Optical absorption in early stage low temperature laser produced plasma

We describe a new technique to measure the UV/visible absorption spectrum of the ablated material during the laser pulse. The technique utilizes the continuum emission from one laser produced plasma as a light source to measure the absorption properties of a second laser produced plasma which is formed on a semi-transparent target with an array of 40 μm holes. A 6 ns, 1064 nm laser was used to ablate a Ag target and the plasma absorption was measured in the range 450–625 nm for a laser fluence of 1 J cm⁻². The total absorption cross-section is (0.5–1.5)×10⁻¹⁷ cm² in the range 450–540 nm. By comparing the measured absorption with a calculation using the plasma spectroscopy code FLYCHK it can be concluded that, in the wavelength region examined here, the absorption is mainly due to bound-bound transitions.     

High order harmonic generation by non-linear reflection of a pedestal-free intense laser pulse on a plasma

We demonstrate the influence of the prepulses and ASE of ultrashort pulses interacting with a solid target by addressing the direct comparison of the harmonic spectra generated by reflection onto a solid target with and without the introduction of a plasma mirror system. Harmonics up to the 20th of the fundamental of the Ti-Sa laser are clearly visible in a situation free from any plasma expansion. PACS 42.65. Ky; 52.38.-r; 52.38.Ph; 52.50.Jm     

Transient reflectivity and transmission changes during plasma formation and ablation in fused silica induced by femtosecond laser pulses

We have studied the plasma formation and ablation dynamics in fused silica upon irradiation with a single 120 fs laser pulse at 800 nm by using fs-resolved pump-probe microscope. It allows recording images of the laser-excited surface at different time delays after the arrival of the pump pulse. This way, we can extract both the temporal evolution of the surface reflectivity and transmission, at 400 nm, for different spatial positions in the spots (and thus for different local fluences) from single series of images. At fluences well above the visible ablation threshold, a fast and large increase of the reflectivity is induced by the formation of a dense free-electron plasma. The maximum reflectivity value is reached within ≈1.5 ps, while the normalized transmission decreases within ≈400 fs. The subsequent temporal evolution of both transient reflectivity and transmission are consistent with the occurrence of surface ablation. In addition, the time-resolved images reveal the existence of a free-electron plasma distribution surrounding the visible ablation crater and thus formed at local fluences below the ablation threshold. The lifetime of this sub-ablation plasma is ≈50 ps, and its maximum electron density amounts to 5.5×10²² cm⁻³.     

A dust plasma

A dust plasma was created using spores in a glass discharge tube. The dust particles become negatively charged, levitate in the positive plasma potential, and behave like massive electrons. A laser beam is used to make the dust visible, revealing plasma structure, such as the wall sheath and the direction of the local electric field     

准分子激光等离子体开关控制脉宽研究

 利用准分子激光等离子体技术,在紫外预电离XeCl准分子激光器上获得了最短1.58 ns的短脉冲激光输出。实验中分析了聚焦到薄膜表面的光束能量密度对所产生的等离子体密度的影响,并对不同等离子体密度及维持时间情况下脉冲压缩效果进行了讨论,给出了激光器谐振腔在稳定腔及非稳腔两种工作方式下的实验结果。激光器在稳定腔工作时,脉宽可压缩至2.87 ns;采用非稳腔结构时,在脉冲能量不变情况下减小聚焦光斑面积,提高入射到薄膜表面的能量密度,得到了最短1.58 ns的短脉冲激光输出。该技术适用于任何其它准分子器件。     

Nd-Glass laser with plasma mirror

A high power pulse Nd-glass laser system with plasma mirror is studied. Plasma is created on the surface of a solid target and the action is superradiantly triggered. Long trains of modelocked pulses are generated if the carbon or metal targets are used and Q-switched pulses are observed with dielectric targets like PVC. Tens of joules of light energy are extracted from the laser medium and absorbed by plasma in both regimes. Spatial and temporal structures of the laser beam are very reproducible, the laser action is insensitive to variations in air pressure inside the target chamber. Stimulated Brillouin back-scattering in the underdense plasma is discussed as the nonlinear mechanism governing the plasma reflectivity.     

