Influence of cutting off position of plasma filament formed by two-color femtosecond laser on terahertz generation

Femtosecond laser filamentation is a method of generating terahertz, which has wide application in terahertz subwavelength resolution imaging. In this paper, the plasma filament formed by femtosecond laser focusing was terminated with an alumina ceramics at different positions and the influence of the cutting off position of the plasma filament on the terahertz wave was studied. The results showed that the terahertz amplitude increases as the position approaches the end of the filament gradually. The stability of amplitude and peak frequency of the terahertz generated by the filament formed by two-color femtosecond laser via a lens with a longer focal length is lower than that through a lens with a shorter focal length, especially the terahertz amplitude at the end of the filament. The study will be helpful for future researchers in the field of THz sub-wavelength imaging utilizing femtosecond laser filament.     

飞秒激光成丝诱导Cu等离子体的温度和电子密度

研究了飞秒激光成丝诱导铜击穿光谱,利用光发射光谱对产生的铜等离子体光谱强度沿着丝长度进行了测量,获得了在不同样品与聚焦透镜间距离的Cu(I)的强度分布.结果显示,由于强度钳箍效应成丝诱导的光谱在较大的透镜样品间距离范围内有较强的辐射强度.另外,利用玻尔兹曼图和斯塔克展宽计算了整个成丝繁衍距离中Cu等离子体温度和电子密度.     

Competition between multiphoton/tunnel ionization and filamentation induced by powerful femtosecond laser pulses in air

In this work we present experiments by focusing 42 femtosecond laser pulses in air using three differentfocal length lenses: f=100, 30 and 5 cm. For the longest focal length, only the filament, which is aweak plasma column,is observed. When the shorter focal length lens is used, a high density plasma isgenerated near the geometrical focus and coexists with a weak plasma channel of the filamemt. Under thetightest focusing condition, filamentation is prevented and only a strong plasma volume appears at tehgeometrical focus.     

基于飞秒激光等离子体丝诱导击穿光谱探测土壤重金属Cr元素含量

本文基于飞秒激光等离子体丝诱导击穿光谱对土壤重金属Cr元素含量进行了实验研究.利用荧光法对等离子体丝的长度进行测量,给出了在不同焦距聚焦透镜作用下土壤中Cr425.5 nm的谱线强度空间分布,实验给出了Cr元素的定标曲线.实验结果表明,土壤中Cr元素浓度分析测量的相对标准偏差小于5%,土壤中重金属Cr元素的检测极限为7.85 ppm.表明飞秒激光等离子体丝诱导击穿光谱技术在土壤重金属Cr元素含量的定量探测方面是完全可行的.     

A laser system for counting cloth filaments

A simple, high speed and high accuracy counter system for cloth filament has been constructed. The principle is based on the laser light attenuation through the cloth. The system is composed of semiconductor laser, objective lens, focusing lens PIN photodiode and processing system including amplifier, Δ V detector, counter, comparator, preset and marker. The counting accuracy is above 99% and the maximum counting speed is beyond 5 kHz, which will be enough for practical use.     

粘纤单丝数目自动检测系统的研究

针对化纤企业生产、检测的需要,研制了一种粘纤单丝数目自动检测系统。系统采用环形光源,从周围均匀照明经静电和高速风两次分离的粘纤,再由成像物镜将分离的粘纤清晰成像至高速面阵CCD传感器靶面上,基于Directshow技术自行编制出图像采集程序。利用自主研发的图像处理软件,处理粘纤图像信息,自动计算其单丝的数目,并给出判别结果。在图像处理方面采用分行消包络阈值分割法和连通域跟踪计数法,解决了粘纤计数中的胶丝重叠、分叉等关键技术难题,保证了计数准确率。该系统具有精度高、检测速度快等特点。     

Biprismainterferenzen mit langsamen Elektronen

Diffraction patterns of low energy electrons (150 eV) were observed with an electron interferometer. An interferometer of special design was developed for this purpose. The electron source is reduced in size by a retardation lens, which simultaneously decelerates the electrons down to about 150 eV. The electron beam is split up by an electrostatic biprism (filament diameter 10 μ ¦) in two coherent waves, which recombine behind the biprism giving a Fresnel diffraction pattern. Then the electrons are accelerated and the interference fringes are electron optically magnified. In comparison with an interferometer for high energy electrons the intensity on the viewing screen is higher.     

