Equation‐of‐state studies of high‐energy‐density matter using intense ion beams at the Facility for Antiprotons and Ion Research

This paper describes an experiment design based on numerical simulations to measure the equation‐of‐state properties of high‐energy‐density (HED) matter using intense particle beams. The simulations are performed using a 2D hydrodynamic computer code, BIG2, while the beam parameters are considered to match the Facility for Antiprotons and Ion Research beam. This study has shown that in such experiments one can generate different phases of HED lead. Similar calculations are planned for other materials.     

Core collapse via coarse dynamic renormalization

In the context of the recently developed "equation-free" approach to computer-assisted analysis of complex systems, we extract the self-similar solution describing core collapse of a stellar system from numerical experiments. The technique allows us to sidestep the core "bounce" that occurs in direct N-body simulations due to the small-N correlations that develop in the late stages of collapse, and hence to follow the evolution well into the self-similar regime.     

Strongly screening electron capture for nuclides ~(52,53,59,60)Fe by the Shell-Model Monte Carlo method in pre-supernovae

The death of massive stars due to supernova explosions is a key ingredient in stellar evolution and stellar population synthesis.Electron capture(EC) plays a vital role in supernova explosions.Using the Shell-Model Monte Carlo method,based on the nuclear random phase approximation and linear response theory model for electrons,we study the strong screening EC rates of ~(52,53,59,60)Fe in pre-supernovae.The results show that the screening rates can decrease by about 18.66%.Our results may become a good foundation for future investigation of the evolution of late-type stars,supernova explosion mechanisms and numerical simulations.     

激光聚变内爆流体不稳定性基础问题研究进展

激光聚变有望一劳永逸地解决人类的能源问题,因而受到国际社会的普遍重视,一直是国际研究的前沿热点。目前实现激光惯性约束聚变所面临的最大科学障碍（属于内禀困难）是对内爆过程中高能量密度流体力学不稳定性引起的非线性流动的有效控制,对其研究涵盖高能量密度物理、等离子体物理、流体力学、计算科学、强冲击物理和高压原子物理等多个学科,同时还要具备大规模多物理多尺度多介质流动的数值模拟能力和高功率大型激光装置等研究条件。作为新兴研究课题,高能量密度非线性流动问题充满了各种新奇的现象亟待探索。此外,流体力学不稳定性及其引起的湍流混合,还是天体物理现象（如星系碰撞与合并、恒星演化、原始恒星的形成以及超新星爆炸）中的重要过程,涉及天体物理的一些核心研究内容。本文首先综述了高能量密度非线性流动研究的现状和进展,梳理了其中的挑战和机遇。然后介绍了传统中心点火激光聚变内爆过程发生的主要流体力学不稳定性,在大量分解和综合物理研究基础上,凝练出了目前制约美国国家点火装置（NIF）内爆性能的主要流体不稳定性问题。接下来,总结了国外激光聚变流体不稳定性实验物理的研究概况。最后,展示了内爆物理团队近些年在激光聚变内爆流体不稳定性基础性问题方面的主要研究进展。该团队一直从事激光聚变内爆非线性流动研究与控制,以及聚变靶物理研究与设计,注重理论探索和实验研究相结合,近年来在内爆重要流体力学不稳定性问题的解析理论、数值模拟和激光装置实验设计与数据分析等方面取得了一系列重要成果,有力地推动了该研究方向在国内的发展。     

Application of intense ion beams to planetary physics research at the Facility for Antiprotons and Ion Research facility

This paper presents detailed 2D hydrodynamic simulations of implosion of a multi‐layered cylindrical target that is driven by an intense uranium beam. The target is comprised of a thick, high‐Z, high‐ρ cylindrical shell that encloses a sample material (Fe in the present case). Two options have been used for the focal spot geometry: an annular form and a circular form. The purpose of this work is to show that an intense heavy‐ion beam can induce the extreme physical conditions in the sample material similar to those that exist in the planetary cores. In this study, we use parameters of the beam that will be generated at the Facility for Antiprotons and Ion Research (FAIR), Darmstadt, in a few years' time. Production of these high‐energy‐density (HED) samples will allow us to study planetary physics in the laboratory. It is to be noted that planetary physics research is an important part of the FAIR HED physics program. A dedicated experiment named LAboratory PLAnetary Sciences (LAPLAS) has been proposed for this purpose. These simulations show that in such experiments an Fe sample can be imploded to the Earth's core conditions and to those in more massive rocky planets called Super‐Earths. Similarly, implosion of hydrogen and water samples will generate the core conditions of solar and extrasolar hydrogen‐rich gas giants and water‐rich icy planets, respectively. The LAPLAS experiments will thus provide very valuable information on the equation of state and transport properties of matter under extreme physical conditions, which will help scientists understand the structure and evolution of the planets in our solar system as well as of the extrasolar planets.     

