Rays on a 10 MeV High Energy Electron Accelerator Using MCNP

The e-beam produced by the high-energy electron accelerator has a relatively small penetrating power. To increase the penetrating power, a converter must convert the e-beam energy into bremsstrahlung x-rays. This research aims to determine the optimum thickness of tantalum, tungsten, and lead as a converter of e-beam energy into bremsstrahlung x-rays at a 10 MeV high-energy electron accelerator. The optimum thickness of tantalum, tungsten, and lead is determined using simulation with MCNPX software. The electron source modeling with an energy of 10 MeV is made in the form of a flat plane with a size of 120 cm by 10 cm at a distance of 1 mm from the converter. The converter has dimensions of 160 cm by 24 cm and its thickness varies from 1 - 7.5 mm. Then two planar detectors are placed at a distance of 2 cm in front and behind the converter. The simulation results show that the optimum thickness for tantalum, tungsten, and lead converter is 2.0 mm, 1.8 mm, and 2.8 mm respectively. The maximum forward scattered bremsstrahlung x-rays energy are 2.1137 MeV, 2.1287 MeV, and 2.1850 MeV, respectively. And the maximum conversion efficiency is 21.137%, 21.287%, and 21.850%, respectively. These results can be used as a reference in the design of the converter for the 10 MeV high-energy electron accelerator.

Optimization of X-Ray Spectra in Medical Sterilization Accelerator through MCNP

A Bremsstrahlung converter for medical apparatuses sterilization has been designed using tantalum and stainless-steel frame for a high energy electron accelerator operating at an incident electron energy of 7.5 MeV. Optimization of the x-ray spectra of the converter is investigated in this paper by adding a beam hardening filter from copper, aluminum, and stainless steel. The MCNP code is used to simulate the particle transport through the converter design. To survey the effect of each filtered spectra, angular dose distribution in the air is measured at radial distances of 50, 75, and 100 cm from the tantalum for the angle of 0° to 90°. A comparison between the unfiltered and the filtered spectra shows a marked decrease in low energy photon intensity up to the energy of 100 keV by all three filter materials with a greater reduction and hardening when combining the filter materials. These findings result in the angular dose distribution of the produced x-ray having a lower intensity at closer radial distances to the target due to the decreased intensity of x-rays with low energy, denoting a better dose homogeneity over the irradiated material.

Electron relay acceleration in wakefields driven by a single laser interacting with multi-stage plasma channels

One limitation in high energy and high-efficiency electron acceleration by laser-driven plasma wakefield is the dephasing due to the accelerated electrons surpassing the acceleration phase of the wake. Here, by utilizing multi-stage plasma channels with different densities, we show in simulations that electrons can jump from a back acceleration bubble into a front one before getting into the deceleration phase and obtain relay acceleration in the front bubble when the laser steps into a new stage of the plasma channel. In our numerical studies, the final maximum energy of the electrons by such relay acceleration can be several times higher than electrons accelerated in a single-stage plasma channel. The defocusing effects on the beam emittance and charge, caused by electrons crossing the high-density electron layer located between the neighboring bubbles, can be suppressed by appropriately connecting the staged channels. The current scheme helps to increase the acceleration energy and efficiency of laser wakefield accelerators.

Effect of nozzle curvature on supersonic gas jets used in laser–plasma acceleration