Multilevel phase-type diffractive lenses in silica glass induced by filamentation of femtosecond laser pulses

Multilevel phase-type diffractive lenses were fabricated by translating a filament of a femtosecond laser pulse into three-dimensional space inside synthesized silica. The profile of the lenses was designed by use of a multi-level approximation to a kinoform lens. Two-level diffractive lenses with multiple layers along the optical axis provided a maximum efficiency of 37.6% at a wavelength of 632.8 nm. A four-level diffractive lens provided a maximum efficiency of 56.9%. The lenses fabricated with filamentation were free from birefringence.     

Effect of the target positions on the rapid identification of aluminum alloys by using filament-induced breakdown spectroscopy combined with machine learning

Filament-induced breakdown spectroscopy (FIBS) combined with machine learning algorithms was used to identify five aluminum alloys. To study the effect of the distance between focusing lens and target surface on the identification accuracy of aluminum alloys, principal component analysis (PCA) combined with support vector machine (SVM) and K-nearest neighbor (KNN) was used. The intensity and intensity ratio of fifteen lines of six elements (Fe, Si, Mg, Cu, Zn, and Mn) in the FIBS spectrum were selected. The distances between the focusing lens and the target surface in the pre-filament, filament, and post-filament were 958 mm, 976 mm, and 1000 mm, respectively. The source data set was fifteen spectral line intensity ratios, and the cumulative interpretation rates of PC1, PC2, and PC3 were 97.22%, 98.17%, and 95.31%, respectively. The first three PCs obtained by PCA were the input variables of SVM and KNN. The identification accuracy of the different positions of focusing lens and target surface was obtained, and the identification accuracy of SVM and KNN in the filament was 100% and 90%, respectively. The source data set of the filament was obtained by PCA for the first three PCs, which were randomly selected as the training set and test set of SVM and KNN in 3:2. The identification accuracy of SVM and KNN was 97.5% and 92.5%, respectively. The research results can provide a reference for the identification of aluminum alloys by FIBS.     

用红外测温仪观察二极管板流引起的阴极降温现象

为了观察由板流引起的灯丝温度下降现象,利用红外测温仪对理想二极管灯丝温度作了精密测量。由于忽略了板流,导致阴极灯丝温度下降,致使测得的金属逸出功存在着大约-1.86%的系统相对误差。实验验证了当灯丝电流和板流较小时,测不出灯丝温度随板流的变化。当灯丝电流为0.74A以上时,可以明显地观察到输出板流后,灯丝温度要比板流为0时低（1～2）K,此结果与预测值十分相符。     

气压对微束射频容性放电模式调制的研究

微束射频容性放电在纳米晶体颗粒等离子体增强气相合成有着潜在的应用前景.本论文利用ICCD、单反相机、高压探头和电流探头等对微束射频容性放电特性进行了实验诊断研究.结果发现:在纯氩气微束射频放电中,随着气压的增加,放电从辉光放电模式向多通道丝状放电模式转换;在99%氩/1%氢混合气体微束射频放电中,丝状放电模式消失,而是从低气压全空间分布的辉光放电模式,到中等气压向轴心收缩的辉光放电模式,最后到高气压的“环状”辉光放电模式;而在纯氢气微束射频放电中,随着气压的增加,放电模式直接从全空间分布的辉光放电模式向“环状”辉光放电模式转换.最后通过射频电场中电子加热、趋肤效应和气体热传导的共同作用解释了产生不同放电模式的物理机制.     

Fluid transport at low Reynolds number with magnetically actuated artificial cilia

By numerical modeling we investigate fluid transport in low-Reynolds-number flow achieved with a special elastic filament or artifical cilium attached to a planar surface. The filament is made of superparamagnetic particles linked together by DNA double strands. An external magnetic field induces dipolar interactions between the beads of the filament which provides a convenient way of actuating the cilium in a well-controlled manner. The filament has recently been used to successfully construct the first artificial micro-swimmer (R. Dreyfus et al., Nature 437, 862 (2005)). In our numerical study we introduce a measure, which we call pumping performance, to quantify the fluid transport induced by the magnetically actuated cilium and identify an optimum stroke pattern of the filament. It consists of a slow transport stroke and a fast recovery stroke. Our detailed parameter study also reveals that for sufficiently large magnetic fields the artificial cilium is mainly governed by the Mason number that compares frictional to magnetic forces. Initial studies on multi-cilia systems show that the pumping performance is very sensitive to the imposed phase lag between neighboring cilia, i.e., to the details of the initiated metachronal wave.     

