半导体断路开关截断过程模拟的缩比模型

在维持电路参数同比变化和通过半导体断路开关(SOS)的电流密度不变的基础上,提出了一种SOS截断特性模拟的缩比模型,并可在Silvaco ATLAS软件中应用。在以不同的缩比率选取等效SOS横截面积的情况下,将原电路中串联的100个二极管等效为若干个二极管,模拟得到了相同的二极管电流和电压波形。模拟结果表明,该模型不仅可以得到正确的SOS瞬态截断过程,而且可将计算速度提高近百倍。通过对SOS截断过程中载流子分布和电场分布变化过程的分析发现,SOS的截断过程发生在n-n+区。     

半导体断路开关电路-流体耦合数值模拟

为了研究掺杂结构为p+-p-n-n+的半导体断路开关的ns脉冲截断机理,根据流体力学方程和全电流方程推导出半导体断路开关内部载流子运动满足的电流-电压关系表达式,提出了一种外电路方程和载流子流体力学方程联立求解的1维耦合数值模型。采用该模型对半导体断路开关的ns脉冲截断过程进行了数值模拟,模拟结果表明:在截断过程中,n-n+结处的载流子数密度首先开始明显降低,并出现高电场,随后p+-p结处也出现类似现象,随着载流子的抽取,高电场区域向p-n结处快速移动,最终在p-n结处完成截断,而基区载流子数密度在截断前后无明显变化。     

截断光束的广义M2因子测量的实验研究

提出了一种可广泛适用于截断光束和非截断光束的广义M2因子测量的实验方法和相应的数据处理方法。在利用广义强度二阶矩计算光束束宽时,以积分角谱范围能够包含光束总能量的75%以上作为积分范围的选取标准,并遵循现有的国际标准化组织(ISO)规定的测量步骤和方法对截断高斯光束的广义M2因子进行了测量,并将不同截断参量时的实验结果与理论计算结果进行了比较,证明了该方法的正确性。研究结果表明,根据该方法测量截断光束M2因子时,只须将非截断光束M2因子测量的数据处理部分进行适当修正,而无需增加其他硬件开销,该方法简便易行且测量步骤仍可遵循国际标准化组织规定。     

耗散子运动方程组的可控截断

为耗散子运动方程组提出精度可控的预截断方案来模拟和研究非微扰的量子耗散动力学．发展了具有类似Caldeira-Leggett主方程形式、在终止阶层采取马尔科夫和高温近似方法的截断方案,根据马尔科夫性质的分析给出确定终止层数的精度判据,并通过电荷转移体系的动力学模拟进行了检验．     

光瞳半径对纯位相调制激光束整形系统的影响

光瞳截断作用对纯位相调制激光束整形系统的系统性能有重要影响. 本文提出了一种关于光瞳半径对近场调制位相和远场系统评价函数影响的定量分析方法. 通过拉格朗日乘数法等方法分析光瞳半径对近场调制位相的影响, 结果表明: 近场调制位相随光瞳半径近似线性增加. 通过建立数学模型, 拟合分析光瞳半径对系统评价函数的影响. 结果表明: 对方形目标光强, 系统评价函数拟合精度确定系数达到99%左右, 光瞳半径为2.5倍高斯光束束腰半径时, 相关系数达到0.997, 光强偏离残差平方均值达到0.0004左右, 光瞳截断作用趋于最小; 对圆形目标光强, 系统评价函数拟合精度确定系数达到97%, 光瞳半径为3倍高斯光束束腰半径时, 相关系数与光强偏离残差平方均值变化幅度均在10^-3量级, 系统评价函数趋于收敛, 光瞳截断作用趋于最小.     

