Numerical simulation of melting and evaporation of a cold foil target irradiated by a pre-pulse

Melting and evaporation of matter due to pre-pulse irradiation of a high-peak-power ultra-short pulse laser onto a cold foil target and the expansion of laser-produced plasma are numerically evaluated using a hydrodynamic code based on CIP (cubic-interpolated propagation) and modified C-CUP (CIP-combined unified procedure) methods. The material properties of the solid, equation of state, laser absorption coefficient, skin depth, and thermal conductivity are consistently implemented. The formation and propagation of laser-produced plasmas are obtained with good numerical stability. PACS 02.70.-c; 52.38.Mf; 52.38.Kd     

Two-surface wave decay: improved analytical theory and effects on electron acceleration

Two-surface wave decay (TSWD), i.e., the parametric excitation of electron surface waves, was recently proposed as an absorption mechanism in the interaction of ultrashort, intense laser pulses with solid targets. We present an extension of the fluid theory of TSWD to a warm plasma that treats boundary effects consistently. We also present test-particle simulations showing localized enhancement of electron acceleration by TSWD fields; this effect leads to a modulation of the current density entering into the target and may seed current filamentation instabilities. PACS 52.38.-r; 52.38.Dx     

Development of a target for laser-produced plasma EUV light source using Sn nano-particles

Nano-structured and tin-based targets have been fabricated by the pulsed-laser ablation method, in order to develop efficient and debris-free targets for the laser-produced plasma extreme ultraviolet (EUV) light source at 13.5 nm. Characteristic spectra that have the radiation peak around 13.5 nm were obtained from CO₂ laser produced plasma using the films as a target. A nano-structured target produced EUV light as intense as a bulk target and a narrower line spectrum at 13.5 nm than a bulk target. PACS 32.30.Rj; 52.38.-r; 52.38.Mf; 61.46.+w; 68.37.-d     

Time-resolved analysis of the X-ray emission of femtosecond-laser-produced plasmas in the 1.5-keV range

Recent experimental results on ion beams produced in high-intensity laser–solid interactions indicate the presence of very intense electric fields in the target. This suggests the possibility of efficiently heating a solid material by means of the fast electrons created during the laser–solid interactions and trapped in the target, rather than by the laser photons themselves. We tested this mechanism by irradiating very small cubic aluminum targets with the LULI 100-TW, 300-fs laser at 1.06-m wavelength. X-ray spectra were measured with an ultra-fast streak camera, coupled to a conical Bragg crystal, providing spectra in the 1.5-keV range with high temporal and spectral resolution. The results indicate the creation of a hot plasma, but a very low coupling between the rapid electrons and the solid. A tentative explanation, in agreement with other experimental results and with preliminary particle-in-cell (PIC) simulations, points out the fatal role of the laser prepulse. PACS 52.50.Jm; 52.38.Ph; 52.38.Kd     

Femtosecond X-ray line emission from multilayer targets irradiated by short laser pulses

The invention of high-power, ultra-short-pulse lasers has opened the way to investigations aimed at the creation of a new type of bright X-ray source for various uses including material science applications and time-resolved X-ray diffraction for biology. The efficiency with which laser energy incident on a solid target is converted into an X-ray emission depends on many factors, including the temporal profile of the laser pulse. Here we report the results of our theoretical and experimental investigations of the line X-ray emission from layered solid targets irradiated by ultra-short laser pulses. The laser prepulse parameters and target thickness are optimized to convert the maximum laser energy into an emission in the selected X-ray line. Multilayer foils are proposed to increase the energy of the K-line emission from laser plasma while simultaneously keeping the X-ray pulse duration at a hundred femtoseconds. The emission is studied both experimentally and theoretically by means of an analytical model and numerical simulations. PACS 52.38.Ph; 52.38.Dx; 52.50.Jm     

All optical electron injector using an intense ultrashort pulse laser and a solid wire target

Energetic electron bunches were generated by irradiating a solid tungsten wire 13 μm wide with 50 femtosecond pulses at an intensity of ∼3×10¹⁸ W/cm². The electron yield, energy spectrum and angular distribution were measured. These energetic electron bunches are suitable for injection into a laser driven plasma accelerator. An all-optical electron injector based on this approach could simplify timing and alignment in future laser-plasma accelerator experiments. PACS 41.75.Ht; 41.75.Lx; 52.38.Kd; 52.38.Ph     

Efficient water-window X-ray pulse generation from femtosecond-laser-produced plasma by using a carbon nanotube target

We adopt a multiwalled carbon nanotube target to increase the efficiency of water-window and [_]K X-ray pulse conversion from femtosecond-laser-produced plasma. The diameter of the carbon nanotubes is around 30 nm and the length is about 12-m. The X-ray fluence enhancement in the water-window region is sevenfold compared with a conventional carbon plate target. Further enhancement can be expected by optimizing the size of the carbon nanotubes. Soft X-ray pulse duration is 26 ps. It is also found that the [_]K X-ray line emission from the Si substrate of the carbon nanotube target was enhanced. This result indicates that by covering various solid materials with carbon nanotubes, enhanced short [_]K X-ray pulses with the corresponding wavelength can be obtained. These results show that carbon nanotubes are very attractive as a target for femtosecond laser-produced-plasma X-ray sources in single-shot X-ray microscopy and time-resolved X-ray diffraction. PACS 52.50.Jm; 52.38.-r; 52.38.Ph; 68.37.Yz; 78.67.-n     