Multi-parametric MRI of the physiology and optics of the in-vivo mouse lens

The optics of the ocular lens are determined by its geometry (shape and volume) and its inherent gradient of refractive index (water to protein ratio), which are in turn maintained by unique cellular physiology known as the lens internal microcirculation system. Previously, magnetic resonance imaging (MRI) has been used on ex vivo organ cultured bovine lenses to show that pharmacological perturbations to this microcirculation system disrupt ionic and fluid homeostasis and overall lens optics. In this study, we have optimised in vivo MRI protocols for use on wild-type and transgenic mouse models so that the effects of genetically perturbing the lens microcirculation system on lens properties can be studied. In vivo MRI protocols and post-analysis methods for studying the mouse lens were optimised and used to measure the lens geometry, diffusion, T1 and T2, as well as the refractive index (n) calculated from T2, in wild-type mice and the genetically modified Cx50KI46 mouse. In this animal line, gap junctional coupling in the lens is increased by knocking in the gap junction protein Cx46 into the Cx50 locus. Relative to wild-type mice, Cx50KI46 mice showed significantly reduced lens size and radius of curvature, increased T1 and T2 values, and decreased n in the lens nucleus, which was consistent with the developmental and functional changes characterised previously in this lens model. These proof of principle experiments show that in vivo MRI can be applied to transgenic mouse models to gain mechanistic insights into the relationship between lens physiology and optics, and in the future suggest that longitudinal studies can be performed to determine how this relationship is altered by age in mouse models of cataract.     

Numerical Investigation of the Dynamics of a Flexible Filament in the Wake of Cylinder

Fluid-structure-interaction problems are ubiquitous, complicated, and not yet well understood. In this paper we investigate the interaction of a leading rigid circular cylinder and a trailing compliant filament and analyze the dynamic responses of the filament in the wake of the cylinder. It is revealed that there exist two flapping states of the filament depending on the cylinder-filament separation distance and the relevant critical distance distinguishing the two states is associated with the Reynolds number and the filament length. It is also found that the drag coefficient of the cylinder is reduced but that of the filament may be increased or decreased depending on its length. Compared with a single filament in a uniform flow, the filament of the same mechanical properties flapping in the wake of the cylinder has a lower frequency and a greater amplitude.     

Effect of different bending shapes on thermal properties of flexible light-emitting diode filament

Heat dissipation is an important part of light-emitting diode(LED) filament research and has aroused constant concern.In this paper, we studied the thermal performance of flexible LED filament by numerical simulation and through experiment.The heat dissipation characteristics of spring-like structure flexible LED filament were computed by finite volume method,and it was found that the chip junction temperature was closely related to the pitch and the bending radius. The effect of inclination angle of lighting LED filament was discussed because it is relevant to the spring-like structure flexible LED filament in geometry. The results demonstrated that the temperature of the filament increases as the inclination angle improves.     

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The high speed CCD visible-light camera system has been installed to detect the dynamic characteristics of ELMs filaments on EAST tokamak. In order to convert the CCD video imaging coordinates into real space coordinates, the CCD system has calibrated using Tsai’s two-stage calibration technique. A raised spike in the grayscale profile, drawn by selecting the grayscale value of the image in the vertical direction of the ELMs filament in the CCD image, represents an ELMs filament structure whose positional coordinates are determined by the apex of the spike. The Sobel edge detection operator was used to examine the edge profile of the filament to obtain the width of the filament. By analyzing the characteristics of the filament width of the ELMs, it is found that the width of ELMs filaments continuously decreases outward from the outermost magnetic surface. This indicates that the filaments continue to dissipate energy and particles as they propagate outward. The width of filaments of Type-I ELMs is greater than that of Type-III ELMs.     

A Non-Energy-Concentrating Lens Using Left-Handed Material

In a planar lens made of left-handed material (LHM), the rays converge to a focal point in the material and once again in free space. The problems exhibited by the planar lens are its great thickness, the long optical path length, and energy concentration in the LHM. We examine a bowl-shaped LHM lens with an index n = −1. In this lens, the back is a concave hemispherical surface and the rays never converge in the LHM, but in free space only. We also designed a quantized bowl-shaped LHM lens composed of multiple layers of thin LHM plates of uniform thickness. A bowl-shaped LHM lens quantized with more than 4 bits has a lens function similar to that of a smooth curved lens. The bowl-shaped lens is characterized by its thinness, its short optical path length, and the absence of energy concentration in the LHM. Furthermore, the quantized bowl-shaped LHM lens can be fabricated. Wave propagation through these lenses is numerically simulated using the finite difference time domain (FDTD) method.     

