光子与相对论麦克斯韦分布电子散射的能谱角度谱研究

光子与相对论麦克斯韦分布电子散射的描述及能谱角度谱计算非常复杂且费时.本文提出了一种光子与相对论麦克斯韦速度分布电子散射的蒙特卡罗(MC)模拟方法,该方法能够细致模拟高温等离子体中任意能量光子与任意温度电子的Compton和逆Compton散射问题.对于散射后光子的能谱和角度谱参数,可以根据电子温度抽样若干不同状态的电子,分别模拟其与光子发生散射,可以得到各次散射后的光子能量和偏转角度,取统计平均后的结果即可获得该光子与该温度电子散射的能谱和角度谱分布.根据该方法编写了光子与相对论电子散射MC模拟程序,开展了高温全电离等离子体中光子与相对论电子散射的能谱角度谱计算和分析,分析结果显示:热运动电子将展宽出射光子能谱,且低能光子与高温电子散射后的蓝移现象明显;出射光子的角度谱很复杂,其决定于入射光子能量、出射光子能量及电子温度.基于该方法计算并以数表形式给出的光子-相对论电子散射能谱角度谱数据,可以供辐射输运数值模拟程序使用.     

多光子非线性Compton散射对等离子体中颗粒成长的影响

应用等离子体颗粒成长模型和多光子非线性Compton模型,研究了多光子非线性Compton散射对等离子体中颗粒成长的影响。提出了将入射光和多光子非线性Compton散射光作为颗粒成长的新机制,对颗粒凝合截面、凝合系数、颗粒收集的电子流和离子流的表达式进行了修正。结果表明,等离子体参数的变化与多光子非线性Compton散射成分、电子散射前的初始速度、入射角、与电子同时作用的光子数等因素有关。散射使生长过程中收集离子机制和凝合机制增强,颗粒的凝合截面增大,颗粒半径较快的非均匀增长。     

Compton散射下激光等离子体纵波色散特性

应用电子与多光子集团非线性Compton散射模型,研究了Compton散射下激光等离子体纵波色散特性.结果表明:长波支纵色散曲线由解析上的长波、数值计算结果和短波组成,长波支和短波支纵色散均随相对论正负电子对特征温度的增大而增大,随Compton散射引起的频率的增量的增大而降低,且单温激光等离子体的色散曲线与散射前的双温等离子体的色散曲线相似.     

Compton散射下激光等离子体纵波色散特性

应用电子与多光子集团非线性Compton散射模型,研究了Compton散射下激光等离子体纵波色散特性.结果表明:长波支纵色散曲线由解析上的长波、数值计算结果和短波组成,长波支和短波支纵色散均随相对论正负电子对特征温度的增大而增大,随Compton散射引起的频率的增量的增大而降低,且单温激光等离子体的色散曲线与散射前的双温等离子体的色散曲线相似.     

锥形激光等离子体中Compton 散射对电子的加速

应用相对论性电子与光子非弹性碰撞模型和经典相对论电动力学理论,结合锥形飞秒强激光等离子体中的光场特性和静电场能,分析、计算了入射的高能电子束与等离子体中的光子发生多光子非线性Compton散射时对电子的加速效应,发现等离子体中的光场会引起电子加速能量的振荡;等离子体中的静电场降低电子的加速效应。用高能电子束与锥形飞秒强激光等离子体中的光子发生双光子非线性Compton散射,是加速电子最为理想的情况。     

Compton散射下离子初始速度对等离子体鞘层厚度的影响

应用相对论理论、Compton散射等离子体离子源和龙格-库塔方法,研究了Compton散射对等离子体鞘层厚度的影响,提出了Compton散射等离子体离子源概念,并将入射光和Compton散射光作为形成等离子体离子初始速度的新机制,给出了Compton散射下鞘层空间内电位梯度随等离子体离子初始速度变化的修正方程,并进行了数值模拟.结果表明,与Compton散射前相比,Compton散射使等离子体鞘层厚度随加速电压增大而明显的非线性增大,这主要是由于Compton散射使离子与电子碰撞频率增大,电子温度升高,使离子初始速度和能量增大的缘故.     