Characteristics of an envelope model for laser–plasma accelerator simulation

Simulation of laser–plasma accelerator (LPA) experiments is computationally intensive due to the disparate length scales involved. Current experiments extend hundreds of laser wavelengths transversely and many thousands in the propagation direction, making explicit PIC simulations enormously expensive and requiring massively parallel execution in 3D. Simulating the next generation of LPA experiments is expected to increase the computational requirements yet further, by a factor of 1000. We can substantially improve the performance of LPA simulations by modeling the envelope evolution of the laser field rather than the field itself. This allows for much coarser grids, since we need only resolve the plasma wavelength and not the laser wavelength, and therefore larger timesteps can be used. Thus an envelope model can result in savings of several orders of magnitude in computational resources. By propagating the laser envelope in a Galilean frame moving at the speed of light, dispersive errors can be avoided and simulations over long distances become possible. The primary limitation to this envelope model is when the laser pulse develops large frequency shifts, and thus the slowly-varying envelope assumption is no longer valid. Here we describe the model and its implementation, and show rigorous benchmarks for the algorithm, establishing second-order convergence and correct laser group velocity. We also demonstrate simulations of LPA phenomena such as self-focusing and meter-scale acceleration stages using the model.     

Proposal for the study of thermophysical properties of high-energy-density matter using current and future heavy-ion accelerator facilities at GSI Darmstadt

The subject of high-energy-density (HED) states in matter is of considerable importance to numerous branches of basic as well as applied physics. Intense heavy-ion beams are an excellent tool to create large samples of HED matter in the laboratory with fairly uniform physical conditions. Gesellschaft für Schwerionenforschung, Darmstadt, is a unique worldwide laboratory that has a heavy-ion synchrotron, SIS18, that delivers intense beams of energetic heavy ions. Construction of a much more powerful synchrotron, SIS100, at the future international facility for antiprotons and ion research (FAIR) at Darmstadt will lead to an increase in beam intensity by 3 orders of magnitude compared to what is currently available. The purpose of this Letter is to investigate with the help of two-dimensional numerical simulations, the potential of the FAIR to carry out research in the field of HED states in matter.     

Strain solitary waves in inhomogeneous wave guides

The paper presents recent results of the research on strain solitary wave (soliton) evolution in elastic wave guides with different types of inhomogeneities. We analyze in calculations, numerical simulations and in experiments how physical or geometrical inhomogeneities affect the parameters of a density soliton propagating in it. In our experiments strain solitons are produced in a wave guide from an initial shock wave generated in the surrounding water by laser evaporation of a metallic target immersed into it nearby the input edge of the wave guide. Strain solitons are recorded in a desired part of the wave guide by means of holographic interferometry that allows to visualize the whole process and to obtain the complete set of data at different stages of the wave evolution.     

The laser polarization as control parameter in the formation of laser-induced periodic surface structures: Comparison of numerical and experimental results

Considering self-organized surface pattering upon multi-pulse femtosecond laser irradiation, in particularly the strong dependence of ripples orientation on the laser polarization, we present numerical simulations from an adopted surface erosion model and compare the result to our experimental data on laser-induced nanostructures formation. We present the surface morphologies obtained by this model for different polarizations of the incident laser electric field and show good agreement with ripple formation produced by laser ablation experiments. The correlation of ripples orientation with laser polarization can be described within a model where the polarization causes a breaking of symmetry at the surface. Further we discuss a time evolution of pattern formation. Our results support the non-linear self-organization mechanism of pattern formation on the surface of solids.     

Dynamics and gravitational wave signature of collapsar formation

We perform 3+1 general relativistic simulations of rotating core collapse in the context of the collapsar model for long gamma-ray bursts. We employ a realistic progenitor, rotation based on results of stellar evolution calculations, and a simplified equation of state. Our simulations track self-consistently collapse, bounce, the postbounce phase, black hole formation, and the subsequent early hyperaccretion phase. We extract gravitational waves from the spacetime curvature and identify a unique gravitational wave signature associated with the early phase of collapsar formation.     

Experimental investigation of high-mach-number 3D hydrodynamic jets at the national ignition facility

The first hydrodynamic experiments were performed on the National Ignition Facility. A supersonic jet was formed via the interaction of a laser driven shock ( approximately 40 Mbar) with 2D and 3D density perturbations. The temporal evolution of the jet's spatial scales and ejected mass were measured with point-projection x-ray radiography. Measurements of the large-scale features and mass are in good agreement with 2D and 3D numerical simulations. These experiments provide quantitative data on the evolution of 3D supersonic jets and provide insight into their 3D behavior.     

