Wavelength Dependence in the Analysis of Carbon Content in Coal by Nanosecond 266 nm and 1064 nm Laser Induced Breakdown Spectroscopy

The wavelength dependence of laser induced breakdown spectroscopy(LIBS) in the analysis of the carbon contents of coal was studied using 266 nm and 1064 nm laser radiations.Compared with the 1064 nm wavelength laser ablation,the 266 nm wavelength laser ablation has less thermal effects,resulting in a better crater morphology on the coal pellets.Besides,the 266 nm wavelength laser ablation also provides better laser-sample coupling and less plasma shielding,resulting in a higher carbon line intensity and better signal reproducibility.The carbon contents in the bituminous coal samples have better linearity with the line intensities of atomic carbon measured by the 266 nm wavelength than those measured by the 1064 nm wavelength.The partial least square(PLS) model was established for the quantitative analysis of the carbon content in coal samples by LIBS.The results show that both of the 266 nm and 1064 nm wavelengths are capable of achieving good performance for the quantitative analysis of carbon content in coal using the PLS method.     

Time-of-flight and UV spectroscopy characterization of laser-generated plasma

A study of laser ablation of different targets (Al, Ti, Mo, Au and polyethylene), in vacuum, by using 3 ns Nd:YAG laser radiation, at 1064 nm wavelength, is reported. The ion emission from the plasma was monitored through time-of-flight (TOF) measurements, performed by using an ion collector placed along the normal to the target surface. The deconvolution of the IC experimental spectra with a Coulomb-Boltzmann-shifted function permitted to evaluate the equivalent ion temperature and the acceleration voltage developed inside the non-equilibrium plasma.The UV plasma emission, detected with an optical spectroscope, permitted to estimate the electronic temperature and density, to evaluate the Debye length and the temperature gradient in the laser-generated plasma plume.     

Influence of sample temperature on the expansion dynamics of laser-induced germanium plasma

In this paper, we investigated the influence of sample temperature on the expansion dynamics and the optical emission spectroscopy of laser-induced plasma, and Ge was selected as the test sample. The target was heated from room temperature(22 °C) to 300 °C, and excited in atmospheric environment by using a Q-Switched Nd:YAG pulse laser with the wavelength of 1064 nm. To study the plasma expansion dynamics, we observed the plasma plume at different laser energies(5.0, 7.4 and 9.4 mJ)and different sample temperatures by using time-resolved image. We found that the heated target temperature could accelerate the expansion of plasma plume. Moreover, we also measured the effect of target temperature on the optical emission spectroscopy and signal-to-noise ratio.     

Numerical simulation of nanosecond laser ablation and plasma characteristics considering a real gas equation of state

Based on the governing equations which include the heat conduction equation in the target and the fluid equations of the vapor plasma,a two-dimensional axisymmetric model for ns-laser ablation considering the Knudsen layer and plasma shielding effect is developed.The equations of state of the plasma are described by a real gas approximation,which divides the internal energy into the thermal energy of atoms,ions and electrons,ionization energy and the excitation energy of atoms and ions.The dynamic evolution of the silicon target and plasma during laser ablation is studied by using this model,and the distributions of the temperature,plasma density,Mach number related to the evaporation/condensation of the target surface,laser transmissivity as well as internal energy of the plasma are given.It is found that the evolution of the target surface during laser ablation can be divided into three stages:(1)the target surface temperature increases continuously;(2)the sonic and subsonic evaporation;and(3)the subsonic condensation.The result of the internal energy distribution indicates that the ionization and excitation energy plays an important role in the internal energy of the plasma during laser ablation.This model is suitable for the case that the temperature of the target surface is lower than the critical temperature.     

