Combining stochastic density functional theory with deep potential molecular dynamics to study warm dense matter

In traditional finite-temperature Kohn–Sham density functional theory(KSDFT),the partial occupation of a large number of high-energy KS eigenstates restricts the use of first-principles molecular dynamics methods at extremely high temperatures.However,stochastic density functional theory(SDFT)can overcome this limitation.Recently,SDFT and the related mixed stochastic–deterministic density functional theory,based on a plane-wave basis set,have been implemented in the first-principles electronic structure software ABACUS[Q.Liu and M.Chen,Phys.Rev.B 106,125132(2022)].In this study,we combine SDFT with the Born–Oppenheimer molecular dynamics method to investigate systems with temperatures ranging from a few tens of eV to 1000 eV.Importantly,we train machine-learning-based interatomic models using the SDFT data and employ these deep potential models to simulate large-scale systems with long trajectories.Subsequently,we compute and analyze the structural properties,dynamic properties,and transport coefficients of warm dense matter.

Spatial and temporal evolution of electromagnetic pulses from solid target irradiated with multi-hundred-terawatt laser pulse inside target chamber

Giant electromagnetic pulses(EMPs) induced by high-power laser irradiating solid targets interfere with various experimental diagnoses and even damage equipment,so unveiling the evolution of EMPs inside the laser chamber is crucial for designing effective EMP shielding.In this work,the transmission characteristics of EMPs as a function of distances from the target chamber center(TCC) are studied using B-dot probes.The mean EMP amplitude generated by picosecond laser-target interaction reaches 561 kV m^(-1),357 kV m^(-1),395 kV m^(-1),and 341 kV m^(-1)at 0.32 m,0.53 m,0.76 m,and 1 m from TCC,which decreases dramatically from 0.32 m to 0.53 m.However,it shows a fluctuation from 0.53 m to 1 m.The temporal features of EMPs indicate that time-domain EMP signals near the target chamber wall have a wider full width at half maximum compared to that close to TCC,mainly due to the echo oscillation of electromagnetic waves inside the target chamber based on simulation and experimentation.The conclusions of this study will provide a new approach to mitigate strong electromagnetic pulses by decreasing the echo oscillation of electromagnetic waves inside the target chamber during laser coupling with targets.

Generation and regulation of electromagnetic pulses generated by femtosecond lasers interacting with multitargets

Ultrashort and powerful laser interactions with a target generate intense wideband electromagnetic pulses(EMPs).In this study,we report EMPs generated by the interactions between petawatt(30 fs,1.4×10^(20) W/cm^(2))femtosecond(fs)lasers with metal flat,plastic flat,and plastic nanowire-array(NWA)targets.Detailed analyses are conducted on the EMPs in terms of their spatial distribution,time and frequency domains,radiation energy,and protection.The results indicate that EMPs from metal targets exhibit larger amplitudes at varying angles than those generated by other types of targets and are enhanced significantly for NWA targets.Using a plastic target holder and increasing the laser focal spot can significantly decrease the radiation energy of the EMPs.Moreover,the composite shielding materials indicate an effective shielding effect against EMPs.The simulation results show that the NWA targets exert a collimating effect on thermal electrons,which directly affects the distribution of EMPs.This study provides guidance for regulating EMPs by controlling the laser focal spot,target parameters,and target rod material and is beneficial for electromagnetic-shielding design.

A Natural Catalytic Converter® for Continuously Inactivating Air and Surface Pathogens with More Effect than Ventilation and Filtration

Study Objective: The purpose of the study is to present independent laboratory testing for a novel technology in air and on surfaces. Since 2020, public health goals have focused on improving indoor air quality. This includes protection from airborne pathogens, such as tuberculosis, RSV, SARS-CoV-2, common cold or influenza viruses, measles, and others. Engineering controls are highly effective at reducing hazardous pathogens found in indoor air and from recontamination of surfaces. This occurs from a continuous cycle of settling of small, sustained airborne pathogens, which may become dehumidified, becoming airborne again, carried by room air currents around indoor spaces, then repeating the cycle. Methods: The novel technology utilizes a catalytic process to produce safe levels of hydrogen peroxide gas that are effective in reducing pathogens in the air and on surfaces. Air testing was performed with the MS2 bacteriophage, the test organism for ASHRAE standard 241, and methicillin-Resistant Staphylococcus aureus (MRSA). Surface testing was performed with SARS-COV-2 (Coronavirus COVID-19) and H1N1 (Influenza). Typical ventilation and filtration does not effectively remove disbursed pathogens from the entire facility, due to inconsistent air circulation and surface deposits of pathogens. Results: MS2 was reduced by 99.9%;MRSA was reduced by 99.9%;SARS-CoV-2 was reduced by 99.9%;H1N1 was reduced by 99.9%. Conclusion: This novel catalytic converter reduces a variety of pathogens in the air (99%) and on surfaces (99%), by actively disinfecting with the introduction of gaseous hydrogen peroxide. This active disinfection provides a strong solution for protecting the entire facility and its occupants.

