Divergence angle consideration in energy spread measurement for high-quality relativistic electron beam in laser wakefield acceleration

The thorough exploration of the transverse quality represented by divergence angle has been lacking yet in the energy spread measurement of the relativistic electron beam for laser wakefield acceleration(LWFA). In this work, we fill this gap by numerical simulations based on the experimental data, which indicate that in a C-shape magnet, magnetic field possesses the beam focusing effect, considering that the divergence angle will result in an increase in the full width at half maxima(FWHM) of the electron density distribution in a uniformly isotropic manner, while the length-to-width ratio decreases. This indicates that the energy spread obtained from the electron deflection distance is smaller than the actual value, regardless of the divergence angle. A promising and efficient way to accurately correct the value is presented by considering the divergence angle(for instance, for an electron beam with a length-to-width ratio of 1.12, the energy spread correct from 1.2% to 1.5%), providing a reference for developing the high-quality electron beam source.

Laboratory observation of electron energy distribution near three-dimensional magnetic nulls

The acceleration of electrons near three-dimensional(3D)magnetic nulls is crucial to the energy conversion mechanism in the 3D magnetic reconnection process.To explore electron acceleration in a 3D magnetic null topology,we constructed a pair of 3D magnetic nulls in the PKU Plasma Test(PPT)device and observed acceleration of electrons near magnetic nulls.This study measured the plasma floating potential and ion density profiles around the 3D magnetic null.The potential wells near nulls may be related to the energy variations of electrons,so we measured the electron distribution functions(EDFs)at different spatial positions.The axial variation of EDF shows that the electrons deviate from the Maxwell distribution near magnetic nulls.With scanning probes that can directionally measure and theoretically analyze based on curve fitting,the variations of EDFs are linked to the changes of plasma potential under 3D magnetic null topology.The kinetic energy of electrons accelerated by the electric field is 6 eV(v_(e)~7v_(Alfvén-e))and the scale of the region where accelerating electrons exist is in the order of serval electron skin depths.

Cross-satellite calibration of high-energy electron fluxes measured by FengYun-4A based on Arase observations

We use the High-energy Electron Experiments(HEP)instrument onboard Arase(ERG)to conduct an energy-dependent cross-satellite calibration of electron fluxes measured by the High Energy Particle Detector(HEPD)onboard FengYun-4A(FY-4A)spanning from April 1,2017,to September 30,2019.By tracing the two-dimensional magnetic positions(L,magnetic local time[MLT])of FY-4A at each time,we compare the datasets of the conjugate electron fluxes over the range of 245–894 keV in 6 energy channels for the satellite pair within different sets of L×MLT.The variations in the electron fluxes observed by FY-4A generally agree with the Arase measurements,and the percentages of the ratios of electron flux conjunctions within a factor of 2 are larger than 50%.Compared with Arase,FY-4A systematically overestimates electron fluxes at all 6 energy channels,with the corresponding calibration factors ranging from 0.67 to 0.81.After the cross-satellite calibration,the electron flux conjunctions between FY-4A and Arase show better agreement,with much smaller normalized root mean square errors.Our results provide a valuable reference for the application of FY-4A high-energy electron datasets to in-depth investigations of the Earth’s radiation belt electron dynamics.

A compact X-band backward traveling-wave accelerating structure

Very high-energy electrons(VHEEs)are potential candidates for FLASH radiotherapy for deep-seated tumors.We proposed a compact VHEE facility based on an X-band high-gradient high-power technique.In this study,we investigated and realized the first X-band backward traveling-wave(BTW)accelerating structure as the buncher for a VHEE facility.A method for calculating the parameters of single cell from the field distribution was introduced to simplify the design of the BTW structure.Time-domain circuit equations were applied to calculate the transient beam parameters of the buncher in the unsteady state.A prototype of the BTW structure with a thermionic cathode-diode electron gun was designed,fabricated,and tested at high power at the Tsinghua X-band high-power test stand.The structure successfully operated with 5-MW microwave pulses from the pulse compressor and outputted electron bunches with an energy of 8 MeV and a pulsed current of 108 mA.

High-flux electron beams from laser wakefield accelerators driven by petawatt lasers

Laser wakefield accelerators （LWFAs） are considered to be one of the most compeuuve next- generation accelerator candidates. In this paper, we will study the potential high-flux electron beam production of an LWFA driven by petawatt-level laser pulses. In our three-dimensional particle-in-cell simulations, an optimal set of parameters gives -40 nC of charge with 2 PW laser power, thus -400 kA of instantaneous current if we assume the electron beam duration is 100 fs. This high flux and its secondary radiation are widely applicable in nuclear and QED physics, industrial imaging, medical and biological studies.