激光诱导镍等离子体的自吸收时间分辨特性研究

激光诱导击穿光谱(LIBS)技术是一种新兴的元素分析技术,但自吸收效应对LIBS测最的影响较大.文中利用Nd:YAG激光器产生的基频1 064 nm脉冲激光在空气中击穿镍靶产生等离子体,观测了四条跃迁对应同一电子组态(3d<'9>(<'2>D)4p-3d<'9>(<'2>D)4s)谱线Ni I 341.476/351.034/351.505/352.454 nm的自吸收现象.实验发现,谱线Ni I 351.034 nm没有出现自吸收现象,其下能级电子组态为3d9(<'2>D)4s的各能级中能量最高的<'3>D,态.对于其他三条谱线,在等离子体辐射初期白吸收较为严重,随着等离子体的演化,自吸收减弱.不同谱线的自吸收持续时间不同,其中谱线Ni I 352.454 nm自吸收最为严重,且当门延时为1 100 ns时仍存在明显自吸收现象,而NiI 341.476/351.505 nm的自吸收持续时间分别为900和500 ns.自吸收现象随着激光脉冲能量的增加而减弱.结果表明通过选择合适的谱线、激光脉冲能量和较长的探测门延时可以有效避免自吸收现象对LIBS测量的影响.文章还对不l司谱线自吸收持续时间小同的原因进行了讨论.     

基于激光等离子体加速的自由电子激光研究新进展

激光等离子体加速器能够在cm尺度内产生GeV量级的高品质电子束,为研制台式化自由电子激光提供驱动源。但是受限于激光等离子体加速中的难点和现有技术发展,电子束的品质难以达到自由电子激光的需求,尤其在稳定性、发散角和能散等方面,阻碍了台式化自由电子激光的研制。介绍了基于激光等离子体加速器的自由电子激光的最新进展,整理了目前高增益自由电子激光实验过程中存在的主要挑战和对应的解决方案与实验进展,并展望未来的发展方向。最近的研究结果证明,通过控制和优化激光等离子体加速器的注入和加速过程产生的高品质电子束可以在指数增益区域实现自发辐射放大,产生高增益的辐射,这也推动基于激光等离子体加速器的自由电子激光研究进入了一个新的阶段。     

Cross-focusing of two laser beams in a plasma

Cross-focusing of two copropagating laser beams in a plasma is investigated using paraxial ray theory. If the lasers have a frequency difference equal to the electron plasma frequency, they can drive a large amplitude plasma wave. The ponderomotive force due to the plasma wave forces the plasma electrons outwards thereby generating a parabolic density profile giving rise to cross-focusing. The results show a decrease in threshold for focusing by two orders of magnitude as compared to focusing due to the ponderomotive force of the laser beams.     

Generation of autonomous long-lived plasma objects in free atmosphere

Results are presented from experimental studies of autonomous long-lived plasma objects in free atmosphere with a visible afterglow lasting 2 s. The experimental setup is described, and the energy conditions ensuring the generation of such plasma objects are determined.     

Electron-beam-controlled CO2 laser with plasma mirror

A new high-power electron-beam-controlled CO₂ laser scheme, based on the use of the molecular properties of a high-temperature laser plasma (plasma mirror) produced by the action of the laser radiation on the surface of a solid target and serving as one of the mirrors of the laser resonator, is investigated in detail. The scheme makes it possible to generate laser pulses of nanosecond duration with power of several dozen gigawatts and efficiency 6–10% and obtain a laser plasma with a temperature of several million degrees. The possibility of practical utilization, in applications, of a plasma mirror as a powerful source of soft x rays is demonstrated.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 116, pp. 118–145, 1980.     

Spectral-time characteristics of combined-target laser plasma

From the analysis of the contours of the spectral lines of target-material atoms, the electron and atomic concentrations and their variation with time are determined in the laser plasma of a combined Cu–Al–Cu target. The time dependences for the concentrations of electrons and atoms in the ground and excited states are explained within the framework of a model that allows for plasma decay being determined by processes of three-particle recombination and ionization as well as by the variation in the particle concentration in plasma expansion. Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 3, pp. 426–432, May–June, 1998.     

Adaptive control of femtosecond laser ablation plasma emission

The influence of temporal pulse shaping on plasma plume generated by ultrafast laser irradiation of aluminum is investigated. Time resolved plasma emission spectroscopy is coupled with a temporal shaping procedure in a closed loop. The ionic emission is enhanced relative to the neutral one via an adaptive optimization strategy. The plasma emission efficiency in case of optimized and ultrashort temporal shapes of the laser pulses are compared, evidencing an enhancement of the ionization degree of the plasma plume. Simplified temporal shapes of the femtosecond laser pulses are extracted from the optimized shape and their corresponding effect on laser induced plasma emission is discussed.     