Photonuclear production of nuclear isomers using bremsstrahlung induced by laser-wakefield electrons

In this study,we theoretically investigate the feasibility of using laser-wakefield accelerated(LWFA)electrons for the photonuclear measurement of nuclear isomers according to the characteristics of the electrons obtained from LWFA experiments conducted at the Compact Laser-Plasma Accelerator(CLAPA)laboratory.The experiments at the CLAPA show that a stable electron beam with an energy of 78-135 MeV and a charge of 300-600 pC can be obtained.The bremsstrahlung spectra were simulated using Geant4,which suggests that a bremsstrahlung source with a peak intensity of 10^(19) photons/s can be generated.Theoretical calculations of isomer production cross sections from the photonuclear reactions on six target nuclei,^(197)Au,^(180)Hf,^(159)Tb,^(115)In,^(103)Rh,and ^(90)Zr,were performed and compared with the available experimental data in EXFOR,which suggest that further experiments are required for a series of photonuclear reaction channels.Flux-averaged cross sections and isomer ratios(IR)resulting from such bremsstrahlung sources are theoretically deduced.The results suggest that IR measurements can be used to constrain nuclear components,such asγstrength function and optical model potential.In addition,the detection of the decay characteristics was evaluated with Geant4 simulations.The use of the LWFA electron beam and its bremsstrahlung for photonuclear studies involving nuclear isomers is anticipated.

不稳定分层大气边界层中沙尘暴的无黏模型研究

实验观测表明,沙尘暴发生之前的大气边界层常常是不稳定分层的。沙尘暴可被视为处于超高Reynolds数的颗粒两相湍流异重流。为了研究异重流坍塌阶段的主控机制,建立了一个无黏模型,发现在重流体或环境流体或两者均存在不稳定密度分层时,表征异重流头部速度的Froude数都会变小。对不同的开闸高度和不稳定分层情况的开闸式异重流进行了数值模拟。尽管在密度界面存在混合,但是模拟所得的头部速度与高度关系同无黏模型的预测相符。另外,野外观测到的沙尘暴头部速度增量同该模型的预测基本一致,显示在该类高Reynolds数湍流中无黏机制的贡献不容忽视。

八叉树网格到非结构混合网格的转换

在数值模拟中,非结构网格的优势是可以采用相同的数值格式统一处理任意复杂的计算区域,但在网格生成过程中难度大,并且不容易控制网格质量。树结构网格可以认为是介于结构网格和非结构网格之间的一种网格,目前已经有相对成熟的方法快速在复杂区域内生成二维四叉树网格和三维八叉树网格。在实际应用中,数值方法往往需要在连接协调的非结构网格上做离散,树结构网格中不同尺寸的网格之间连接不是协调的,在应用上会受到很多限制。文章实现了树结构网格到非结构混合网格的转换,这种转换在二维情况下就是将四叉树网格转换为非结构三角形和四边形的混合网格,三维情况下则将八叉树网格转换为非结构混合网格。这一转换过程的难点在于需要考虑数千种不同的八叉树单元,并给出能实现连接协调的非结构混合网格划分。可以出现的网格单元包括六面体、三棱柱、金字塔和四面体这4种不同情况。通过特别的分类,实现了程序的自动生成,这种程序自动生成技术一方面可以避免人工编写大量程序时的失误,另一方面也使得对数以千计的不同情况的处理成为可能。通过对几个简单网格的测试,对网格数据转换方法做了初步的验证。

Design and calibration of an elliptical crystal spectrometer for the diagnosis of proton-induced x-ray emission(PIXE)

Laser-driven proton-induced x-ray emission(laser-PIXE) is a nuclear analysis method based on the compact laser ion accelerator. Due to the transient process of ion acceleration, the laser-PIXE signals are usually spurted within nanoseconds and accompanied by strong electromagnetic pulses(EMP), so traditional multi-channel detectors are no longer applicable.In this work, we designed a reflective elliptical crystal spectrometer for the diagnosis of laser-PIXE. The device can detect the energy range of 1 keV–11 ke V with a high resolution. A calibration experiment was completed on the electrostatic accelerator of Peking University using samples of Al, Ti, Cu, and ceramic artifacts. The detection efficiency of the elliptical crystal spectrometer was obtained in the order of 10-9.

