气体压强对纳秒激光诱导空气等离子体特性的影响

环境气体的压强对激光诱导等离子体特性有重要影响.基于发射光谱法开展了气体压强对纳秒激光诱导空气等离子体特性影响的研究,探讨了气体压强对空气等离子体发射光谱强度、电子温度和电子密度的影响.实验结果表明,在10-100 kPa空气压强条件下,空气等离子体发射光谱中的线状光谱和连续光谱依赖于气体压强变化,且原子谱线和离子谱线强度随气体压强的变化有明显差别.随着空气压强增大,激光击穿作用区域的空气密度增加,造成激光诱导击穿空气几率升高,从而等离子体辐射光谱强度增大.空气等离子体膨胀区域空气的约束作用,增加了等离子体内粒子间的碰撞几率以及能量交换几率,并且使离子-电子-原子的三体复合几率增加,因此造成原子谱线OⅠ777.2 nm与NⅠ821.6 nm谱线强度随着气体压强增大而增大,在80 kPa时谱线强度最高,随后谱线强度缓慢降低.而离子谱线N Ⅱ 500.5 nm谱线强度在40 kPa时达到最大值,气体压强大于40 kPa后,谱线强度随压强增加而逐渐降低.空气等离子体电子密度均随压强升高而增大,在80 kPa后增长速度变缓.等离子体电子温度在30 kPa时达到最大值,气体压强大于30 kPa后,等离子体电子温度逐渐降低.研究结果可为不同海拔高度的激光诱导空气等离子体特性的研究提供重要实验基础,为今后激光大气传输、大气组成分析提供重要的技术支持.     

基于等离子体锥形空间约束对土壤中Cr的LIBS研究

为了提高激光诱导击穿光谱(LIBS)技术对土壤中重金属元素的检测灵敏度,降低检测限,以国家标准土壤中Cr元素为研究对象,实验时,在LIBS装置中与样品接近的聚焦透镜下方安装一个锥形罩,锥形罩的小端面直径为20 mm,大端面直径为45 mm,目的是对等离子体发射信号形成空间约束,并且在一定程度上约束等离子体本身。实验得到CrⅠ425.44 nm的最佳延迟时间为1.3 μs,相对标准偏差低于10%。与无锥形空间约束的装置相比,Cr的特征光谱强度增强了7%以上。以土壤中的Cr在60～400 μg·g-1之间的浓度建立定标曲线,有空间约束下定标曲线的线性拟合相关系数为0.997 71,Cr的检测限为18.85 μg·g-1,而没有空间约束下定标曲线的线性拟合相关系数为0.991 22,Cr的检测限为36.99 μg·g-1。由此表明锥形空间约束能够提高目标元素的灵敏度和光谱强度,在LIBS技术检测土壤中Cr元素时有很好的辅助作用。     

High Coupling Efficiency Generation in Water Confined Laser Plasma Propulsion

High coupling efficiency generation in water confined laser plasma propulsion is investigated. It is found that the coupling efficiency is enhanced over thirty times in water confined ablation compared to that of direct ablation. From calculation of the ablation pressure induced by the plasma on the target surface, it is realized that high coupling efficiency is attributed to the confinement of the water layer on the plasma expansion.     

Numerical and theoretical calculation of breakdown voltage in the electrical discharge for rare gases

This work has been devoted to a numerical and analytical calculus of the voltage breakdown in electrical discharge for several rare gases such as argon, krypton, neon, xenon and helium. It was performed using a fluid model 2D, which is based on the numerical solution of the two Boltzmann equations (equation of continuity and momentum), coupled to Poisson's equation to measure the breakdown voltage according to the product of the electrode spacing and the pressure. This study allowed a better comprehension of the physical phenomena occurring in the discharges. We, thus, developed a calculation, based on the empirically Paschen's law, allowing the determination of the breakdown voltage, which describes the transition from insulating gas to the conductive state. Paschen's curves of the different gases are plotted and a comparison between numerical and experimental as well as analytical results is also presented and analyzed.     

样品温度和空间约束两种方法相结合对激光诱导击穿光谱的影响

升高样品温度和采用空间约束能提高激光诱导击穿光谱的信号强度,两种技术的结合可以进一步提高激光诱导击穿光谱的光谱强度.本文在空气环境中研究了升高样品温度和空间约束效应两种方法相结合对激光诱导击穿光谱的影响,测量了激光诱导铝等离子体的时间分辨光谱.实验结果表明:升高样品温度能增加激光诱导击穿光谱的信号强度,高温样品能耦合更...     

