共查询到19条相似文献,搜索用时 156 毫秒
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采用能够较为清晰、完整描述强飞秒激光等离子体通道内带电粒子产生过程及其演化的物理模型,进一步研究了飞秒光丝中等离子体密度的时间演化特征。计算结果表明:对于不同时间线型的脉冲,在等离子体通道形成过程中,氧气分子的电离贡献率及氮气分子的贡献率明显不同,不同线型的脉冲对高效维持高密度等离子体的寿命具有较大的影响。有效控制成丝脉冲线型能够达到对等离子体通道的高效利用。长脉冲、短波长虽能够获得较高密度等离子体通道,但其存活寿命却完全受限于通道的后期演化。 相似文献
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对超短超强激光脉冲在大气中传播时形成的电离通道的寿命进行了理论研究.综合考虑了通道中自由电子,正离子,负离子的复合,自由电子和中性分子的吸附以及在后续 激光作用下的退吸附过程.推导出了退吸附激光强度恒定时通道中带电离子密度的速率方程 的解析解.计算结果表明,通过引入退吸附激光抑制电子和中性分子的吸附作用能够在微秒 的时间尺度上将电子密度维持在1012—1013cm-3的水平,在相同的波长 和平均功率下,短脉冲序列的退吸附效果要略好于连续激光
关键词:
等离子体通道
复合
吸附
退吸附
寿命 相似文献
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实验研究了高功率纳秒量级激光脉冲在空气中聚焦时的能量透过率随输入激光脉冲能量变化的规律,发现在纳秒激光脉冲聚焦半径相同的情况下,激光脉冲的能量透过率随入射激光脉冲能量的变化可分为三种情况:当入射激光脉冲能量较低时,激光脉冲能量全部通过;当入射激光脉冲能量增大后,激光脉冲的能量透过率由近100%迅速减小;当入射激光脉冲的能量进一步增加时,激光脉冲的能量透过率继续缓慢变小.用临界自由电子密度以及所对应的临界时间点对上述实验现象进行了理论分析得到了如下结论:当自由电子密度未达到临界自由电子密度时,多光子电离过程起主要作用,而当自由电子密度超过临界自由电子密度后,逆韧致吸收过程起主要作用,临界时间点是入射激光脉冲与空气作用过程中自由电子密度达到临界自由电子密度的时刻.入射激光脉冲能量决定了临界时间点在脉冲作用时间上的位置,临界时间点的位置决定了激光脉冲的能量透过率.可以通过测量激光脉冲的能量透过率来计算出临界自由电子密度,从而确定出激光脉冲在空气中聚焦时的能量透过特性.
关键词:
临界自由电子密度
临界时间点
多光子电离
逆韧致吸收 相似文献
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激光在厚原子蒸气介质中传播时会产生脉冲形变和延迟现象,这会直接影响原子多步光电离过程中的电离率和选择性.从原子蒸气激光同位素分离的实际出发,对厚原子蒸气介质中的原子光电离过程进行了研究,利用密度矩阵方法描述原子的光电离过程,利用Maxwell方程描述激光在厚介质中的传播,建立了介质中同时存在两种同位素的激光传播电离方程组,考察了原子蒸气参数和激光参数对厚介质中平均电离率和平均选择性的影响.研究结果表明:对于较厚的原子蒸气介质,激光功率的增加使平均电离率升高,平均选择性下降;对于相对较薄的介质,适当地降低激光功率可以同时提高平均电离率和平均选择性.存在一个正的激光延时使原子蒸气中目标同位素的平均电离率达到最大.尽量延长激光脉冲的宽度不仅可以同时提高目标同位素的平均电离率和平均选择性,还可以降低对激光脉冲之间相对延时的控制精度. 相似文献
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针对宁西京教授提出激光脉冲延时方案,在含时薛定谔方程理论框架下探讨了各种参数对激光共振电离效率的影响;并以镥原子共振电离为例,探讨了该方案用于激光共振电离质谱(LRIMS)同位素分析的可行性.结果表明:由于超精细结构能级激励拉比频率很难满足匹配条件,该方案很难用于宽带激光激励电离LRIMS同位素分析中.在理论分析基础上,给出了基于窄带激光激励电离的LRIMS同位素分析的光致电离方案. 相似文献
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为了进一步理解极端条件下物质的电离特性, 特别是超短超强激光脉冲辐照超薄靶时等离子体的形成与分布, 本文以超薄碳膜为例, 细致研究了超短超强激光脉冲辐照下原子的离化过程. 分析和比较了强激光场直接作用电离和靶内静电场电离等两种场致电离形式, 在碰撞电离可以忽略的情况下, 发现更多的电离份额是来自靶内静电场的电离方式. 研究了激光脉冲强度对电离的影响, 发现激光脉冲强度越强, 电离速度越快, 产生的高价态离子所占比例也越高.当激光强度为1×1020 W/cm2时, 尽管该强度高于电离生成C+6所需要的激光强度阈值, 但该激光脉冲并不能将整个靶电离成C+6离子, 对此本文进行了详细的分析. 在研究激光脉冲宽度的影响时, 发现激光脉宽越小, 电离速度越快, 但越小的激光脉冲电离获得的高价态离子越少. 相似文献
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采用分子动力学方法模拟研究了激光诱导金属薄膜的电离过程,对激光等离子体形成早期原子的运动轨迹、薄膜表面的温度变化以及原子的电离特性进行了详细分析,并探究了脉冲激光参数对原子电离过程的影响.结果表明,在激光照射过程中,薄膜表面先熔化而后又气化,气化的原子继续吸收激光能量继而电离.激光的峰值功率密度越大,原子电离速率越快,电离数目越多,薄膜表面的温度越高.脉冲宽度越小,原子电离速率越快,薄膜表面的温度越高,但原子的电离数目先增加后减小. 相似文献
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采用三维粒子动力学模拟方法研究了甲烷团簇在超短强激光脉冲激励下的爆炸动力学行为,重点讨论了几种典型的内电离机理对团簇爆炸过程中离子的价态和动能的影响.研究表明,在激光脉冲强度比较小的情况下,团簇中的原子主要是在光场作用下通过隧道电离的方式发生电离.当激光场进一步增强时,势垒压低电离是电离的主要方式.在相同的较高激光强度下,团簇更容易通过势垒压低电离达到高的电离价态.团簇发生电离后,其内部库仑电场的点火电离效应和内部滞留自由电子的碰撞电离效应也将增强团簇的再次电离过程.