Development of a silver ion source using nanosecond pulses of a Nd:YAG laser at different wavelengths

A silver ion source was designed by focusing the fundamental and harmonics of Q-switched Nd:YAG laser pulses onto a silver target and simultaneously applying an electric potential in an argon environment. The silver ions were detected at a distance of 2 cm from the target surface using a Faraday cup ion probe after letting them pass through a retarding mesh grid (copper electrode). We aim to produce and characterize the silver ions generated by the laser radiation of different wavelengths and pulse energy, ambient gas pressure and the electrode spacing under applied electric field. In addition to this, the effect of laser radiation on plasma under vacuum and at different argon gas pressures was investigated. The velocity distribution function of the plasma emitted from the silver target was investigated under argon discharge. These measurements demonstrated clearly that the velocity distribution function and current signals depend on laser power, laser wavelength and argon pressure. We observed a ten fold increase in the plume current with increase in the applied voltage and ion velocity in the presence of a laser field. The surface morphology of the laser irradiated samples was investigated using reflection optical microscopy.     

Laser induced plasma plume imaging and surface morphology of silicon

Shot-to-shot variation in the characteristics of laser produced plasma plume and surface profile of N-type silicon (1 1 1) are investigated. In order to produce plasma, a Q-switched Nd: YAG laser (1064 nm, 10 mJ, 9-14 ns) is tightly focused on silicon target in air at room temperature. Target was exposed in such a way that number of laser shots was increased from point to point in ascending order starting from single shot at first point. Target was moved 2 mm after each exposure. In order to investigate shot-to-shot variation in the time integrated emission intensity regions within the plasma plume, a computer controlled CCD based image capture system was employed. Various intensity regimes were found depending strongly on the number of incident laser pulses. Plasma plume length was also found to vary with the number of pulses. The topographic analysis of the irradiated Si was performed by Scanning Electron Microscope (SEM) which shows the primary mechanisms like thermal or non-thermal ablation depend on the number of shots. Surface morphological changes were also studied in terms of ripple formation, ejection, debris and re-deposition of material caused by laser beam at sample surface. The micrographs show ripples spacing versus wavelength dependence rule [Λ ≈ λ/(1 − sin θ)]. Intensity variations with number of shots are correlated with the surface morphology of the irradiated sample.     

The Enhanced Effect of Optical Emission from Laser Induced Breakdown Spectroscopy of an Al-Li Alloy in the Presence of Magnetic Field Confinement

In this paper,the influence of magnetic field strength on laser-induced breakdown spectroscopy(LIBS) has been investigated for various pressures.The plasma plume was produced by employing Q-switch Nd:YAG laser ablation of an Al-Li alloy operating at a 1064 nm wavelength.The results indicated that the LIBS intensity of the Al and Li emission lines is boosted with an increase of magnetic strength.Typically,the intensity of the Al Ⅰ and Li Ⅰ spectral emissions can be magnified by 1.5-3 times in a steady magnetic field of 1.1 T compared with the field-free case.Also,in this investigation we recorded time-resolved images of the laser-produced plume by employing a fast ICCD camera.The results show that the luminance of the plasma is enhanced and the time of persistence is increased significantly,and the plasma plume splits into two lobes in the presence of a magnetic field.The probable reason for the enhancement is the magnetic confinement effect which increases the number density of excited atoms and the population of species in a high energy state.In addition,the electron temperature and density are also augmented by the magnetic field compared to the field-free case.     

Spatial and Excitation Variations for Different Applied Voltages in an Atmospheric Neon Plasma Jet

A neon plasma jet was generated in air,driven by a 9 kHz sinusoidal power supply.The characteristics of the plasma plume and the optical spectra with plasma propagation for different applied voltages were investigated.By increasing the applied voltage,the plasma plume first increases and then retracts to become short and bulky.The shortened effect of Ne plasma plume(about 10 mm) for the further voltage increasing is more apparent than that of He(about3 mm) and Ar(about 1 mm).Emission intensity of the N_2(337 nm) increases with the applied voltage,gradually substituting the emission intensity of Ne(702 nm and 585 nm) as the noticeable radiation.At the nozzle opening,the Ne(702 nm) emission dominates,while the Ne(585 nm)emission is most noticeable around the tip of the plasma plume.The spatial distribution of the three spectral lines indicates that Ne(702 nm) emission decreases dramatically with plasma propagation while Ne(585 nm) and N_2(337 nm) emissions reach their maxima at the middle of the plasma plume.The results indicate that the Ne(702 nm) emission is much more sensitive to the average electron temperature and the density of the high-energy electrons,so it changes greatly at the tube nozzle and little at the tip region as the voltage increases.The population of high-energy electrons,the average electron temperature,the collision with air molecules and the Penning effect between Ne metastables and air molecules may explain their different variations with plasma propagating and voltage increasing.     