Supersonic gas jets produced by converging–diverging nozzles are commonly used as targets for laser–plasma acceleration (LPA) experiments. A major point of interest for these targets is the gas density at the region of interaction where the laser ionizes the gas plume to create a plasma, providing the acceleration structure. Tuning the density profiles at this interaction region is crucial to LPA optimization. A “flat-top” density profile is desired at the line of interaction to control laser propagation and high-energy electron acceleration, while a short high-density profile is often preferred for acceleration of lower-energy tightly focused laser–plasma interactions. A particular design parameter of interest is the curvature of the nozzle's diverging section. We examine three nozzle designs with different curvatures: the concave “bell,” straight conical, and convex “trumpet” nozzles. We demonstrate that for mm-scale axisymmetric nozzles that, at mm-scale distances from the nozzle exit, curvature significantly impacts shock formation and the resulting gas jet density field and, therefore, is an essential parameter in LPA gas jet design. We show that bell nozzles are able to produce focused regions of gas with higher densities. We find that the trumpet nozzle, similar to straight and bell nozzles, can produce flat-top profiles if optimized correctly and can produce flatter profiles at the cost of slightly wider edges. An optimization procedure for the trumpet nozzle is derived and compared to the straight nozzle optimization process. We present results for different nozzle designs from computational fluid dynamics simulations performed with the program ANSYS Fluent and verify them experimentally using neutral density interferometry.

Separation of Flux/Dose Contributions from Multiple Transport Pathways in Monte Carlo Shielding Analyses of a Consolidated Interim Spent Nuclear Fuel Storage Facility

The characteristics of the radiation field around a consolidated interim spent nuclear fuel storage facility were investigated comprehensively through Monte Carlo simulations. Neutron and gamma-ray flux/dose contributions from multiple transport pathways, including direct, streaming, skyshine, groundshine, and multishine, were isolated using a modified version of the method that was originally developed by Oh et al. [J. Korean Phys. Soc., Vol. 69, 1057 (2016)] for the evaluation of neutron skyshine from a high-energy electron accelerator. The application of the methodology was demonstrated in this paper, and the flux/dose contributions of individual pathways were examined and compared. The results provided additional insight into how the radiation propagated from the source to off-site locations. The modified method for separating five transport pathways can provide valuable information for shielding optimization during the design phase and is generally applicable to Monte Carlo shielding analyses of other nuclear facilities.

Optimization of the electron beam dump for a GeV-class laser electron accelerator

The advances of laser-driven electron acceleration offer the promise of great reductions in the size of high-energy electron accelerator facilities. Accordingly, it is desirable to design compact radiation shielding for such facilities. A key component of radiation shielding is the high-energy electron beam dump. In an effort to optimize the electron beam dump design, different material combinations have been simulated with the FLUKA Monte Carlo code in the range of 1–40 GeV. The studied beam dump configurations consist of alternating layers of high-Z material (lead or iron) and low-Z material (high-density concrete or borated polyethylene) in either three-layer or five-layer structures. The designs of various beam dump configuration have been compared and it has been found that the iron and concrete stacking in a three-layer structure with a thick iron layer results in the lowest dose at 1, 10, and 40 GeV. The performance of the beam dump exhibits a strong dependence on the selected materials, the stacking method, the beam dump thickness, as well as the electron energy. This parametric study provides general insights that can be used for compact shielding design of future electron accelerator facilities.

Fabrication of large-area uniform carbon nanotube foams as near-critical-density targets for laser–plasma experiments

Abstract Carbon nanotube foams (CNFs) have been successfully used as near-critical-density targets in the laser-driven acceleration of high-energy ions and electrons. Here we report the recent advances in the fabrication technique of such targets. With the further developed floating catalyst chemical vapor deposition (FCCVD) method, large-area ( $>25\kern0.5em {\mathrm{cm}}^2$ ) and highly uniform CNFs are successfully deposited on nanometer-thin metal or plastic foils as double-layer targets. The density and thickness of the CNF can be controlled in the range of $1{-}13\kern0.5em \mathrm{mg}/{\mathrm{cm}}^3$ and $10{-}200\kern0.5em \mu \mathrm{m}$ , respectively, by varying the synthesis parameters. The dependence of the target properties on the synthesis parameters and the details of the target characterization methods are presented for the first time.

