Time-resolved spectroscopy of collinear femtosecond and nanosecond dual-pulse laser-induced Cu plasmas

In this paper,we investigate the time-resolved spectroscopy of collinear femtosecond(fs)and nanosecond(ns)dual-pulse(DP)laser-induced plasmas.A copper target was used as an experimental sample,and the fs laser was considered as the time zero reference point.The interpulse delay between fs and ns laser beams was 3μs.First,we compared the time-resolved peak intensities of Cu(I)lines from Cu plasmas induced by fs+ns and ns+fs DP lasers with collinear configuration.The results showed that compared with the ns+fs DP,the fs+ns DP laser-induced Cu plasmas had stronger peak intensities and longer lifetimes.Second,we calculated time-resolved plasma temperatures using the Boltzmann plot with three spectral lines at Cu(I)510.55,515.32 and 521.82 nm.In addition,time-resolved electron densities were calculated based on Stark broadening with Cu(I)line at 521.82 nm.It was found that compared with ns+fs DP,the plasma temperatures and electron densities of the Cu plasmas induced by fs+ns DP laser were higher.Finally,we observed images of ablation craters under the two experimental conditions and found that the fs+ns DP laser-produced stronger ablation,which corresponded to stronger plasma emission.

Time-resolved measurement of atomic emission enhancement by fs-ns dual-pulsed laser-induced breakdown spectroscopy

Time-resolved measurement of atomic emission enhancement is performed by using a 500-fs KrF laser pulse incident upon a high density supersonic O2 gas jet, synchronized with an orthogonal ns frequency-doubled Nd：YAG laser pulse. The ultra-short pulse serves as an igniter of the gas jet, and the subsequent ns-laser pulse significantly enhances the atomic emission. Analysis shows that the contributions to the enhancement effect are made mainly by the bremsstrahlung radiation and cascade ionization.

Ablation characteristics of insulator under high-temperature gas dual-pulse erosion

This study numerically simulated and investigated the flow field characteristics of a typical dual-pulse solid rocket motor with a soft pulse separation device through thermal insulation ablation under high-temperature dual-pulse erosion.The ablation rate of ethylene-propylene-diene monomer(EPDM)insulator was measured after the experiment.Experimental results were analyzed through scanning electron microscopy and microcomputed tomography.The ablation mechanism of the EPDM insulator under the operation conditions of a dual-pulse solid rocket motor was evaluated by analyzing the results.The results reveal that the internal flow field of the motor with a soft pulse separation device is uniform.The original charred layer existing on the EPDM insulator surface in the first pulse combustor is the decisive factor affecting the final ablation rate of the dual-pulse motor during the second pulse operation,and the ablation characteristic region is easily formed with the exfoliation of the charred layer.The ablation rate difference of the insulator increases with gas velocity.

Control and data acquisition system for collinear laser spectroscopy experiments

A control and data acquisition system was implemented for the recently developed collinear laser spectroscopy setup.This system is dedicated to data recording,storage,processing,monitoring of the beam intensity and energy,and visualization of various spectra.In comparison to the conventional resonance nuclear reaction system,the key technique is the precise synchronization of the detected counts with the actual scanning voltage(or probing laser frequency).The functions of the system were tested by measuring the hyperfine structure spectra of stable calcium(e.g.,^(40)Ca^(+))and radioactive potassium(e.g.,^(38)K)in the bunched and continuous modes,respectively.This system will be routinely applied and further improved in subsequent laser spectroscopy experiments on unstable isotopes at the Beijing Radioactive Ion-beam Facility(BRIF).

