沉积粒子能量对薄膜早期生长过程的影响

利用Monte Carlo(MC)模型研究了能量粒子对薄膜生长的初始阶段岛膜的形貌和岛的尺寸的影响,沉积粒子的能量范围为:0—0.7eV.在模型中考虑了原子沉积、吸附原子扩散和蒸发等过程,并详细考虑了临近和次临近原子的影响.结果表明,在所采用的参量范围内不同的基底温度情况下,能量粒子的影响有很大的区别.低基底温度情况下,沉积粒子强烈地影响着薄膜的生长过程中,岛膜的形貌、数量和尺寸随能量粒子的能量增加而有很大的变化.分析表明,这些变化都是由于能量粒子的介入使得表面吸附粒子的扩散能力增强所致 关键词： 薄膜生长 Monte Carlo方法 扩散     

局域热源对磁岛热输运的影响

研究托卡马克等离子体磁岛内的热输运行为.应用局域高斯热源对磁岛加热来模拟电子回旋共振加热.对同时存在背景热源与局域高斯热源的情况,观察局域高斯热源对径向电子温度分布及热输运产生的变化,分析局域高斯热源对温度扰动及磁岛约束能量的影响.     

六方晶格基底上薄膜生长的Monte Carlo模拟研究

利用Monte Carlo（MC）方法,模拟研究了六方晶格基底上薄膜生长的初始阶段岛的形貌和岛的尺寸与薄膜覆盖度以及入射粒子沉积速率之间的关系. 结果表明在基底温度为300K时,岛的形貌主要表现为分形生长,随着薄膜覆盖度的增加,岛的分形枝簇变大,岛的数目不断减少. 在同样的温度下,随着入射粒子沉积速率的增大,薄膜表面的形貌逐步由少数聚集型岛核分布状态向众多各自独立的离散型岛核分布状态过渡. 进一步研究得出,薄膜覆盖度和入射粒子沉积速率对粒子扩散能力的影响最终导致岛的形貌发生了改变.     

耗散气球模动力理论

本文从迴旋动力理论出发,较全面地研究了碰撞、热离子成分(中性束热离子或聚变α粒子)、粒子逆磁漂移、粒子拉莫尔半径和粒子捕获等动力效应对耗散气球模的影响。流体极限下的数值结果表明,离子有限拉莫尔半径、捕获粒子对气球模起稳定作用,碰撞则对气球模起解稳作用。此外,动力理论也得出了磁流体理论关于剪切能改善气球模稳定性的结论。 关键词：     

超薄膜生长的Monte Carlo模拟研究

利用Monte Carlo(MC)方法模拟研究了薄膜生长的初始阶段岛的形貌和岛的尺寸与基底温度和入射粒子剩余能量之间的关系.模型中考虑了粒子的沉积、吸附粒子的扩散和蒸发等过程.结果表明当基底温度从200K变化到260K时,岛的形貌经历了一个从分散生长逐渐过渡到分形生长的过程,并且在较低温度(200K)下,随入射粒子剩余能量的增加,岛的形貌也经历了同样的变化过程.进一步研究证明,随着基底温度的升高或入射粒子剩余能量的增加,沉积粒子的扩散能力显著增强,从而使岛的形貌发生了改变.     

钙原子4p_(1/2,3/2)ng(J=3)自电离Rydberg光谱的研究

利用Monte Carlo(MC)方法模拟研究了薄膜生长的初始阶段岛的形貌和岛的尺寸与基底温度和入射粒子剩余能量之间的关系.模型中考虑了粒子的沉积、吸附粒子的扩散和蒸发等过程.结果表明当基底温度从200K变化到260K时,岛的形貌经历了一个从分散生长逐渐过渡到分形生长的过程,并且在较低温度(200K)下,随入射粒子剩余能量的增加,岛的形貌也经历了同样的变化过程.进一步研究证明,随着基底温度的升高或入射粒子剩余能量的增加,沉积粒子的扩散能力显著增强,从而使岛的形貌发生了改变.     

