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.     

Comparison of Pd plasmas produced at 532 nm and 1064 nm by a Nd:YAG laser ablation

A comparison between the laser ablation of a palladium target in vacuum, by using 1064 nm and 532 nm Nd:YAG laser wavelengths, with an intensity of about 10⁹ W/cm², is reported. Nanosecond pulsed ablation produces high non-isotropic emission of neutrals and ions. For both wavelengths, mass quadrupole spectrometry and time-of-flight measurements allow estimation of the atomic and ionic species emitted from the plasma and of their energy distributions.Ions show Coulomb-Boltzmann-shifted distributions depending on their charge state. Surface profiles of the ablated craters permitted to study the ablation threshold and yields of palladium in vacuum vs. the laser fluence. The plasma temperature and density was evaluated by the experimental data. A special regard is given to the ion acceleration process occurring inside the plasma, due to the high electrical field generated inside the plasma.     

Present Status and Future Prospects of Laser Fusion Research at ILE Osaka University

Reviewed are the present status and future prospects of the laser fusion research at the ILE Osaka.The Gekko XII and Peta Watt laser system have been operated for investigating the implosion hydrodynamics,fast ignition, and the relativistic laser plasma interactions and so on.In particular,the fast ignition experiments with cone shell target have been in progress as the UK and US-Japan collaboration programs.In the experiments,the imploded high density plasmas are heated by irradiating 500 J level peta-watt laser pulse.The thermal neutron yield is found to increase by three orders of magnitude by injecting the peta-watt laser into the cone shell target.The Rayleigh-Taylor instability experiment results are also reviewed is this paper.     

Laser-induced plasma generated by a 532nm pulsed laser in bulk water:unexpected line-intensity variation with water temperature and the possible underlying physics

The effect of the matrix temperature on laser-induced plasma generated in bulk water by using a532 nm pulsed laser beam has been studied.Ca Ⅰ and Ⅱ emission line intensities were recorded for an aqueous solution of CaCl_2 in the temperature range of 7℃-70℃.The emission line intensities did not follow the matrix temperature in our experiments.Maximum intensities were observed at ～18℃ for both lines.Herein,a possible mechanism responsible for the observed variation in intensity is suggested,in which laser-produced bubbles play important roles.Bubble formation is essential to ignite plasma in the liquid and more feasible at the higher liquid temperature.However,the abundant bubbles at the higher temperature can scatter the incident laser beam more effectively to decrease the energy delivered for the laser-induced plasma.Thus,these two roles have effects on the optical emission intensities in opposite ways.The validity of the suggested mechanism is discussed based on the plasma temperature,temperature dependence of the refractive index of water,plasma electron density,scattered light intensity,and plasma ignition threshold energy.Our result indicates that the temperature of the liquid is also an important parameter to be considered in the laser-induced breakdown spectroscopy analysis of bulk liquid samples and its application in deep-sea exploration.     

“Water window” compact, table-top laser plasma soft X-ray sources based on a gas puff target

We have developed compact, high repetition, table-top soft-X-ray sources, based on a gas puff target, emitting in “water window” spectral range at λ = 2.88 nm from nitrogen gas target or, in 2-4 nm range of wavelengths, from argon gas target. Double stream gas puff target was pumped optically by commercial Nd:YAG laser, energy 0.74 J, pulse time duration 4 ns. Spatial distribution of laser-produced plasma was imaged using a pinhole camera. Using transmission grating spectrometer, argon and nitrogen emission spectra were obtained, showing strong emission in the “water window” spectral range. Using AXUV100 detector the flux measurements of the soft-X-ray pulses were carried out and are presented.These debris free sources are table-top alternative for free electron lasers and synchrotron installations. They can be successfully employed in microscopy, spectroscopy and metrology experiments among others.     

Recent results on Nova

On the Nova Laser at LLNL, we have recently demonstrated many of the key elements required for assuring that the next proposed laser, the National Ignition Facility (NIF) will reach ignition. In particular, we have achieved a drive of 300 eV in laser heated hohlraums; have shown good understanding and control of symmetry in hohlruams; created large NIF-Scale plasmas with plasma and irradiation conditions relevant to NIF targets that showed low levels of plasma instabilities; demonstrated a good understanding of hydrodynamic instability and subsequent pusher/fuel mix in implosions by means of spectroscopic tracers; and performed integrated implosion experiments that have performed well even under stringent convergences of order 25, which is well into the NIF ignition target regime.     