Effects of pulse energy ratios on plasma characteristics of dual-pulse fiber-optic laser-induced breakdown spectroscopy

Laser-induced plasmas of dual-pulse fiber-optic laser-induced breakdown spectroscopy with different pulse energy ratios are studied by using the optical emission spectroscopy(OES)and fast imaging.The energy of the two laser pulses is independently adjusted within 0–30 m J with the total energy fixed at 30 m J.The inter-pulse delay remains 450 ns constantly.As the energy share of the first pulse increases,a similar bimodal variation trend of line intensities is observed.The two peaks are obtained at the point where the first pulse is half or twice of the second one,and the maximum spectral enhancement is at the first peak.The bimodal variation trend is induced by the change in the dominated mechanism of dual-pulse excitation with the trough between the two peaks caused by the weak coupling between the two mechanisms.By increasing the first pulse energy,there is a transition from the ablation enhancement dominance near the first peak to the plasma reheating dominance near the second peak.The calculations of plasma temperature and electron number density are consistent with the bimodal trend,which have the values of 17024.47 K,2.75×10^(17)cm;and 12215.93 K,1.17×10^(17)cm;at a time delay of 550 ns.In addition,the difference between the two peaks decreases with time delay.With the increase in the first pulse energy share,the plasma morphology undergoes a transformation from hemispherical to shiny-dot and to oblate-cylinder structure during the second laser irradiation from the recorded images by using an intensified charge-coupled device(ICCD)camera.Correspondingly,the peak expansion distance of the plasma front first decreases significantly from 1.99 mm in the single-pulse case to 1.34 mm at 12/18(dominated by ablation enhancement)and then increases slightly with increasing the plasma reheating effect.The variations in plasma dynamics verify that the change of pulse energy ratios leads to a transformation in the dual-pulse excitation mechanism.

非共线多裂纹结构的剩余强度分析与评估

针对老龄飞机的广布疲劳损伤(Widespread Fatigue Damage,WFD)评估问题,通过试验方法研究了不同结构参数对非共线多裂纹结构剩余强度的影响,修正原有剩余强度评估方法中的缺陷,提出改进的净截面屈服准则和改进的Irwin塑性区连通准则,获得更准确的评估结果。改进的净截面屈服准则评估结果误差小,适用范围广,可以评估铆钉孔间距和铆钉孔直径对结构剩余强度的影响;改进的Irwin塑性区连通准则在铆钉孔间距大于10mm时,有较准确的评估结果,还可以评估裂纹分布位置对剩余强度的影响。

45号钢腐蚀程度的超声非线性检测实验研究

腐蚀会显著降低金属零件的使用性能,检测金属零件的腐蚀程度对确保其安全使用具有重要意义。超声非线性技术已被应用于检测材料微损伤、残余应力和黏结状态等方面,其能否用于检测腐蚀程度倍受关注。本文搭建了基于超声非线性技术的临界折射纵波(criticalrefraction longitudinal wave,简称LCR波)混频检测系统,检测不同腐蚀程度的45号钢试件的非线性系数,随后根据超声非线性效应的机理结合材料的微观形貌对差频非线性系数的变化原因进行分析。实验结果表明:差频非线性系数对材料腐蚀程度十分敏感,随着45号钢试件腐蚀程度增加,差频非线性系数随之增加。超声非线性技术可以有效地实现对金属材料腐蚀程度的表征。

小麦TaPOD家族的全基因组鉴定及表达分析

过氧化物酶(Peroxidase,POD)家族成员在调节植物生长发育和响应逆境胁迫中起重要作用。为系统探究小麦(Triticum aestivum L.)TaPOD基因家族的功能及其表达模式,本研究利用生物信息学的方法鉴定了小麦TaPOD基因家族成员,对其理化性质、启动子顺式作用元件、进化特征做了预测分析,并通过小麦转录组和实时荧光定量PCR(Real Time Quantitative,RT-qPCR)分析了其在不同组织、外源激素及逆境胁迫下的表达模式。结果表明,目前基因组测序小麦中包含659个TaPOD基因家族成员,蛋白质序列长度在206~518个氨基酸之间;系统发育分析表明,小麦TaPOD家族成员分为I~VII组且每组成员数量不等;序列比对显示小麦TaPOD家族成员具有5个保守基序,且基因结构等方面存在很大差异,预示着功能存在多样性;染色体定位发现其数量在小麦的21条染色体上分布不均匀,其中2B染色体上数量最多;通过种内共线性分析发现,小麦TaPOD基因共有396个重复事件,同源性较高且进化过程非常保守,主要通过片段复制和串联复制进行扩增,且Ka/Ks比率显示仅有4对家族成员受到了正向的自然选择压力;顺式作用元件分析表明,上游2 kb区域中存在23种与生长发育和抗逆性相关的结合元件;基因表达模式分析显示,86.5%TaPOD基因在小麦根系中表达量较高;通过RT-qPCR检测发现TaPOD基因表达量与激素诱导和非生物胁迫密切相关。上述结果为深入研究TaPOD基因在调控小麦生长发育与逆境胁迫中的功能提供初步的理论基础。

