Numerical simulation for all-optical Thomson scattering X-ray source

Energy spectra, angular distributions, and temporal profiles of the photons produced by an all-optical Thomson scat- tering X-ray source are explored through numerical simulations based on the parameters of the SILEX-I laser system （800 nm, 30 fs, 300 TW） and the previous wakefield acceleration experimental results. The simulation results show that X-ray pulses with a duration of 30 fs and an emission angle of 50 mrad can be produced from such a source. Using the optimized electron parameters, X-ray pulses with better directivity and narrower energy spectra can be obtained. Besides the electron parameters, the laser parameters such as the wavelength, pulse duration, and spot size also affect the X-ray yield, the angular distribution, and the maximum photon energy, except the X-ray pulse duration which is slightly changed for the case of ultrafast laser-electron interaction.     

一种基于Logistic模型的突变过程性分析方法及其应用

传统突变检测方法往往将突变检测的关键放在"突变点"上而忽略突变事件发生、发展乃至消亡的过程. 本文提出一种新的检测突变过程的方法, 其基于Logistic模型推导出能够反应均值突变过程的分段函数, 利用分段函数对实际时间序列进行参数提取, 借助参数的物理意义, 研究分析完整的突变过程, 从而加深对突变过程性的理解和认识, 为进一步研究突变的形成机理、影响因素乃至发展趋势奠定重要的科学基础. 文章还对太平洋年代际振荡 (PDO) 指数进行突变过程性检测, 发现1940/1942, 1977, 1987年开始序列发生突变, 突变持续时间锁定在某些固定值, 并且系统相图表明一般性系统存在三种基本的状态. 关键词： 均值突变 参数提取 突变过程 突变持续时间     

江淮流域夏季降水对前冬持续时间长短的响应

利用NCEP/NCAR再分析资料计算得到江淮流域1961–2011年前冬持续时间, 分析其时空变化特征, 并进一步探究它与后期江淮流域夏季降水的关系. 结果表明, 江淮流域前冬持续时间存在着明显的年际和年代际变化, 前冬持续时间显著偏长年份比偏短年份的前冬温度低、气压高、北风强, 表明温度、气压、经向风可能是反映江淮流域前冬持续时间的关键因子, 且不同区域各气象要素对季节长度的影响存在一定差异; 1961–2011年江淮流域前冬持续时间与该区域夏季降水呈显著正相关关系; 统计分析亦发现, 前冬持续时间显著偏长(偏短)的代表年份中, 夏季降水以偏多(偏少)为主; 对典型代表年份环流场进行合成分析发现, 前冬持续时间显著偏长时, 乌拉尔山与鄂霍次克海地区夏季易形成阻塞形势, 进而会对江淮流域夏季降水产生影响; 最后利用奇异值分解从空间场相关的角度探讨了两者的联系, 发现江淮流域夏季降水与前冬持续时间存在非常显著的关系. 关键词： 江淮流域 季节划分 前冬持续时间 夏季降水     

三爆轰管-脉冲爆轰发动机的近场噪声特性

为研究三爆轰管脉冲爆轰发动机噪声的形成机理和特性,设计了正三角形组合的三爆轰管PDE实验系统,分别在与中心轴线成0°、30°、60°及90°方向上,对8个不同距离处的声压进行了测量,结果表明:不同距离处噪声幅值最大值均在30°方向上;爆轰噪声的参考半径为3倍“名义管径”;爆轰噪声的A持续时间随着r(距离三爆轰管名义“中点”的直线距离)的增加近似线性减小,并随着角度的增大而减小。B持续时间与声压值的大小成反比关系,并且随着轴向距离的增加而增加。在不同的方位角上,峰值声压越大,B持续时间越小。     

换向持续时间和换向点位置对往复运动齿轮齿条弹流润滑的影响

往复运动齿轮齿条的润滑失效通常发生在换向死点位置附近,因此研究齿轮齿条换向点位置和换向持续时间对换向过程中润滑油膜的影响具有重要的实际意义。根据齿轮齿条换向瞬间的运动几何关系,建立了换向过程齿轮齿条弹流润滑的瞬态数值模型。采用Ree-Eyring润滑流体,应用多重网格法和多重网格积分法等数值方法,计算得到了齿轮齿条往复运动过程中换向点位置附近一对啮合轮齿间的压力、膜厚和温度,并与前人的实验结果进行了对比验证。分析了不同换向持续时间和换向点位置对一对啮合轮齿间压力、膜厚和温度的影响。齿轮齿条换向过程中油膜厚度明显降低,缩短换向持续时间虽然可以增大齿轮齿条的润滑膜厚,但会导致瞬间油温升高,因此换向持续时间存在最优值。通过比较不同换向死点位置的膜厚发现,当换向死点在单齿啮合后的双齿啮合区时,啮合轮齿间具有较理想的润滑膜厚。无论换向持续时间长短,润滑膜厚的最小值都在换向死点位置,换向死点位置是往复运动齿轮齿条润滑失效的危险点。研究结果为往复运动齿轮齿条的润滑设计提供了理论依据。     