Numerical investigation of a fast-axial-flow CW CO2 laser

This paper presents the results of the calculation of the parameters of the active medium of a fast-axial-flow CO₂ laser using numerical methods in the framework of a one-dimensional approximation of the set of continuity equations, Bernoulli equation, equation of gas state, energy equation and multi-temperature rate equations with regard to diffusion for the gas flow in the cylindrical discharge tube. The spatial distribution of the small-signal gain and gas temperature along the gas flow direction have been calculated for a given set of initial conditions, namely, gas flow velocity, gas pressure and the tube diameter. In addition, the dependence of small-signal gain, the asymmetric stretch vibrational temperature of CO₂ (T₃) and the gas temperature on the discharge current were studied.     

Short-channel drain current model for asymmetric heavily / lightly doped DG MOSFETs

Performance characteristics of an excimer laser (XeCl) with single-stage magnetic pulse compression suitable for material processing applications are presented here. The laser incorporates in-built compact gas circulation and gas cooling to ensure fresh gas mixture between the electrodes for repetitive operation. A magnetically coupled tangential blower is used for gas circulation inside the laser chamber for repetitive operation. The exciter consists of C–C energy transfer circuit and thyratron is used as a high-voltage main switch with single-stage magnetic pulse compression (MPC) between thyratron and the laser electrodes. Low inductance of the laser head and uniform and intense pre-ionization are the main features of the electric circuit used in the laser. A 250 ns rise time voltage pulse was compressed to 100 ns duration with a single-stage magnetic pulse compressor using Ni–Zn ferrite cores. The laser can generate about 150 mJ at ～100 Hz rep-rate reliably from a discharge volume of 100 cm ³. 2D spatial laser beam profile generated is presented here. The profile shows that the laser beam is completely filled with flat-top which is suitable for material processing applications. The SEM image of the microhole generated on copper target is presented here.     

吸气式激光推进中激光能量沉积过程的数值模拟

 结合辐射输运方程,在流体力学方程组的能量方程中加入包括空气吸收的激光能量以及高温气体向周围辐射损失的能量源项,转化为辐射流体力学方程组,建立了用于模拟吸气式激光推进中能量沉积过程的物理力学模型和计算方法。该辐射流体力学计算程序可以很好地模拟激光能量沉积过程中空气对激光能量的吸收、等离子体对激光的屏蔽作用以及激光维持的爆轰波的传播规律,计算得到激光能量的沉积效率约为57%,激光维持的爆轰波的传播速度与同等条件下的理论和实验结果吻合得较好。     

Application of proton radiography in experiments of relevance to inertial confinement fusion

Multi-Mev proton beams generated by target normal sheath acceleration (TNSA) during the interaction of an ultra intense laser beam (I≥10¹⁹ W/cm²) with a thin metallic foil (thickness of the order of a few tens of microns) are particularly suited as a particle probe for laser plasma experiments. The proton imaging technique employs a laser-driven proton beam in a point-projection imaging scheme as a diagnostic tool for the detection of electric fields in such experiments. The proton probing technique has been applied in experiments of relevance to inertial confinement fusion (ICF) such as laser heated gasbags and laser-hohlraum experiments. The data provides direct information on the onset of laser beam filamentation and on the plasma expansion in the hohlraum’s interior, and confirms the suitability and usefulness of this technique as an ICF diagnostic.     

激光加热金属板流固耦合数值模拟

 数值模拟研究了气流和激光辐照双重作用下的金属平板温度场。流体控制方程为3维雷诺平均N-S方程,固体控制方程为能量方程,湍流粘性系数求解使用k-ε两方程模型。采用流固耦合计算方法,使用动网格模型模拟气流的“冲刷效应”,较完整地模拟了激光辐照金属材料的物理变化过程,计算得到金属平板的温度分布及流场分布。结果表明:在较低气流速度及激光功率下,气流的冷却效应占主导地位;当气流速度和激光功率上升到一定程度后,气流的“冲刷效应”和冷却效应共同决定金属平板的温度分布。     

