Plasma ignition threshold disparity between silver nanoparticle-based target and bulk silver target at different laser wavelengths

Plasma ignition threshold of nanoparticle-based and bulk silver targets was measured in air. The plasma was initiated by a Nd:YAG laser at wavelengths of 355, 532, and 1064 nm. The plasma ignition was monitored utilizing the prominent Ag I line at 546.5 nm. Lower ignition thresholds of the nanoparticle-based silver target were estimated at 0.4?±?0.02, 0.34?±?0.04, and 0.27?±?0.035 J cm~(-2) coupled with the different laser wavelengths, respectively. In contrast, the bulk silver target plasma exhibited an order of magnitude higher ignition threshold. A three orders of magnitude enhanced emission intensity from the nano-based target over the bulk target was achieved at lower levels of laser irradiation. A reduction of the thermal diffusion length of the nanosilver was assumed in order to theoretically predict this reduction in the plasma threshold. In addition, the effect of self-reversal on the resonance lines was taken into consideration.     

自然光照条件下植被几何光学四分量的提取算法

目的 几何光学四分量是指在太阳光照条件下传感器所能观测的4个光学分量，即光照植被、光照土壤、阴影植被和阴影土壤。四分量是构成遥感几何光学模型的重要内容。在近地表遥感应用中，相机俯视拍照是提取四分量的一个途径。准确快速地从图像数据中提取四分量对植被冠层结构参数反演和植被长势监测具有重要意义。方法 植被与土壤二分量的识别是四分量提取的基础。目前大多数二分类算法在自然光照条件复杂时分类误差较大。本文基于卷积神经网络（CNN）和阈值法实现了多种二分类和四分量提取算法。阈值法中，使用SHAR-LABFVC （shadow-resistant algorithm：LABFVC）实现植被与土壤的二分类，并在此基础上应用二次阈值分割获取四分量，称为二次阈值法；基于CNN的方法中，采用U-Net架构，并使用RGB和RGBV数据进行训练得到U-Net和U-Net-V模型，前者完成二分类和四分量任务，后者只完成四分量提取实验。最后，对一种结合U-Net与阈值法的混合算法进行四分量提取实验。结果 本文在18幅图像（1 800个子图）数据上进行了实验，结果表明，与目视解译得到的四分量真值相比较，U-Net-V和混合法精度最高，具有相近的均方根误差（RMSE）（0.06和0.07）和相关系数（0.95和0.94）；二次阈值法与U-Net模型精度略低于上述两种算法，RMSE分别是0.08和0.09，相关系数均为0.88。在二分类实验中，U-Net的分类正确率是91%，SHAR-LABFVC为85%。结论 通过对比实验表明，在二分类问题中，U-Net可以更好地应对复杂自然光照条件下的数字图像。在四分量提取实验中，混合法和U-Net-V的结果优于U-Net与二次阈值法，可以用于提取四分量。     

考虑应力敏感的致密气藏水平井产能计算方法

除启动压力梯度外致密气藏还存在应力敏感,在实验确定应力敏感参数的基础上,引入虚拟裂缝概念,利用保角变换方法,考虑应力敏感和启动压力梯度建立了气藏水平井产能方程,同时分析了应力敏感、启动压力梯度、水平井长度等对产能的影响。结果表明:产能方程与试采产能的误差小于7%,验证了产能方程的可靠性;应力敏感和启动压力梯度均使产能降低,其中应力敏感占主要作用;应力敏感在低井底流压时对产能影响严重,启动压力梯度在高井底流压时对产能影响较大;水平井长度是影响产能的主控因素。该研究丰富了致密气藏水平井产能计算方法,并为水平井长度和生产压差的优化提供了理论指导。     

Sediment-associated metals levels along the sewer-natural treatment wetland continuum,Phnom Penh,Cambodia

Sediment deposited within open sewers of Phnom Penh and the natural wetland that treats the waste was sampled and analyzed by X-ray fluorescence (XRF). Mean levels of Pb, Zn (P < 0.05), and Cu (P = 0.08) were greater in the sewer samples than the wetland, while As was not different (P>0.2), and Mn was greater (P < 0.06) in the wetland. Pb, Cu, and Zn levels generally were lower at the wetland outlet and in quiescent areas, suggesting deposition occurs along the sewer/wetland continuum. Background (natural) levels of Mn and As are higher in soils/sediments of this region and levels in the wetland likely reflect these background levels than enrichment from anthropogenic activity. Although 60% of all metals levels in the wetland samples exceeded USEPA threshold effect concentrations, results of recent studies that analyzed fish and vegetables from the wetland showed a small health risk associated with consumption. The XRF was a good tool for environmental analysis in a developing country.     