介质阻挡放电系统复合气隙中三种放电丝的光谱研究

介质阻挡放电系统（DBD）作为一个典型的非平衡气体放电系统,不仅在工业生产如低温等离子体生产和发光等方面被广泛应用,而且该系统表现出的非线性现象、自组织现象也吸引人们的关注。DBD系统中放电丝的等离子体参量受诸多因素影响,为了探究DBD系统的放电条件对等离子体参量的影响,该实验重新设计放电单元以保证在其他实验条件相同的情况下,对放电气隙间距和气体组分与等离子体参数之间的关系展开研究。本实验的放电单元为一个平板型玻璃框架气隙,该气隙由三个厚度均为1.2 mm,放电区域边长分别为40,30和20 mm的正方形玻璃框架复合而成,因此该放电气隙有三个放电区域,将此复合气隙放置于可调节气体成分和压强的真空室内,可以同时产生三种放电气隙间距分别为1.2,2.4和3.6 mm的等离子体放电丝。高速录像机拍摄的瞬时照片表明三种放电丝均为随机放电丝,即其放电类型均为流光放电。在垂直于放电气隙平面的方向设置光路,使用聚焦透镜获得清晰的成像,移动光纤探头实现空间分辨并采集数据。实验用光谱仪采集三种等离子体的氮分子第二正带系(C3Π_u→B3Π_u) 谱线,根据谱线强度计算得到各类放电丝的分子振动温度;利用谱线中包含的氮分子离子N+₂第一负带系谱线(391.4 nm)和氮分子第二正带系394.1 nm谱线强度的比值反应放电丝中电子平均能量;改变气室内氩气的含量,得到了三种等离子体的分子振动温度和电子平均能量的变化趋势。实验结果表明：在氩气含量0%~60%区间内,随着氩气含量的增加,三种等离子体的分子振动温度均先升高后降低,整体趋势表现为相同氩气含量下放电气隙间距越小分子振动温度越高,即1.2 mm气隙厚度中的放电丝的分子振动温度最高,2.4 mm气隙厚度次之,3.6 mm气隙厚度的最低;随氩气含量的增加放电丝的平均电子能量先升高后降低,氩气含量相同时气隙厚度越小的放电丝的电子平均能量越高,即1.2 mm气隙厚度中放电丝的电子平均能量最高,2.4 mm气隙厚度的次之,3.6 mm气隙厚度中的最低。实验结果对于研究DBD系统中等离子体参量、工业生产等方面具有重要的参考意义。     

EAST 装置边缘局域模细丝宽度的高速CCD可见光相机系统诊断

利用高速CCD 可见光相机系统研究了EAST 上的边缘局域模(ELMs)细丝动力学特性。采用Tsai 两步标定法对CCD 相机进行标定,以实现将CCD 图像坐标转换为真实空间坐标。选取CCD 图像中与边缘局域模细丝垂直方向上的图像灰度值,绘制了灰度分布图。灰度分布图中的一个凸起的尖峰代表了一条边缘局域模细丝结构,细丝的位置坐标由尖峰顶点确定。运用Sobel 边缘检测算子检测细丝边缘轮廓,从而得到细丝的宽度。通过对边缘局域模细丝宽度特性的分析,发现边缘局域模细丝在从最外层封闭磁面向外传播的过程中宽度不断减小。这表明细丝在向外传播的过程中其能量和粒子不断耗散。I-型边缘局域模的细丝宽度比III-型边缘局域模细丝的宽度更大。     

Elasticity of a Filament with Kinks

Using the wormlike chain model, we analytically study the elasticity of a filament with kinks. We calculate the position probability density function and the force constant of a kinked filament with a general kink angle. Then, using the mathematical induction, we obtain the positional-orientational probability density function of a filament with regular kinks. For this filament, we compute the force constant in two different directions. In longitudinal direction of the filament, the force constant is proportional to the inverse of the number of the segments, i.e., 1 / m, while in transverse direction, it is proportional to \(1/m^3\).