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.

The Boundary Element Method for Ordinary State-Based Peridynamics

The peridynamics(PD),as a promising nonlocal continuum mechanics theory,shines in solving discontinuous problems.Up to now,various numerical methods,such as the peridynamic mesh-free particlemethod(PD-MPM),peridynamic finite element method(PD-FEM),and peridynamic boundary element method(PD-BEM),have been proposed.PD-BEM,in particular,outperforms other methods by eliminating spurious boundary softening,efficiently handling infinite problems,and ensuring high computational accuracy.However,the existing PD-BEM is constructed exclusively for bond-based peridynamics(BBPD)with fixed Poisson’s ratio,limiting its applicability to crack propagation problems and scenarios involving infinite or semi-infinite problems.In this paper,we address these limitations by introducing the boundary element method(BEM)for ordinary state-based peridynamics(OSPD-BEM).Additionally,we present a crack propagationmodel embeddedwithin the framework ofOSPD-BEM to simulate crack propagations.To validate the effectiveness of OSPD-BEM,we conduct four numerical examples:deformation under uniaxial loading,crack initiation in a double-notched specimen,wedge-splitting test,and threepoint bending test.The results demonstrate the accuracy and efficiency of OSPD-BEM,highlighting its capability to successfully eliminate spurious boundary softening phenomena under varying Poisson’s ratios.Moreover,OSPDBEMsignificantly reduces computational time and exhibits greater consistencywith experimental results compared to PD-MPM.

Diagnosis of indirectly driven double shell targets with point-projection hard x-ray radiography

We present an application of short-pulse laser-generated hard x rays for the diagnosis of indirectly driven double shell targets. Coneinserted double shell targets were imploded through an indirect drive approach on the upgraded SG-II laser facility. Then, based on thepoint-projection hard x-ray radiography technique, time-resolved radiography of the double shell targets, including that of their near-peakcompression, were obtained. The backlighter source was created by the interactions of a high-intensity short pulsed laser with a metalmicrowire target. Images of the target near peak compression were obtained with an Au microwire. In addition, radiation hydrodynamicsimulations were performed, and the target evolution obtained agrees well with the experimental results. Using the radiographic images, arealdensities of the targets were evaluated.

Simulations of hot electron transport in radiation-ablated plasma

The transport of hot electrons in inertial confinement fusion(ICF)is integrated issue due to the coupling of hydrodynamic evolution and many physical processes.A hot electron transport code is developed and coupled with the radiation hydrodynamic code MULTI1D in this study.Using the code,the slowing-down process and ablation process of the hot electron beam are simulated.The ablation pressure scaling law of hot electron beam is confirmed in our simulations.The hot electron transport is simulated in the radiation-ablated plasmas relevant to indirect-drive ICF,where the spatial profile of hot electron energy deposition is presented around the shock compressed region.It is shown that the hot electron can prominently increase the total ablation pressure in the early phase of radiation-ablated plasma.So,our study suggests that a potential-driven symmetric mechanism may occur under the irradiation of asymmetric hot electron beam.The possible degradation from the hot electron transport and preheating is also discussed.

Efficient ion acceleration driven by a Laguerre–Gaussian laser in near-critical-density plasma

Laser-driven ion accelerators have the advantages of compact size,high density,and short bunch duration over conventional accelerators.Nevertheless,it is still challenging to generate ion beams with quasi-monoenergetic peak and low divergence in experiments with the current ultrahigh intensity laser and thin target technologies.Here we propose a scheme that a Laguerre–Gaussian laser irradiates a near-critical-density(NCD)plasma to generate a quasi-monoenergetic and low-divergence proton beam.The Laguerre–Gaussian laser pulse in an NCD plasma excites a moving longitudinal electrostatic field with a large amplitude,and it maintains the inward bowl-shape for dozens of laser durations.This special distribution of the longitudinal electrostatic field can simultaneously accelerate and converge the protons.Our particle-in-cell(PIC)simulation shows that the efficient proton acceleration can be realized with the Laguerre–Gaussian laser intensity ranging from 3.9×10^(21)W·cm^(-2)–1.6×10^(22)W·cm^(-2)available in the near future,e.g.,a quasi-monoenergetic proton beam with peak energy~115 MeV and divergence angles less than 5°can be generated by a 5.3×10^(21)W·cm^(-2)pulse.This work could provide a reference for the high-quality ion beam generation with PWclass laser systems available recently.

