Plasma properties of laser—ablated Si target in air

In plasma emission spectra produced by pulsed laser ablation of Si target in air under the assumption of local thermodynamic equilibrium(LTE),the electron temperature and the electron number density are calculated.respectively,It seems that LTE is valid in early stage of the laser induced plasma evolution.     

Micro—beam XRF localization by a laser beam

A new method for micro-beam XRF localiztion is presented.A laser beam along with an incident X-ray hits on the surface of a sample.The micro region on the sample that reached by X-ray beam can be localized by means of the visible spot of the laser beam.This method is suitable for X-ray microprobes using an X-ray tube or synchrotron radiation as excitation sources.     

Experimental observation of hot electrons produced by laser with small scale—length and shorter wavelength

The experiments on gold-disk and hohlraum targets irradiated by laser beams with wavelength of 0.35(Xingguang--Ⅱ） and 0.530μm(Shenguang-I) are performed.The characteristics of hot electrons and the SRS( Stimulated Raman light scattering)produced by shorter wavelength laser are experimentally studied.Associated with the measurement of backwared SRS,the production mechanism of hot electrons is preliminarily analyzed in laser plasma with shorter wavelength.     

Transmission Degradation of Femtosecond Laser Pulses in Opened Cone Targets

Irradiated by femtosecond laser pulses with different energies, opened cone targets behave very differently in the transmission of incident laser pulses. The targets, each with an opening angle of 71 and an opening of 5 μm, are fabricated using standard semiconductor technology. When the incident laser energy is low and no pre-plasma is generated on the side walls of the cones, the cone target acts like an optical device to reflect the laser pulse, and 15% of the laser energy can be transmitted through the cones. In contrast, when the incident laser energy is high enough to generate pre-plasmas by the pre-pulse of the main pulse that fills the inner cone, the cone with the plasmas will block the transmission of the laser, which leads to a decrease in laser transmission compared with the low-energy case with no plasma. Simulation results using optical software in the low-energy case, and using the particle-in-cell code in the high-energy case, are primarily in agreement with the experimental results.     

The acceleration mechanism of shock wave induced by millisecond-nanosecond combined-pulse laser on silicon

The velocity variation law of shock wave induced by millisecond-nanosecond combined-pulse laser has been investigated experimentally. The pulse delay and laser energy are important experimental variables. The method of laser shadowgraphy is used in the experiment.Experimental results show that when the pulse delay is 2.4 ms, the ms and ns laser energy density is 301 J cm~(-2) and 12 J cm~(-2), respectively, the velocity of shock wave is 1.09 times faster than that induced by single ns pulse laser. It is inferred that the shock wave propagates in the plasma is faster than that in air. When the ms and ns laser energy density is 414.58 and 24 J cm~(-2), the velocity of shock wave shows rising trend with pulse delay in a range of 1.4 msΔt 0.8 ms. It is indicated that with the increase of ns laser energy, the laser energy absorbed by laser-supported absorption wave increases. The mechanism of inverse bremsstrahlung absorption acts with target surface absorption simultaneously during the ns laser irradiation. Thus, the phenomenon of the double shock wave is induced. The numerical results of the phenomenon were accordance with experiment. The results of this research can provide a reference for the field of laser propulsion.     

The Electron Trajectory in a Relativistic Femtosecond Laser Pulse

In this report, we start from Lagrange equation and analyze theoretically the electron dynamics in electromagnetic field. By solving the relativistic government equations of electron, the trajectories of an electron in plane laser pulse, focused laser pulse have been given for different initial conditions. The electron trajectory is determined by its initial momentum, the amplitude, spot size and polarization of the laser pulse. The optimum initial momentum of the electron for LSS （laser synchrotron source） is obtained. Linear polarized laser is more advantaged than circular polarized laser for generating harmonic radiation.     

Red Shift and Broadening of Backward Harmonic Radiation from Electron Oscillations Driven by Femtosecond Laser Pulse

The characteristics of backward harmonic radiation due to electron oscillations driven by a linearly polarized fs laser pulse are analysed considering a single electron model. The spectral distributions of the electron＇s backward harmonic radiation are investigated in detail for different parameters of the driver laser pulse. Higher order harmonic radiations are possible for a sufficiently intense driving laser pulse. We have shown that for a realistic pulsed photon beam, the spectrum of the radiation is red shifted as well as broadened because of changes in the longitudinal velocity of the electrons during the laser pulse. These effects are more pronounced at higher laser intensities giving rise to higher order harmonics that eventually leads to a continuous spectrum. Numerical simulations have further shown that by increasing the laser pulse width the broadening of the high harmonic radiations can be controlled.     

Analysis of surface damage produced by pulsed laser ablation on metal Al and semiconductor Si

The suraface morphological changesd produced by Nd:YAG pulsed laser ablation of metal Al and semiconductor Si were carefully examined and analyzed by using scanning elkectron microscope.The formation mechanism of the droplets was discussed.and the reasons for formation of the microcracks on the laser irradiated area of the target surface were analyzed by calculating the thermal stress,the vapor pressure and the shock pressure induced by the laser supported detonation.     

Experimental studies of nonlinear laser—plasma interactions and hot electrons

Observation shows that the stimulated Raman scattering (SRS) in a cavity target irradiated by 1.053um laser is the dominant mechanism in producing hot electrons.In cavity targets,stimulated Brillouin scattering (SBS) and SRS are main abnormal absorption processes,they can scatter about 0.4 of the laser light energy fraction.     