石墨烯热学性能及表征技术

2004年石墨烯的发现立刻引起了全球科技界的高度关注,掀起了从碳纳米管问世以来对于碳族材料的又一个研究高潮,人们迅速开展了针对石墨烯的制备、性能表征、甚至应用的研究工作.从石墨烯问世到目前,主要研究工作集中在石墨烯电学性能的研究,特别是集中在用石墨烯制备超级电容器方面.相比之下,人们对于石墨烯热学性能的研究还比较少.然而,鉴于石墨烯具有极高的热导率和负的热膨胀系数,以及作为热界面材料的工程应用价值,对其热学性能的研究正逐渐成为研究的一个重要分支.以石墨烯热学性能如热导率到热膨胀系数为研究对象,全面总结国际上的发展现状.内容涉及单层石墨烯、多层石墨烯和石墨烯泡沫.研究手段包括理论研究、数值模拟和实验测定3个方面.在综合研究成果的基础上,最后对于存在的问题和可能的发展方向给出了合理的建议.

强激光等离子体相互作用驱动高次谐波与阿秒辐射研究进展

对超快过程的探测和控制决定了人类在微观层面认识和改造物质世界的能力.阿秒光源可完成对组成物质的电子运动及其关联效应进行超高时空分辨的探测和操控,为人类认识微观世界提供了全新手段,被认为是激光科学史上最重要的里程碑之一.世界主要科技强国都将阿秒科学列为未来10年重要的科技发展方向.利用强激光与物质相互作用产生高次谐波是突破飞秒极限实现高亮度阿秒脉冲辐射的重要方案之一,成为了近年来激光等离子体领域的研究热点.本文聚焦强激光与等离子体相互作用中的高次谐波和阿秒脉冲辐射,主要介绍其产生机制、研究进展和前沿应用,并对未来的发展趋势和创新突破进行展望.

Recent advances on 3D printing graphene-based composites

3D printing or additive manufacturing (AM) has revolutionized the way of manufacturing by designing complex structures in a customized feature which cannot be realized by traditional processing methods. Incoming materials are trying to adopt 3D printing techniques which directly fabricate sophisticated entities with multifunctionality like mechanical, electrical, thermal and magnetic properties etc. For the realization of advanced materials, 3D printing techniques are emerging from single material to composite materials manufacturing by simply introducing the nano- and micro-reinforcements with the matrix. In this review, we provide an outline of 3D printing graphene-based composites according to various AM techniques including fused deposition modeling (FDM), direct ink writing (DIW), stereolithography (SLA) and selective laser sintering (SLS). First a brief introduction of various AM techniques is given to get a basic understanding of the principles of 3D printing, and then the fabrication process, structural characteristics and applications of different 3D printing techniques for graphene-based composites are summarized. In addition, some effective simulation and characterization methods are also included. We hope that this review would clarify the potential of AM techniques for composite materials and can open new prospects for designing of novel materials.

Change in Continuous Detonation Wave Propagation Mode from Rotating Detonation to Standing Detonation

We perform a three-dimensional numerical simulation based on a one-step chemical reaction model to investigate changes in the mode of H2-Air detonation wave propagation from rotating detonation wave （RDW） mode to standing detonation wave mode. The physical characteristics of an RDW with injection velocity of 500 m/s are analyzed to investigate the physical mechanisms involved. We find that with increasing injection velocity, the detonation wave gradually changes from perpendicular to the head wall to parallel to the head wall. When the injection velocity exceeds the Chapman-Jouguet velocity VCJ （about 1984 m/s）, the detonation wave changes orientation to become perpendicular to the fuel injection direction, and the rotating mode changes accordingly to a standing mode. Finally, the plane detonation characteristic triple-wave structures can be found from the standing mode.

