Interactions of a Femtosecond Intense Laser with Rare Gas Clusters in a Dense Jet

A study on the interactions of high intensity (- 10^16 W/cm^2) femtosecond laser pulses with rare gas clusters in a dense jet is performed. Energy absorption by Ar and Xe clusters is measured and it can be as high as 90%. Very energetic ions produced in the laser interaction with a dense cluster jet are detected by time-of-flight spectrometry and the maximum ion energy of Xe is up to 1.3 MeV. The average ion energies are found to increase with increasing cluster size and get saturated gradually. The average ion energies also show a strong directionality and the average ion energy in the direction parallel to the laser polarization vector is 40% higher than that perpendicular to it. The findings are discussed in terms of a model of charge-dependent ion acceleration.     

Experimental Investigation of Interaction of Ultra-Short Pulse Laser With Atomic Clusters

Effects of experimental conditions to laser absorption coefficient at the interaction of ultra-short pulse laser with atomic clusters are investigated. Big clusters are easily formed for high Z rare gases at same backing gas pressure. The size of cluster …     

Simulation of radiation damages in molybdenum by combining molecular dynamics and OKMC

In this paper,radiation defects in bcc molybdenum with the primary knock-on atom(PKA) energies of2-40 keV are simulated by the molecular dynamics.The binding energy of single point defect-to-defect clusters increases with the cluster size.The stabiUty and mobility of point defects and defect clusters are analyzed.The interstitial-type clusters are found to be easily migrating along the 111 direction with low barriers(0.01-0.10 eV).Then,the object kinetic Monte Carlo is used to gain insight into the long-term defect evolution in the cascade.The simulation results indicate that Stage I almost occurs at annealing temperature of 100K,which corresponds to the correlated recombination resulting from the motion of small interstitial clusters(n ≤2).The formation of substage partly as result of the small vacancy clusters motion.At about 460 K,the Stage II starts because of uncorrected recombination due to an emitting mechanism of larger clusters.Size distribution of the clusters at the cascade quenching stage is positively correlated with the PKA energies,affecting notably the subsequent annealing process.     

Establishment of a Cluster Physics Research Device

Carbon cluster beam of MeV C_2-C_4 has been successfully accelerated on HI-13 tandem accelerator,researches of interaction between these particles and plastic track detector CR-39 has been lead out. Sincethe mass energy product of analysis magnet on HI-13 tandem accelerator is 200 MeV·Amu, only lowenergy C_2-C_4 cluster beam could be deflected and analyzed in the experimental hall. In order to obtainhigher energy C_2-C_4 or bigger clusters as well as other types of clusters (such as Si_n and Au_n, etc.), a beam     

Light and heavy clusters in warm stellar matter

Light and heavy clusters are calculated for warm stellar matter in the framework of relativistic mean-field models, in the single-nucleus approximation. The cluster abundances are determined from the minimization of the free energy. In-medium effects of light cluster properties are included by introducing an explicit binding energy shift analytically calculated in the Thomas–Fermi approximation, and the coupling constants are fixed by imposing that the virial limit at low density is recovered. The resulting light cluster abundances come out to be in reasonable agreement with constraints at higher density coming from heavy-ion collision data. Some comparisons with microscopic calculations are also shown.     

Structure and optical properties of the Ge and Sn quantum-dots

Embedded cluster samples Ge:CaF2 and Sn:CaF2 with different sizes less than 8nm were produced and their optical properties were examined .Electron diffraction revals that the cluster‘ structures are crystallites and compared with the bulk Germanium and Stannum,lattice constants expand about 4.7% in the case of Ge:CaF2 and 5.2% of Sn:CaF2.Optical absorption and photoluminescence spectra of these samples show good agreement with the Quantum-Confinement-Feffect(QCE) theory,With the decrasing of the cluster‘s size,the energy gap broadens and the absorption edge blueshift is observed,and the absorption edge also shifts with the changing of the fraction of Sn and Ge,The XPS results show that in the cluster state,the binding energy becomes higher.     

Study of Temperature Distribution Along an Artificially Polluted Insulator String

Insulator becomes wet partially or completely,and the pollution layer on it becomes conductive,when collecting pollutants for an extended period during dew,light rain,mist,fog or snow melting.Heavy rain is a complicated factor that it may wash away the pollution layer without initiating other stages of breakdown or it may bridge the gaps between sheds to promote flashover.The insulator with a conducting pollution layer being energized,can cause a surface leakage current to flow (also temperature-rise).As the surface conductivity is non-uniform,the conducting pollution layer becomes broken by dry bands(at spots of high current density),interruptibng the flow of leakage current.Voltage across insulator gets concentrated across dry bands.and causes high electric stress and breakdown(dry band arcing).If the resistance of the insulator surface is sufficiently low,the dry band arcs can be propagated to bridge the terminals causing flashover.The present paper concerns the evaluation of the temperature distribution along the surface of an energized artificially polluted insulator string.     