ArF准分子激光系统的能量效率特性

为深入理解ArF准分子激光系统的运转机制,进而获得优化ArF准分子激光系统设计的理论及方向性指导,利用一维流体模型,以气体高压放电等离子体深紫外激光辐射过程为主要对象,研究了放电抽运ArF准分子激光系统的动力学特性,梳理了ArF准分子激光系统的能量传递过程,深入研究了等离子体放电机理,从能量沉积效率、ArF*粒子形成过程、激光输出三个方面,分析了动力学过程中影响能量效率的主要因素,提出了相应的改进优化措施.仿真结果表明,氟气及相关粒子在系统运转过程中有重要作用,工作气体中氟气的组分比例对能量效率影响较大,偏离最佳点会导致激光系统能量效率的下降.相关结论为ArF准分子激光系统的优化设计和稳定可靠运转提供了重要的理论参考依据.     

超短脉冲激光在电介质材料中传输及破坏深度微观理论计算

基于Fokker-Planck方程和激光传输方程建立超短脉冲激光在电介质材料中的传输及材料破坏理论模型,计算材料内不同位置处导带电子数密度及激光电场强度随时间的变化,进而得到激光的反射率、透射率及沉积率随激光能量密度的变化特征.选取导带电子数密度阈值作为材料破坏的判断条件,计算了不同激光能量密度下的破坏深度,发现破坏深度随激光能量密度的变化曲线呈现先增长后减小,讨论了激光能量沉积特性对破坏深度的影响.计算最大破坏深度随激光脉宽的变化发现,激光脉宽越短则最大破坏深度越小. 关键词： 超短脉冲激光 电介质材料 破坏深度 微观理论模型     

Propagation of intense laser pulses through inhomogeneous ionizinggas profiles

By simultaneously and self-consistently solving Maxwell's equations, the Ammosov-Delone-Krainov (ADK) field ionization equation, and the relativistic cold plasma equations, we have investigated the propagation of intense, ultrashort laser pulses through spatially inhomogeneous longitudinal gas gradients. Along with highly accurate calculations of the spatial and temporal beam profiles of the pulse at the end of various gradients, we have also determined simple scaling rules for the location of the vacuum-gas interface in order to minimize the pulse distortion at the focus. We show the benefits of using either preionized or low-Z gases, and we discuss the implications of this work for plasma-channel laser wakefield acceleration     

Spatial and Temporal Characteristics of an Excimer Laser-Induced Lead Plasma Emission

A laser-induced plasma was generated from a lead target using an ArF excimer laser (λ = 193 nm) and characterized by time-resolved and time-integrated spatially resolved spectrometry. The ambient atmosphere (gas composition and pressure) influenced the emission intensity for both atomic and ionic lines. The emission of laser-induced lead plasma varied with time as well as the location in the plasma. Lead ion emission decayed more rapidly than lead atomic line emission. High excitation temperatures and nonlinear optical phenomena were observed in the laser-induced lead plasma. Gas breakdown and the subsequent shielding effect of the incident laser beam in different ambient gas compositions and pressures were discussed to explain the different efficiency of metal ion for matioa in the plasma. The experimental results show that spatial discrimination of the laser-induced plasma emission is desirable for direct spectrochemical analysis.     

Influence of Gap Spacing between Dielectric Barriers in Atmospheric Pressure Discharges

The breakdown activity in helium atmospheric pressure dielectric barrier discharge (DBD) plasma is strongly modified by introducing small impurities (nitrogen (N₂) and air in ppm), although its precise implications for the behavior of discharge plasma is not evident under several constraints. In this simulation study, we investigate the influence of gap spacing between the dielectric barriers to explore the dynamic modification in the structure of discharge plasma in distinct phases of the discharge current pulse using a two‐dimensional fluid model in He‐air gas mixture. Specifically, the impact of nitrogen and air impurities is contrasted by exploring the spatial distributions of electrons in the breakdown phase under similar operating conditions. The filamentary mode of DBD plasma in He‐N₂ is transformed into uniform glow discharge in He‐air gas mixture by the dominant effect of Penning ionization. Finally, the outcomes of two‐dimensional fluid model are validated by comparing with three‐dimensional fluid model to provide the reliability of numerical simulations. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Self-Focusing and de-Focusing of Intense Left- and Right-Hand Polarized Laser Pulse in Hot Magnetized Plasma in the Presence of an External Non-Uniform Magnetized Field