高能量粒子再分布模型在CFETR上的应用

在中国聚变工程实验堆(CFETR)设计参数下,利用一种高能量粒子(EP)物理简化模型分析了磁流体动力学(MHD)不稳定中新经典撕裂模(NTM)引起的粒子再分布和损失。相位矢量旋转法利用EP相空间中小间隔区域内跟踪探测粒子对来判定粒子轨道随机与否,即记录连接矢量旋转角的方法标记该区域是否存在KAM面破缺,以此为据修正相空间的粒子分布函数和判断粒子损失。利用相空间探测粒子对相位矢量旋转法标记一定NTM幅度下的随机区域分布并迭代得到稳态分布剖面。对捕获粒子区域的共振岛探测结果表明,低频或零频率扰动不会与高能量α粒子形成有效共振,MHD扰动和有效碰撞效应可能使捕获-通行轨道边界附近形成损失通道。     

反常输运与非线性自持磁岛的自洽分析

从沿磁力线的等离子体电流与粒子径向输运相关的新经典ＭＨＤ型的广义欧姆定律出发，提出了分析反常输运与非线性自持磁岛的自洽理论模型，基于该模型导出了反常扩散系数及反常电子热导。结果表明，非线性磁岛链可由磁岛调制的反常输动维持，反过来又通过磁岛短路效应维持反常输运。     

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通过求解二维热输运方程,数值分析了横越单磁岛的热输运现象。仅有背景加热时,当磁岛宽度小于临界磁岛宽度时,磁岛对能量约束影响不明显;当磁岛宽度大于临界磁岛宽度时,磁岛降低的能量损失随着磁岛宽度呈线性增大,有效径向热导系数在磁岛区域呈高斯分布且最大值在有理面处。当等离子体背景加热与ECRH共同加热时,ECRH对磁岛区域的温度分布及能量约束均有别于背景热源,这为进一步研究ECRH抑制撕裂模的问题提供了基础。     

不同晶格基底上薄膜生长的Monte Carlo模拟研究

利用Monte Carlo（MC）方法模拟研究了四方和六方晶格基底上薄膜生长的初始阶段岛的形貌和岛的尺寸与基底温度和入射粒子剩余能量之间的关系. 模型中考虑了粒子的沉积、吸附粒子的扩散和蒸发等过程. 结果表明,基底晶格结构对薄膜生长具有明显影响. 当基底温度为300K、入射粒子剩余能量为0时,四方晶格基底上薄膜的生长已经呈现明显的凝聚生长,随着基底温度或入射粒子剩余能量的增加,岛的数目变少、岛的平均尺寸变大. 对于六方晶格基底,当入射粒子剩余能量为0、温度从300K升高到350K时,岛的形貌从分散生长向分形生长转变;当基底温度为300K、入射粒子剩余射能量从0上升到0.05eV时,岛由分散生长向分形生长转变.     

循环床内气固两相流中稠密颗粒间碰撞的数值模拟

循环床内部的流动属于复杂不均匀的稠密气固两相流，稠密颗粒间的相互作用是影响颗粒运动和浓度分布的不可忽视的重要因素。本文采用直接模拟Monte－Carlo算法（DSMC算法）来模拟颗粒间的相互碰撞过程，并与随机轨道模型结合起来综合考虑湍流和颗粒碰撞对颗粒运动和浓度分布的影响，模拟结果预报了床内分层流动结构和颗粒在稀相区的不均匀分布，计算结果与实验定性符合。     

A numerical study on the formation of diffusion flame islands in a turbulent hydrogen jet lifted flame