Atomic mobility in energetic cluster deposition

This paper tries to outline the influence of atomic mobility on the initial fabrication of thin films formed by LECBD. Based on our recent studies on low-energy cluster beam deposition (LECBD) by molecular dynamics simulation, two examples, the deposition of small carbon clusters on Si and diamond surfaces and AI clusters on Ni substrate, were mainly discussed. The impact energy of the cluster ranges from 0.1 eV to 100 eV. In the former case,the mobility and the lateral migration of surface atoms, especially the recoil atoms, are enhanced with increasing the impact energy, which promote the film to be smoother and denser. For the latter case, the transverse kinetic energy of cluster atoms, caused mainly by the collision between moving cluster atoms, dominates the lateral spread of cluster atoms on the surface, which is contributive to layer-by-layer growth of thin films. Our result is consistent with the experimental observations that the film structure is strongly dependent on the impact energy. In addition, it elucidates that the atomic mobility takes a leading role in the structure characteristic of films formed by LECBD.     

Configuration of 2D Finite Clusters in Plasma Sheath

Finite clusters with a small number of charged particles immersed in a plasma environment have been numerically simulated with a dynamic method. Finite Coulomb clusters are systems of a small number of charged particles, N = 1 to 100, confined by a potential produced by plasma 2D-sheath. Under the action of net force each particle is in an equilibrium position and together they form finite Coulomb clusters. The results of our study show the configuration of Coulomb clusters do not depend on their initial state. After theoretically studied and tested by using the Monte Carlo technique we also prove the system energy is the determinant parameter of the configuration. In addition, the effect of the external magnetic field on the cluster configuration is analyzed.     

Experimental System Foundation of Laser Cluster Interaction

In order to invest absorption mechanism of laser-clusters interaction, system of cluster generation, system of laser cluster synchronization and a set of TOF spectrometer were developed. The system of cluster generation is made up of solenoid valve and po…     

利用磁控溅射与液氮冷凝相结合产生金属及其化合物团簇

利用磁控溅射与液氮冷凝相结合的团簇产生设备成功地制备了多种金属及金属化合物等团簇，团簇尺寸大小为２－３０ｎｍ，其微结构为晶体结构。     

Hardening and microstructural evolution in A533B steels under neutron irradiation and a direct comparison with electron irradiation

A533B steels irradiated at 290 °C up to 10 mdpa in the Kyoto University Reactor were examined by hardness, positron annihilation and atom probe measurements. Dose dependent irradiation hardening and formation of Cu-rich clusters were confirmed in medium Cu (0.12% and 0.16%Cu) steels whereas neither hardening nor cluster formation was detected in low Cu (0.03%Cu) steel. No microvoids were formed in any of the steels. Post-irradiation annealing in medium Cu steels revealed that the hardening recovery at temperatures above 350-400 °C could be attributed to compositional changes and dissociation of the Cu-rich clusters. Compared to electron irradiation at almost the same dose and dose rate, KUR irradiation caused almost the same hardening and produced Cu-rich clusters, more solute-enriched with larger size and lower density. Considering lower production of freely-migrating vacancies in neutron irradiation, the results suggested that cascades enhance the formation of Cu-rich clusters.     

MD simulation of cluster formation during sputtering

The cluster ejection due to cluster impact on a solid surface is studied through molecular dynamics (MD) simulations. Simulations are performed for Cu cluster impacts on the Cu(1 1 1) surface for cluster energy 100 eV/atom, and for clusters of 6, 13, 28 and 55 atoms. Interatomic interactions are described by the AMLJ–EAM potential. The vibration energy spectrum is independent of the incident cluster size and energy. This comes from the fact that sputtered clusters become stable through the successive fragmentation of nascent large sputtered clusters. The vibration energy spectra for large sputtered clusters have a peak, whose energy corresponds to the melting temperature of Cu. The exponent of the power-law fit of the abundance distribution and the total sputtering yield for the cluster impacts are higher than that for the monatomic ion impacts with the same total energy, where the exponent δ is given by Y_n∝n^δ and Y_n is the yield of sputtered n-atom cluster. The exponent δ follows a unified function of the total sputtering yield, which is a monotonic increase function, and it is nearly equal to δ −3 for larger yield.     

Mobility of self-interstitial atom clusters in vanadium, tantalum and copper

Molecular dynamics (MD) simulations were performed to investigate the mobility of isolated self-interstitial atoms (SIAs) and their clusters in V, Ta and Cu. The migration of an isolated SIA is accompanied by rotation of a dumbbell axis to the close-packed direction of metals. The migration of an SIA cluster strongly depends on its structure. A relatively smaller-size cluster can migrate with simultaneous rotation of the axes of SIA pairs in the cluster to the same close-packed direction, which is the glissile configuration of the cluster. The transformation to the glissile configuration takes place more frequently than the dumbbell rotation of an isolated SIA in V and Ta, while it takes place less frequently in Cu. The smaller cluster can still change its diffusion direction. A greater-size cluster in the bcc metals, on the other hand, has the thermally stable form of densely-packed, parallel crowdions. It migrates without any changes of diffusion direction. The migration behavior of 7-SIAs clusters in Ta was also evaluated as a function of tensile and compressive strains.     