Experimental Study of the X-Ray Radiation Source at Approximately Constant Radiation Temperature

An X-ray radiation source with approximately constant radiation temperature is realized by irradiating golden hohlraum with a shaped laser pulse. A simple theoretical model based on power balance is used to design the shape of the drive laser pulse. Experiments are carried out on the Shenguang III prototype laser facility, and the experimentM results are presented for radiation sources with the flat-top lasting about 2.5 ns at two different peak temperatures of about 150 eV and 170 eV, respectively, including the the drive laser pulses and the time integrated possible improvements are discussed. time histories of the temperatures, the shapes of radiation spectra. The validity of the model and     

不锈钢放射性模拟样片在空气中的激光去污技术

采用APEX放电型准分子激光器作为激光光源,用低放废液制备不锈钢放射性模拟样片,在激光去污装置上开展激光去污工艺参数的研究。通过考察不同的辐照强度、脉冲频率、辐照角度及扫描速率参数对不锈钢放射性模拟样片表面去污因子DF的影响,获得了激光去污工艺参数。在此参数下对模拟样片表面的去污因子大于200,剥离厚度小于20μm。     

Focusing of Intense Laser via Parabolic Plasma Concave Surface

Since laser intensity plays an important role in laser plasma interactions,a method of increasing laser intensity- focusing of an intense laser via a parabolic plasma concave surface- is proposed and investigated by three-dimensional particle-in-cell simulations.The geometric focusing via a parabolic concave surface and the temporal compression of high harmonics increased the peak intensity of the laser pulse by about two orders of magnitude.Compared with the improvement via laser optics approaches,this scheme is much more economic and appropriate for most femtosecond laser facilities.     

FPGA单粒子效应的脉冲激光试验方法研究

通过测试基于静态随机存储器（SRAM）的现场可编程门阵列（FPGA）芯片的单粒子效应,研究脉冲激光的试验方法,评估脉冲激光试验单粒子效应的有效性。研究表明,激光光斑聚焦深度和激光注量是影响脉冲激光单粒子效应试验的重要因素。试验发现,脉冲激光在较高能量时,单个激光脉冲会触发多个配置存储位发生单粒子翻转,造成芯片饱和翻转截面偏大。激光辐照芯片时,观察到芯片的内核工作电流以1～2 mA的幅度逐渐增加,在此期间器件工作正常。试验获得了Virtex 2 FPGA芯片的静态单粒子翻转截面和翻转阈值。通过对比激光与重离子的试验结果发现,二者在测试器件单粒子翻转方面基本一致,脉冲激光可有效研究芯片的单粒子效应特性。     

Spatial and temporal evolution of electromagnetic pulses generated at Shenguang-Ⅱ series laser facilities

The distribution and sources of EMPs produced at Shenguang-Ⅱ(SG-Ⅱ)series laser facilities are systematically investigated.The results indicate that the EMP amplitudes in the SG-Ⅱ ps PW laser are very strong,one order higher than those from the SG-Ⅱ laser facility.EMPs outside the target chamber decrease exponentially with the distance from the measuring points to the target chamber center at the two laser facilities.Moreover,EMPs can be remarkably reduced when the picosecond laser together with the nanosecond laser is incident to targets compared to the SG-Ⅱps PW laser alone.The resulting conclusions are expected to offer experimental supports for further effective EMPs shielding design and achievement in high-power laser facilities.     

激光吸收光谱法实时监测原子蒸气密度

采用固体激光器泵浦环形染料激光器作为光源,通过激光吸收光谱法对钆原子蒸气密度进行实时监测。应用光纤远距离传输提高光路稳定性,采用多步吸收光程技术,并引入参考光消除激光功率不稳定因素影响。实验结果表明：采用该方法建立的原子蒸气密度实时监测系统标准误差约为4%,可为激光同位素分离过程提供可靠数据,从而提高分离效率。     

激光有一定带宽时的光电离

研究了激光有一定带宽情况下的光电离。首先推导了三步光电离的公式，然后从含有激光带宽的方程组出发，计算了不同情况下的电离几率对于激光带宽的依赖关系，以计算结果分析了激光带宽的选取原则。     

Nanosecond laser ablation of Thoria fuel pellets for microstructural study

A two-dimensional numerical model has been developed simulating the process of laser based surface etching of Thoria targets via pulsed laser ablation enabling their surface preparation for subsequent metallographic investigation. The heat conduction equation solved by an explicit finite difference method provides simulated data on the temperature distribution at the surface and within the target, melt depth and evaporation rate from the target as a function of time, during and after the laser pulse. Calculations have been performed for laser and target parameters corresponding to experimental conditions matching our reported experimental observations on pulsed laser etching of Thoria pellets via laser ablation. The calculated maximum surface temperature reached by the laser treated Thoria target exceeds the estimated value of thermodynamic critical temperature of Thoria. Thus, our results on simulation of pulsed laser ablation for an average laser flux of 10 J/cm² delivered by a 10 ns Nd:YAG laser pulse corresponding to a peak laser intensity of 3.87 × 10⁹ W/cm² suggest, that explosive boiling could probably be an additional material-removal mechanism other than normal boiling and evaporation when surface etching Thoria with such intense laser radiation. Since explosive boiling is usually accompanied by intense material ejection, this mechanism of material-removal should be avoided to ensure minimum induced target damage associated with the technique of laser based etching. Our calculations thus help us to make a proper choice of laser parameters facilitating subsequent metallographic investigation of laser etched Thoria fuel pellets, at the same time, minimizing unwanted associated thermal effects such as target damage through crater formation, as has been experimentally observed.     

1.55μm电流调制半导体激光器混沌的自适应控制

通过加入调制电流的方式使1.55μm半导体激光系统处于混沌状态，再加入参数自适应控制，将激光输出控制在所需的输出强度上。用该方法可根据不同的控制目标选择不同的控制强度，在控制范围内能很快获得所需稳定的输出激光。研究结果对改善实际激光系统稳态输出的快速性、输出功率的灵活可调性和能量转换效率有较好的参考价值。