Controlled Levitation of Dust Particles in RF+DC Discharges

Experiments and particle‐based kinetic simulations were performed to obtain the equilibrium levitation height of dust particles in plane parallel electrode discharges in low pressure argon gas, established by combined RF and DC excitation. The computed values were compared to experimental data. The good overall agreement of the simulation results and the experimental data verifies our gas discharge, dust charging, as well as dust force balance models. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

模拟颗粒凝并过程的快速Monte Carlo方法

对常规异权值Monte Carlo(MC)方法进行改进,基于强核函数思想,通过颗粒群单重遍历即可求得强核函数最大值,采用接受-拒绝法随机搜寻凝并对,并利用搜寻过程中拒绝和接受的所有凝并对的信息来估计凝并事件的等待时间(时间步长),从而避免颗粒群的双重遍历,以提高MC的效率.对典型工况的模拟结果显示该快速方法计算代价仅为O(N_s),能够显著提高计算效率,同时保持足够的计算精度,较好地协调计算代价与计算精度之间的矛盾.     

蒙特卡罗粒子输运问题的全局降方差方法

精确获得中子通量的时间-空间-能量精细分布对于一类非定常粒子输运蒙特卡罗模拟至关重要.以零方差理论为基础,采取化整为零的策略,把蒙特卡罗方法与离散纵标方法相耦合,用近似重要函数指导蒙特卡罗模拟,给出一种实现蒙特卡罗全局降方差的计算方案.数值算例表明,该计算方案获得了全局降方差的效果.     

粒子输运蒙特卡罗模拟现状概述

蒙特卡罗(MC)方法发展已有60多年历史,广泛应用于核科学及相关领域.MC方法超强的几何处理能力,使用精密的连续点截面参数,能够模拟各种复杂几何系统内的中子、光子、电子、α粒子、质子及其耦合输运问题.随着计算机的快速发展,通过大规模并行计算,MC方法及程序已成为模拟各种粒子输运问题的首选工具.     

非均匀大气中γ射线输运的蒙特卡罗模拟方法

γ射线在高空大气中的输运问题常采用蒙特卡罗方法进行模拟,但由于大气是非均匀连续介质,通常需要根据高度上的密度变化将大气进行分层处理.高空大气密度变化非常剧烈,精细的分层会带来很大的计算量.针对非均匀连续介质中的粒子输运问题,利用质量距离抽样方法代替传统的步长抽样方法,提出一种可直接模拟粒子在非均匀连续介质中输运的蒙特卡罗方法,实现γ射线在非均匀大气中输运的蒙特卡罗模拟,并与MCNP程序的计算结果进行比较,结果符合较好.     

连续能量蒙特卡罗方法组件均匀化中的临界效应

研究蒙特卡罗均匀化中的临界效应,应用B_N理论修正临界效应.简化压水堆模型和临界试验装置的验证表明:在连续能量蒙特卡罗方法均匀化中,B_N方法很好的考虑了群常数的临界效应,曲率基模成功度量了堆芯的有限性.     

Effect of exchange interaction in ferromagnetic superlattices:A Monte Carlo study

The Monte Carlo simulation is used to investigate the magnetic properties of ferromagnetic superlattices through the Ising model. The reduced critical temperatures of the ferromagnetic superlattices are studied each as a function of layer thickness for different values of exchange interaction. The exchange interaction in each layer within the interface and the crystal field in the unit cell are studied. The magnetic coercive fields and magnetization remnants are obtained for different values of exchange interaction, different values of temperature and crystal field with fixed values of physical parameters.     

Transient yielding of CO2 over oxidized ruthenium surfaces: Monte Carlo simulations

We develop Monte Carlo simulations to study the catalytic oxidation of CO over a surface of ruthenium. The catalyst is exposed to a continuous flux of CO molecules and its surface is pre-covered with an amount of oxygen atoms. Recent experiments performed on this system [R. Blume, W. Christen, H. Niehus, J. Phys. Chem. B 110 (2006) 13912] have shown that three different reaction mechanisms can account for the experimental results. Two of them are based on the Langmuir-Hinshelwood mechanism, where CO molecules are adsorbed at oxygen-free defect sites before reactions take place. The third one proceeds via the Eley-Rideal mechanism, which is almost time independent, and reactions occur at non-defect sites. In our model, we consider a semi-infinite cubic lattice to mimic the surface of the catalyst and oxygen atoms are incorporated into the layers below the surface. A fraction of defects is created at the topmost layer and at the first subsurface layer. Oxygen atoms can diffuse over the surface as well as between adjacent layers of the system. We also assumed a temperature dependent reaction rate that is related to the residence time of CO at the surface. Comparisons are made between the CO₂ yielding at defect-rich and smooth surfaces as a function of temperature.     