种子注入式太赫兹波参量产生源研究

种子注入式太赫兹波参量产生源(is-TPG)基于非线性晶体中的受激电磁偶子散射,具有线宽窄、相干性好、室温运转、可调谐等优点。本文基于5%摩尔百分比氧化镁掺杂的同成分铌酸锂晶体(MgO:CLN),对连续/脉冲种子注入式太赫兹波参量产生源(is/ips-TPG)调谐范围、输出能量、阈值等方面进行了研究。相比于is-TPG系统,ips-TPG系统调谐范围范围扩大约80%。is-TPG系统与ips-TPG系统3dB带宽分别为1THz与1.41THz,占各自调谐范围的51%和39%。ips-TPG系统相比is-TPG系统3dB带宽范围扩大了41%。在相近的平均功率密度下,由于ips-TPG具有更高的峰值功率密度降低了阈值,产生了更强的THz波输出,最大转换效率为1.01×10^(-5)。不同形式种子光的注入对太赫兹波参量辐射源有着显著的影响,实验结果表明ips-TPG系统的输出特性更为突出在实际应用中具有更高的价值。本文最后对其未来发展进行了分析和展望。

一维六方压电准晶狭长体中双半无限共线裂纹问题的D-B模型

基于解析函数理论与保角映射方法,通过构造一维六方压电准晶狭长体中双半无限共线裂纹问题的D-B(Dugdale-Barenblatt)模型,研究了一维六方压电准晶狭长体中双半无限共线裂纹的弹塑性断裂力学问题,得到了塑性区尺寸及场强度因子的解析解,推广了材料中裂纹问题的已有结果,扩大了准晶压电材料弹塑性断裂力学问题的研究范围。

青海省北川河径流量与水文地貌特征的关系分析

以北川河流域5个水文站径流量观测资料为基础,利用30 m分辨率的DEM数据提取了与径流量相关的水文地貌特征因子,通过对多年平均径流量与水文地貌特征因子之间的相关分析,经过共线性诊断筛选水文地貌特征因子,建立了线性回归方程.结果表明:北川河流域从上游到下游地形趋于平坦,水系发育程度更高,汇流能力更强;影响多年平均径流量主要的宏观地形因子贡献率分别是地形湿度指数(50.66%)和地表粗糙度(44.33%),主要的微观地形因子贡献率分别是剖面曲率(36.25%)、高程(27.26%)和坡向变率(24.31%),主要的水系形态指标贡献率分别是节点连接率(54.16%)和面积高程积分(32.95%).上述结果为深入理解北川河流域的水文地貌关系提供依据,为研究其他小流域的不同水文节点,特别是上下游的水文地貌特征以及径流量与水文地貌特征的关系分析提供思路参考.

