End Of Laser Pulse Research Articles

First large capsule implosions in a frustum-shaped hohlraum

We report on the first indirect-drive implosions driven by a dual conical frustum-shaped hohlraum denoted “frustraum” and the experimental tuning campaigns leading up to two layered implosions. The campaign used 1.2 and 1.4 mm inner radius high density carbon (HDC) capsules and represented the largest HDC capsules to be imploded on the National Ignition Facility via indirect drive. Several techniques were successfully implemented to control the Legendre mode 2 capsule symmetry of the implosions, including changing the wall angle of the frustraum, which is not possible with cylindrical hohlraums. A mode 4 feature was observed and its implications for hotspot mix discussed. Two layered implosions were conducted with 1.2 mm inner radius capsules, the latter of which achieved the highest layered capsule absorbed energy on the National Ignition Facility using only 1.74 MJ of laser energy. The layered implosion results, along with generalized Lawson parameters, suggest that increasing the energy absorbed by the capsule at the expense of long coast times makes it more challenging to achieve ignition and that further reducing coast time (time between end of laser pulse and bang time) closer to the 1 ns level is warranted to improve the areal density and make it easier to achieve the hotspot temperature, alpha heating, and yield amplification required for ignition.

Modeling of Electron and Lattice Temperature Distribution Through Lifetime of Plasma Plume

When employing shorter (sub picosecond) laser pulses, in ablation kinetics the features appear which can no longer be described in the context of the conventional thermal model. Meanwhile, the ablation of materials with the aid of ultra-short (sub picosecond) laser pulses is applied for micromechanical processing. Physical mechanisms and theoretical models of laser ablation are discussed. Typical associated phenomena are qualitatively regarded and methods for studying them quantitatively are considered. Calculated results relevant to ablation kinetics for a number of substances are presented and compared with experimental data. Ultra-short laser ablation with two-temperature model was quantitatively investigated. A two-temperature model for the description of transition phenomena in a non-equilibrium electron gas and a lattice under picosecond laser irradiation is proposed. Some characteristics are hard to measure directly at all. That is why the analysis of physical mechanisms involved in the ablation process by ultra-short laser pulses has to be performed on the basis of a theoretical consideration of `indirect' experimental data. For Copper and Nickel metal targets, the two-temperature model calculations explain that the temperature of the electron subsystem increased suddenly and approached a peak value at the end of laser pulse. In addition, the temperature profile of lattice temperature subsystem evolution slowly, and still increasing after the end of laser pulse. A good agreement prevails when a comparison between the present results and published results.

Numerical study of effect of magnetic field on laser-driven Rayleigh-Taylor instability

Rayleigh-Taylor instability (RTI) is a fundamental physical phenomenon in fluids and plasmas, and plays a significant role in astrophysics, space physics, and engineering. Especially in inertial confinement fusion (ICF) research, numerous experimental and simulation results have identified RTI as one of the most significant barriers to achieving fusion. Understanding the origin and development of RTI will be conducive to formulating mitigation measures to curb the growth of instability, thereby improving the odds of ICF success. Although there have existed many theoretical and experimental studies of RTI under high energy density, there are few experiments to systematically explore the influence of magnetic fields on the evolution of magnetized RTI. Here, a new experimental scheme is proposed based on the Shenguang-II laser facility on which the nanosecond laser beams are used to drive modulation targets of polystyrene (CH) and low-density foam layers. A shock wave is generated after the laser’s CH modulation layer has been ablated, and propagates through CH to low-density foam. Moreover, Richtmyer-Meshkov instability is triggered off when the shock wave accelerates the target. When the laser pulse ends, the shock wave evolves into a blast wave, causing the system to decelerate, resulting in RTI in the reference system of the interface. In this paper the open-source radiation MHD simulation code (FLASH) is used to simulate the RTI generated by a laser-driven modulation target. The evolution of RTI under no magnetic field, under Biermann self-generated magnetic field, and under different applied magnetic fields are systematically investigated and compared with each other. The simulation results show that the Biermann self-generated magnetic field and the applied magnetic field parallel to flow direction do not change the interface dynamics in the evolution process of RTI. Nevertheless, the applied magnetic field perpendicular to flow direction can stabilize RTI and the Kelvin-Helmholtz vortex at the tail of the RTI spike. Magnetic pressure plays a decisive role. The present results provide a reference for the follow-up study of target physics related to ICF and deepen the understanding of the fluid mixing process.