氮等离子体枪中电子输运行为的蒙特卡罗模拟

采用蒙特卡罗模拟,对氮等离子体枪的两个环形电极间的氮气辉光放电过程中电子的输运过程进行研究.计算在不同平均电场与粒子数密度的比值(E/N)下,电子与氮分子发生不同碰撞的概率、出射电子的平均能量、方向角分布和电子的能量分布.结果表明,电子能量近似服从玻尔兹曼分布.随着E/N的升高,电子平均能量升高,发生激发、离化、电离和离化电离碰撞的概率增大;非均匀分布的电场使分子获得更高的离化率,同时显著增强出射电子的能量.模拟结果为等离子体应用设计提供了参考依据.     

应用Monte Carlo方法模拟爆轰产物状态方程的研究进展

 应用Monte Carlo方法模拟爆轰产物状态方程,区别于建立在球形分布分子势基础上的传统爆轰产物状态方程计算方法,从原子水平上直接模拟实际炸药的爆轰产物状态方程,克服了传统方法中混合产物状态方程不精确的缺点。综述了该方法的历史背景及发展现状。对模拟中宏观量的描述、统计平均的计算、势函数的选取及边界条件进行了详尽描述。     

蒙特卡罗粒子输运软件JMCT抽样工具库设计

为通用型蒙特卡罗粒子输运程序JMCT设计了抽样工具库,通过两种技术途径提供各分布的抽样。一是针对各种常见分布提供特定抽样子程序;二是提供一个通用型的抽样子程序,可以实现任意离散分布和任意一维有限区间上连续分布的自动抽样。在设计任意一维有限区间上连续分布的自动抽样工具时利用了部分开源代码,利用其功能提供给用户最大的方便性。对抽样工具库的检验表明,其可以正确、方便地实现各种输运模拟中常见分布的抽样。     

混浊介质多光子激发荧光显微成像的蒙特卡罗模拟

将蒙特卡罗方法与几何光线追踪技术相结合用于研究样品混浊特性对多光子激发蒌光显微成像的影响。推导了模拟多层样品显微成像的数学模型,给出了用程序框图表示的基本仿真系统骨架,对传统蒙特卡罗方法的改进有效地增强了对聚焦光束的模拟能力,初步的模拟结果说明了混浊介质散射特性对显微成像的显著影响。     

A Kinetic Code System to Study Impurity Deposition and Fuel Retention in Gaps Between Divertor Tiles

For the purpose of investigating impurity deposition and fuel retention in gaps between divertor tiles, we combine a three‐dimensional Monte Carlo impurity transport code ITCD with a Particle‐In‐Cell Monte Carlo collision code plus a plasma‐surface interaction (PSI) code. The simulation results show that a potential hill formed near the entrance of the gap can prevent the ionized eroded carbon species from entering the gap. It is found that carbon sources for the co‐deposition in gaps are mainly located at the plasma‐closest edges of the gap, and this finding indicates that special measures associated with the engineering issues should be taken to minimize fuel retention in gaps. The smaller scale of carbon sources inside the gap and the existence of the potential hill near the entrance of the gap are the reasons why the eroded carbon species spatial density is lower inside the gap than outside the gap. Through the study on several scenarios with different substrate and plasma temperatures, the simulation with the combined code can deepen the understanding of fuel retention in the codeposited layers in gaps (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

输运问题蒙特卡罗模拟方法回顾及展望北大核心CSCD

蒙特卡罗(MC)方法具有复杂几何处理能力强,方法通用灵活,核数据完备,模拟忠实于物理过程等特点,成为中子学数值模拟的首选方法之一。在核能领域,MC方法得益于计算机的快速发展,在辐射屏蔽、反应堆堆芯临界安全分析、乏燃料后处理、放射性废物处置、核设施退役、核事故应急、放射性石油测井、核医学等领域均有广泛应用。对MC方法及软件输运计算做简要回顾,并对未来发展进行展望。     

在BEPCⅡ/BESⅢ上寻找胶子球的Monte Carlo研究

在即将建造的BEPCⅡ BESⅢ上利用MonteCarlo模拟研究了J ψ→γX→γηη′(η′→γρ0 ,ρ0 →π+ π-,η→γγ)过程.研究显示BESⅢ的良好性能和大统计量的J ψ事例样本为寻找胶子球态的一些候选者并确认其存在提供了可能.基于所研究的衰变道,为BESⅢ电磁量能器晶体BGO和CsI的选择及磁场强度的选择提供了数据.     