移动闭塞条件下线路弯道对列车交通流影响的研究

在NaSch模型的基础上,针对铁路线路特点提出了一种用于模拟铁路线路弯道的多列车追踪运行的元胞自动机模型.应用该模型,进行了计算机数值模拟,研究了具有不同弯道半径、不同外轨超高和不同弯道长度的线路弯道对铁路交通流的影响.模拟结果表明:建立的模型能够准确的反映出特殊线路交通流情况,线路弯道对列车的行车安全有较大的影响,同时再现了时走时停的交通波现象,根据仿真结果得出了随着弯道半径的增大,列车的晚点时间逐渐减少,合理选择弯道半径、外轨超高值和弯道长度,可以显著提高线路的通行能力,减低轮轨磨耗,保证列车的安全、舒适运行.所得结果对铁路线路选线设计和运营管理有一定的指导意义.     

分形理论在分子动力学模拟中的应用

本文基于分形理论提出了一个假说,认为实际流体分子的无规运动可以用分数布朗函数作为概率密度函数来描写,而其分数维数可根据分子运动的图像确定。本文以流体Ar作为对象进行了分子动力学模拟,根据分子动力学模拟的结果提取了运动的分数维数,构造了描述分子无规运动的分数布朗函数,并对所提出的假说进行了验证。     

利用冰水同心球模型反演冰水混合云

将云看作由众多冰水同心球粒子所构成,建立冰水同心球模型。模拟冰水混合云,计算可见光(0.66μm)和近红外波段(1.64μm和2.14μm)的反射函数和光学厚度,建立查找表。选取MODIS卫星数据中1通道、6通道和7通道数据,用最小方差拟合法反演光学厚度、有效粒子半径,和MOD06数据中的光厚度和有效粒子半径对比结果较一致,实验结果表明提出的冰水混合云的反演方法具有合理性。     

激光诱导介质击穿中的脉冲截断问题

强激光与介质相互促进作用过程中,当自由电子的密度达到介质的损伤阈值时,介质将被击穿,脉冲的后续能量被激光等离子体强烈吸收而引发截断,这个时间点定义为截断时间点.脉冲的截断时间点对激光脉冲能量的传输有很大的影响.理论研究了入射激光脉冲能量对脉冲截断时间点分布的影响.定量分析和模拟了截断时间点对脉冲能量透过率的影响.模拟结果和实验结果符合很好.研究发现:激光脉冲能量透过率由脉冲截断时间点的位置决定.当脉冲截断时间点分布在脉冲前沿、峰值和后沿时,激光脉冲能垦的透过率分别对应小于50%,等于50%和大于50%.在线测量激光脉冲能量的透过率也可作为一种在线检测光学元件是否发生破坏的方法.     

自仿射Sierpinski地毯中的粘滞指进

构造了自仿射Sierpinski地毯.在自仿射Sierpinski地毯中,认为喉管半径服从截断瑞利分布,采用逐次超松弛技术,模拟了自仿射Sierpinski地毯中的粘滞指进.计算了粘滞指进分维,结果表明,当粘滞比M→∞时,指进图样与在自仿射Sierpinski地毯中DLA模拟的结果类似：当M=1时,驱替流体在长标度范围内具有紧凑的结构,且具有稳定的位移. 关键词：     

An axial flow cyclone to remove nanoparticles at low pressure conditions

In this study, the axial flow cyclone used in Tsai et al. (2004) was further tested for the collection efficiency of both solid (NaCl) and liquid (OA, oleic acid) nanoparticles. The results showed that the smallest cutoff aerodynamic diameters achieved for OA and NaCl nanoparticles were 21.7 nm (cyclone inlet pressure: 4.3 Torr, flow rate: 0.351 slpm) and 21.2 nm (5.4 Torr, 0.454 slpm), respectively. The collection efficiencies for NaCl and OA particles were close to each other for the aerodynamic diameter ranging from 25 to 180 nm indicating there was almost no solid particle bounce in the cyclone. The 3-D numerical simulation was conducted to calculate the flow field in the cyclone and the flow was found to be nearly paraboloid. Numerical simulation of the particle collection efficiency based on the paraboloid flow assumption showed that the collection efficiency was in good agreement with the experimental data with less than 15% of error. A semi-empirical equation for predicting the cutoff aerodynamic diameter at different inlet pressures and flow rates was also obtained. The semi-empirical equation is able to predict the cutoff aerodynamic diameter accurately within 9% of error. From the empirical cutoff aerodynamic diameter, a semi-empirical square root of the cutoff Stokes number, , was calculated and found to be a constant value of 0.241. This value is useful to the design of the cyclone operating in vacuum to remove nanoparticles.     