On-chip stackable dielectric laser accelerator

In this paper,we propose a novel stacked laser dielectric acceleration structure.This structure is based on the inverse Cherenkov effect and represented by a parametric design formulation.Compared to existing dielectric laser accelerators relying on the inverse Smith–Purcell effect,the proposed structure provides an extended-duration synchronous acceleration field without requiring the pulse front tilting technique.This advantage significantly reduces the required pulse duration.In addition,the easy to integrate layered structure facilitates cascade acceleration,and simulations have shown that low-energy electron beams can be cascaded through high gradients over extended distances.These practical advantages demonstrate the potential of this new structure for future chip accelerators.

Laser performance upgrade for precise ICF experiment in SG-Ⅲ laser facility

The SG-Ⅲlaser facility(SG-Ⅲ)is the largest laser driver for inertial confinement fusion(ICF)researches in China,which has 48 beamlines and can deliver 180 kJ ultraviolet laser energy in 3 ns.In order to meet the requirements of precise physics experiments,some new functionalities need to be added to SG-Ⅲand some intrinsic laser performances need upgrade.So at the end of SG-Ⅲ's engineering construction,the 2-year laser performance upgrade project started.This paper will introduce the newly added functionalities and the latest laser performance of SG-Ⅲ.With these function extensions and performance upgrade,SG-Ⅲis now fully prepared for precise ICF experiments and solidly paves the way towards fusion ignition.

Generation of 88 as Isolated Attosecond Pulses with Double Optical Gating

Isolated attosecond pulses with a duration of 88 as are generated in the spectral range of 29–72 eV using double optical gating technique.The gate width is set to be shorter than half the optical cycle to avoid carrier envelop phase stabilization of the 4.2 fs driving laser pulses centered at 800 nm.The attosecond pulse duration is measured with the technique of frequency resolved optical gating for complete reconstruction of attosecond bursts.

A novel superconducting magnetic levitation method to support the laser fusion capsule by using permanent magnets

A novel magnetic levitation support method is proposed, which can relieve the perturbation caused by traditional support methods andprovide more accurate position control of the capsule. This method can keep the perfect symmetry of the octahedral spherical hohlraum and hasthe characteristics in stability, tunability and simplicity. It is also favorable that all the results, such as supporting forces acting on the super-conducting capsule, are calculated analytically, and numerical simulations are performed to verify these results. A typical realistic design isproposed and discussed in detail. The superconducting coating material is suggested, and the required superconducting properties are listed.Damped oscillation of the floating capsule in thin helium gas is discussed, and the restoring time is estimated.

High-order optical vortex harmonics generated by relativistic femtosecond laser pulse

Harmonics generated by an intense femtosecond Gaussian laser pulse normally incident on a spiral-shaped thin foil target were studied. By using two-dimensional （2D） particle-in-cell （PIC） simulation, we observed evident odd harmonics signals in the reflection direction and found that the reflected field has a helical structure determined by the target shape. This method provides a new way to generate an intense ultraviolet vortex with high-order topological charge by use of ultrahigh intense laser-driven harmonics.

Enhanced laser-induced plasma channels in air

Plasma is a significant medium in high-energy density physics since it can hardly be damaged. For some applications such as plasma based backward Raman amplification （BRA）, uniform high-density and large-scale plasma channels are required. In the previous experiment, the plasma transverse diameter and density are 50-200 μm and 1-2 x 10^19 cm-3, here we enhance them to 0.8 mm and 8 x 10^19 cm-3, respectively. Moreover, the gradient plasma is investigated in our experiment. A proper plasma gradient can be obtained with suitable pulse energy and delay. The experimental results are useful for plasma physics and nonlinear optics.

Simulations on the multi-shell target ignition driven by radiation pulse in Z-pinch dynamic hohlraum

We present the first simulation results of a multi-shell target ignition driven by Z-pinch dynamic hohlraum radiation pulse.The radiation pulse is produced with a special Z-pinch dynamic hohlraum configuration,where the hohlraum is composed of a single metal liner,a low-Z plastic foam,and a high-Z metallic foam.The implosion dynamics of a hohlraum and a multi-shell target are investigated separately by the one-dimensional code MULTI-IFE.When the peak drive current is 50 MA,simulations suggest that an x-ray pulse with nearly constant radiation temperature(-310 eV)and a duration about 9 ns can be obtained.A small multi-shell target with a radius of 1.35 mm driven by this radiation pulse is able to achieve volumetric ignition with an energy gain(G)about 6.19,where G is the ratio of the yield to the absorbed radiation.Through this research,we better understand the effects of non-uniformities and hydrodynamics instabilities in Z-pinch dynamic hohlraum.