航天器表面环境散射返回流TPMC模拟

介绍试验粒子Monte Carlo(test particle Monte Carlo,TPMC)方法,并采用该方法对4种航天器表面出气分子形成的环境散射返回流进行数值模拟.其中,圆球出气表面的计算结果与已有的DSMC(direct simulation Monte Carlo)结果一致,验证了方法的正确性.此外,对不同出气和来流条件下圆形平板、凸半球和凹半球3种航天器简化表面出气分子形成的环境散射返回流进行计算,结果表明:出气表面外形是影响返回通量比的一个重要因素;圆形平板和凹半球出气表面的返回通量比远大于凸半球表面的;凹半球表面的出气分子会直接和出气表面碰撞形成直接流污染,且其量级远大于返回流污染.因此,在航天器设计中尽可能使用凸形表面作为敏感的出气表面可以有效降低出气分子污染.     

Monte Carlo Simulations of Reflected Spectra Derived from Tissue Phantom with Double-Peak Particle Size Distribution

We made a tissue phantom with double-peak particle size distribution, which has polystyrene particles of cell nuclear size and mitochondrial size, and measured the spectrum from the tissue phantom using a single optical fiber. In this paper we investigate the characterization method for the tissue phantom with double-peak particle size distribution by comparing the measured spectra with the calculated ones using the Monte Carlo (MC) method. It is first shown that the Mie phase function characterizes better than the Henyey-Greenstein (H-G) phase function in MC calculation. Next, we compare the measurement spectra with those obtained by modeling as single-peak, conventional modeling for particle size distribution, and for double-peak particle size distribution. The single-peak modeling is found to cause considerable error for the tissue phantom with double-peak particle size distribution, which seems to simulate a biological tissue. We suggest that if one simulates the particle size distribution of a biological tissue by conventional modeling, the accuracy of estimation will be lower.     

聚合物中多重光散射传导的Monte Carlo数值模拟

基于米氏散射(Mie scattering)理论,建立填充分散粒子群的聚合物对光散射传导的Monte Carlo数学模型.在此基础上,编写了一套仿真模拟程序.通过模拟单个光子在聚合物中的多重散射运动过程,把问题扩展到以激光束或线状光为入射源,得到在聚合物板块内的光传导情况,并且在计算机上图像化地重现整个物理过程,对输出光强的分布情况进行模拟统计分析.模拟结果表明,利用体散射机制,可以将点光源和线光源转换为平面光输出,输出光的状态可以通过对比计算结果实施有效控制.     

Particle Size Distribution of Nanospheres by Monte Carlo Fitting of Small Angle X‐Ray Scattering Curves

A method for recovering the size distribution of spherical particles from small angle scattering data by using a Monte Carlo interference function fitting algorithm is presented. The method is based on the direct simulation of the small angle scattering data upon the assumption of non‐interacting hard sphere ensembles (“dilute” solution approximation). The algorithm for retrieving the particle size distribution does not require any additional parameters apart from the input of the scattering data. The fitting strategy necessarily implies positive particle size distributions, while preserving the advantage of the indirect transformation method for data desmearing. Furthermore, the present approach does not use any regularisation procedures of the best fit solution and favours smooth particle size distributions. The Monte Carlo procedure has been tested against several simulated cases with various types of mono‐ and bi‐modal size distributions and different noise levels. In the special case of non‐interacting spheres, the Monte Carlo fitting algorithm had the same retrieving ability as the well assessed indirect transformation, structure interference and maximum entropy methods. Finally, the algorithm was applied to retrieve the distribution of spherical nanopowders produced by gas‐to‐particle conversion both as free powder and as reinforcing second‐phase agent in polymer nanocomposites.     

非视线紫外通信大气传输特性的蒙特卡罗模拟

基于蒙特卡罗方法建立了紫外光非视线传输多次散射模型,利用单次散射近似法和实验方法验证了模型的有效性,并利用该模型完成了非视线紫外光通信大气传输特性的模拟.模拟时光波长取紫外光通信的最佳工作波段(250 nm附近),分析了不同传输距离下能见度、风、雨、雾等参量对系统能量透射比的影响.结果指出,系统能量透射比随传输距离增大而剧烈减小,在天气较差传输条件下能量衰减得更快;风力大小的变化对通信系统影响不大.较近距离通信传输时(一两百米),通信系统受天气条件的影响较小.