The Effect of Ion Motion on Laser-Driven Plasma Wake in Capillary

The effect of ion motion in capillary-guided laser-driven plasma wake is investigated through rebuilding a two-dimensional analytical model.It is shown that laser pulse with the same power can excite more intense wakefield in the capillary of a smaller radius.When laser intensity exceeds a critical value,the effect of ion motion reducing the wakefield rises,which becomes significant with a decrease of capillary radius.This phenomenon can be attributed to plasma ions in smaller capillary obtaining more energy from the plasma wake.The dependence of the difference value between maximal scalar potential of wake for two cases of ion rest and ion motion on the radius of the capillary is discussed.

Excitation and power spectrum analysis of electromagnetic radiation for the plasma wake of reentry vehicles

The plasma wake of reentry vehicles has the advantages of extensive space range and long traceability,which provides new possibilities for the detection and monitoring of reentry vehicles.Based on the Zakharov model,this work investigates the excitation and power spectrum characteristics of electromagnetic radiation for the plasma wake of a typical reentry vehicle.With the aid of parametric decay instability,the excitation condition of electromagnetic radiation for a typical plasma wake is evaluated first.The power spectrum characteristics of electromagnetic radiation,including the effects of both the flight parameters and incident wave parameters are analyzed in detail.The results show that when the phenomenon of excited electromagnetic radiation occurs,plasma wakes closer to the bottom of the vehicle and with faster speeds require higher incident frequencies and thresholds of the electric field.As the frequency of the incident wave increases,peaks appear in the power spectra of plasma wakes,and their magnitudes increase gradually.The frequency shifts of the secondary peaks are equal,whereas,the peaks of the downshifted spectral lines are generally larger than those of the upshifted spectral lines.The work in this paper provides a new idea and method for the tracking of reentry vehicles,which has potential application value in the field of reentry vehicle detection.

Efficient ion acceleration driven by a Laguerre–Gaussian laser in near-critical-density plasma

Laser-driven ion accelerators have the advantages of compact size,high density,and short bunch duration over conventional accelerators.Nevertheless,it is still challenging to generate ion beams with quasi-monoenergetic peak and low divergence in experiments with the current ultrahigh intensity laser and thin target technologies.Here we propose a scheme that a Laguerre–Gaussian laser irradiates a near-critical-density(NCD)plasma to generate a quasi-monoenergetic and low-divergence proton beam.The Laguerre–Gaussian laser pulse in an NCD plasma excites a moving longitudinal electrostatic field with a large amplitude,and it maintains the inward bowl-shape for dozens of laser durations.This special distribution of the longitudinal electrostatic field can simultaneously accelerate and converge the protons.Our particle-in-cell(PIC)simulation shows that the efficient proton acceleration can be realized with the Laguerre–Gaussian laser intensity ranging from 3.9×10^(21)W·cm^(-2)–1.6×10^(22)W·cm^(-2)available in the near future,e.g.,a quasi-monoenergetic proton beam with peak energy~115 MeV and divergence angles less than 5°can be generated by a 5.3×10^(21)W·cm^(-2)pulse.This work could provide a reference for the high-quality ion beam generation with PWclass laser systems available recently.

Electron trajectory evaluation in laser-plasma interaction for effective output beam

Using the ellipsoidal cavity model, the quasi-monoenergetic electron output beam in laser-plasma interaction is described. By the cavity regime the quality of electron beam is improved in comparison with those generated from other methods such as periodic plasma wave field, spheroidal cavity regime and plasma channel guided acceleration. Trajectory of electron motion is described as hyperbolic, parabolic or elliptic paths. We find that the self-generated electron bunch has a smaller energy width and more effective gain in energy spectrum. Initial condition for the ellipsoidal cavity is determined by laser-plasma parameters. The electron trajectory is influenced by its position, energy and cavity electrostatic potential.