This paper presents a numerical study on the formation of diffusion flame islands in a hydrogen jet lifted flame. A real size hydrogen jet lifted flame is numerically simulated by the DNS approach over a period of about 0.5 ms. The diameter of hydrogen injector is 2 mm, and the injection velocity is 680 m/s. The lifted flame is composed of a stable leading edge flame, a vigorously turbulent inner rich premixed flame, and a number of outer diffusion flame islands. The relatively long-term observation makes it possible to understand in detail the time-dependent flame behavior in rather large time scales, which are as large as the time scale of the leading edge flame unsteadiness. From the observation, the following three findings are obtained concerning the formation of diffusion flame islands. (1) A thin oxygen diffusion layer is developed along the outer boundary of the lifted flame, where the diffusion flame islands burn in a rather flat shape. (2) When a diffusion flame island comes into contact with the fluctuating inner rich premixed flame, combustion is intensified due to an increase in the hydrogen supply by molecular diffusion. This process also works for the production of the diffusion flame islands in the oxygen diffusion layer. (3) When a large unburned gas volume penetrates into the leading edge flame, the structure of the leading edge flame changes. In this transformation process, a diffusion flame island comes near the leading edge flame. The local deficiency of oxygen plays an important role in this production process.     

A Monte-Carlo method for coulomb collisions in hybrid plasma models

A procedure for implementing Coulomb collisions into hybrid (particle-fluid) plasma models is outlined which is rapid in execution due to the use of approximate expressions for the collision integrals and conserves energy and momentum exactly. Particles undergo dynamic friction and diffusion in velocity-space at rates consistent with the velocity-dependent Fokker–Planck diffusion coefficients and there are no assumptions made about the shape or size of the particle distribution function. The method is tested against the analytical theory of test particle slowing in a background plasma and the thermal equilibration of a Maxwellian distribution.     

Fast dimer diffusion near the island: A computational study

Performing atomic scale simulations, we study the diffusion of small Co clusters on Cu(111) surface in the presence of Co islands. It is revealed that mesoscopic relaxations at the island-substrate interface play an important role during the diffusion events of the clusters. The diffusion barrier of small Co clusters near the islands increases with increasing cluster size. Especially, we find that dimer diffusion near the island with B step is as fast as monomer’s, which might be responsible for the formation of dendritic shaped islands at low temperature.     

Diffusion in a collisional standard map

Test particle evaluation of the diffusion coefficient in the presence of magnetic field fluctuations and binary collisions is presented. Chaotic magnetic field lines originate from resonant magnetic perturbations (RMPs). To lowest order, charged particles follow magnetic field lines. Drifts and interaction (collisions) with other particles decorrelate particles from the magnetic field lines. We model the binary collision process by a constant collision frequency. The magnetic field configuration including perturbations on the integrable Hamiltonian part is such that the single particle motion can be followed by a collisional version of a Chirikov-Taylor (standard) map. Frequent collisions are allowed for. Scaling of the diffusion beyond the quasilinear and subdiffusive behaviour is investigated in dependence on the strength of the magnetic perturbations and the collision frequency. The appearance of the so called Rechester-Rosenbluth regime is verified. It is further shown that the so called Kadomtsev-Pogutse diffusion coefficient is the strong collisional limit of the Rechester-Rosenbluth formula. The theoretical estimates are supplemented by numerical simulations.     

A unified gas-kinetic scheme for continuum and rarefied flows

With discretized particle velocity space, a multiscale unified gas-kinetic scheme for entire Knudsen number flows is constructed based on the BGK model. The current scheme couples closely the update of macroscopic conservative variables with the update of microscopic gas distribution function within a time step. In comparison with many existing kinetic schemes for the Boltzmann equation, the current method has no difficulty to get accurate Navier–Stokes (NS) solutions in the continuum flow regime with a time step being much larger than the particle collision time. At the same time, the rarefied flow solution, even in the free molecule limit, can be captured accurately. The unified scheme is an extension of the gas-kinetic BGK-NS scheme from the continuum flow to the rarefied regime with the discretization of particle velocity space. The success of the method is due to the un-splitting treatment of the particle transport and collision in the evaluation of local solution of the gas distribution function. For these methods which use operator splitting technique to solve the transport and collision separately, it is usually required that the time step is less than the particle collision time. This constraint basically makes these methods useless in the continuum flow regime, especially in the high Reynolds number flow simulations. Theoretically, once the physical process of particle transport and collision is modeled statistically by the kinetic Boltzmann equation, the transport and collision become continuous operators in space and time, and their numerical discretization should be done consistently. Due to its multiscale nature of the unified scheme, in the update of macroscopic flow variables, the corresponding heat flux can be modified according to any realistic Prandtl number. Subsequently, this modification effects the equilibrium state in the next time level and the update of microscopic distribution function. Therefore, instead of modifying the collision term of the BGK model, such as ES-BGK and BGK–Shakhov, the unified scheme can achieve the same goal on the numerical level directly. Many numerical tests will be used to validate the unified method.     