Laser ablation of lithium and lithium/cadmium alloy studied by time-of-flight mass spectrometry

Ablation of solid lithium and lithium/cadmium alloy was performed by a 308-nm, nanosecond excimer laser. Analysis of the atomic and molecular composition of the plume in vacuum and in nitrogen atmosphere was performed by means of a linear time-of-flight mass spectrometer. Several ionic masses were observed and systematically studied with respect to the laser fluence, laser beam spot size, background pressure, and target composition. PACS 52.38.Mf; 52.50.Jm; 82.80.Rt     

Ablation plasma ion implantation using a dc power supply

Experiments are reported in which ablation plasma ion implantation (APII) has been demonstrated using a dc power supply. The ability to use a dc power supply for APII has been accomplished by using a perpendicular orientation between the target and the substrate. This perpendicular orientation significantly reduces the arcing between the target and the substrate, in contrast to previous experiments using a parallel target–substrate orientation. With this new technique a KrF laser may be fired during the dc high voltage, accelerating full-energy ions. Initial experiments using dc APII have shown that Ti is deposited and implanted onto the Si substrate, with the highest concentration of Ti located beneath the surface of the film. The deposition/implantation of Ti ions onto Si was verified by X-ray photoelectron spectroscopy. PACS 52.38.Mf     

Development of a two-color interferometer for observing wide range electron density profiles with a femtosecond time resolution

A two-color interferometer for preformed plasma characterization is developed. We observe the electron density distribution of preformed plasmas on a 5 μm-thick copper target irradiated with a high-intensity Ti:sapphire laser. The two-color interferometer extended the observable electron density region using a fundamental (800 nm) probe beam to cover the lower density region and a second harmonic (400 nm) probe beam to cover the higher density region, simultaneously. This characterization of the electron density distribution of preformed plasmas with femtosecond time resolution significantly contributes to the understanding of high-intensity laser–thin-foil interactions during high-energy electron, ion, and X-ray generation. PACS 52.38.-r; 52.50.Jm; 52.70.-m     

Study of forward accelerated fast electrons in ultrashort Ti <Emphasis Type="BoldItalic">K</Emphasis><Subscript><Emphasis Type="BoldItalic">α</Emphasis></Subscript> sources

The results of an experiment aimed at studying hot electrons emerging from a target rear side in ultrashort laser-based [ _]Kα sources are described. In particular, forward accelerated fast electrons propagating through a Ti foil are found to be emitted in a cone perpendicular to the target surface. The energy of these electrons is estimated as well as their divergence. A comparison of the experimental findings with the results of a PIC simulation is also reported, aimed at identifying the physical processes responsible for the production of this forward propagating electron population. PACS 52.38.-r; 52.38.kd; 52.38.Ph     

Spectroscopic measurements of excited particles in a N2 gas RF plasma-assisted carbon laser ablation

In this work, we investigated a carbon plasma plume produced by laser ablation of a graphite target in a nitrogen gas environment. The spatial distributions of C and N atoms were measured by time-resolved absorption spectroscopy. The spatial distributions of the relative densities of CN radicals, C₂, and C₃ molecules were measured using time-resolved emission spectroscopy. We determined that nitrogen gas produced an increase in carbon atom and molecule densities in the ablation plume. It was observed that the addition of RF plasma to the plume increased the CN radicals and C atom densities, and decreased the C₂ and C₃ molecule densities. The RF plasma changed the evolution of various fractional species of C, N, CN, C₂, and C₃ in the ablation plume. The chemical reactions with and without RF plasma were explained using the evolution and density of the fractional species of C, N, CN, C₂, and C₃in the plume. PACS 52.38.Mf; 42.62.Fi; 33.20.-t; 81.05.Uw     

Numerical study of ultra-short laser ablation of metals and of laser plume dynamics

Numerical modeling is used to investigate the physical mechanisms of the interaction of ultra-short (sub-picosecond) laser pulses with metallic targets. The laser–target interaction is modeled by using a one-dimensional hydrodynamic code that includes the absorption of laser radiation, the electronic heat conduction, the electron-phonon or electron–ion energy exchange, as well as a realistic equation of state. Laser fluences typical for micromachining are considered. The results of the 1D modeling are then used as the initial conditions for a 2D plasma expansion model. The dynamics of laser plume expansion in femtosecond regime is investigated. Calculations show that the plasma plume is strongly forward directed. In addition, a two-peaked axial density profile is obtained for 400 nm laser wavelength. The calculation results agree with the experimental observations. PACS 52.38.Mf; 02.60.Cb     