Characteristics of a Very High Energy Electron Beam in a Laser Wakefield Accelerator for Cancer Therapy

A cancer treatment device using a very high energy electron beam has been studied as a new radiation therapy method because of its deep penetration depth, dose characteristics that are better than those of the existing X-ray method, and easy adjustment of the irradiation direction compared to existing equipment. For this treatment, compact high-energy electron accelerators are essential, and a laser wakefield accelerator is a good candidate. In this study, very high energy electrons were accelerated using a laser wakefield accelerator, and the dose characteristics due to these electrons were measured to confirm the possibility of treatment. Electron beam with an energy of 94 MeV were accelerated by using a 16-TW high-power laser, and the dose profile was measured with a tough phantom by using a Gafchromic film. With an 18 nC charge, the peak dose was 120 cGy at a 40 mm depth. The measured dose profile was compared with that obtained using a Monte Carlo simulation with low and high energy electron beams, and the results indicate a quasi-mono-energetic property of the electron beam.

Evaluation of Skyshine from an Accelerator Facility: Dependence on Distance and Angle.

Neutron skyshine from Linac Coherent Light Source II 4 GeV electron beam operation at SLAC National Accelerator Laboratory can contribute to prompt radiation exposure to the public at distances far beyond the accelerator tunnel housing. One of the shielding design requirements at SLAC is that the annual dose to a member of the public is no more than 0.05 mSv y. This study uses Monte Carlo code FLUKA to simulate the generation of neutrons from 4 GeV electron beam losses on a thick copper target inside a generalized geometry of the Linac Coherent Light Source II Beam Transport Hall accelerator tunnel section. The effective dose from neutron skyshine was characterized as a function of both distance from the tunnel wall (up to 1 km away) and angle relative to the beam direction (between 0° and 180°). This new methodology for evaluating neutron skyshine dose is applicable to high-energy GeV-range electron accelerator facilities both at SLAC and elsewhere.

Response of PE films to low energy electron beam irradiation

In the past few years low energy electron beam units with energies in the range from 80 to 300 keV have been developed as a convenient and non-expensive alternative to high-energy electron accelerators. Some of these applications like surface decontamination or sterilization of medical supplies need the measurement of the dose in order to be in compliance with regulatory agencies. The demand to have dosimetry systems capable to deliver the irradiation dose using a simple procedure and having an appropriate calibration procedure will grow as these applications increase. Having a dosimeter with a response independent of the energy of the incident radiation would allow to calibrate it using high energy radiation sources and then use it at low-energy electron irradiations. In this paper the response of polyethylene (PE) thin films irradiated with low-energy electrons in the interval from 100 to 200 keV has been investigated with the main purpose of using them as dosimeters in low-energy electron irradiations. The response of the films was obtained from their infrared spectrum measured at their surface using the attenuated total reflectance technique (ATR). Analysis of the data obtained using the statistical technique ANOVA was performed to determine whether the energy of the radiation affects the response of the film. The results show that the response of the films depends on the energy of the electron beam used. The conclusion of this research points out that in order to use the PE film in low-energy electron irradiations the films need to be calibrated under the same conditions in which they will be used.

Relativistic electron acceleration by surface plasma waves excited with high intensity laser pulses

The process of high energy electron acceleration along the surface of grating targets (GTs) that were irradiated by a relativistic, high-contrast laser pulse at an intensity $I=2.5\times 10^{20}~\text{W}/\text{cm}^{2}$ was studied. Our experimental results demonstrate that for a GT with a periodicity twice the laser wavelength, the surface electron flux is more intense for a laser incidence angle that is larger compared to the resonance angle predicted by the linear model. An electron beam with a peak charge of ${\sim}2.7~\text{nC}/\text{sr}$ , for electrons with energies ${>}1.5~\text{MeV}$ , was measured. Numerical simulations carried out with parameters similar to the experimental conditions also show an enhanced electron flux at higher incidence angles depending on the preplasma scale length. A theoretical model that includes ponderomotive effects with more realistic initial preplasma conditions suggests that the laser-driven intensity and preformed plasma scale length are important for the acceleration process. The predictions closely match the experimental and computational results.

