Stochastic motion of the untrapped particle in electrostatic mode

The stochastic energy diffusion of the untrapped particle in the electrostatic mode is investigated analytically.We find that the equilibrium electrostatic field of periodical structure plays the same role as the usual focusing magnetic field to lead the test particle to stochastic motion.The resonance overlapping criterion for the random state is given,and also the Fokker-Planck-Kolmogorov approach to diffusion is considered for our system.     

Stochastic motion of the untrapped particle in electrostatic mode

The stochastic energy diffusion of the untrapped particle in the electrostatic mode is investigated analytically. We find that the equilibrium electrostatic field of periodical structure plays the same role as the usual focusing magnetic field to lead the test particle to stochastic motion. The resonance overlapping criterion for the random state is given, and also the Fokker-Planek-Kohnogorov approach to diffusion is considered for our system.     

Ultrarelativistic particle acceleration in collisionless shock waves

This paper describes the theory and particle simulations of ultrarelativistic particle acceleration caused by shock waves in a collisionless magnetized plasma.     

Role of stochastic heating in wakefield acceleration monitored by optical injection

It is shown that stochastic heating can play an important role in Laser Wake Field Acceleration. When considering low density plasma interacting with a high intensity wave perturbed by a low intensity counterpropagating wave, stochastic heating can provide electrons with the right momentum for trapping in the wake field. The influence of stochastic acceleration on the trapping of electrons is compared to the one of cold injection by considering several polarizations of the colliding pulses. For some value of the plasma density and pulse duration, a transition from an injection due to stochastic acceleration to a cold injection dominated regime – regarding the trapped charge – has been observed from PIC code simulations. When the plasma density exceeds some value, stochastic heating becomes important and is necessary in some circumstances to get electrons trapped into the wakefield.     

随机过程数值仿真的精度实验与分析

为了考核随机过程数值仿真中采用的混沌多项式展开,小波动态自适应网格等的可靠性和近似程度,本文构造了一个具有解析解的随机过程,随后进行数值仿真。比较了数值仿真结果与解析解,证明两者是相互一致的。说明所采用的数值仿真方法是成立和近似度是可以接受的。所进行比较的随机过程解析解是相对比较简单的,当对更复杂的情况需要进一步数值实验验证。在文中并讨论了影响数值仿真精度和增加数值仿真工作难度的因素。     

Geotropic tracers in turbulent flows: a proxy for fluid acceleration

We investigate the statistics of orientation of small, neutrally buoyant, spherical tracers whose centre of mass is displaced from the geometrical centre. If appropriate-sized particles are considered, a linear relation can be derived between the horizontal components of the orientation vector and the same components of acceleration. Direct numerical simulations are carried out, showing that such relation can be used to reconstruct the statistics of acceleration fluctuations up to the order of the gravitational acceleration. Based on such results, we suggest a novel method for the local experimental measurement of accelerations in turbulent flows.     

激光加速高能质子实验研究进展及新加速方案

利用超强激光与等离子体相互作用来加速高能离子是激光等离子体物理及加速器物理领域的研究热点.经过了近20年的发展,激光离子加速已取得丰硕成果,催生了一批新的应用.本文概述了国内外激光离子加速所取得的标志性实验研究进展,围绕高能质子的产生这一关键问题进行了深入的探讨,介绍了近几年来发展的有潜力的新加速方案.     

超强激光脉冲下等离子体块的整体加速研究

为了克服激光加速中强流离子束空间电荷效应对粒子输运的影响,提出一种利用两块不同密度的固体靶先后和一束强度约为1022 W/cm2、脉冲长度为5T(T为激光周期)的超强脉冲激光相互作用的方案,实现了中性等离子体块的加速。通过一维PIC数值模拟研究发现,在合适的参数下,加速后的电子与质子几乎以相同的速度共同飞行长达60（为激光波长）的距离,其中质子与电子的能量分别为GeV和100 MeV量级。     

Simulation of Stochastic Differential Equations Through the Local Linearization Method. A Comparative Study

A new local linearization (LL) scheme for the numerical integration of nonautonomous multidimensional stochastic differential equations (SDEs) with additive noise is introduced. The numerical scheme is based on the local linearization of the SDE's drift coefficient by means of a truncated Ito–Taylor expansion. A comparative study with the other LL schemes is presented which shows some advantanges of the new scheme over other ones.     