Numerical studies on the radiation uniformity of Z-pinch dynamic hohlraum

Radiation uniformity is important for Z-pinch dynamic hohlraum driven fusion. In order to understand the radiation uniformity of Z-pinchdynamic hohlraum, the code MULTI-2D with a new developed magnetic field package is employed to investigate the related physical processeson Julong-I facility with drive current about 7e8 MA. Numerical simulations suggest that Z-pinch dynamic hohlraum with radiation temperaturemore than 100 eV can be created on Julong-I facility. Although some X-rays can escape out of the hohlraum from Z-pinch plasma and electrodes, the radiation field near the foam center is quite uniform after a transition time. For the load parameters used in this paper, the transitiontime for the thermal wave transports from r = 1 mm to r = 0 mm is about 2.0 ns. Implosion of a testing pellet driven by cylindrical dynamichohlraum shows that symmetrical implosion is hard to achieve due to the relatively slow propagation speed of thermal wave and the compressionof cylindrical shock in the foam. With the help of quasi-spherical implosion, the hohlraum radiation uniformity and corresponding pelletimplosion symmetry can be significantly improved thanks to the shape modulation of thermal wave front and shock wave front.

Ultrafast structural dynamics using time-resolved x-ray diffraction driven by relativistic laser pulses

Based on a femtosecond laser plasma-induced hard x-ray source with a high laser pulse energy(>100 mJ)at 10 Hz repetition rate,we present a time-resolved x-ray diffraction system on an ultrafast time scale.The laser intensity is at relativistic regime(2×10^(19)W/cm^(2)),which is essential for effectively generating K_(α)source in high-Z metal material.The produced copper K_(α)radiation yield reaches to 2.5×10^(8)photons/sr/shot.The multilayer mirrors are optimized for monochromatizating and two-dimensional beam shaping of Kαemission.Our experiment exhibits its ability of monitoring the transient structural changes in a thin film SrCoO_(2.5)crystal.It is demonstrated that this facility is a powerful tool to perform dynamic studies on samples and adaptable to the specific needs for different particular applications with high flexibility.

Mechanical properties of irradiated multi-phase polycrystalline BCC materials

Structure materials under severe irradiations in nuclear environments are known to degrade because of irradiation hardening and loss of ductility,resulting from irradiation-induced defects such as vacancies,interstitials and dislocation loops,etc.In this paper,we develop an elastic-viscoplastic model for irradiated multi-phase polycrystalline BCC materials in which the mechanical behaviors of individual grains and polycrystalline aggregates are both explored.At the microscopic grain scale,we use the internal variable model and propose a new tensorial damage descriptor to represent the geometry character of the defect loop,which facilitates the analysis of the defect loop evolutions and dislocation-defect interactions.At the macroscopic polycrystal scale,the self-consistent scheme is extended to consider the multiphase problem and used to bridge the individual grain behavior to polycrystal properties.Based on the proposed model,we found that the work-hardening coefficient decreases with the increase of irradiation-induced defect loops,and the orientation/loading dependence of mechanical properties is mainly attributed to the different Schmid factors.At the polycrystalline scale,numerical results for pure Fe match well with the irradiation experiment data.The model is further extended to predict the hardening effect of dispersoids in oxide-dispersed strengthened steels by the considering the Orowan bowing.The influences of grain size and irradiation are found to compete to dominate the strengthening behaviors of materials.