Compton散射下离子初始速度对等离子体鞘层厚度的影响

应用相对论理论、Compton散射等离子体离子源和龙格-库塔方法,研究了Compton散射对等离子体鞘层厚度的影响,提出了Compton散射等离子体离子源概念,并将入射光和Compton散射光作为形成等离子体离子初始速度的新机制,给出了Compton散射下鞘层空间内电位梯度随等离子体离子初始速度变化的修正方程,并进行了数值模拟。结果表明,与Compton散射前相比,Compton散射使等离子体鞘层厚度随加速电压增大而明显的非线性增大,这主要是由于Compton散射使离子与电子碰撞频率增大,电子温度升高,使离子初始速度和能量增大的缘故。     

Compton散射对激光等离子体通道寿命的影响

为了研究多光子非线性Compton散射对飞秒激光退吸附作用对等离子体通道寿命的影响,应用多光子非线性Compton散射模型和强飞秒激光在空气中产生等离子体通道内带电粒子动力学模型,提出了多光子非线性Compton散射是影响等离子体辐射阻尼和通道寿命的一个重要机制,给出了等离子体通道中带电粒子的修正动力学方程,并进行了数值计算和实验研究.结果表明:散射削弱了自由电子对氧分子的吸附和正负离子的复合,有效地补偿了等离子体的辐射阻尼效应,后续激光脉宽和间隔以及Compton散射是影响等离子体辐射阻尼和通道寿命的关键因素,理论计算与实验结果较好的吻合.     

精确计算电子重整化链图传播下的Compton散射微分截面

采用电子与光子电磁相互作用最小耦合模型,对电子重整化链图传播下Compton散射微分截面作了严格解析计算,获得精确理论结果;并将该计算结果与电子树图和重整化单圈图传播下Compton散射微分截面作对比分析,获得了有关辐射修正的重要信息.此研究结果对精确描述Compton散射现象以及对电磁相互作用所呈现的复杂内部过程的深入探讨,都将从一个重要研究侧面给出理论计算研究方面某些可供借鉴与参考之处.     

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应用电子和多光子集团非弹性碰撞模型和冷等离子体模型，研究了飞秒强激光与线性等离子体发生多光子非线性Compton散射时，散射激光与入射激光形成的飞秒耦合激光场对线性等离子体层中光场和电子密度分布的影响。研究发现，在耦合激光的有质动力作用下，电子密度分布和离子密度分布比Compton散射前的偏离更加严重，电子密度的变化比离子密度的变化更快，产生的静电场更强。即使耦合激光场非常弱，电子的运动仍表现出相对论效应，仍有静电场存在。     

Direct Monte Carlo simulation of scattering processes of kV electrons in aluminum; comparison of theoretical N(E) spectra with experiment

A Monte Carlo simulation of the scattering processes of kV electrons penetrating into aluminum was performed. The simulation is based on the use of different types of differential cross-sections for individual elastic and inelastic scattering: (i) Elastic scattering; the differential cross-sections derived by partial wave expansion method. (ii) Inelastic scattering; Gryzinski's excitation function for inner-shell electron excitation, Streitwolfs excitation function for conduction electron excitation, and Quinn's mean free path for plasmon excitation. For verification the energy loss spectra obtained from the Monte Carlo calculations were then compared with experiment done with commercial type Auger microprobes, JAMP-3, for angle of incidence 45° and JAMP-10 for normal incidence at primary electron energies of 1.5 and 3.0 keV, respectively. The results show satisfactory agreement between theory and experiment.     

利用轫致辐射谱回推超热电子温度

超短超强激光打靶产生的超热电子与固体靶相互作用时会产生轫致辐射X射线。利用蒙特卡罗方法,对电子在固体靶中传输产生的轫致辐射X射线进行了模拟。1 MeV电子束与固体靶作用产生的轫致辐射谱模拟结果表明,轫致辐射谱高能段斜率受靶厚度及靶材料的影响不明显。麦克斯韦分布的电子束及单能电子束与30 m铜靶作用的模拟结果显示,两种电子源产生的轫致辐射谱在电子束能量或温度较高时基本一致。给出了一种利用轫致辐射谱斜率反推超热电子温度的定标方法。模拟了不同温度下超热电子产生的轫致辐射光子的能量角分布及光子数角分布,结果显示辐射光子能量通量和光子数随着电子温度的提高越来越向前倾,并给出了另外一种由轫致辐射能量角分布反推超热电子温度的定标关系。     

逆Compton散射的MonteCarlo模拟

研究模拟均匀、各向同性介质内的逆康普顿散射过程的Monte Carlo方法,分析电子速度分布乘抽样法和光子散射方向分布乘抽样法的抽样效率,完善电子速度分布的直接抽样法、光子散射能量分布的乘加抽样法.通过对这四种抽样方法抽样费用的分析和比较,得出各种方法的最适条件,该方法可以模拟更高能的辐射.     