An electrostatic model of the cutoff probe

Although a circuit model of the previously developed cutoff probe for plasma diagnostics elucidates the basic physics and contributes to the development of the cutoff probe, a theoretical validation of the circuit model has yet to be accomplished. For theoretical validation, this paper proposes a one-dimensional electrostatic model, or 1dESM, of the cutoff probe, which is based on electrostatic field analysis in a finite two-wire approximation. The transmission spectrum S₂₁ calculated by the 1dESM shows a good agreement with that from a three-dimensional full electromagnetic wave simulation for various electron densities and pressures. Based on the 1dESM, the formation mechanism of the S₂₁ of the cutoff probe was analyzed. Theoretical validation of the circuit model was then achieved by comparing the circuit model with the 1dESM. This paper is believed to contribute to a better understanding of the cutoff probe and to the development of cutoff probe models.     

Simulation of QCD jets: The choice of the cutoff

In view of the simulation ofe ⁺ e ^? annihilation events corresponding to QCD multi-jet final states, we examine the dependence of the results on the value of the cutoff, which delimits the perturbative from the non-perturbative regimes. We do this by studying in some detail related effects for the lowest order perturbative contributions (including single gluon emission). We conclude that event simulation is considerably affected by the choice of the cutoff. Up to energies where only two and three jet configurations are dominant, though, this does not prevent from having quantitative tests of QCD, if suitable kinematical cuts are applied.     

无截断薄膜模型与Dirac场的黑洞熵

计算黑洞熵的砖墙模型被改进为薄膜模型后其物理思想更直接而明了,且突出了事件视界作为静态或稳态黑洞特征面的重要性.但为避免发散,薄膜模型同样需要引入紫外截断因子.截断因子的引入非常人为,至今没有给以合理的解释.有文献将广义不确定关系引入黑洞熵的计算而不需要任何截断便可避免发散.本文以静态球对称黑洞Dirac场的熵的计算为例,阐述了无截断薄膜模型及其与有截断薄膜模型的本质区别. 关键词： 黑洞熵 无截断薄膜模型 广义不确定关系 Dirac场     

激光共聚焦近红外荧光扫描系统光学设计

为了实现对近红外荧光的高分辨率扫描,设计了工作在近红外光谱区的激光共聚焦光学系统。采用结构简单的凹凸双透镜物镜实现了照明光路和发射光路的设计,并采用Zemax软件进行了光学设计和仿真。实验表明:照明光路的聚焦弥散斑小于1 m,照明针孔处的聚焦光斑小于40 m,满足照明针孔的尺寸要求;发射针孔处的聚焦光斑小于10 m,满足探测针孔尺寸要求;同时照明光路和发射光路的MTF曲线的截止频率都分别满足其衍射极限分辨率的要求,照明光路在全视场空间分辨率420 lp/mm处MTF0.08,发射光路在全视场空间频率400 lp/mm处MTF0.07。     

多注速调管π模双间隙腔截止波导输出回路研究

 用数值模拟的方法研究了多注速调管π模双间隙腔加载截止波导两节滤波器型输出回路的特性，并重点研究了π模双间隙腔与截止段的耦合结构对输出腔间隙阻抗频率特性的影响，以及耦合接头和电容性销钉对截止波导滤波器的调谐作用。研究结果表明：π模双间隙腔连接截止波导段的耦合口的尺寸主要影响输出腔的外观品质因数，而耦合接头的粗细和电容性销钉的大小和插入深度对截止波导滤波器的调谐具有重要影响。另外，由于耦合接头相当于在截止段中引入了一个附加电感，这使滤波器谐振电路的品质因数变大，谐振峰变窄。因此，π模双间隙腔加载截止波导滤波器型输出回路并不比单间隙腔加载截止波导滤波器型输出回路有更好的带宽优势。     