Coupling Effects in Multistage Laser Wake-field Acceleration of Electrons

Staging laser wake-field acceleration is considered to be a necessary technique for developing full-optical jitter-free high energy electron accelerators. Splitting of the acceleration length into several technical parts and with independent laser drivers allows not only the generation of stable, reproducible acceleration fields but also overcoming the dephasing length while maintaining an overall high acceleration gradient and a compact footprint. Temporal and spatial coupling of pre-accelerated electron bunches for their injection in the acceleration phase of a successive laser pulse wake field is the key part of the staging laser-driven acceleration. Here, characterization of the coupling is performed with a dense, stable, narrow energy band of <3% and energy-selectable electron beams with a charge of ~1.6 pC and energy of ~10 MeV generated from a laser plasma cathode. Cumulative focusing of electron bunches in a low-density preplasma, exhibiting the Budker–Bennett effect, is shown to result in the efficient injection of electrons, even with a long distance between the injector and the booster in the laser pulse wake. The measured characteristics of electron beams modified by the booster wake field agree well with those obtained by multidimensional particle-in-cell simulations.

Enhanced pair production in collisions of intense pulsed lasers with a high-energy electron beam

Production of electron-positron pairs by a relativistic electron beam propagating through an intense laser field is reconsidered by use of the Monte Carlo method within the quantum electrodynamics theory. The scenario of the SLAC experiment is analyzed [D. Burke et al., Phys. Rev. Lett. 79, 1626 (1997)], on the basis of which a scheme for pair production by a high-energy electron beam colliding with a composed laser field is proposed. The laser field consists of two components with different frequencies. We show that this composed laser field can facilitate pair creation by the double-photon Breit-Wheeler process and could significantly amplify the production probability. Our predictions can be tested by utilizing presently available high-energy electron accelerators and high-intensity laser facilities.

Particle Acceleration in Kinetic Simulations of Nonrelativistic Magnetic Reconnection with Different Ion–Electron Mass Ratios

Abstract By means of fully kinetic particle-in-cell simulations, we study whether the proton-to-electron mass ratio m i /m e influences the energy spectrum and underlying acceleration mechanism during magnetic reconnection. While kinetic simulations are essential for studying particle acceleration during magnetic reconnection, a reduced m i /m e is often used to alleviate the demanding computing resources, which leads to artificial scale separation between electron and proton scales. Recent kinetic simulations with high mass ratios have suggested new regimes of reconnection, as electron pressure anisotropy develops in the exhaust region and supports extended current layers. In this work, we study whether different m i /m e changes the particle acceleration processes by performing a series of simulations with different mass ratio (m i /m e = 25–400) and guide field strength in a low-β plasma. We find that mass ratio does not strongly influence reconnection rate, magnetic energy conversion, ion internal energy gain, plasma energization processes, ion energy spectra, and the acceleration mechanisms for high-energy ions. Simulations with different mass ratios are different in electron acceleration processes, including electron internal energy gain, electron energy spectrum, and the acceleration efficiencies for high-energy electrons. We find that high-energy electron acceleration becomes less efficient when the mass ratio gets larger because the Fermi-like mechanism associated with particle curvature drift becomes less efficient. These results indicate that when particle curvature drift dominates high-energy particle acceleration, the further the particle kinetic scales are from the magnetic field curvature scales (∼d i ), the weaker the acceleration will be.

Radiological clearance of equipment from high-energy electron accelerators: The example of LEP superconducting acceleration system

The lifespan of a particle accelerator includes its final decommissioning and the disposal of potentially activated materials, one of the major challenges faced at accelerator laboratories. This paper discusses the computational and experimental techniques adopted for clearing from regulatory control the superconducting acceleration system of the former CERN Large Electron Positron collider (LEP), consisting of nearly 450 tons of valuable metals. The paper first briefly recalls the major sources of activation at high-energy electron accelerators and then discusses the general process, the computational tools employed, the methods to determine the amount of radionuclides that cannot be measured by γ-spectrometry, the selection of samples representative of the whole equipment and the outcome of the operational phase. The process ended up with the clearance of 95% of the material. The aim of this work is to provide a practical example of an approach that could be applicable to similar projects targeting radiological clearance of high-energy particle accelerator equipment.