Moment-generated radiative-transfer functions for relativistic Maxwellian electrons

Coupling the relativistic expressions for various momentum moments of Maxwellian electron distributions to an invariant representation of Compton photon scattering permits fast and accurate approximations of the effective transfer kernels appearing in the photon transport equation. The method is detailed, earlier approaches are discussed, and numerical results and comparisons are presented.     

Approximate and exact photon-maxwellian electron cross-sections and a Monte Carlo sampling scheme

Fast and accurate approximate methods for computing temperature-corrected photon-Maxwellian electron cross-sections, using distribution averaged electron energies and scattering angles, are presented. Use is made of the covariant (frame independent) picture of scattering for relativistic electrons. The schemes are motivated and detailed, predictions are obtained and compared with exact values, and appropriate limiting forms that retain the Thomson and Klein-Nishina cross-section structure are also recovered. A two-parameter fit to the total cross-sections in a Klein-Nishina picture is detailed using an invariant scattering kernel. This particular fit is useful in extending existing sampling schemes which employ the static probability density. Extensions of Kahn's technique (for sampling the Klein-Nishina density) arepresented for a temperature-corrected set of probability densities obtained from the fitted cross-sections. The methods are amenable to further pointwise (Monte Carlo) or multigroup (S_n) cross-section processing. An exact numerical scheme employing adaptive quadrature techniques, as implemented in the production code module MAXWELL is also described.     

Monte Carlo方法模拟低能电子在Al中的散射

基于文献[1]的工作,电子在固体中的弹性散射用Mott微分截面计算;非弹性散射分为单电子激发和等离子激发并由Streitwolf、Gryzinski及Quinn的截面描述.模拟了低能电子在Al块样及薄膜中的散射过程,对不同能量低能电子作用下Al的背散射系统、能谱又透射系数作了计算,结果与实验符合较好.也对背散射电子、低能损背散射电子表面分布作了计算,结果表明低能损背散射电子具有较好的空间分辨率.     

Inference of the electron temperature in inertial confinement fusion implosions from the hard X‐ray spectral continuum

Using the free‐free continuum self‐emission spectrum at photon energies above 15 keV is one of the most promising concepts for assessing the electron temperature in inertial confinement fusion (ICF) experiments. However, these photons are due to suprathermal electrons whose mean free path is much larger than the thermal one, making their distribution deviate from Maxwellian in a finite‐size hotspot. The first study of the free‐free X‐ray emission from an ICF implosion is conducted, accounting for the kinetic modifications to the electron distribution. These modifications are found to result in qualitatively new features in the hard X‐ray spectral continuum. Inference of the electron temperature as if the emitting electrons are Maxwellian is shown to give a lower value than the actual one.     

Monte Carlo calculation of electron spectra produced in water phantoms by 20 keV-1 GeV photons

Abstract

A Monte Carlo calculation for the energy spectra of electrons and positrons produced in infinite and semiinfinite water phantoms by photons ranging in energies from 20 keV to 1 GeV are presented. The dominant processes considered are the photoelectric effect, Auger effect, Compton effect, pair, and triplet production. Bremsstrahlung produced by electrons and positrons with energies greater than 1 MeV is also included. The effect of multiple Compton scattering, not considered in the earlier calculation, for the infinite phantom for photon energies higher than 2 MeV has been incorporated. For a semi-infinite phantom, multiple Compton scattering and backscattering through the top are considered. The results are compared with the earlier calculations for the first-collision spectrum. It is found that the inclusion of multiple Compton scattering significantly increases the average number of electrons/cm³ per photon/cm² at all energies considered whereas bremsstrahlung reduces the number of high energy electrons and produces more low energy electrons in the spectrum.     

Stability analysis and time-step limits for a Monte Carlo Compton-scattering method

A Monte Carlo method for simulating Compton scattering in high energy density applications has been presented that models the photon–electron collision kinematics exactly [E. Canfield, W.M. Howard, E.P. Liang, Inverse Comptonization by one-dimensional relativistic electrons, Astrophys. J. 323 (1987) 565]. However, implementing this technique typically requires an explicit evaluation of the material temperature, which can lead to unstable and oscillatory solutions. In this paper, we perform a stability analysis of this Monte Carlo method and develop two time-step limits that avoid undesirable behavior. The first time-step limit prevents instabilities, while the second, more restrictive time-step limit avoids both instabilities and nonphysical oscillations. With a set of numerical examples, we demonstrate the efficacy of these time-step limits.