Numerical simulation of laser ablation of molybdenum target for laser-induced breakdown spectroscopic application

Laser-induced breakdown spectroscopy has been recognized as a significant tool for element diagnostics in plasma–wall interaction. In this work, a one-dimensional numerical model is developed to simulate the laser ablation processes of a molybdenum(Mo) target in vacuum conditions. The thermal process of the interaction between the ns-pulse laser with wavelength of 1064 nm and the Mo target is described by the heat conduction equation. The plasma plume generation and expansion are described by Euler equations, in which the conservation of mass density, momentum and energy are included. Saha equations are used to describe the local thermal equilibrium of electrons, Mo atoms,Mo~+ and Mo~(2+) Plasma shielding and emission are all considered in this model. The mainly numerical results are divided into three parts, as listed below.Firstly, the rule of the plasma shielding effect varying with laser intensity is demonstrated quantitatively and fitted with the Nelder function. Secondly, the key parameters of plasma plume,such as the number density of species, the propagation velocity and the temperature, are all calculated in this model. The results indicate that the propagation velocity of the plume center increased with time in a general trend, however, one valley value appeared at about 20 ns due to the pressure gradient near the target surface leading to negative plasma velocity. Thirdly, the persistent lines of a Mo atom in the wavelength range from 300 nm to 600 nm are selected and the spectrum is calculated. Moreover, the temporal evolutions of Mo's spectral lines at wavelength of 550.6494 nm,553.3031 nm and 557.0444 nm are given and the results are compared with experimental data in this work.     

Energy and flux measurements of laser-induced silver plasma ions by using Faraday cup

Silver(Ag)plasma has been generated by employing Nd∶YAG laser(532 nm,6 ns)laser irradiation.The energy and flux of ions have been evaluated by using Faraday cup(FC)using time of flight(TOF)measurements.The dual peak signals of fast and slow Ag plasma ions have been identified.Both energy and flux of fast and slow ions tend to increase with increasing irradiance from 7 GW cm-2 to 17.9 GW cm-2 at all distances of FC from the target surface.Similarly a decreasing trend of energies and flux of ions has been observed with increasing distance of FC from the target.The maximum value of flux of the fast component is 21.2×1010cm-2,whereas for slow ions the maximum energy and flux values are 8.8 keV,8.2×1012 cm-2 respectively.For the analysis of plume expansion dynamics,the angular distribution of ion flux measurement has also been performed.The overall analysis of both spatial and angular distributions of Ag ions revealed that the maximum flux of Ag plasma ions has been observed at an optimal angle of～15°.In order to confirm the ion acceleration by ambipolar field,the self-generated electric field(SGEF)measurements have also been performed by electric probe;these SGEF measurements tend to increase by increasing laser irradiance.The maximum value of 232 V m-1 has been obtained at a maximum laser irradiance of 17.9 GW cm-2.     

Application of High Energy Protons in Inertial Confinement Fusion by Using the Fast Ignitor Concept for DT Fusion

A short-laser-pulse driven ion flux is examined as a fast ignitor candidate for inertial confinement fusion. The main mechanism for ion acceleration is charge separation in a plasma due to high-energy electrons driven by the laser inside the target. Another very new branch of fast ignition research is the investigation of the use of laser generated proton beams. In the present paper aims to provide insights into the feasibility of the fast ignition concept with high energy beams of protons generated in laser–plasma interactions. The optimum parameters of an ion beam and laser pulse that are suitable for an ignition spark in a hot precompressed DT fuel are estimated as a rough guide. Also, in this paper we estimate the radius of Deuterium–Tritium (DT) fuel pellet that is equal to the protons range in DT plasma.     

The Potential of Fast Ignition and Related Experiments with a Petawatt Laser Facility

A model of energy gain induced by fast ignition of thermonuclear burn in compressed deuterium-tritium fuel, is used to show the potential for 300× gain with a driver energy of 1 MJ, if the National Ignition Facility (NIF) were to be adapted for fast ignition. The physics of fast ignition has been studied using a petawatt laser facility at the Lawrence Livermore National Laboratory. Laser plasma interaction in a preformed plasma on a solid target leads to relativistic self-focusing evidenced by x-ray images. Absorption of the laser radiation transfers energy to an intense source of relativistic electrons. Good conversion efficiency into a wide angular distribution is reported. Heating by the electrons in solid density CD₂ produces 0.5 to 1 keV temperature, inferred from the D-D thermo-nuclear neutron yield.     

Nova target physics program and the Nova upgrade laser

Summary Recent advances in ICF target design and performance have made possible the achievement of ignition and gain with 1–2 MJ laser drive energy, as against the 5–10 MJ necessary to achieve high gain in the earlier designs. Ignition and propagating burn can be achieved at the lower energy by increasing the hohlraum temperature and, thereby increasing the pressure driving the imploding fusion capsule. Nova experiments continue to address the target physics of radiatively driven targets, such as laser-plasma interaction physics, the efficiency of laser light conversion to X-rays, hohlraum characterization and design, hydrodynamic stability, and implosion physics. Recent experiments on Nova have also demonstrated 1.3 times higher hohlraum temperature than previously predicted. This latter demonstration is the key achievement leading to the Nova Upgrade proposal. These combined results, together with those from experiments to study the interaction of high-power laser light with target plasmas, indicate that the capsule drive and symmetry conditions required for ignition and net gain can be achieved with a properly designed upgrade of the existing Nova facility.Success in the Nova Upgrade objective would firmly establish target and driver requirements for achieving high yield and high gain and would support a decision to construct a Laboratory Microfusion Facility (LMF) for defense applications and an Engineering Test Facility (ETF) for energy applications by the end of the first decade of the next century. Nova Upgrade experiments would focus on the target physics necessary to determine the minimum driver energy required to achieve ignition and high-gain laser fusion. The thermonuclear yield produced (up to 20 MJ) would be used to study the effects of fusion microexplosions on potential LMF and ETF reactor chamber materials. This information would permit development of the most efficient and least costly designs for the LMF and the ETF.In collaboration with W. H. Lowdermilk, N. Frank, C. D. Henning, John R. Murray, M. T. Tobin, J. R. Smith, E. K. Storm, J. D. Lindl, J. D. Kilkenny, J. T. Hunt, and J.B. Trenholme.     