Recent progress of laser driven particle acceleration at Peking University

Recently, radiation pressure acceleration （RPA） has been proposed and extensively studied, which shows that circularly polarized （CP） laser pulses can accelerate mono-energetic ion bunches in a phase-stable-acceleration （PSA） way from ultrathin foils. It is found that self-orgizing proton beam can be stably accelerated to GeV in the interaction of a CP laser with a planar target at 1022 W/cm2. A project called Compact LAser Plasma proton Accelerator （CLAPA） is approved by MOST in China recently. A prototype of laser driven proton accelerator （1 to 15 MeV/1 Hz） based on the PSA mechanism and plasma lens is going to be built at Peking University in the next five years. It will be upgraded to 200 MeV later for applications such as cancer therapy, plasma imaging and fast ignitiou for inertial confine fusion.     

Successive particle acceleration by use of two tightly focused ultra-short intense laser pulses

We suggest a scheme of electron acceleration by use of two tightly focused ultra-short intense laser pulses at a 100TW level. Electrons obtain a preliminary acceleration with a small angular spread by the longitudinal ponderomotive force of the first pulse. They are then injected and further accelerated to hundreds of MeV by the second laser pulse.     

Determination of nonlinear optical properties using the Voigt function: Stochastic considerations

In this work, the absorptive and dispersive profiles of a molecular system have been calculated through a methodology based on the averaging of the coherence obtained by the resolution of the optical stochastic Bloch equations. To this aim, a generalized Lorentzian approximation to the Voigt function has been used as a probability distribution, which allows a more generalized analysis of the interactions between the solvent and the molecule. This has been modeled with the parameters of the standard organic dye Green Malaquite, which exhibits a nonlinear behavior under the interaction with a high intensity electric field.     

湍流模型在内流流场数值模拟中的应用

本文将Spalart-Allmaras湍流模型方程推导为守恒形式,并应用于内流湍流流场计算.采用此-方程湍流模型分别对扩压器流场以及NASA 67转子流场进行了数值模拟.结果表明常用在外流计算中的Spalart-Allmaras湍流模型在内流计算中仍具有较高的精度、数值稳定性和效率.     

自适应光学系统随机并行梯度下降算法

随机并行梯度下降(SPGD)算法可以对系统性能指标直接优化来校正畸变波前。对基于SPGD算法的61单元自适应光学系统进行仿真模拟,分析了对不同初始静态畸变波前的校正能力,并比较了不同性能指标情况下的算法增益系数、扰动幅度值的选取及校正情况。仿真结果表明:算法收敛速度很大程度上依赖于增益系数和扰动幅度值,对畸变较大的波前,随机扰动幅度在0.50~0.85范围内,性能指标采用焦斑平均半径比采用斯特列尔比取得的校正效果好。     

超强激光驱动的辐射反作用力效应与极化粒子加速

光强超过10~(22) W/cm~2的极端超强激光将光与物质的相互作用推进到辐射主导区域,激发高能伽马光子辐射,产生明显的辐射反作用力效应.辐射反作用力可以显著影响强场中带电粒子的动力学行为,并从根本上改变了极端强场区域的激光等离子体相互作用规律.如何理解和验证辐射反作用力效应是强场物理研究的核心内容之一.本文结合该方向的国内外研究进展,论述了辐射反作用力的经典形式与强场量子电动力学的理论计算与模拟方法,详细讨论了单粒子在强场中的反射、量子随机辐射、自旋-辐射耦合等效应,介绍了激光等离子体相互作用中的电子冷却、辐射俘获、高效伽马辐射等机制,并给出了目前辐射反作用力效应的实验验证方法与进展.针对自旋在强场量子电动力学方面的效应,介绍了激光加速产生极化粒子源的方法.     

不同湍流模型对热等离子体射流模拟的影响

本文应用大气压等离子体射流传热与流动的三维数学模型,在相同初始条件下,计算得到了采用不同湍流模型时氩等离子体射流对称轴线上的温度、速度及空气质量分数分布,并与文献中同等条件下的实验结果进行了比较,结果表明采用标准κ-ε模型和Realizableκ-ε模型时与实验结果相差较大,而采用RNGκ-ε湍流模型时模拟结果与实验数...     

Particle simulation on electron acceleration process by the laser ponderomotive force in inhomogeneous underdense plasma layers

The mechanism of electron ponderomotive acceleration due to increasing group velocity of laser pulse in inhomogeneous underdense plasma layers is studied by two-dimensional relativistic parallel particle-in-cell code. The electrons within the laser pulse move with it and can be strongly accelerated ponderomotively when the duration of laser pulse is much shorter than the duration of optimum condition for acceleration in the wake. The extra energy gain can be attributed to the change of laser group velocity. More high energy electrons are generated in the plasma layer with descending density profile than that with ascending density profile. The process and character of electron acceleration in three kinds of underdense plasma layers are presented and compared.