Molecular dynamics simulations of cluster distribution from femtosecond laser ablation in aluminum

Femtosecond laser ablation and plume evolution of aluminum is investigated for various inhomogeneous laser pulses. For the simulations of the atoms the molecular dynamics code IMD is used. The ablated gas-phase is scanned by a cluster algorithm (DBSCAN), from which we gain a cluster size distribution of the ablated material. Per single pulse, only a small portion of the total volume evaporates into the gas phase. Therefore??to have reasonable statistics??we have to deal with huge samples (6×10⁷?atoms). The ablation threshold is determined by comparing the depth of the holes to the applied fluence. Angular and velocity distributions of the plume are compared to experiments.     

Film growth mechanisms in pulsed laser deposition

This paper reviews our recent studies of the fundamentals of growth morphology evolution in Pulsed Laser Deposition in two prototypical growth modes: metal-on-insulator island growth and semiconductor homoepitaxy. By comparing morphology evolution for pulsed laser deposition and thermal deposition in the same dual-use chamber under identical thermal, background, and surface preparation conditions, and varying the kinetic energy by varying the laser fluence or using an inert background gas, we have isolated the effect of kinetic energy from that of flux pulsing in determining the differences between morphology evolution in these growth methods. In each growth mode analytical growth models and Kinetic Monte Carlo simulations for thermal deposition, modified to include kinetic energy effects, are successful at explaining much of what we observe experimentally.     

Shock compression of [001] single crystal silicon

Silicon is ubiquitous in our advanced technological society, yet our current understanding of change to its mechanical response at extreme pressures and strain-rates is far from complete. This is due to its brittleness, making recovery experiments difficult. High-power, short-duration, laser-driven, shock compression and recovery experiments on [001] silicon (using impedance-matched momentum traps) unveiled remarkable structural changes observed by transmission electron microscopy. As laser energy increases, corresponding to an increase in peak shock pressure, the following plastic responses are are observed: surface cleavage along {111} planes, dislocations and stacking faults; bands of amorphized material initially forming on crystallographic orientations consistent with dislocation slip; and coarse regions of amorphized material. Molecular dynamics simulations approach equivalent length and time scales to laser experiments and reveal the evolution of shock-induced partial dislocations and their crucial role in the preliminary stages of amorphization. Application of coupled hydrostatic and shear stresses produce amorphization below the hydrostatically determined critical melting pressure under dynamic shock compression.     

Features of electrical discharges in air triggered by laser

A numerical study of laser-triggered discharges in air at atmospheric pressure is presented for an ultraviolet laser in small gaps. Two models, one for the ionization of the air by the laser pulse and the second for the streamer evolution have been computed. From results of numerical simulations the influence of the laser parameters such as energy, pulse duration and beam radius is analyzed and electron distributions are obtained for different small gaps. Electric field, streamer velocity and evolution of the ionized volume are calculated by means of streamer simulations. This paper shows the main features of the laser-triggered discharges and also the importance of using numerical simulations in a laser-triggered experiment.     

A study of process parameters during pulsed Nd:YAG laser notching of C70S6 fracture splitting connecting rods

This paper reports on the laser notching technology in C70S6 steel for fracture splitting connecting rod using a Nd:YAG pulsed laser. The effects of process parameters on starting notch (SN) dimension and morphology were investigated by both finite element method (FEM) simulations and physical experiments for various process parameters. Optical microscopy and the scanning electron microscope (SEM) were used for the measurement of SN dimension and the observation of SN morphology in the experiment. The results were compared with the predictions. It was found that the FEM simulations results showed good consistency with the experiments, which indicates that the finite element model is feasible and reliable. Based on the principal findings from the two methods, optimum ranges of process parameters for different fracture splitting connecting rods were predicted, which are a flexibly adjusting notch depth, a curvature radius less than 0.08 mm and an opening angle within the range 18–26°. The results indicate that the predicting ranges are suitable for making good SNs, which has also been proved by the fracture splitting experiments.     