Plasma ignition threshold disparity between silver nanoparticle-based target and bulk silver target at different laser wavelengths

Plasma ignition threshold of nanoparticle-based and bulk silver targets was measured in air. The plasma was initiated by a Nd:YAG laser at wavelengths of 355, 532, and 1064 nm. The plasma ignition was monitored utilizing the prominent Ag I line at 546.5 nm. Lower ignition thresholds of the nanoparticle-based silver target were estimated at 0.4?±?0.02, 0.34?±?0.04, and 0.27?±?0.035 J cm~(-2) coupled with the different laser wavelengths, respectively. In contrast, the bulk silver target plasma exhibited an order of magnitude higher ignition threshold. A three orders of magnitude enhanced emission intensity from the nano-based target over the bulk target was achieved at lower levels of laser irradiation. A reduction of the thermal diffusion length of the nanosilver was assumed in order to theoretically predict this reduction in the plasma threshold. In addition, the effect of self-reversal on the resonance lines was taken into consideration.     

Spatial Resolution Measurements of C,Si and Mo Using LIBS for Diagnostics of Plasma Facing Materials in a Fusion Device

Recently,a laser-induced breakdown spectroscopic(LIBS) system has been developed for in situ measurements of the chemical compositions of plasma facing materials(PFMs)in the Experimental Advanced Superconducting Tokamak(EAST).In this study,a LIBS system,which was used in a similar optical configuration to the in situ LIBS system in EAST,has been developed to investigate the spatial distribution of PFM elements at 1CP4 Pa.The aim of this study was to understand the nature of the spatial distribution of atoms or ions of different elements in the plasma plume and optimize the signal to background ratio for the in situ LIBS diagnosis in EAST.The spatial profiles of the LIBS signals of C,Si,Mo and the continuous background were measured.Moreover,the influence of laser spot size and laser energy density on the LIBS signals of C,Si,Mo and H was also investigated.The results show that the distribution of the C,Si and Mo peaks' intensities first increased and then decreased from the center to the edge of the plasma plume.There was a maximum value at R≈1.5 mm from the center of the plasma plume.This work aims to improve the understanding of ablating plasma dynamics in very low pressure environments and give guidance to optimize the LIBS system in the EAST device.     

Micro-impulse and plasma plume produced by irradiating aluminum target with nanosecond laser pulses in double-pulse scheme

The micro-impulse generated by ablating an aluminum target in double-pulse laser bursts with different interpulse delays was investigated using a torsion pendulum. The plasma plume was simultaneously visualized using high-speed photography to analyze the coupling mechanism of the ablation impulse. The experiment was carried out using a pulsed laser with a pulse width of 8 ns and a wavelength of 1064 nm. The experimental results show that an impulse with an interpulse delay of 60 ns is roughly 60% higher than that with no delay between the two pulses, when the energy of both laser pulses is 50 mJ. Therefore, double-pulse schemes could enhance the ablation impulse under certain conditions. This is because the ablation of the first laser pulse changes the optical properties of the aluminum target surface, increasing the absorptivity. However, the ablation impulse is reduced with a time delay of 20 ns when the energy of both laser pulses is 100 mJ or 150 mJ. It can be concluded that the plasma produced by ablating the aluminum with the first pulse shields the second laser pulse. To summarize, the experimental results show that different delay times in a double-pulse scheme have a significant effect on the ablation impulse. The study provides a reference for the optimization of the parameters when laser ablation propulsion with a double-pulse scheme is applied in the fields of space debris removal, laser ablation thrusters, and so on.     