Likelihood ratio tests for the structural change of an AR(p) model to a Threshold AR(p) model

This article considers the likelihood ratio (LR) test for the structural change of an AR model to a threshold AR model. Under the null hypothesis, it is shown that the LR test converges weakly to the maxima of a two‐parameter vector Gaussian process. Using the approach in Chan and Tong (1990)and Chan (1991), we obtain a parameter‐free limiting distribution when the errors are normal. This distribution is novel and its percentage points are tabulated via a Monte Carlo method. Simulation studies are carried out to assess the performance of the LR test in the finite sample and a real example is given.     

Reaction Characteristic for Various Scale Explosive under Mild Impact

This work presents a varying trend of impact ignition threshold denoted by minimum impact velocity to trigger an ignition when the scale of the explosive changes. The effects of explosive scale factors on impact-induced reaction degree were investigated using Steven tests and numerical simulation for polymer-bonded explosive-C03 (a cyclotetramethylene tetranitramine [HMX]-based explosive) impacted by projectiles of various velocities. Two scale factors—that is, axial thickness and radius—were studied through various scale samples including Φ98 mm × 13 mm, Φ98 mm × 39 mm, Φ140 mm × 13 mm, and Φ140 mm × 39 mm. The velocities of projectiles and the impact and ignition processes were analyzed using a high-speed camera. The pressure histories were measured by embedded manganin pressure gauges and poly vinylidene fluoride stress gauges. The reaction overpressures of the explosive were obtained by blast pressure gauges to evaluate the reaction degree. The effects of explosive scale factor on reaction degree and characteristics under mild impact were summarized. In a certain range (larger than the diameter of the impact projectile), different sample diameters do not influence the velocity threshold, but the thickness of the samples does; that is, the velocity threshold increases with the thickness of the sample. The study also indicates that the ignition and explosion in Steven tests are mainly triggered by the overlapping of direct impact and reflected stress waves. Our numerical simulations results of pressure and ignition times are consistent with the experimental data. The obtained knowledge can be used to evaluate the safety of different scale HMX-based explosives under accidental impact or falls.     

A set‐membership approach to integrated trade‐off design of robust fault detection system

In order to improve fault detection (FD) performance, integrated design of residual generation and evaluation is investigated in this paper for trade‐offs between fault detection rate and false alarm rate (FAR). A set‐membership approach is proposed in residual evaluation by adopting a threshold ellipsoid, which enables more design freedom than a conventional threshold value. With the set‐based definitions of fault detection rate and FAR, the integrated design problem is formulated by maximizing the FD performance under a predefined FAR. The joint optimal selection of a residual generator and a threshold ellipsoid is equivalently transformed into a simplified optimization problem of determining an optimal threshold ellipsoid for any given residual generator. A suboptimal solution for the set‐membership‐based integrated FD system design is obtained based on approximated computation of the FD performance. Monte Carlo simulations show the performance improvement of the proposed method compared with an existing integrated design method. Copyright © 2016 John Wiley & Sons, Ltd.     

自适应连续多级分区与初始阈值估计的快速模板匹配方法

归一化互相关测度在光照改变时比采用绝对差之和测度（SAD）要稳定，但是归一化互相关测度的缺陷在于它的计算量非常大。为此，提出了一种结合自适应连续多级分区和初始阈值估计的基于归一化互相关（NCC）的快速模板匹配算法。根据模板图像中不同模块的梯度值，将模板图像进行逐级分区，通过分区顺序将互相关之和分为不同的层，得到各层互相关的上界，运用柯西-施不等式得到上界间的关系，形成自适应连续多级分区淘汰方法。同时，为了加快匹配速度，利用初始阈值估计产生一个较大的边界阈值，以淘汰初始搜索时的大量非匹配点，减少搜索点数目。实验结果表明:所提出的算法具有较好的鲁棒性，且算法的执行速度优于传统算法。