Effect of compressibility on the hypervelocity penetration

We further consider the effect of rod strength by employing the compressible penetration model to study the effect of compressibility on hypervelocity penetration.Meanwhile, we define different instances of penetration efficiency in various modified models and compare these penetration efficiencies to identify the effects of different factors in the compressible model. To systematically discuss the effect of compressibility in different metallic rod-target combinations, we construct three cases, i.e., the penetrations by the more compressible rod into the less compressible target, rod into the analogously compressible target, and the less compressible rod into the more compressible target. The effects of volumetric strain, internal energy, and strength on the penetration efficiency are analyzed simultaneously. It indicates that the compressibility of the rod and target increases the pressure at the rod/target interface. The more compressible rod/target has larger volumetric strain and higher internal energy. Both the larger volumetric strain and higher strength enhance the penetration or anti-penetration ability. On the other hand, the higher internal energy weakens the penetration or anti-penetration ability. The two trends conflict, but the volumetric strain dominates in the variation of the penetration efficiency, which would not approach the hydrodynamic limit if the rod and target are not analogously compressible. However, if the compressibility of the rod and target is analogous, it has little effect on the penetration efficiency.

基于数字化植物表型平台(D3P)的田间小麦冠层光截获算法开发

冠层光截获能力是反映作物品种间差异的重要功能性状,高通量表型冠层光截获对提高作物改良效率具有重要意义。本研究以小麦为研究目标,利用数字化植物表型平台(D3P)模拟生成了100种冠层结构不同的小麦品种在5个生育期的三维冠层场景,记录了从原始冠层结构中提取的绿色叶面积指数(GAI)、平均倾角(AIA)和散射光截获率(FIPAR_(dif))信息作为真实值,进一步利用上述三维小麦场景开展了虚拟的激光雷达(LiDAR)模拟实验,生成了对应的三维点云数据。基于模拟的点云数据提取了其高度分位数特征(H)和绿色分数特征(GF)。最后,利用人工神经网络(ANN)算法分别构建了从不同LiDAR点云特征(H、GF和H+GF)输入到FIPAR_(dif)、GAI和AIA的反演模型。结果表明,对于GAI、AIA和FIPAR_(dif),预测精度从高到低对应的点云特征输入为GF+H> H> GF。由此可见,H特征对提高目标表型特性的估算精度起到了重要作用。输入GF+H特征,在中等测量噪音(10%)情况下,FIPAR_(dif)和GAI的估算均获得了满意精度,R^2分别为0.95和0.98,而AIA的估算精度(R^2=0.20)还有待进一步提升。本研究基于D3P模拟数据开展,算法的实际表现还有待通过田间数据进一步验证。尽管如此,本研究验证了D3P协助表型算法开发的能力,展示了高通量LiDAR数据在估算田间冠层光截获和冠层结构方面的较高潜力。

Numerical simulation of two-dimensional granular shearing flows and the friction force of a moving slab on the granular media

This paper studies some interesting features of two-dimensional granular shearing flow by using molecular dynamic approach for a specific granular system. The obtained results show that the probability distribution function of velocities of particles is Gaussian at the central part, but diverts from Gaussian distribution nearby the wall. The macroscopic stress along the vertical direction has large fluctuation around a constant value, the non-zero average velocity occurs mainly near the moving wall, which forms a shearing zone.. In the shearing movement, the volume of the granular material behaves in a random manner. The equivalent fl＇iction coefficient between moving slab and granular material correlates with the moving speed at low velocity, and approaches constant as the velocity is large enough.

球墨铸铁齿轮断齿的原因分析及防止措施

介绍了球墨铸铁齿轮铸件的结构及原生产工艺,详细阐述了铸件在使用过程中出现的断齿问题,通过对客户反馈的图片及铸件实物分析,确定齿轮断齿的原因是球化衰退,并从孕育处理、操作细节、浇注温度及浇注时间方面进行了原因分析,提出了防止球化衰退的措施:(1)加强孕育控制,将75SiFe孕育剂加入量增至0.9%~1.1%,分3次孕育,并增加BaSiFe孕育剂进行随流孕育;(2)将球化处理温度控制在1 470~1 500℃;(3)控制浇注时间在10 min以内。经过半年的生产实践,齿轮断齿问题得到了解决。

Thermal transport by electrons and ions in warm dense aluminum:A combined density functional theory and deep potential study

We propose an efficient scheme that combines density functional theory(DFT)with deep potentials(DPs),to systematically study convergence issues in the computation of the electronic thermal conductivity of warm dense aluminum(2.7 g/cm^(3)and temperatures ranging from 0.5 eV to 5.0 eV)with respect to the number of k-points,the number of atoms,the broadening parameter,the exchange-correlation functionals,and the pseudopotentials.Furthermore,we obtain the ionic thermal conductivity using the Green–Kubo method in conjunction with DP molecular dynamics simulations,and we study size effects on the ionic thermal conductivity.This work demonstrates that the proposed method is efficient in evaluating both electronic and ionic thermal conductivities of materials.