6-16 Progress of Electron Accelerators in 2014 at IMP

In 2014 some research works have been carried out by the Electron Accelerator Group at IMP, including thehigh energy electron radiography (HEER), the electron accelerator design with short pulse for HEER, and a newtype low energy electrostatic electron accelerator for industrial applications.The verification experiments of HEER have been accomplished on THU electron accelerator in 2013, so moreexperimental works were proposed to continue with complicated parameters. This year, a specialized beam linefor HEER experiment was designed and built, which consists of two dipole magnets, ten quadrapole magnets andsome vacuum chambers with diagnostic probes.

Sterilization complexes based on ILU-type electron accelerators

This paper describes the industrial electron accelerators of the ILU type and their usage for sterilization.The ILU machines produced by Budker Institute of Nuclear Physics have energy range of 0.8-10 MeV and beam power up to 100 kW,and they are working in industries all over the world.The ILU-10 and ILU-14 machines are described as well as the industrial sterilization facility based on ILU-6,ILU-10 and ILU-14 machines.

Electron Acceleration in the Cone-shaped Extra-intense Stationary Laser Field

A technique of electron acceleration in the cone shaped stationary laser field is proposed. An electron acceleration in this laser is studied, which shows that there is no electron bunching but there exists electron capture in this laser field.

Surface segregation of InGaAs films by the evolution of reflection high-energy electron diffraction patterns

Surface segregation is studied via the evolution of reflection high-energy electron diffraction （RHEED） patterns under different values of As4 BEP for InGaAs films. When the As4 BEP is set to be zero, the RHEED pattern keeps a 4x3/（nx3） structure with increasing temperature, and surface segregation takes place until 470 ℃ The RHEED pattern develops into a metal-rich （4x2） structure as temperature increases to 495℃. The reason for this is that surface segregation makes the In inside the InGaAs film climb to its surface. With the temperature increasing up to 515℃, the RHEED pattern turns into a GaAs（2x4） structure due to In desorption. While the As4 BEP comes up to a specific value （1.33 x 10-4 Pa-1.33 x 10-3 Pa）, the surface temperature can delay the segregation and desorption. We find that As4 BEP has a big influence on surface desorption, while surface segregation is more strongly dependent on temperature than surface desorption.

Non-Maxwellian electron distributions resulting from direct laser acceleration in near-critical plasmas

The irradiation of few-nm-thick targets by a finite-contrast high-intensity short-pulse laser results in a strong pre-expansion of these targets at the arrival time of the main pulse.The targets decompress to near and lower than critical densities with plasmas extending over few micrometers,i.e.multiple wavelengths.The interaction of the main pulse with such a highly localized but inhomogeneous target leads to the generation of a short channel and further self-focusing of the laser beam.Experiments at the Glass Hybrid OPCPA Scaled Test-bed(GHOST)laser system at University of Texas,Austin using such targets measured non-Maxwellian,peaked electron distribution with large bunch charge and high electron density in the laser propagation direction.These results are reproduced in 2D PIC simulations using the EPOCH code,identifying direct laser acceleration(DLA)[1]as the responsible mechanism.This is the first time that DLA has been observed to produce peaked spectra as opposed to broad,Maxwellian spectra observed in earlier experiments[2].This high-density electrons have potential applications as injector beams for a further wakefield acceleration stage as well as for pump-probe applications.

Evaluating Acceleration Ability of Electrons of SiO_2 and ZnS in ZnS∶Er Electroluminescence

In layered optimization scheme and solid state cathodoluminescence, silicon oxide plays a very important role in obtaining high energy hot electrons to excite luminescent centers or organic luminescent ma terials. The acceleration ability of electrons of SiO2 and ZnS was compared through the variation of emission intensity based on ZnS：Er phosphor during the reverse of polarity of sinusoidal voltage. The ratio of maximum emission intensity under positive and negative half period is 2.18. This result not only demonstrates that parts of primary electron comes from electrode, but electrons in conduction band of SiO2 can be heated to higher energy than that of ZnS.

Influence of spatiotemporal coupling on the capture-and-acceleration-scenario vacuum electron acceleration by ultrashort pulsed laser beam

This paper investigates the properties of the ultrashort pulsed beam aimed to the capture-and-acceleration-scenario （CAS） vacuum electron acceleration. The result shows that the spatiotemporal distribution of the phase velocity, the longitudinal component of the electric field and the acceleration quality factor are qualitatively similar to that of the continuous-wave Gaussian beam, and are slightly influenced by the spatiotemporal coupling of the ultrashort pulsed beam. When the pulse is compressed to an ultrashort one in which the pulse duration TFWHM 〈 5T0, the variation of the maximum net energy gain due to the carrier-envelope phase is a crucial disadvantage in the CAS acceleration process.