In this paper, self-focusing of an intense circularly polarized laser beam in the presence of a non-uniform positive guide magnetic field with slope constant parameter δ in hot magnetized plasma, using Maxwell’s equations and relativistic fluid momentum equation is investigated. An envelope equation governing the spot-size of laser beam for both of left- and right-hand polarizations has been derived, and the effects of the plasma temperature and magnetic field on the electron density distribution of hot plasma with respect to variation of normalized laser spot-size has been studied. Numerical results show that self-focusing is better increased in the presence of an external non-uniform magnetic field. Moreover, in plasma density profile, self-focusing of the laser pulse improves in comparison with no non-uniform magnetic field. Also, with increasing slope of constant parameter of the non-uniform magnetic field, the self-focusing increases, and subsequently, the spot-size of laser pulse propagated through the hot magnetized plasma decreases.     

激光聚焦位置对半球腔约束等离子体光谱增强特性的影响

光谱信号增强是提高激光诱导击穿光谱技术分析性能的重要手段之一,对等离子体进行空间约束由于装置简单且约束效果好而常被采用,等离子体的特性会直接影响空间约束的效果,而等离子体的特性与实验系统中激光的聚焦情况密切相关,为研究激发光源的聚焦情况对半球形空腔约束等离子体光谱增强特性的影响,通过控制透镜到样品之间的距离(LTSD)来改变激光的聚焦位置,分别在无约束和有半球形空腔约束两种实验条件下,烧蚀合金钢产生等离子体,采集15个不同LTSD位置时等离子体的时间演变光谱,得到谱线强度和增强倍数随着LTSD和采集延时的二维空间分布图。研究结果发现：无约束情况下,谱线强度分别在LTSD为94和102 mm时出现峰值,在采集延时小于8 μs时,谱线强度的最大值在LTSD为94 mm的位置,采集延时大于8 μs后,谱线强度的最大值出现在LTSD为102 mm的位置;当用半球空腔约束等离子体,谱线强度先后在采集延时范围为4~10和12~15 μs出现第一次增强和第二次增强。谱线强度出现第二次增强的主要原因是被半球腔内壁反射的冲击波与等离子体相互作用后会继续向前传播,遇到另一侧的腔壁再次被反射,进而对等离子体产生二次压缩。分析增强倍数随LTSD和采集延时的二维变化关系发现,第一次增强的最大增强倍数随LTSD的变化没有明显规律,增强倍数在2~6之间波动;谱线第二次增强时的增强倍数相对较高,最大增强倍数随着LTSD变化呈现出先增大再减小,然后再小幅增加后降低的变化规律,在LTSD为96 mm时达到最大值,两条谱线的最大增强倍数约为6倍。分析出现最大增强倍数对应的延迟时间发现,第一次增强出现的最优延迟时间在6~9 μs之间变化,当LTSD在85~93 mm范围时,最优延迟时间保持不变,当LTSD在94~105 mm时,出现先降低再增大的变化规律;第二次增强出现的延迟时间主要在14~15 μs,随着LTSD的变化没有明显的变化规律。     

Investigation of Hydrodynamic Properties of Hot Dense Plasma

Hydrodynamic properties of hot dense plasma bunch have been studied in terms of their application to the problem of inertial confinement fusion. The two-temperature hydrodynamic equations taking into account the electron heat conduction and kinetics of the charge composition are presented. The following model problems are considered: expansion of a plasma bunch in a vacuum, compression of the plasma by shock waves, interaction of the plasma bunch with a laser radiation and a beam of heavy ions. The some problems arising in the problem of inertial confinement fusion are analyzed on the basis of the developed numerical methods.     

超短脉冲激光与微小水滴相互作用中电子密度和光场的时空分布

为了研究超短激光脉冲和液滴相互作用过程中电子密度和光场的变化,基于非线性麦克斯韦方程组和电离速率方程,构建了激光等离子体非线性瞬态时域耦合模型,对飞秒激光脉冲击穿微米量级水滴时的电子密度和光场的时空分布进行了计算.结果显示水滴的击穿阈值最小可达2 TW/cm~2,为同等条件下无边界水介质击穿阈值的1/4.随着脉冲能量增强,水滴内自由电子密度峰值区域逆着激光入射方向移动,且入射光越强,水滴对光传播的屏蔽越明显.光束在水滴出射端外部汇聚,汇聚点的光功率密度可达入射光的5倍,且时域波形出现压缩和变形.另外,水滴对激光能量的吸收系数随光强增大而增大,并最终趋于饱和.