Surface-charge-governed electrolyte transport in carbon nanotubes

The transport behavior of pressure-driven aqueous electrolyte solution through charged carbon nanotubes(CNTs) is studied by using molecular dynamics simulations. The results reveal that the presence of charges around the nanotube can remarkably reduce the flow velocity as well as the slip length of the aqueous solution, and the decreasing of magnitude depends on the number of surface charges and distribution. With 1-M KCl solution inside the carbon nanotube, the slip length decreases from 110 nm to only 14 nm when the number of surface charges increases from 0 to 12 e. This phenomenon is attributed to the increase of the solid–liquid friction force due to the electrostatic interaction between the charges and the electrolyte particles, which can impede the transports of water molecules and electrolyte ions. With the simulation results,we estimate the energy conversion efficiency of nanofluidic battery based on CNTs, and find that the highest efficiency is only around 30% but not 60% as expected in previous work.

Dense pair plasma generation and its modulation dynamics in counter-propagating laser field

With two-dimensional quantum electrodynamics（QED） particle-in-cell simulations, a dense electron-positron（e^-e~＋） pair generation from laser-solid interactions is demonstrated. When the interaction of two linearly polarized laser pulses with a thin target enters into the relativistic transparency regime, a stable standing wave（SW） field can be formed by the overlap of the two counter-propagating laser pulses directly. The present study aims to clarify the effects of the S W field on the dynamics of e^-e^＋ pair plasmas. Our results indicate that under the combined effect of the SW field and radiation reaction（RR） effect, the created e^-e^＋ pairs can be trapped into the electric field nodes when the field strength is strong.The trapping effect contributes to the generation of γAV≥400 and ultra-dense pair plasmas in the two-side irradiation scheme. Despite different laser intensities, these pair plasmas have a Maxwellian spectral distribution with a peak energy of 150 MeV. Besides, the periodical modulation of the average energy, spatial, phase-space, and angular patterns of the e^-e^＋ pair plasmas can be triggered. In the angular patterns, as long as the SW field exists, pair plasmas can be pinched along the laser polarization direction. These results may offer a better understanding of the laser-solid interactions in the experiments when 10-PW laser facilities come into operation in the future.

Analysis of electromagnetic pulses generation from laser coupling with polymer targets: Effect of metal content in target

Powerful lasers interacting with solid targets can generate intense electromagnetic pulses(EMPs).In this study,EMPs produced by a pulsed laser(1 ps,100 J)shooting at CH targets doped with different titanium(Ti)contents at the XG-III laser facility are measured and analyzed.The results demonstrate that the intensity of EMPs first increases with Ti doping content from 1%to 7%and then decreases.The electron spectra show that EMP emission is closely related to the hot electrons ejected from the target surface,which is confirmed by an analysis based on the target–holder–ground equivalent antenna model.The conclusions of this study provide a new approach to achieve tunable EMP radiation by adjusting the metal content of solid targets,and will also help in understanding the mechanism ofEMPgeneration and ejection of hot electrons during laser coupling with targets.

Droplet impact on wetted structured surfaces

In this study,we numerically investigate the droplet impact onto a thin liquid film deposited on a structured surface with square pillars and cavities.The time evolution of crown geometry is strongly affected by the surface structure.When the thickness of the liquid film is larger than the structure height,the expanding speed of the crown base radius is independent of the structure width.However,if the liquid film thickness is equal to the structure height,the crown base expands slower as the structure width increases.Surface structures have strong effects on the crown height and radius,and can prevent ejected filament from breaking into satellite droplets for certain cases.For the liquid film with the thickness equal to the pillar height,both the crown height and the radius exhibit non-monotonic behaviors as the pillar width increases.There exists one pillar width which produces the smallest crown height and the largest crown radius.

Dense pair plasma generation by two laser pulses colliding in a cylinder channel

An all-optical scheme for high-density pair plasmas generation is proposed by two laser pulses colliding in a cylinder channel. Two dimensional particle-in-cell simulations show that, when the first laser pulse propagates in the cylinder, electrons are extracted out of the cylinder inner wall and accelerated to high energies. These energetic electrons later run into the second counter-propagating laser pulse, radiating a large amount of high-energy gamma photons via the Compton back-scattering process. The emitted gamma photons then collide with the second laser pulse to initiate the Breit-Wheeler process for pairs production. Due to the strong self-generated fields in the cylinder, positrons are confined in the channel to form dense pair plasmas. Totally, the maximum density of pair plasmas can be 4.60 × 10^27 m%-3, for lasers with an intensity of 4 × 10^22 W.cm^-2. Both the positron yield and density are tunable by changing the cylinder radius and the laser parameters. The generated dense pair plasmas can further facilitate investigations related to astrophysics and particle physics.