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。     

Experimental Study on 308nm Laser Interaction with Materials

1. IntroductionIn the interaction Of laser pulse with solid materials, a short~wavelength laser has a higher energycoupling efficiency due to its larger photon energy.This kind of laser interaction with materials has become. more attractive in recent years 11-5]. A laserpulse focused on a target can produce high heatingrates and high temperature. With the increased laserintensity, the target material could be melt and vaporized, and the laser-produced pulse on the targetbecomes observable. At…     

Laser damage thresholds of ITER mirror materials and first results on in situ laser cleaning of stainless steel mirrors

A laser ablation system has been constructed and used to determine the damage threshold of stainless steel, rhodium and single-, poly- and nanocrystalline molybdenum in vacuum, at a number of wavelengths between 220 nm and 1064 nm using 5 ns pulses. All materials show an increase of the damage threshold with decreasing wavelength below 400 nm. Tests in a nitrogen atmosphere showed a decrease of the damage threshold by a factor of 2–3. Cleaning tests have been performed in vacuum on stainless steel samples after applying mixed Al/W/C/D coatings using magnetron sputtering. In situ XPS analysis during the cleaning process as well ex situ reflectivity measurements demonstrate near complete removal of the coating and a substantial recovery of the reflectivity. The first results also show that the reflectivity obtained through cleaning at 532 nm may be further increased by additional exposure to UV light, in this case 230 nm, an effect which is attributed to the removal of tungsten dust from the surface.     

Thermal Transport Effect in Tokamaks and Block Ignition for Laser Fusion

Measurements of thermal conduction in tokamaks parallel to the magnetic field were up to 20 times less than the classical values. This was explained by the quantum correction of the collision frequency of electrons with ions. This stowing effect of heat is applied to re-evaluate the ignition threshold for the energy flux density E* for the ignition of solid state density deuterium tritium using nonlinear (ponderomotive) laser force driven space charge neutral plasma blocks.     

15 TW激光与氮气作用产生稳定电子束的实验研究

利用15 TW激光脉冲，系统研究了基于电离化注入的激光尾波场加速。实验中，研究了等离子体密度、相互作用位置、激光脉宽以及激光能量对电子束的电荷量、发散角、指向性、能量以及产生概率的影响。将约400 mJ、25 fs的激光脉冲聚焦在喷嘴前沿，等离子体密度约9×10¹⁸ cm^-3时，电子的产生概率高达100%，获得了水平（竖直）发散角（6.5±0.5） mrad（（5.3±0.3） mrad）、水平（竖直）指向稳定性±1.2 mrad （±0.7 mrad）、峰值能量（135±8） MeV和电荷量（13.5±2.0） pC（＞50 MeV）的稳定电子束，为其应用奠定了基础。     

High temperature silver ion implantation into glass

Soda-lime-silicate glass has been implanted with 55 keV Ag⁺ ions at five different temperatures: room temperature, 100, 225, 350 and 600°C. Rutherford backscattering spectroscopy was used to provide depth profiles for the implants. All samples show a low retention of silver and this varies with temperature. In the room temperature and 100°C implants the silver diffuses to, and is lost from the surface. This also occurs in the higher temperature implants, but in these cases there is a significant amount of inward diffusion by the silver. This diffusion extends to at least 500 nm: greatly in excess of the predicted range for a standard implant. Optical measurements on the samples show that those with the inward diffusion have formed an enhanced-index waveguide.     

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.     

Simple reactor for the synthesis of silver nanoparticles with the assistance of ethanol by gas–liquid discharge plasma

Atmospheric pressure plasma technology is gaining increasing importance because it is a simple and tunable synthesis process for the production of metallic nanoparticles. In addition to the development of the power supply, improving the reactor is also one of the main strategies to enhance the utility. In this study, a simple reactor for the gas–liquid discharge plasma induced by argon gas was applied to synthesize silver nanoparticles from silver nitrate (AgNO₃) in solution. An AC power supply with a peak voltage of 3.5 kV was used. The frequency and on-time were set to 50 kHz and 2.5 μs, respectively. The oscilloscope showed that the rising time was approximately 2 μs. The ethanol was used as the source for the reactive reducing agent. No more additional components existed in the solution during the discharge and neither of the electrodes was in contact with the treated solution. The temperature increased by 10 °C within 1min without a cooling system. Carbon was the main impurity and was expected to be produced from the decomposition of the organics under the plasma. The elevated temperature decreased the organic by-products by evaporation and could also decrease the production of carbon. Transmission electron microscopy showed that the spherical silver nanoparticles with a size of approximately 10 nm were synthesized with a crystal structure and that a low concentration of ethanol prefers the production of the mono-dispersed colloid.     

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.