关键词:
超短强激光脉冲
甲烷团簇
内电离 相似文献
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Deng S Barnes CD Clayton CE O'Connell C Decker FJ Fonseca RA Huang C Hogan MJ Iverson R Johnson DK Joshi C Katsouleas T Krejcik P Lu W Mori WB Muggli P Oz E Tsung F Walz D Zhou M 《Physical review letters》2006,96(4):045001
The propagation of an intense relativistic electron beam through a gas that is self-ionized by the beam's space charge and wakefields is examined analytically and with 3D particle-in-cell simulations. Instability arises from the coupling between a beam and the offset plasma channel it creates when it is perturbed. The traditional electron hose instability in a preformed plasma is replaced with this slower growth instability depending on the radius of the ionization channel compared to the electron blowout radius. A new regime for hose stable plasma wakefield acceleration is suggested. 相似文献
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The propagation characteristics of an intense laser beam in a preformed plasma channel with the flat-bottom leaky density profile are investigated in detail. The evolution equation of the laser spot size is derived by employing variational technique. Seven propagation modes of the laser spot size are identified and some numerical results are presented. By comparison, we find that the results in this Letter may be more realistic since the flat-bottom leaky plasma channel comes closer to the practical plasma channel. 相似文献
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De Wispelaere E Malka V Hüller S Amiranoff F Baton S Bonadio R Casanova M Dorchies F Haroutunian R Modena A 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》1999,59(6):7110-7120
We report on a detailed study of channel formation in the interaction of a nanosecond laser pulse with a He gas jet. A complete set of diagnostics is used in order to characterize the plasma precisely. The evolution of the plasma radius and of the electron density and temperature are measured by Thomson scattering, Schlieren imaging, and Mach-Zehnder interferometry. In gas jets, one observes the formation of a channel with a deep density depletion on axis. Because of ionization-induced defocusing which increases the size of the focal spot and decreases the maximum laser intensity, no channel is observed in the case of a gas-filled chamber. The results obtained in various gas-jet and laser conditions show that the channel radius, as well as the density along the propagation axis, can be adjusted by changing the laser energy and gas-jet pressure. This is a crucial issue when one wants to adapt the channel parameters in order to guide a subsequent high-intensity laser pulse. The experimental results and their comparison with one-dimensional (1D) and two-dimensional hydrodynamic simulations show that the main mechanism for channel formation is the hydrodynamic evolution behind a supersonic electron heat wave propagating radially in the plasma. It is also shown from 2D simulations that a fraction of the long pulse can be self-guided in the channel it creates. The preliminary results and analyses on this subject have been published before [V. Malka et al., Phys. Rev. Lett. 79, 2979 (1997)]. 相似文献
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P K KARMAKAR 《Pramana》2011,76(6):945-956
The pulsational mode of gravitational collapse (PMGC) in a hydrostatically bounded dust molecular cloud is responsible for
the evolution of tremendous amount of energy during star formation. The source of free energy for this gravito-electrostatic
instability lies in the associated self-gravity of the dispersed phase of relatively huge dust grains of solid matter over
the gaseous phase of background plasma. The nonlinear stability of the same PMGC in an infinite dusty plasma model (plane
geometry approximation for large wavelength fluctuation in the absence of curvature effects) is studied in a hydrostatic kind
of homogeneous equilibrium configuration. By the standard reductive perturbation technique, a Korteweg–de Vries (KdV) equation
for investigating the nonlinear evolution of the lowest order perturbed self-gravitational potential is developed in a time-stationary
(steady-state) form, which is studied analytically as well as numerically. Different nonlinear structures (soliton-like and
soliton chain-like) are found to exist in different situations. Astrophysical situations, relevant to it, are briefly discussed. 相似文献