3D short-range wetting and nonlocality

Analysis of a microscopic Landau-Ginzburg-Wilson model of 3D short-ranged wetting shows that correlation functions are characterized by two length scales, not one, as previously thought. This has a simple diagrammatic explanation using a nonlocal interfacial Hamiltonian and yields a thermodynamically consistent theory of wetting in keeping with exact sum rules. For critical wetting the second length serves to lower the cutoff in the spectrum of interfacial fluctuations determining the repulsion from the wall. We show how this corrects previous renormalization group predictions for fluctuation effects, based on local interfacial Hamiltonians. In particular, lowering the cutoff leads to a substantial reduction in the effective value of the wetting parameter and prevents the transition being driven first order. Quantitative comparison with Ising model simulation studies due to Binder, Landau, and co-workers is also made.     

Droplet formation by rapid expansion of a liquid

Molecular dynamics of two- and three-dimensional liquids undergoing a homogeneous adiabatic expansion provides a direct numerical simulation of the atomization process. The Lennard-Jones potential is used with different force cutoff distances; the cluster distributions do not depend strongly on the cutoff parameter. Expansion rates, scaled by the natural molecular time unit (about a picosecond), are investigated from unity down to 0.01; over this range the mean droplet size follows the scaling behavior of an energy balance model which minimizes the sum of kinetic plus surface energy. A second model which equates the elastic stored energy to the surface energy gives better agreement with the simulation results. The simulation results indicate that both the mean and the maximum droplet size have a power-law dependence upon the expansion rate; the exponents are -2d/3 (mean) and -d/2 (maximum), where d is the dimensionality. The mean does not show a dependence upon the system size, whereas the maximum does increase with system size, and furthermore, its exponent increases with an increase in the force cutoff distance. A mean droplet size of 2.8/eta(2), where eta is the expansion rate, describes our high-density three-dimensional simulation results, and this relation is also close to experimental results from the free-jet expansion of liquid helium. Thus, one relation spans a cluster size range from one atom to over 40 million atoms. The structure and temperature of the atomic clusters are described.     

Entropy-Driven Cutoff Phenomena

In this paper we present, in the context of Diaconis’ paradigm, a general method to detect the cutoff phenomenon. We use this method to prove cutoff in a variety of models, some already known and others not yet appeared in literature, including a non-reversible random walk on a cylindrical lattice. All the given examples clearly indicate that a drift towards the opportune quantiles of the stationary measure could be held responsible for this phenomenon. In the case of birth-and-death chains this mechanism is fairly well understood; our work is an effort to generalize this picture to more general systems, such as systems having stationary measure spread over the whole state space or systems in which the study of the cutoff may not be reduced to a one-dimensional problem. In those situations the drift may be looked for by means of a suitable partitioning of the state space into classes; using a statistical mechanics language it is then possible to set up a kind of energy-entropy competition between the weight and the size of the classes. Under the lens of this partitioning one can focus the mentioned drift and prove cutoff with relative ease.     

Grain size distributions for competitive growth with nucleation

The paper introduces and discusses an idealized competitive growth model with nucleation for the microstructure formation during dense branching phase separation in thin Al/Ge films. Grain size and grain length distributions for the new model are obtained analytically and by simulation. These distributions exhibit a characteristic scaling form similar to cluster size distributions in many other growth models. The cutoff functions in these scaling forms and their influence on the determination of effective exponents are studied in detail. It is found that nucleation introduces a new length scale into the other-wise selfsimilar competitive growth model. This length scale appears only inside the cutoff function and diverges algebraically as the nucleation rate vanishes. We find both analytically and by simulation that the cutoff functions can exhibit stretched exponential behaviour exp(–x ) for large arguments. Our analytical and simulation results for grain size and grain length distributions are in excellent quantitative agreement.