Self-modulation and anomalous collective scattering of laser produced intense ion beam in plasmas

The collective interaction between intense ion beams and plasmas is studied by simulations and experiments,where an intense proton beam produced by a short pulse laser is injected into a pre-ionized gas.It is found that,depending on its current density,collective effects can significantly alter the propagated ion beam and the stopping power.The quantitative agreement that is found between theories and experiments constitutes the first validation of the collective interaction theory.The effects in the interaction between intense ion beams and background gas plasmas are of importance for the design of laser fusion reactors as well as for beam physics.

The effect of cavity density on the formation of electrostatic shock in the lunar wake: 1-D hybrid simulation

One-dimensional hybrid simulations are carried out to study the plasma refilling process in the lunar wake.Previous theoretical and simulation studies have shown that ion-ion acoustic(ⅡA)instability can be initiated and electrostatic shock can be formed under the condition Te>>Ti.We find that the time evolution ofⅡA instability and the formation of electrostatic shock strongly depend on initial cavity density.The initial position of the electrostatic shock is dependent on the ratio between initial cavity density and background solar wind density,i.e.,the farther away the initial position,the lower is the ratio.When the initial cavity density is low enough,the density and electric field profile across the wake become much complex.Meanwhile,the back-to-back electrostatic shock is unstable in the case of lower cavity densities;at the late evolution stage,a new shock-like structure can be formed at the central region of the lunar wake.

临近空间高超声速飞行器RCS特性研究

首次系统地对升力体外形临近空间飞行器雷达散射截面(RCS)特性开展了研究,主要研究内容包括本体及绕流RCS特性研究,亚密湍流尾迹RCS特性研究和层流尾迹的RCS特性研究等。结果显示:等离子体绕流将降低飞行器后向RCS;亚密湍流尾迹对低频段RCS影响明显,对高频段影响不明显。另外,双站雷达对临近空间高超声速飞行器也会有较好的探测效果。

高速模型尾迹流场及其电磁散射特性相似性实验研究

高速目标再入大气层或在临近空间飞行时,空气电离形成的等离子体鞘套和尾迹对目标的雷达散射特性会产生影响.为了研究不同模型尾迹流场及其电磁散射特性规律和相似性,以氧化铝球模型为研究对象,在弹道靶设备上开展了双尺度参数相同的条件下高速球模型尾迹流场及其电磁散射相似性实验研究.由二级轻气炮发射模型,模型直径分别为8.0、10.0、12.0、15.0 mm,速度约6 km/s,靶室压力分别为6.3,5.0,4.2,3.3 k Pa,采用阴影照相系统测量模型激波脱体距离、电子密度测量系统测量模型尾迹的电子密度分布、X波段单站雷达系统测量在视角为40?的模型及流场的雷达散射截面(RCS)分布.实验结果表明:在速度不变、双尺度参数相同的条件下,随着模型尺寸的增加,激波脱体距离逐渐增加,激波脱体距离与模型直径之比近似相同;不同模型尾迹电子密度测量曲线的趋势和数量级一致,表明不同模型的尾迹流场适用于双缩尺律;不同尺寸模型尾迹的总体RCS与分布RCS均不相同,表明不同模型尾迹的电磁散射不适用于二元缩尺律;高速球模型全目标电磁散射能量分布在模型及其绕流区域、等离子体尾迹区域;高速球模型全目标电磁散射能量在模型及绕流场区域出现1个强散射中心,在模型湍流尾迹区域出现多个散射中心;高速球模型尾迹的RCS测量信号呈现随机性分布特性,幅度脉动和频率脉动均没有周期性;随着模型尺寸的增加,模型尾迹的总体RCS增加,尾迹脉动频率的变化范围减小.

超视距雷达探测高超声速飞行器可行性分析

在前期开展再入飞行器RCS(雷达散射截面)特性试验和理论研究的基础上,对典型临近空间高超声速飞行器RCS特性开展了研究,分析了绕流和尾迹对飞行器本体RCS特性的影响。研究表明等离子体流场在头身部绕流、近尾尾迹和部分远尾尾迹的最大电子密度将远高于电离层最高电子密度,更高于典型天波超视距雷达工作频段对应的临界电子密度。因而等离子体尾迹将会对3~30 MHz频段电磁波产生较强的散射,使得等离子体尾迹的RCS远远大于飞行器本体的RCS。利用临近空间高超声速飞行器尾迹RCS的这一特点,有可能实现对临近空间高超声速飞行器的超视距探测。