Synergism in sputtering of copper nanoclusters on graphite substrate at low energy Cu2 bombardment

Molecular dynamics simulation of Cu cluster sputtering by 50-200 eV/atom Cu₂ dimers and Cu single atoms has been performed. The clusters were located on a (0 0 0 1) graphite surface and consisted of 13-195 atoms. Synergy features were identified in the sputtering yield and energy distributions of sputtered particles calculated for the cases of cluster bombardment with Cu dimers and monomers at the same velocity. The reason for the nonlinear effects in surface cluster sputtering is the overlapping of collision cascades generated by each of the dimer atoms.     

基于AKMC方法研究Mn对Cu团簇析出的影响

Cu团簇析出是影响FeCu合金体系安全服役的主要因素之一。本文基于原子动力学蒙特卡罗（AKMC）方法研究了热老化条件下,合金元素Mn对Fe-1.34at.%Cu-Xat.%Mn（X为0、0.27、1.2、3、5）合金中Cu团簇析出的影响。研究结果表明,随着体系中Mn含量的增加,析出形核的纳米级Cu团簇的数密度不断增加;Mn含量对形核Cu团簇的平均尺寸影响不大。团簇中主要成分为Cu;团簇中Mn含量随初始基体中Mn含量的增加而增加。合金元素的成团参与百分比决定了团簇平均尺寸。Cu-Vac复合体较强的扩散能力及其较远的扩散距离是造成Cu团簇析出的主要因素;而Mn的加入会抑制Cu-Vac复合体的可移动距离和可移动能力,从而提高Cu团簇的数密度。合金元素Mn的加入会影响形核Cu团簇的尺寸、数密度,从而影响Cu团簇引起的硬化效应。     

Experiment and simulation of cluster emission from 5 keV Ar → Cu

The abundance distribution of neutral Cu_n clusters sputtered by 5 keV Ar impact from a polycrystalline Cu surface is measured using single-photon laser post-ionization. Molecular dynamics computer simulation is used to gain insight into the cluster sputtering process. Three different codes and two potentials are used to check the sensitivity of the results on numerics and physical input. Differences in the results obtained by the various codes and the different potentials used are discussed. While the total sputter yield is consistent with experiment, the fraction of atoms bound in clusters, and in particular the dimer fraction, are overestimated by at least a factor of 4. This is also true for a many-body potential which has been fitted to describe both bulk Cu and dimers. In detail, the simulation shows that larger clusters are emitted at later times from the target. Clusters originate mainly from regions of the surface, which are around the melting temperature of bulk Cu. Large clusters are emitted preferably from ion impacts with a high individual sputter yield. Finally, we simulate sputtering from a model Cu material with an artificially decreased cohesive energy. Here, drastic high-yield events (up to Y=78) can be observed, which produce clusters abundantly.     

核材料辐照损伤的并行空间分辨随机团簇动力学模拟

核反应堆中关键材料的辐照损伤演化对其性能有重要影响，且跨越多个量级的时空尺度。空间分辨随机团簇动力学是近年来发展的1种模拟核材料辐照损伤行为的有效方法，它避免了传统团簇动力学在缺陷种类和计算复杂性方面的限制，且能考虑缺陷的空间依赖性。在概述了空间分辨随机团簇动力学基本原理的基础上，论述了自主开发的大规模并行空间分辨随机团簇动力学程序MISA-SCD1.0的实现方式与关键技术，并将其应用于反应堆压力容器模型合金中富Cu团簇的析出模拟，验证了程序的正确性并测试了并行性能。结果表明，MISA-SCD1.0能获得与实验结果和类似模拟结果吻合的Cu析出过程，且具有较高的并行效率和良好的扩展性。     

Irradiation induced clustering in low copper or copper free ferritic model alloys

In order to determine the formation mechanism of solute cluster in ferritic steels under irradiation, Cu and Cu free ferritic model alloys (FeCuMnNiP and FeMnNiP) were irradiated with electrons and ions. Their microstructures were characterized by atom probe tomography and the evolution of the population of point defects was estimated with a model of cluster dynamic. The comparison between experimental results and predictions of the model suggests that the formation of the solute clusters is heterogeneous, on the point defect clusters. The comparison between the experimental results obtained on these two alloys shows that the absence of copper slows down the kinetics of precipitation. Moreover, by comparing the results obtained on the FeCuMnNiP alloy with previous results obtained on binary alloy FeCu, it appears that the presence of manganese, nickel and/or phosphorus slows down the kinetics of precipitation. Finally, the comparison between this study and results obtained by other authors on similar alloys irradiated with neutrons reveals a strong flux effect on the kinetic of precipitation of solute clusters.