低信噪比巡天数据中特殊恒星光谱的搜寻方法

特殊恒星是金属丰度异常的恒星,其中包含的信息对于研究宇宙起源、太阳系的演变以及生命的演化都有着重要的意义。因此,特殊恒星的搜寻是国内外巡天项目中的重要目标。恒星光谱中包含着恒星的化学成分、物理性质以及运动状态等丰富的信息,它是开展恒星研究的重要依据。恒星的识别、分类以及特殊恒星的发现主要依据的是恒星光谱数据。随着LAMOST和SDSS等国内外大规模数字巡天项目的深入展开,恒星光谱的数据量达到了前所未有的高度,如此大的数据量为特殊恒星的发现提供了强有力的支撑。因此如何利用这些数据快速准确地发现特殊、稀少甚至于未知类型的恒星光谱是天文学研究的重要问题。数据挖掘是结合模式识别、机器学习、统计分析及相关专家背景知识,从数据中提取出隐含的过去未知的有价值的潜在信息的技术,其在处理大数据方面有着天然的优势,越来越多的数据挖掘方法被应用到巡天数据处理及分析之中。目前针对特殊恒星搜寻的数据挖掘算法主要包含随机森林、聚类分析以及异常值检测等,但随着巡天深度的拓展,观测的目标越来越暗,进而观测光谱的信噪比也随之变低。低信噪比光谱中存在着大量的无用信息,直接利用相关算法对其进行分析处理得到的结果往往存在很大的偏差。因此,如何从大量低信噪比恒星光谱巡天数据中有效地搜寻出特殊的恒星光谱,是当前面临的一个重要问题。由于低信噪比恒星光谱本身的特点,对于从中搜寻特殊恒星光谱的工作开展较少。为了解决此问题,在仔细研究光谱数据处理方法的基础上,针对低信噪比巡天数据中特殊恒星光谱的搜寻,提出了一种以主成分分析（PCA）和基于密度峰值聚类为基础的方法。该方法首先选取O,B,A,F,G,K和M各种类型的高信噪比恒星光谱,进行波长统一和流量插值后,利用主成分分析得到特征光谱;然后利用方差贡献率最大的前几个特征光谱对低信噪比的恒星光谱进行重构得到高信噪比的光谱;最后利用重构之后的高信噪比光谱进行聚类,聚类分析中得到的离群数据即为所要搜寻的特殊恒星光谱。在聚类时,考虑到恒星光谱数据本身的特点,采用了一种基于密度峰值的聚类方法来进行聚类及离群点的挖掘。实验表明,该方法能够在低信噪比的恒星光谱巡天数据中准确地搜寻出数量相对较少的特殊恒星。同时,也可应用于诸如LAMOST、SDSS等各种银河系巡天的光谱数据分析与挖掘中。     

Consequences of nuclear electron capture in core collapse supernovae

The most important weak nuclear interaction to the dynamics of stellar core collapse is electron capture, primarily on nuclei with masses larger than 60. In prior simulations of core collapse, electron capture on these nuclei has been treated in a highly parametrized fashion, if not ignored. With realistic treatment of electron capture on heavy nuclei come significant changes in the hydrodynamics of core collapse and bounce. We discuss these as well as the ramifications for the postbounce evolution in core collapse supernovae.     

激光内爆靶的四分幅X射线阴影成像诊断实验和理论模拟

本文报道X射线阴影成像诊断技术的新发展,以及应用新发展的四分幅X射线阴影成像诊断于激光内爆微球靶动力过程的研究结果。我们的一维Lagrangian流体理论模拟程序(WL程序)成功地模拟出实验结果。诊断实验和理论模拟的密切结合是了解内爆动力学过程的必要途径。 关键词：     

High Energy Density Physics Studies at the Facility for Antiprotons and Ion Research: the HEDgeHOB Collaboration

The forthcoming F acility for A ntiprotons and I on R esearch (FAIR) at Darmstadt, is going to be a unique accelerator facility that will deliver high quality, strongly bunched, well focused, intense beams of heavy ions that will lead to unprecedented specific power deposition in solid matter. This will generate macroscopic samples of H igh E nergy D ensity (HED) matter with fairly uniform physical conditions. These samples can be used to study the thermophysical and transport properties of HED matter. Extensive theoretical work has been carried out over the past decade to design numerous dedicated experiments to study HED physics at the FAIR, which has provided the basis for the HEDgeHOB ( H igh E nergy D ensity Matter Ge nerated by H eavy I o n B eams) scientific proposal. This work is still in progress as the feasibility studies for more experimental schemes are being carried out. Another, very important research area that will benefit tremendously from the FAIR facility, is the production of radioactive beams. A superconducting fragment separator, Super–FRS is being designed for the production and separation of rare radioactive isotopes. Unlike the HED targets, the Super–FRS production target should not be destroyed or damaged by the beam, but should remain intact during the long experimental campaign. However, the high level of specific power deposited in the production target by the high intensity ion beam at FAIR, could cause serious problems to the target survival. These HED issues related to the Super–FRS production target are also discussed in the present paper (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)