星光掩星探测技术的轨道模拟结果分析

星光掩星技术是利用恒星光谱进行地球及其他行星大气痕量成分密度、温度、气溶胶等测量的有效手段。该探测原理主要是根据不同的大气成分在恒星光谱的不同位置上表现出不同的吸收特征,具体表现在:紫外波段可进行臭氧、氧气、氢气等的测量,可见光谱段可探测二氧化氮、三氧化氮、氧气等,红外可探测水蒸气、气溶胶、甲烷、二氧化碳、氧气等。星光掩星的实现过程为:当LEO卫星和恒星分别位于地球的两侧时,恒星发射的光经过地球大气的吸收、散射等作用,被另一侧的LEO所接收,即构成掩星观测。根据光谱流量得到恒星的视星等范围,给出恒星在天球坐标系中的分布和不同的光谱型,以及利用各光谱型可探测的大气成分,再利用恒星和LEO卫星在地固坐标系中的相对位置,进行恒星-LEO星光掩星轨道观测模拟,基本流程为:首先读取LEO卫星的轨道位置以及目标恒星的位置,设置24 h的模拟时间,其次判断是否处于掩星状态,当掩星开始时,计算并输出掩星发生的经纬度、速度等,直至模拟时间结束。其中涉及恒星从天球坐标系转换到地固系的过程, LEO卫星轨道、掩星切点经纬度等的计算。根据模拟流程,计算并分析掩星事件的日观测量、全球分布、持续时间以及漂移速度等,得到以下结果:(1)目标恒星在全天区都有一定数量的分布且具有不同的光谱型,可进行臭氧、二氧化氮等成分的探测;(2)在对星光掩星进行24 h的轨道模拟过程中,日观测量为5 563次,其中包括2 737次上升掩星, 2 826次下降掩星;(3)从全球分布来看,掩星事件主要分布在低纬度,两极最少,其他纬度数量相当,且经度方向分布均匀;(4)根据方位角的分布,正常掩星占比为78.25%,持续时间平均为1.5 min,切点水平漂移在18~600 km;(5)21.75%的侧面掩星事件,其较正常掩星来说,持续时间长,切点的水平漂移速度大,方位角变化也大。该结果为卫星轨道设计和探测载荷设计提供理论指导。     

Characterizations of Stress and Strain Variation in Three-Dimensional Forming of Laser Micro-Manufacturing

A micro-manufacturing technology is presented to form three-dimensional metallic micro-structures directly. Micro grid array structures are replicated on a metallic foil surface, with high spatial resolution in micron levels. The numerical simulation results indicate that the material deformation process is characterized by an ultrahigh strain rate. With increasing pulse duration, the sample absorption strain energy increases, and the sample deformation degree enlarges. The stress state of the central point fluctuates between tensile stress and compression stress. The stress state of the angular point is altered from compressive stress to tensile stress due to geometry and loading conditions. The duration length of pulse stress has an effect on the stress state, as with the increase of pulse duration, fluctuation in the stress state decreases. Therefore, laser micro-manufacturing technology will be a potential laser micro forming method which is characterized by low cost and high efficiency.     

Generation of 8.5-nJ pulse compensation-free from all-fiber dispersion Yb-doped laser

We report the generation of an 8.5-nJ chirped pulse from a mode-locked all-fiber Yb-doped laser. Mode- locking is achieved through nonlinear polarization evolution （NPE） along with spectral filtering. The laser delivers 135 mW of average output power with positively chirped 10.9-ps pulses. The pulse repetition rate is 15.9 MHz, which results in an energy of 8.5 nJ per pulse. The externally dechirped pulse duration is 223 fs, and the pulse energy is 6 n J, which corresponds to the peak power of -27 kW.     

Self-starting harmonic mode-locked Tm-Bi co-doped germanate fiber laser with carbon nanotube-based saturable absorber

We report a ring cavity passively harmonic mode-locked fiber laser using a newly developed thuliumbismuth co-doped fiber(TBF) as a gain medium in conjunction with a carbon nanotube(CNT)-based saturable absorber.The TBF laser generates a third harmonic mode-locked soliton pulse train with a high repetition rate of 50 MHz and a pulse duration of 1.86 ps.The laser operates at 1 901.6 nm with an average power of 6.6 mW,corresponding to a pulse energy of 0.132 nJ,at a 1 552 nm pump power of 723.3 mW.     

Effect of water temperature on pulse duration and energy of stimulated Brillouin scattering

The water temperature has a strong effect on the kinematic viscosity, which is inversely proportional to the phonon lifetime and the gain coefficient. The higher the temperature is, the smaller the kinematic viscosity is, and the larger the phonon lifetime is. At a low pump power and a short focal length, we can derive a single-peak stimulated Brillouin scattering (SBS) pulse. The duration of this single-peak SBS pulse depends mainly on the phonon lifetime of water. With the increase of the water temperature, the duration of such a single-peak SBS pulse will become longer, and the SBS energy will become higher for the gain coefficient, which is related to the phonon lifetime. Therefore, varying the medium temperature can lead to the changes of SBS pulse duration and SBS energy.