On experimental and numerical study of the dynamics of a liquid metal jet hit by a laser pulse

In this paper, we present an experimental and numerical study of the laser pulse impact on the liquid metal jet target. The jet motion was recorded with the stroboscopic ultrafast shadow photography. Simulations were carried out in a two-step approach. At the first step, we simulated laser interaction with the target using the radiation hydrodynamics code 3DLINE, which accounts for a number of effects: liquid–gas phase transition, dynamics of ionization, radiation transfer, laser reflection, refraction and absorption. However, this code cannot be used on a deeply refined mesh near the liquid surface and does not account for the surface tension, which strongly affects liquid motion on the microsecond timescale after the laser pulse ends. Therefore, for the second step we employed the OpenFOAM solver, based on the volume-of-fluid method, which overcomes these limitations. The simulated target dynamics is found to be in a fairly good agreement with the experiment.

Study of nano particle stripping and composition inspection on wafer surface

Nano-scale particle stripping and inspection on silicon wafer are critical issues for Integrated Circuit(IC) manufacture industry. As more new materials are used in IC manufacture, not only particle itself but also its composition should be inspected. Particles are mainly adhered by the van der waals force. One of potential particle desorption method is laser cleaning which is environment friendly. However, the mechanism of laser cleaning is not clear and more studies should be done for laser ablation. In this paper, the kinetic process of nano particle on silicon wafer induced by nanosecond pulsed laser as well as the on-line detection method of particle composition were studied. A potential method of nano particle dynamic analysis and particle composition inspection were presented. A dual nanosecond pulse laser system both wavelengths at 532 nm is designed in which one laser pumps the particles away from wafer surface almost without damage, the other laser breakdowns the particles in air above the wafer surface to obtain the emission lines of the contaminated particles of 300 nm Cu by a spectroscopy with CCD. Particle motion trail in <i>z</i> direction was observed after laser cleaning by analyzing particle spectral features. The particle dynamic model after stripping was established in which the resistance of air collision and gravity were included. And the model parameters were obtained by calculation using experimental results. The initial velocity of particle at the end of laser pulse and the average acceleration during laser interaction were calculated which were 7.6 m/s and 7.6 × 10<sup>8</sup> m/s<sup>2</sup> respectively. The sensitivity of the dual laser system was evaluated which was between 2.1 × 10<sup>13</sup> to 5.1 × 10<sup>13</sup> atoms/cm<sup>2</sup>. As result, it is found that the gravity of the particle should not be ignored and the velocity divergence between different stripping particles is existed. The study not only provides a methodology for the study of laser-induced removal of nano particles on the wafer surface and laser induced nano particle dynamics, but also provides a potential method for the inspection of particle composition and pollution source monitoring on line in integrated circuit manufacture process. As the results were not the optimum one and further study should be done in which a better laser power density should be used.

Flow characteristics of the molten pool in pulsed laser welding of thin sheet with filler wire

A 3D transient model has been developed to analysis the molten pool flow characteristic in pulsed laser welding thin sheet with filler wire. The free surface is tracked by VOF method, and the filler metal is simplified as droplets. The flow characteristics in welding of Hastelloy C-276 sheets with 0.5 mm in thickness are analyzed with this model. The calculated weld appearance fits well with the experimental one. Before the droplet contacts with the molten pool, the liquid metal driven by surface tension flows towards the center and downward at the upper surface. The max velocity at the center is about 0.97 m/s. At the trailing portion of lower surface, two eddies with similar spin direction are generated due to the outward surface flow and the downward flow. The filler metal has significant influence on the convection pattern in the molten pool, and results in the disappearance of flow characteristics dominated by surface tension. The filler metal flows downward and outward rapidly after the droplet contact with molten pool. The maximum velocity is about 1.74 m/s and appears at the location where the droplet enters into the molten pool. The droplet has an impact effect on the molten pool and the convection pattern is changed. The molten pool shows oscillation characteristic due to the combined effect of droplet impact and surface tension. The volume of the molten pool dose not decrease right after the laser pulse ends, the lower part of the molten pool expands for about 3–4 milliseconds. And this phenomenon is related to the pulse effect of the laser beam, the flow pattern of the molten pool and the thermal properties of the material. Due to high pulse frequency of the laser beam and low thermal conductivity of the material, the molten pool always exists during welding process.