湍流等离子体尾迹雷达散射截面的计算及其影响因素分析

针对一阶畸变波Born近似模型 ,深入分析了湍流内外尺度和电子数密度脉动值对雷达散射截面的影响 ,并且给出了关于湍流外尺度的一个经验的 ,能被工程上较好使用的公式。在以上分析的基础上 ,计算了几种高程条件下再入小钝头锥体等离子尾迹的雷达散射截面 ,与已有实验结果进行了对比分析。分析和计算结果表明 ,湍流的外尺度和局部电子数密度值对雷达散射截面值影响较大 ;

返回舱再入段雷达散射特性研究

"神舟"飞船返回舱在再入过程中,由于产生等离子体鞘套和尾流,其雷达散射截面(RCS)出现剧烈变化,对地面测量雷达的跟踪产生影响。文中以C波段雷达的实测数据为依据,对返回舱RCS突增和缩减的机理进行了分析;重点阐述了厘米波雷达以反射式跟踪方式跟踪再入体时,会出现短时间目标丢失现象的必然性。

低轨航天器致空间等离子体尾流

建立起一个低轨道航天器引起的等离子体尾流的二维计算机模式，结果表明：（１）尾流的宽度随表面负电位值的增加而变窄．（２）随离子马赫数增加尾流位垒变宽．（３）对正电位表面情况尾流近区有电子聚集，并且离子的放空更远．

再入飞行器尾迹流场及其雷达散射效应研究

对再入飞行器等离子体尾迹及其雷达散射特性进行了分析、研究和大量的计算。讨论了物形、流场各因素对尾迹雷达散射截面的影响。流场计算使用准一维粘性尾迹方程，以修正基尔方法（多值法）求解，用一阶Ｂｏｒｎ近似完成亚密雷达散射截面（ＲＣＳ）计算。计算中使用８组元混合空气、１４个非平衡化学反应模型，考虑５种不同尺度的小钝头锥形物体，沿再入轨道取６５至３４公里，共１３个高程的飞行条件。通过计算得到了再入体尾迹各流场参数、电子密度分布及湍流亚密尾迹的ＲＣＳ。结果说明再入钝锥细长体粘性尾迹的转捩特性对于等离子体的散射性质具有决定性的作用；再入弹头尾迹等离子体对地面单站雷达发射波的回波主要来源于尾迹湍流亚密的非相干散射；对确定的波长，当环境雷诺数达到临界值之后，可能出现ＲＣＳ的突增现象；不同物形及来流条件造成尾迹转捩位置的改变，从而影响ＲＣＳ的数值及其沿轨道的分布；改变尾迹颈部初值会引起ＲＣＳ值的明显变化。

高超声速圆球绕流流场及其近尾尾迹流场电磁散射特性分析

采用分段线性电流密度递归卷积时域有限差分(PLJERC-FDTD)方法计算高超声速导电金属球绕流流场及其近尾尾迹流场电磁散射特性,分析等离子体绕流流场RCS的频率特性、双站散射特性、极化特性及随飞行高度、飞行马赫数、入射角的变化关系。计算表明,前向散射方向是全方位散射中RCS取得最大值的方向;马赫数较大(本文Ma≥14)时,入射波频率增大、马赫数增大及飞行高度降低,绕流流场前向RCS增大。马赫数较小(本文Ma≤10)时,飞行马赫数、高度及入射角变化对绕流流场UHF、L、S波段后向RCS和双站RCS影响很小;在UHF、L波段,绕流流场及本体的后向RCS差距较小。马赫数较大时,大范围过密等离子体尾迹的形成使得电磁波垂直轴线入射时绕流流场增大了目标本体的后向RCS;在L、S波段,绕流流场后向RCS曲线可用一条直线逼近。