ELECTRON DIFFUSION IN A NON-UNIFORM ELECTRIC FIELD AND A UNIFORM MAGNETIC FIELD

A Monte Carlo simulation technique has been used to model the electron transport beharlot, especially the electron diffusion motion, in the cathode fall region of a glow discharge under the influence of a non-uniform electric field and a transverse magnetic field perpen-dicular to the cathode sheath electric field. Three types of collisions (elastic, excitation and ionization) are taken into account in our model. The electron free flying time is determined by the electron-neutral atom collision frequency. We focus attention on the electron diffusion distance and velocity. The electron-neutral atom collision processes and the electron drift velocity are also studied. The results indicate that with the increase of the magnetic field the electron diffusion distance increases and the electron diffusion velocity decreases. The results Mso show that the collision processes are enhanced by the magnetic field, this is in agreement with the experimental result. However, the axial magnetic field does not affect the electron transport behavior.     

On the theory of surface diffusion: kinetic versus lattice gas approach

The present paper extends the results of a recent analytic kinetic theory of particle-on-substrate diffusion. The approach treats explicitly the molecule–surface interaction and takes into account inter-molecular interaction within the hard particle approximation. The physics influencing the diffusion pre-exponential factor and mechanisms determining the density dependence of collective diffusivity are discussed. The kinetic results are compared with those of the traditional lattice gas hopping models. Analytical expressions for jump rates in the low density limit are derived, and the density dependence of effective jump rates at finite occupancy is discussed. It is shown how the traditional hopping model oversimplifies the picture of diffusion by neglecting the collision part of the hopping process.     

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The fluid dynamics model was used to study the influence of the existence of the collisions on the dust particles in a dusty plasma sheath. The main collisions in this study consist of the ionization collision, the collision between electron and neutral particle, the collision between ion and neutral particle, the collision between electron and dust particle, the collision between ion and dust particle. Numerical calculation results are obtained by the fourth-order-Rung-Kutta method. It is shown that both the dust particles’ density and corresponding electricity quantity increase as the following collisions’ frequency enhances, such as ionization collision, the collision between electron and neutral particle, the collision between ion and neutral particle. The charge reduces due to the collisions concluding electron-dust particle and ion-dust particle. In addition, the effect produced by ions’ collision is much more obviously compared with electrons’ collision.     

碰撞对尘埃等离子体鞘层中尘埃颗粒影响

在一维平板鞘层中应用流体模型研究了尘埃等离子体鞘层中碰撞对尘埃颗粒密度和带电量的影响。 研究所涉及的碰撞主要有电离碰撞,电子、离子分别与中性粒子的碰撞,以及电子、离子分别与尘埃颗粒的碰撞。通过采用四阶龙格库塔法,得到了数值解。结果表明,随着电离碰撞或者电子、离子分别与中性粒子碰撞的频率 增加,都将导致鞘层中尘埃颗粒的数密度增大,数密度的极值位置向鞘边位置靠近,尘埃颗粒带电量增多。电子、离子与尘埃颗粒的碰撞,使得尘埃带电量减小。此外,从研究的结果来看,由离子产生的碰撞要比电子产生的碰 撞对尘埃颗粒的影响明显得多。