Target charging in short-pulse-laser–plasma experiments

Interaction of high-intensity laser pulses with solid targets results in generation of large quantities of energetic electrons that are the origin of various effects such as intense x-ray emission, ion acceleration, and so on. Some of these electrons are escaping the target, leaving behind a significant positive electric charge and creating a strong electromagnetic pulse long after the end of the laser pulse. We propose here a detailed model of the target electric polarization induced by a short and intense laser pulse and an escaping electron bunch. A specially designed experiment provides direct measurements of the target polarization and the discharge current in the function of the laser energy, pulse duration, and target size. Large-scale numerical simulations describe the energetic electron generation and their emission from the target. The model, experiment, and numerical simulations demonstrate that the hot-electron ejection may continue long after the laser pulse ends, enhancing significantly the polarization charge.

Effects of an absorptive coating on the dynamics of underwater laser-induced shock process

The effects of an absorptive coating on the dynamics of underwater laser-induced shock process have been observed from the end of laser pulse to hundreds of microseconds after irradiation by time-resolved imaging techniques. A laser pulse of 13 ns at 1,064 nm was focused by a 40-mm focal length lens onto the surface of epoxy-resin blocks immersed in water to induce the shock process in the confining regime. A custom-designed time-resolved photoelasticity imaging technique and a high-speed laser stroboscopic videography technique in photoelasticity mode were used to analyze the evolution of shock waves in the water phase, the strength of stress waves in the solid phase, the oscillation of cavitation bubbles, and the generation of bubble-collapse-induced shock waves. We showed that black paint coating enhances the strength of laser-induced stress wave inside the solid, drives faster shock waves traveling in the water phase, and produces higher-energy cavitation bubbles. We propose that even at power densities of 1 GW/cm2 and above, an absorptive coating can intensify the shock process by enhancing the absorption of laser energy by plasma.

The Influence of spot size on the expansion dynamics of nanosecond-laser-produced copper plasmas in atmosphere

Laser produced copper plasmas of different spot sizes in air were investigated using fast photography and optical emission spectroscopy (OES). The laser energy was 33 mJ. There were dramatic changes in the plasma plume expansion into the ambient air when spot sizes changed from ∼0.1 mm to ∼0.6 mm. A stream-like structure and a hemispherical structure were, respectively, observed. It appeared that the same spot size resulted in similar expansion dynamics no matter whether the target was located in the front of or behind the focal point, although laser-induced air breakdown sometimes occurred in the latter case. Plasma plume front positions agree well with the classic blast wave model for the large spot-size cases, while an unexpected stagnation of ∼80 ns occurred after the laser pulse ends for the small spot size cases. This stagnation can be understood in terms of the evolution of enhanced plasma shielding effects near the plasma front. Axial distributions of plasma components by OES revealed a good confinement effect. Electron number densities were estimated and interpreted using the recorded Intensified Charge Coupled Device (ICCD) images.

Infrared long nanosecond laser pulse ablation of silicon: Integrated two-dimensional modeling and time-resolved experimental study