太阳风作用下航天器表面充电效应分析

针对太阳风等离子体中航天器的充电过程,基于欧洲航天局开发的航天器表面充电仿真软件,建立了航天器的数值计算模型,模拟了太阳风等离子体与航天器的相互作用,实现了不同表面材料时航天器表面电位以及尾迹结构的计算分析,给出了近日环境下航天器周围的空间电位结构以及等离子体密度分布情况。研究结果表明:表面材料的选择将影响空间势垒深度以及尾迹结构,表面充电至更低电位的表面材料具有较小尾迹区域;航天器向阳面电位分布主要由光电子以及二次电子主导,而背阳面的电位分布则主要受到尾迹效应的影响。以上结果对太阳风环境航天器探测和评估等相关工作以及工程研究具有一定的理论参考价值。

再入尾迹湍流对雷达散射截面影响分析

通过探讨高超声速再入湍流尾迹等离子体场中电磁波的散射机制 ,推导出在工程上描述湍流亚密等离子体雷达散射的一阶畸变波Born近似模型 ,分析了该模型在充分发展湍流尾迹等离子体场中的适用性 ,完成了适用于三维尾迹等离子体场的程序设计 .以已有的湍流尾迹等离子体流场数据为基础 ,分析了再入尾迹湍流等离子体流动对雷达散射截面的影响 .选取考察的几个有代表性的因素为 :湍流模型、转捩过程、湍流尺度、电子组分脉动初始条件等 .由结果可以看到 ,湍流转捩过程和湍流尺度对雷达散射截面值影响不大 ;电子组份脉动强度初始值影响较明显 ;湍流模型在特定条件下影响亦不大 .

极轨航天器尾区带电效应研究

当航天器运行在低能、高密度的低轨等离子体环境中时,由于航天器轨道速度大于离子的热速度而远小于电子的热速度,因此电子可较容易地进入航天器尾部并沉积到表面材料上,从而形成一负电位势垒产生所谓的"尾区效应"。特别是在地球极区的背景等离子体环境中,由于高能极光电子的注入,大倾角极轨航天器尾区介质材料将会发生严重的充电过程。通过对拟合极光电子的微分通量谱分布分析,计算了极光电子收集电流和背景等离子体中离子的收集电流。

日冕背景下的等离子体尾场效应

计算了高能脉冲电子束在冷背景和热背景等离子体条件下产生的等离子体尾场（ＰＷＦ）大小，讨论了高能电子束的速度、密度、长度对等离子体尾场分布的影响．在这基础上，研究了太阳耀斑脉冲相产生的向外逃逸高能脉冲电子束在日冕背景等离子体条件下激发的等离子体尾场分布以及对其捕获电子的加速．

再入飞行器湍流尾迹流场研究

再入飞行器湍流尾迹流场状况，直接关系到飞行器的雷达散射特性．对再入飞行器湍流尾迹等离子体场理论模型，试图通过湍流模式理论来麦达，即使用ｋ－ε－ｇ模型方程来封闭平均化的全Ｎａｖｉｅｒ－Ｓｔｏｋｅｓ方程，从而推确获得流动平均场和脉动场信息．使用的Ｎ－Ｓ平均方程由质量加权平均过程产生，湍流模型方程也经过可压缩性修正．真实气体效应重点考察空气处于局部热化学平衡状态．流动控制方程运用一个二阶ＴＶＤ格式的有限体积法求解．以一典型小钝锥体零攻角再入飞行为例，计算了在两种高程（Ｈ＝４０ｋｍ和 Ｈ＝３０ ｋｍ）条件下的高超声速湍流尾迹流场．获得的尾迹流场参数与流动物理状况符合，并且湍流脉动参数与己有相应的实验结果定性一致。

航天器尾区带电效应实验研究

通过极区极光椭圆区的极轨轨道是特殊的低轨轨道,在该轨道环境中既有低温、稠密的背景等离子体,又有高能极光电子的注入,因此当背景离子密度减小或极光电子通量增加时,通过极区的航天器尾区表面介质材料将被充至负几百伏甚至负几千伏。介绍了航天器尾区带电过程的实验方案设计,建立了航天器尾区带电实验航天器缩比模型,同时测量了不同实验等离子体环境下航天器模型的尾区离子饱和电流,测量了有高能电子注入时的缩比模型的尾区内介质表面的充电电位。