Pulsed laser ablation of SiC in a nitrogen atmosphere: formation of CN

Optical emission lines from the plasma generated by a laser ablation process have been investigated to gather information on the nature of the chemical species present. In particular, the experiments were carried out during the laser ablation of a ceramic sintered SiC target, both in vacuum and in presence of controlled nitrogen atmosphere. Time integrated and spatially resolved emission spectra are dominated by the atomic emission lines from silicon and carbon species, either neutral, or singly ionized. When the ablation process was carried out in a nitrogen gas background direct evidence of the formation of the CN molecular specie was found. Fast photography imaging of the expanding plume revealed the formation of a shock wave at nitrogen pressure above 13.3 Pa, with the consequent heating of the shocked region and enhancement of the kinetics of ionization and excitation. Since the C₂ specie was absent, a CN formation mechanism involving atomic carbon and nitrogen in the presence of a shock wave is suggested. PACS 52.38.Mf; 52.50.Jm, 47.40.-x     

Strong temperature effect on X-ray photo-etching of polytetrafluoroethylene using a 10 Hz laser-plasma radiation source based on a gas puff target

The first results of experiments on direct photo-etching of heated PTFE using a 10 Hz X-ray source based on a laser-irradiated gas puff target are presented. X-ray radiation in the wavelength range from 6 to 20 nm was produced as a result of irradiation of a double-stream gas puff target with Nd:YAG laser pulses of energy 0.8 J and time duration 3 ns. The resulting X-ray pulses with energy of about 100–200 mJ were used to irradiate samples of PTFE to create microstructures by direct photo-etching. Strong enhancement of the photo-etching process was observed for samples heated up to 300 °C. PACS 52.38.Ph; 81.65.Cf; 61.82.Pv     

Propagation of ion shock in solid DT target with nonlinear force-driven plasma blocks

The plasma block (piston) with pressure P ₁ is generated as a result of the nonlinear (ponderomotive) force in laser–plasma interaction. The plasma block can be used for the ignition of a fusion flame front in a solid density deuterium–tritium (DT) target by compressing the fuel that creates an ion shock propagating with velocity u _{ion? shock} in the inside of a solid DT target. The ignition is achieved by creating an ion shock during the final stages of the implosion. We estimated the effect of an ion shock in solid DT target at an early stage with no compression and at the last stage with compression, where density increases by a factor of solid-state density. According to the theoretical model, a large target with a very thin layer of fuel (high-aspect ratio target) would be ideal to obtain the very strong shocks. Results indicate that the maximum compression even by an infinitely strong single shock can never produce more than four times the initial density of DT fuel. The results reported that the threshold ignition energy in a solid DT target is reduced by a factor of 4.     

Characterization of laser-induced plasma plume: Comparison between Al and Al2O3 targets

Characterization of the plasma plume produced by laser ablation from Al and Al₂O₃ targets was carried out on the basis of the line profile analysis of Al(I) (²P°–²S) emission. The spatial distribution and density parameters of electrons and Al atoms in the plume were obtained by comparing observed spectral line profiles with a theoretical calculation. The results showed different behavior for the Al and Al₂O₃ targets. The Al atoms from the Al₂O₃ target were populated in a smaller region than those from the Al target. PACS 52.38.MF; 52.70.Kz; 52.25.Os     

On the atomic particle concentration ratio in a multicomponent laser plasma plume

The occupancy distribution for the excited states of atoms and ions is derived based on emission spectroscopy data for an erosion plasma arising when laser radiation strikes a solid CuSbS₂ target. The ratios between the copper and sulfur concentrations in the laser plume at distances of 1 and 7 mm from the target are compared. It is shown that the copper atom-to-sulfur ion concentration ratio in the laser plasma decreases with distance from the target.     

Generation of monoenergetic ultrashort electron pulses from a fs laser plasma

Ultrashort high-energy electron beams are generated by focusing fs Ti:sapphire laser pulses on a thin metal tape at normal incidence. At laser intensities above 10¹⁶ W/cm² , the fs laser plasma ejects copious amounts of electrons in a direction parallel to the target surface. These electrons are directly detected by means of a backside illuminated X-ray CCD, and their energy spectrum is determined with an electrostatic analyzer. The electrons were observed for two laser polarization directions, parallel and perpendicular to the observation direction. At the maximum applied intensity of 2×10¹⁷ W/cm², the energy distribution peaks at around 35 keV with a hot tail detectable up to about 300 keV. The number of electrons per shot at 35 keV is about 5×10⁸ per sterad per keV. Quasi-monoenergetic electron pulses with a relative energy spread of 1% were produced by using a 50-m slit in the beam path after the analyzer. This approach offers great potential for time-resolved studies of plasma, liquid, and surface structures with atomic-scale spatial resolution. PACS 41.75.Fr; 52.38.Kd; 52.70.Nc     

Observation of a hot high-current electron beam from a self-modulated laser wakefield accelerator

A highly relativistic electron beam produced by a 50 TW laser-plasma accelerator has been characterized by photonuclear techniques. The beam has large divergence that increases with plasma density. The electron yield also increases with plasma density and reaches up to 4x10(11) electrons ( >10 MeV), with beam current approaching the Alfvén limit. Effective electron temperatures exceeding 8 MeV are found, leading to an order of magnitude higher photonuclear activation yield than in solid target experiments with the same laser system.