Nanosecond (ns) laser ablation may provide good solutions to many industrial silicon micromachining applications. However, most of the previous work is on lasers in ultraviolet (UV) or visible spectral ranges, and ns laser ablation of silicon at the infrared (IR) wavelength has not been well understood, particularly for long ns pulses with durations on the order of ∼100ns. IR ns lasers often have lower costs and less external energy consumption for the same laser energy output than UV or visible lasers, which is desirable for many practical applications. This paper aims to understand the mechanism of IR ns laser ablation of silicon, by combining time-resolved experimental observations with physics-based modeling study. The observation is through a ns-gated intensified charged-coupled devices (ICCD) camera coupled with a microscope tube, while the model is based on two-dimensional (2D) gas dynamic equations for the gaseous phases coupled with the condensed phase heat transfer equation through the Knudsen layer relations. The research shows that the material removal mechanism under the studied laser ablation conditions is surface vaporization in the early stage (yielding a plasma plume above the target), followed by subsequent liquid ejection. The measured plasma front propagation matches reasonably well with the model prediction. The experimentally observed spatial distribution of the plasma radiation intensity is consistent with and has been understood through the model. The study also shows that the observed liquid ejection is induced by the total surface pressure difference between the near-boundary region of the target melt pool and the other remaining region of the pool. The pressure difference is mainly due to the surface vaporization flux drop after laser pulse ends.

Backward growth of plasma induced by long nanosecond laser pulse ablation

An interesting physical phenomenon of laser-induced plasma backward growth has been found for long nanosecond (ns) laser pulse ablation, which has been rarely reported. Time-resolved observation shows that the plasma bottom appears to be detached from the ablated target surface shortly after laser pulse ends, and then the bottom grows backwards towards the target. This phenomenon is understood through an experimentally verified, physics-based model. The study shows that the plasma backward growth is fundamentally the backward growth of high-temperature region bottom boundary without any significant backward motion of material, and the underlying gas dynamic process has also been revealed.

Time-Resolved Experimental Study of Silicon Carbide Ablation by Infrared Nanosecond Laser Pulses

Silicon carbide, due to its unique properties, has many promising applications in optics, electronics, and other areas. However, it is difficult to micromachine using mechanical approaches due to its brittleness and high hardness. Laser ablation can potentially provide a good solution for silicon carbide micromachining. However, previous studies of silicon carbide ablation by nanosecond laser pulses at infrared wavelengths are very limited on material removal mechanism, and the mechanism has not been well understood. In this paper, experimental study is performed for silicon carbide ablation by 1064 nm and 200 ns laser pulses through both nanosecond time-resolved in situ observation and laser-ablated workpiece characterization. This study shows that the material removal mechanism is surface vaporization, followed by liquid ejection (which becomes clearly observable at around 1 μs after the laser pulse starts). It has been found that the liquid ejection is very unlikely due to phase explosion. This study also shows that the radiation intensity of laser-induced plasma during silicon carbide ablation does not have a uniform spatial distribution, and the distribution also changes very obviously when the laser pulse ends.

Constraints of two-colour TiRe-LII at elevated pressures

The main objective of this work is to investigate the influence of high-pressure conditions on the determination of primary particle size distributions of laser-heated soot particles using pyrometrically determined temperature decays. The method is based on time-resolved laser-induced incandescence measurements carried out at two different wavelengths (two-colour TiRe-LII). The LII signals are transferred into a particle ensemble averaged (effective) temperature using Planck’s thermal radiation formula. Assuming that all particles within the size distribution possess a unique temperature at the end of the laser pulse, the size distribution can be determined by numerically simulating the measured temperature decay. From our investigations, for pressures up to a few bars it is obvious that this strategy can be successfully applied if standard laser pulses of nano-second duration are used as an LII-excitation source. At higher pressures the time scales of heat conduction are decreased to such an extent that a unique temperature for all particles within the ensemble cannot be assumed at the end of the nano-second laser pulse. However, further investigations show that the presented two-colour TiRe-LII technique can be successfully adopted under technical high-pressure conditions as well, if the pulse duration of the TiRe-LII-excitation source is reduced into the pico-second range.

A simple two-stage model for the formation and expansion of the plasma induced by high intensity nanosecond laser metal ablation in vacuum

A simple two-stage model for the formation and expansion of the plasma induced by high intensity nanosecond laser metal ablation in vacuum has been developed. To simulate the first stage of plasma formation and initial expansion within the laser pulse, the one-dimensional hydrodynamic equations are supplemented by a wide-range equation of state and solved numerically. For the second stage of plasma expansion after the end of laser pulse, a three-dimensional semi-analytical model is applied, the input of which comes from the output of the model in the first stage. In this way, a self-closed calculation is realized. This two-stage model is easy to apply and has very small computational cost. Although simplified and approximate, its predictions, however, are reasonably accurate, which has been shown by the good agreements between simulations and experimental measurements for the plasma front location, temperature and electron number density.