激光等离子体温度时间演化特性

 用脉冲CO₂激光的10.6 μm光束击穿空气产生等离子体,使用光谱仪和ICCD采集等离子体辐射光谱,在局部热力学平衡近似下,利用相对谱线强度法对激光等离子体温度进行了计算。当激光器单脉冲能量为35 J时,选择NⅡ399.5 nm和NⅡ500.5 nm两条线状谱的相对谱线强度计算了不同延迟时间下等离子体温度。实验结果表明:在等离子体的不同位置,等离子体温度均随时间经历了明显的上升到饱和再到下降的过程,等离子体前沿的温度最先达到饱和,距离靶面最近的位置温度达到饱和所需时间最长。     

用于等离子体诊断的ps激光探针

 ps激光探针作为激光等离子体诊断的探针光源，它是通过两次倍频和两次受激喇曼散射，将波长为1 054nm、脉宽约为1ns激光转换成波长为308nm、脉宽小于30ps的紫外光。研究结果表明：探针光系统输出能量大于1mJ，脉宽小于30ps，均匀性较好，运行成功率大于90%，满足了激光等离子体诊断的要求。     

Subnanosecond collapse and pinch dynamics at the plasma focus

Compression and pinch phase at the plasma focus were investigated by streak camera observations of the visible emission. The duration of maximum local compression provides no evidence for magnetic confinement of the focus plasma. With increasing hard X-ray intensity a decreasing of the temporal evoution of the pinch phase is observed, indicating enhanced diffusion of magnetic field energy into the locally compressed plasma.     

Oscillating two-stream instability of laser wakefield-driven plasma wave

The laser wakefield-driven plasma wave in a low-density plasma is seen to be susceptible to the oscillating two-stream instability (OTSI). The plasma wave couples to two short wavelength plasma wave sidebands. The pump plasma wave and sidebands exert a ponderomotive force on the electrons driving a low-frequency quasimode. The electron density perturbation associated with this mode couples with the pump-driven electron oscillatory velocity to produce nonlinear currents driving the sidebands. At large pump amplitude, the instability grows faster than the ion plasma frequency and ions do not play a significant role. The growth rate of the quasimode, at large pump amplitude scales faster than linear. The growth rate is maximum for an optimum wave number of the quasimode and also increases with pump amplitude. Nonlocal effects, however reduce the growth rate by about half.     

激光等离子体相互作用中的两级质子加速（英文）北大核心CSCD

研究了激光辐射压驱动的两级质子加速的相关问题。当超短超强激光脉冲与处在背景等离子体前方的薄固体平靶相互作用时,在固体靶后部形成一个电子层-离子层组成的双层结构。在激光的不断推进下,双层结构在背景等离子体里以一定速度传播,可以看成运动在背景等离子体中的电场。这样,在背景等离子体中的质子被这个运动电场捕获并能加速到很高的能量。通过二维PIC模拟方法和理论分析研究了质子加速的相关问题。研究结果表明,被加速质子的最大能量达到20GeV。     

激光等离子体相互作用的数值模拟

 基于激光等离子体相互作用的复杂物理过程的数学模型，采用PIC方法分别研究了P极化和S极化非均匀短脉冲强激光入射均匀分布的稠密等离子体时引起的空泡、成丝等物理现象。模拟了激光脉冲在真空中的3维传播形貌。由3维密度分布图发现：激光产生的巨大的有质动力向两侧推动粒子，形成等离子体密度通道；当激光脉冲入射等离子体区域后，纵向加速的电子速度峰值出现在电流峰值处。     

Self-action of laser radiation in cluster plasma

We have analytically and numerically studied the self-action dynamics of laser radiation in a plasma with ionized gas clusters. Based on the simplified model of a cluster in the form of a superposition of two charged (electron and ion) bunches, we analyze the nonlinearity mechanisms. We refine the electrodynamic cluster model by the molecular dynamics method. The polarization behavior of the plasma bunch in the main part of the laser pulse is shown to be the same as that in the simplified model. We investigate the self-action dynamics of laser radiation under conditions when the nonlinearity of the stratified medium is determined by the anharmonicity of the electron motion in the cluster, while the group velocity dispersion is determined by both the background plasma and the ionized clusters. Since the characteristic field for the electron nonlinearity depends strongly on the cluster size, the peculiarities of the self-action dynamics result from plasma bunch expansion. The spatiotemporal evolution of the wave field is shown to be accompanied by pulse self-compression near the trailing edge.