A simplified approximate model of compressible hypervelocity penetration

A simplified approximate model considering rod/target material's compressibility is proposed for hypervelocity penetration.We study the effect of shockwaves on hypervelocity penetration whenever the compressibility of the rod is much larger,analogously,and much less than that of the target,respectively.The results show that the effect of shockwaves is insignificant up to 12 km/s,so the shockwave is neglected in the present approximate model.The Murnaghan equation of state is adopted to simulate the material behaviors in penetration and its validity is proved.The approximate model is finally reduced to an equation depending only on the penetration velocity and a simple approximate solution is achieved.The solution of the approximate model is in agreement with the result of the complete compressible model.In addition,the effect of shockwaves on hypervelocity penetration is shown to weaken material's compressibility and reduce the interface pressure of the rod/target,and thus the striking/protective performance of the rod/target is weakened,respectively.We also conduct an error analysis of the interface pressure and penetration efficiency.With a velocity change of 1.6 times the initial sound speed for the rod or target,the error of the approximate model is very small.For metallic rod-target combinations,the approximate model is applicable even at an impact velocity of 12 km/s.

The effects of compressibility and strength on penetration of long rod and jet

The approximate compressible model is adopted to study the effects of strength and compressibility on the penetration by WHA long rod and copper jet into semi-infinite target in detail. For WHA rod penetrating PMMA at 2 km/s <V <5 km/s, the compressibility has a significant effect on the penetration efficiency. We clarify how compressibility affects the penetration efficiency by changing the stagnation pressures of the rod and target. For WHA rod penetrating 4340 Steel and 6061-T6 Al at 2 km/s < V < 10 km/s, the effect of strength is strong and the effect of compressibility is negligible at lower impact velocity, whilst the effect of strength is weak and the effect of compressibility becomes stronger at higher impact velocity. For the copper jet penetrating 4030 Steel, 6061-T6 Al and PMMA. the virtual origin model is adopted, and the compressibility and strength are implicitly considered by the linear relation between the penetration velocity and impact velocity. The effects of compressibility and target resistance on penetration efficiency are studied. The results show that the target resistance has a significant effect on the penetration efficiency. Howver PMMA is much more compressible than copper and the huge difference of compressibility has a significant effect on the penetration by hypervelocity copper jet into PMMA.

连续爆轰发动机中的起爆、湮灭、再起爆和稳定全过程的流场演化机制

为了研究连续爆轰发动机中的起爆、湮灭、再起爆和稳定全过程的流场演化机制,采用基于开源计算流体力学平台OpenFOAM的自定义求解器BYRFoam,使用带Laval尾喷管的空心圆筒燃烧室的几何构型及阵列式小孔进气。分析了起爆和湮灭的过程以及再起爆现象产生的原因,采用时空分布图的分析方法总结了起爆、湮灭、再起爆和稳定全过程的流场演化机制。结果表明,起爆初期的斜激波会在尾喷管收缩段反射形成反射激波,反射激波进入新鲜气体,形成与初始爆轰波方向相反的新爆轰波,这会导致爆轰波的湮灭。湮灭之后,燃烧室头部的高压强会阻碍新鲜气体的进入,之后已燃气体逐渐通过尾喷管排出燃烧室,燃烧室头部压强降低,逐渐进入燃烧室的新鲜气体为后续的再起爆创造了条件。连续爆轰波再起爆的原因有两个:一是尾喷管收缩段处的反射激波进入新鲜气体层并转化为爆轰波;二是已经形成的爆轰波与阵列式进气相互作用,形成反传波,反传波进入新鲜气体并转化为爆轰波。再起爆之后,经过爆轰波、激波和新鲜气体的相互作用和演化,爆轰波最终稳定传播。