Ultrafast dynamics of femtosecond laser-induced high spatial frequency periodic structures on silicon surfaces

Femtosecond laser-induced periodic surface structures(LIPSS)have been extensively studied over the past few decades.In particular,the period and groove width of high-spatial-frequency LIPSS(HSFL)is much smaller than the diffraction limit,making it a useful method for efficient nanomanufacturing.However,compared with the low-spatial-frequency LIPSS(LSFL),the structure size of the HSFL is smaller,and it is more easily submerged.Therefore,the formation mechanism of HSFL is complex and has always been a research hotspot in this field.In this study,regular LSFL with a period of 760 nm was fabricated in advance on a silicon surface with two-beam interference using an 800 nm,50 fs femtosecond laser.The ultrafast dynamics of HSFL formation on the silicon surface of prefabricated LSFL under single femtosecond laser pulse irradiation were observed and analyzed for the first time using collinear pump-probe imaging method.In general,the evolution of the surface structure undergoes five sequential stages:the LSFL begins to split,becomes uniform HSFL,degenerates into an irregular LSFL,undergoes secondary splitting into a weakly uniform HSFL,and evolves into an irregular LSFL or is submerged.The results indicate that the local enhancement of the submerged nanocavity,or the nanoplasma,in the prefabricated LSFL ridge led to the splitting of the LSFL,and the thermodynamic effect drove the homogenization of the splitting LSFL,which evolved into HSFL.

应用遥感测绘的土地资源利用时空变化分析

为实现对土地资源的均衡配置,平衡地区经济发展与土地利用情况,基于遥感测绘设计了土地资源利用时空变化分析方法。首先,以减少旁向偏差为前提,采用共线方程求解的方式,确定土地资源测绘区域的定点坐标;然后,设定遥感测绘中不同波段的参数范围,获取土地资源中的地物标志图像,并归一化处理图像数据;最后,基于特定范围选择评断指数,构建土地利用变化分析模型,以年度为时段动态描述土地利用变化速度,引入转移矩阵表示土地利用结构和方向,区分土地变化的主导与次要类型,实现对土地资源利用时空变化的具体分析。实验表明:以不同的土地类型作为测试对象,该方法均能够直观地反映各种土地利用类型的年度变化情况,具有实际应用价值。

城市轨道交通与市域(郊)铁路融合发展关键技术研究

基于多网融合理念,结合都市圈总体空间结构、产业经济、客流分布特点,布局的国铁干线、城际铁路、市域(郊)铁路、城市轨道交通等,实现区际通道、都市圈市域、城区轨道等服务功能完备、融合高效的西安市多层次综合轨道交通。本文以西安市多层次轨道交通网络规划过程方案为依托,结合铁路项目及地铁工程的功能定位、客流特点、运输组织和技术标准的不同,从运营组织、限界、车辆、轨道、车站建筑、区间隧道、供电制式、信号系统、控制中心等多方面综合分析,总结了城市轨道交通与市域郊铁路局部共线方案的研究思路,为都市圈轨道交通规划建设提供参考。

Exact analytic solutions for an elliptic hole with asymmetric collinear cracks in a one-dimensional hexagonal quasi-crystal

Using the complex variable function method and the technique of conformal mapping, the anti-plane shear problem of an elliptic hole with asymmetric colfinear cracks in a one-dimensional hexagonal quasi-crystal is solved, and the exact analytic solutions of the stress intensity factors （SIFs） for mode Ⅲ problem are obtained. Under the limiting conditions, the present results reduce to the Griffith crack and many new results obtained as well, such as the circular hole with asymmetric collinear cracks, the elliptic hole with a straight crack, the mode T crack, the cross crack and so on. As far as the phonon field is concerned, these results, which play an important role in many practical and theoretical applications, are shown to be in good agreement with the classical results.

Identification of novel soybean oil content-related genes using QTL-based collinearity analysis from the collective soybean genome