Electron acceleration by tightly focused radially polarized few-cycle laser pulses

Within the framework of plane-wave angular spectrum analysis of the electromagnetic field structure, a solution valid for tightly focused radially polarized few-cycle laser pulses propagating in vacuum is presented. The resulting field distribution is significantly different from that based on the paraxial approximation for pulses with either small or large beam diameters. We compare the electron accelerations obtained with the two solutions and find that the energy gain obtained with our new solution is usually much larger than that with the paraxial approximation solution.

Electron Acceleration in Wakefield and Supra-Bubble Regimes by Ultraintense Laser with Asymmetric Pulse

Electron acceleration in plasma driven by circular polarized ultraintense laser with asymmetric pulse axe investigated analytically and numerically in terms of oscillation-center Hamiltonian formalism. Studies include wakefield acceleration, which dominates in blow-out or bubble regime and snow-plow acceleration which dominates in supra-bubble regime. By a comparison with each other it is found that snow-plow acceleration has lower acceleration capability. In wakefield acceleration, there exists an obvious optimum pulse asymmetry or/and pulse lengths that leads to the high net energy gain while in snow-plow acceleration it is insensitive to the pulse lengths. Power and linear scaling laws for wakefield and snow-plow acceleration respetively are observed from the net energy gain depending on laser field amplitude. Moreover, there exists also an upper and lower limit on plasma density for an effective acceleration in both of regimes.

Measurment of the energy spectrum of an electron beam extracted from an accelerator

The electron energy spectrum is one of the most important characteristics of an electron beam that is extracted from a linear accelerator. The most direct way to determine an electron spectrum would be to use a magnetic spectrometer and this method could also give results with high precision and effectiveness. In this article we describe our design of a new multi-layer absorption method, which is based on the depth-dose curves method that can be used in most irradiation accelerators, and adds the Monte Carlo simulation and iterative algorithm in order to reconstruct the electron energy spectrum. In this article the energy spectrum was measured using these two methods, and good results were acquired. These results could be crosschecked, which made the results more reliable.

Performance of an electron linear accelerator for the first photoneutron source in China

A compact 15.0-MeV, 1.5-kW electron linear accelerator(LINAC) was successfully constructed to provide an electron beam for the first photoneutron source at the Shanghai Institute of Applied Physics, Shanghai,China. This LINAC consists of five main parts: a thermal cathode grid-controlled electron gun, a pre-buncher, a variable-phase-velocity buncher, a light-speed accelerating structure, and a high-power transportation beamline. A digital feedforward radio frequency compensator is adopted to reduce the energy spread caused by the transient beam loading effect. Furthermore, a real-time electron gun emission feedback algorithm is used to keep the beam stable. After months of efforts, all the beam parameters successfully met the requirements of the facility. In this paper, the beam commissioning process and performance of the LINAC are presented.

Electron Acceleration by a Focused Gaussian Laser Pulse in Vacuum

By numerically solving the relativistic equations of motion of a single electron in laserfields modeled by a Gaussian laser beam, we get the trajectory and energy of the electron. Whenthe drifting distance is comparable to or even longer than the corresponding Rayleigh length, theevolution of the beam waist cannot be neglected. The asymmetry of intensity in acceleration anddeceleration leads to the conclusion that the electron can be accelerated effectively and extracted bythe longitudinal ponderomotive force. For intensities above 10～(19) Wμm～2/cm～2, an electron's energygain about MeV can be realized, and the energetic electron is parallel with the propagation axis.

Electron Acceleration by Microwave Radiation Inside a Rectangular Waveguide

Electron dynamics in the fields associated with a transverse magnetic （TM） wave propagating inside a rectangular waveguide is analytically studied. The relativistic momentum and energy equations for an electron are solved, which was injected initially along the propagation direction of the microwave. Expressions of the acceleration gradient and deflection angle are obtained. In principle, it is shown that the electron can be accelerated in this condition and there is no deflection when the electron is injected from the centre of the waveguide front. However, it is found that the acceleration gradient and deflection angle depend strongly on the parameters of the microwave （intensity, frequency, etc.） and the dimensions of the waveguide.

Direct acceleration of electrons using two crossed Laguerre-Gaussian laser beams in vacuum

The basic physical characteristics of electrons accelerated by two linearly polarized and circularly symmetric crossed Laguerre-Gaussian （LG） laser beams with equal frequency and amplitude in vacuum are studied in detail. The condition, under which electrons can be accelerated effectively, and the energy gain are discussed.