Dependence of plasma structure and propagation on microwave amplitude and frequency during breakdown of atmospheric pressure air

The structure and propagation of the plasma in air breakdown driven by high-power microwave have attracted great interest. This paper focuses on the microwave amplitude and frequency dependence of plasma formation at atmospheric pressure using one two-dimensional model,which is based on Maxwell's equations coupled with plasma fluid equations. In this model, we adopt the effective electron diffusion coefficient, which can describe well the change from free diffusion in a plasma front to ambipolar diffusion in the bulk plasma. The filamentary plasma arrays observed in experiments are well reproduced in the simulations. The density and propagation speed of the plasma from the simulations are also close to the corresponding experimental data. The size of plasma filament parallel to the electric field decreases with increasing frequency, and it increases with the electric field amplitude. The distance between adjacent plasma filaments is close to one-quarter wavelength under different frequencies and amplitudes. The plasma propagation speed shows little change with the frequency, and it increases with the amplitude. The variations of plasma structure and propagation with the amplitude and frequency are due to the change in the distribution of the electric field.     

快点火物理及其对数值模拟的要求

快点火(fast ignition)是一种新的惯性约束聚交点火方式。实验和理论研究表明其点火环节是非常复杂和困难的问题。研究快点火需要深入地进行数值模拟。报告主要从分析物理出发,探讨快点火对数值模拟的要求,同时结合实际情况进行讨论。快点火主要包括三个过程,即内爆预压缩、超强激光在次临界等离子体中和在超临界密度等离子体中的传播(成道和打洞)、超热电子的产生及其在介质,特别是稠密介质中的传输和高温点火区的形成。研究认为:研究预压缩不仅需要一维、二维,而且需要三维激光靶耦合总体程序;超热电子需要包括电磁场的Fokker-Planck方程描述;点火过程的等离子体流体力学则需要考虑电子、离子双流运动方程,而且应包括电磁场。PIC程序可用来研究局部的细节,并提供上述方程所需要的参数。此外,报告还简述了近两年来的快点火实验和一些国家的未来的计划。     

Characterization of Carbon Plasma Evolution Using Laser Ablation TOF Mass Spectrometry

In this work, a time-of-flight (TOF) mass spectrometer has been used to investigate the distribution of intermediate species and formation process of carbon clusters. The graphite sample was ablated by Nd:YAG laser (532 nm and 1064 nm). The results indicate that the maximum size distribution shifted towards small cluster ions as the laser fluence increased, which happened because of the fragmentation of larger clusters in the hot plume. The temporal evolution of ions was measured by varying the delay time of the ion extraction pulse with respect to the laser irradiation, which was used to provide distribution information of the species in the ablated plasma plume. When the laser fluence decreased, the yield of all of the clusters obviously dropped.     

Ablation characteristics of carbon-doped glycerol irradiated by a 1064 nm nanosecond pulse laser

The ablation characteristics of carbon-doped glycerol were investigated in laser plasma propulsion using a pulse laser with 10 ns pulse width and 1064 nm wavelength.The results showed that with the incident laser intensity increasing,the target momentum decreased.Results still indicated that the strong plasma shielded the consumption loss and resulted in a low coupling coefficient.Furthermore,the carbon-doping gave rise to variations in the laser focal position and laser intensity,which in turn reduced the glycerol splashing.Based on the glycerol viscosity and the carbon doping,a high specific impulse is anticipated.     