Soybean is a global principal source of edible plant oil. As more soybean oil-related quantitative trait loci(QTLs) have been located in the collective genome, it is urgent to establish a classification system for these distributed QTLs. A collinear platform may be useful to characterize and identify relationships among QTLs as well as aid in novel gene discovery. In this study, the collinearity MCScan X algorithm and collective soybean genomic information were used to construct collinearity blocks, to which soybean oil-related QTLs were mapped. The results demonstrated that 666 collinearity blocks were detected in the soybean genome across 20 chromosomes, and 521 collinearity relationships existed in 231 of the 242 effective soybean oil-related QTLs. This included 214 inclusion relationships and 307 intersecting relationships. Among them, the collinearity among QTLs that are related to soybean oil content was shown on a maximum of seven chromosomes and minimum of one chromosome, with the majority of QTLs having collinearity on two chromosomes. Using overlapping hotspot regions in the soybean oil QTLs with collinearity, we mined for novel oil content-related genes. Overall, we identified 23 putatively functional genes associated with oil content in soybean and annotated them using a number of annotation databases. Our findings provide a valuable framework for elucidating evolutionary relationships between soybean oil-related QTLs and lay a foundation for functional marker-assisted breeding relating to soybean oil content.

Exact solutions of two semi-infinite collinear cracks in piezoelectric strip

Using the complex variable function method and the conformal mapping technique, the fracture problem of two semi-infinite collinear cracks in a piezoelectric strip is studied under the anti-plane shear stress and the in-plane electric load on the partial crack surface. Analytic solutions of the field intensity factors and the mechanical strain energy release rate are derived under the assumption that the surfaces of the crack are electrically impermeable. The results can be reduced to the well-known solutions for a purely elastic material in the absence of an electric load. Moreover, when the distance between the two crack tips tends to infinity, analytic solutions of a semi-infinite crack in a piezoelectric strip can be obtained. Numerical examples are given to show the influence of the loaded crack length, the height of the strip, the distance between the two crack tips, and the applied mechanical/electric loads on the mechanical strain energy release rate. It is shown that the material is easier to fail when the distance between two crack tips becomes shorter, and the mechanical/electric loads have greater influence on the propagation of the left crack than those of the right one.

THE BEHAVIOR OF TWO COLLINEAR CRACKS IN MAGNETO-ELECTRO-ELASTIC COMPOSITES UNDER ANTI-PLANE SHEAR STRESS LOADING

In this paper, the behavior of two collinear cracks in magneto-electro-elastic compos- ite material under anti-plane shear stress loading is studied by the Schmidt method for permeable electric boundary conditions. By using the Fourier transform, the problem can be solved with a set of triple integral equations in which the unknown variable is the jump of displacements across the crack surfaces. In solving the triple integral equations, the unknown variable is expanded in a series of Jacobi polynomials. Numerical solutions are obtained. It is shown that the stress feld is independent of the electric feld and the magnetic fux.

Commissioning of a high-resolution collinear laser spectroscopy apparatus with a laser ablation ion source

Collinear laser spectroscopy is a powerful tool for studying the nuclear spins,electromagnetic moments,and charge radii of exotic nuclei.To study the nuclear properties of unstable nuclei at the Beijing Radioactive Ion-beam Facility(BRIF)and the future High Intensity Heavy-ion Accelerator Facility(HIAF),we developed a collinear laser spectroscopy apparatus integrated with an offline laser ablation ion source and a laser system.The overall performance of this state-of-the-art technique was evaluated,and the system was commissioned using a bunched stable ion beam.The high-resolution optical spectra for the 4s ^(2)S_(1/2)→4p^(2)P_(3/2)(D2)ionic transition of ^(40;42;44;48)Ca isotopes were successfully measured.The extracted isotope shifts relative to ^(40)Ca showed excellent agreement with the literature values.This system is now ready for use at radioactive ion beam facilities such as the BRIF and paves the way for the further development of higher-sensitivity collinear resonance ionization spectroscopy techniques.

SINGULARITIES AND EXPLICIT SOLUTIONS OF COLLINEAR CRACK PROBLEM IN ANISOTROPIC MEDIA

The singularities of collinear cracks both in anisotropic single medium and at the interface of anisotropic bimaterials are studied by combining Stroh formalism and the analytic function method. The formulae for calculating the field potential and stress intensity factor (SIF) are obtained. It is found that the field potentials are explicitly related to material matrix L and the in-plane and anti-plane fields can be separately calculated when orthotropic bimaterials are considered.