Impulse-coupling coefficients from a pulsed-laser ablation of semiconductor GaAs

Impulse-coupling coefficients from 1.06 - μm, 10-ns Nd: YAG pulsed-laser radiation to GaAs targets with different areas were measured using the ballistic pendulum method in the laser power density ranging from 4.0×10 8 to 5.0×10 9 W·cm −2. A detonation wave model of the plasma was established theoretically. The expansion process of plasma after the laser pulse ends is described in detail, and the impulse-coupling coefficients from pulsed laser with different energies to GaAs with different areas were calculated using the given model. It is found that the theoretical results agree well with the experimental data.

RADIATIVE MECHANISMS IN LASER-PRODUCED PLASMAS IN Xe

In the present paper the LPP in Xenon at nearly atmospheric pressure is studied by frequency as well as temporal resolved spectroscopy. Experimental evidence confirms that the radiative recombination in the aftermath of the laser pulse, is the main mechanism by which the plasma dissipates its energy. It is also evident that the plasma observed is heated after the laser pulse ends, by a radiative wave evolving from the laser spot which also provides electrons. The evidence is supported by a careful analysis of the temporal evolution of ionized species, by the temporal evolution of the spectral shifts of the line spectrum and its spectral width, and by a careful analysis of the continuum emitted after the line spectrum in the plasma during the initial time-stage.

鉄のエキシマレーザ加工時に誘起されるプラズマの解析 エキシマレーザによるアブレーション加工の基礎的研究 （第１報）

Emission spectra in KrF excimer laser induced plasma were measured with time resolution of the order of tens ns in laser materials processing of iron target; excitation temperature of the laser-induced plasma was determined by a Boltzmann plot of Fe (I), and electron density was determined from an intensity ratio of Fe(I) and Fe(II) lines. At a power density of 8×102 W/m2, the temperature and electron density were approximately 17000-18000 K and 5×1026 m-3 at the end of laser pulse of 30 ns, respectively. The absorption of KrF excimer laser was also measured in the laser induced plasma by using the probe laser beam travelling parallel to the target surface, which is split from the main KrF excimer laser. The absorption coefficient in the plasma thus obtained reached as high as approximetely 4×103 m-1 at the end of laser pulse duration. This value agreed well with the value calculated from the inverse-Bremsstrahlung theory when the excitation temperature and electron density obtained by the spectral method are used. This indicates that considerable amount of laser energy can be absorbed in the laser induced plasma without reaching the target via the absorption in the plasma at the later part of the laser pulse.

-charge-induced acceleration of ions emitted by laser-irradiated surfaces

Pulsed-laser-irradiated surfaces sometimes emit positive ions at energies several volts higher than one would expect, even at modest (<0.1 J/cm2) fluences. A mechanism that can account for this phenomenon is discussed. Intense surface photoemission of electrons during the laser pulse leads to the formation of a space-charge layer near the surface. If the laser fluence were constant, the ions would accelerate and subsequently decelerate as they pass through this steady potential well. As the laser pulse ends, however, some ions may undergo extended acceleration as this space-charge layer moves away from the surface. The maximum possible ion acceleration is analytically calculated and the acceleration for a range of realistic experimental parameters is numerically predicted.

Time-resolved spectroscopy of laser-generated microplasmas

Time-resolved spectrograms have been obtained of the radiation emitted by various target materials when placed at the focus of a 1 MW Q-switched ruby laser beam. A small, highly ionized plasma is formed near the target surface. During each pulse of laser light strong continuous emission occurs, extending well into the ultra-violet. Ge IV and Sn IV lines have been recorded: these appear only briefly. Less highly ionized atoms are more persistent: lines from neutral atoms are emitted for several microseconds after the laser pulse ends. It is shown that absorption of the laser beam by free-free transitions is an effective plasma heating process above a critical `take-off' temperature which depends on the laser beam power density and frequency and on the ionization potential of the target material.