The Effect of an External Magnetic Field on the Plume Expansion Dynamics of Laser-Induced Aluminum Plasma

In this work,we investigated the plasma morphology induced by a Nd:YAG laser with the aim of improving the understanding of the formation and dynamics of the plasma in two cases,with and without a magnetic field.Single laser pulse production of a plasma in the absence and presence of a magnetic field was performed with an aluminum target in air.A fast photography technique was employed to obtain information about the expansion dynamics and confinement of the aluminum plasma in each case.The generation of the laser plasma was allowed to expand at two locations with different magnetic field strengths,which correspond to the strength 0.58 T in the center of two magnetic poles and 0.83 T at a distance of 4 mm from the upper pole(N).The plume showed lateral confinement at longer delays when the target was placed at the center of the two poles.When the target was placed at a distance of 4 mm from the upper pole it was observed that the plume was divided into two lobes at the initial stage and traveled towards the center of the magnetic field with further elapse of time.     

Investigation into the thermal effect of the LIPS-200 ion thruster plume

The distribution of the thermal effects of the ion thruster plume are essential for estimating the influence of the thruster plume, improving the layout of the spacecraft, and for the thermal shielding of critical sensitive components. In order to obtain the heat flow distribution in the plume of the LIPS-200 xenon ion thruster, an experimental study of the thermal effects of the plume has been conducted in this work, with a total heat flow sensor and a radiant heat flow sensor over an axial distance of 0.5–0.9 m and a thruster angle of 0°–60°. Combined with a Faraday probe and a retarding potential analyzer, the thermal accommodation coefficient of the sensor surface in the plume is available. The results of the experiment show that the xenon ion thruster plume heat flow is mainly concentrated within a range of 15°. The total and radial heat flow of the plume downstream of the thruster gradually decreases along the axial and radial directions, with the corresponding values of 11.78 kW m⁻² and 0.3 kW m⁻² for the axial 0.5 m position, respectively. At the same position, the radiation heat flow accounts for a very small part of the total heat flow, approximately 3%–5%. The thermal accommodation factor is 0.72–0.99 over the measured region. Furthermore, the PIC and DSMC methods based on the Maxwell thermal accommodation coefficient model (EX-PWS) show a maximum error of 28.6% between simulation and experiment for LIPS-200 ion thruster plume heat flow, which, on the one hand, provides an experimental basis for studying the interaction between the ion thruster and the spacecraft, and on the other hand provides optimization of the ion thruster plume simulation model.     

Modified model for harmonic generation free electron laser

The longitudinal modulation to the electron beam by a coherent seed laser pulse is widely used for generating fully coherent, short wavelength radiation in various harmonic generation free electron laser (FEL)schemes. After introducing the density modulation, mierostruetures down to attosecond scale are produced over the distance of one seed laser wavelength. In order to take into account the mierostructures in the theoretical and numerical analysis, in the frame of undulator period averaged approach, a modified model for harmonic generation FEL is developed in this paper. With the modified model, three harmonic generation FEL examples are investigated by employing Shanghai soft X-ray FEL (SXFEL) parameters. In FEL schemes with ultra-high harmonic generation and ultra-short pulse, the modified model presents some interesting aspects which are helpful for understanding of radiation pulse evolution, bunching efficiency and noise propagation issues.     

Numerical simulation of laser-induced plasma in background gas considering multiple interaction processes

Laser-induced plasma is often produced in the presence of background gas, which causes some new physical processes. In this work, a two-dimensional axisymmetric radiation fluid dynamics model is used to numerically simulate the expansion process of plasma under different pressures and gases, in which the multiple interaction processes of diffusion, viscosity and heat conduction between the laser ablated target vapor and the background gas are further considered, and the spatio-temporal evolutions of plasma parameters(species number density, expansion velocity,size and electron temperature) as well as the emission spectra are obtained. The consistency between the actual and simulated spectra of aluminum plasma in 1 atm argon verifies the correctness of the model and the numerical simulation, thus providing a refinement analysis method for the basic research of plasma expansion in gases and the application of laser-induced breakdown spectroscopy.     

Investigation of laser plasma dynamics using the X-ray spectra of multicharged ions

Results are reported on the study of the structure of optically thick laser plasmas from the intensity of X-ray spectral lines. The analysis of plasma images provides information about the average velocity of the plasma expansion, the hydrodynamic efficiency and the rate of the target evaporation.