Laser-target Interaction Research Articles

Sources and space-time distribution of the electromagnetic pulses in experiments on inertial confinement fusion and laser-plasma acceleration.

When high-energy and high-power lasers interact with matter, a significant part of the incoming laser energy is transformed into transient electromagnetic pulses (EMPs) in the range of radiofrequencies and microwaves. These fields can reach high intensities and can potentially represent a significative danger for the electronic devices placed near the interaction point. Thus, the comprehension of the origin of these electromagnetic fields and of their distribution is of primary importance for the safe operation of high-power and high-energy laser facilities, but also for the possible use of these high fields in several promising applications. A recognized main source of EMPs is the target positive charging caused by the fast-electron emission due to laser-plasma interactions. The fast charging induces high neutralization currents from the conductive walls of the vacuum chamber through the target holder. However, other mechanisms related to the laser-target interaction are also capable of generating intense electromagnetic fields. Several possible sources of EMPs are discussed here and compared for high-energy and high-intensity laser-matter interactions, typical for inertial confinement fusion and laser-plasma acceleration. The possible effects on the electromagnetic field distribution within the experimental chamber, due to particle beams and plasma emitted from the target, are also described. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 2)'.

Progress and opportunities for inertial fusion energy in Europe.

In this paper, I consider the motivations, recent results and perspectives for the inertial confinement fusion (ICF) studies in Europe. The European approach is based on the direct drive scheme with a preference for the central ignition boosted by a strong shock. Compared to other schemes, shock ignition offers a higher gain needed for the design of a future commercial reactor and relatively simple and technological targets, but implies a more complicated physics of laser–target interaction, energy transport and ignition. European scientists are studying physics issues of shock ignition schemes related to the target design, laser plasma interaction and implosion by the code developments and conducting experiments in collaboration with US and Japanese physicists, providing access to their installations Omega and Gekko XII. The ICF research in Europe can be further developed only if European scientists acquire their own academic laser research facility specifically dedicated to controlled fusion energy and going beyond ignition to the physical, technical, technological and operational problems related to the future fusion power plant. Recent results show significant progress in our understanding and simulation capabilities of the laser plasma interaction and implosion physics and in our understanding of material behaviour under strong mechanical, thermal and radiation loads. In addition, growing awareness of environmental issues has attracted more public attention to this problem and commissioning at ELI Beamlines the first high-energy laser facility with a high repetition rate opens the opportunity for qualitatively innovative experiments. These achievements are building elements for a new international project for inertial fusion energy in Europe.This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 1)’.

Solvents-dependent selective fabrication of face-centered cubic and hexagonal close-packed structured ruthenium nanoparticles during liquid-phase laser ablation

Ruthenium nanoparticles (Ru NPs) with face-centered cubic (fcc) structure possess higher catalytic activity than that with hexagonal close-packed (hcp) structure. However, a high temperature above 1800 K is needed for the formation of the metastable fcc Ru phase. In this study, we present a tunable fabrication strategy of fcc and hcp Ru NPs by laser ablation of Ru target in solvents. In methanol, ethanol or acetone organic solvent, both fcc and hcp Ru NPs encapsulated in carbon layer could be obtained, while in deionized water only pure hcp Ru NPs formed. The extreme conditions, that is, the laser-target interaction induced high temperature and high-pressure plasma plume (4000–5000 K, 10–15 GPa) together with its subsequent quenching process, favored the formation of metastable fcc phase. Significantly, the graphite carbon layers sourced from the thermal decomposition of solvent molecules prevent the further evolution of metastable fcc phase into stable hcp phase. Clarification of the solvents and pulse energy effects promise the tunable fabrication of Ru NPs with desired crystallographic structure during laser ablation in liquids (LAL).

High-Intensity Laser-Driven Oxygen Source from CW Laser-Heated Titanium Tape Targets

The interaction of high-intensity laser pulses with solid targets can be used as a highly charged, energetic heavy ion source. Normally, intrinsic contaminants on the target surface suppress the performance of heavy ion acceleration from a high-intensity laser–target interaction, resulting in preferential proton acceleration. Here, we demonstrate that CW laser heating of 5 µm titanium tape targets can remove contaminant hydrocarbons in order to expose a thin oxide layer on the metal surface, ideal for the generation of energetic oxygen beams. This is demonstrated by irradiating the heated targets with a PW class high-power laser at an intensity of 5 × 1021 W/cm2, showing enhanced acceleration of oxygen ions with a non-thermal-like distribution. Our new scheme using a CW laser-heated Ti tape target is promising for use as a moderate repetition energetic oxygen ion source for future applications.

Time-resolved characterization of ultrafast electrons in intense laser and metallic-dielectric target interaction.

High-intensity ultrashort laser pulses interacting with thin solid targets are able to produce energetic ion beams by means of extremely large accelerating fields set by the energetic ejected electrons. The characterization of such electrons is thus important in view of a complete understanding of the acceleration process. Here, we present a complete temporal-resolved characterization of the fastest escaping hot electron component for different target materials and thicknesses, using temporal diagnostics based on electro-optical sampling with 100 fs temporal resolution. Experimental evidence of scaling laws for ultrafast electron beam parameters have been retrieved with respect to the impinging laser energy (0.4-4 J range) and to the target material, and an empirical law determining the beam parameters as a function of the target thickness is presented.

Study of femtosecond laser pulse induced shockwave in aluminum-coated dielectric target

The influence of the preplasma on laser induced shockwave in the laser and aluminum-coated planar dielectric target interaction at vacuum has been investigated with the shadowgraphy method. While the laser irradiate on the aluminum-coated dielectric target at intensity of about 1017 W/cm2, the metallic layers absorb laser energy, evaporate and ionize into plasma, it is verified that the scale length of laser-produced plasma is dramatically dependent on the contrast ratio of femtosecond-laser while the main laser pulse energy is almost kept. The characteristics of laser induced shock wave in nanosecond time scale were studied. In the nanosecond time scale, shock wave is only observed in the case of relatively short plasma scale length. This result can be explained by the dissipation of the shock wave during its propagation in the preplasma. In addition, we performed numerical simulation with MULTI2D to get an insight into the propagation of shock wave in the overdense plasma [R. Ramis, J. Meyer-ter-Vehn, and J. Ramírez, Comput. Phys. Commun. 180, 977 (2009)].

Laser-driven generation of collimated quasi-monoenergetic proton beam using double-layer target with modulated interface

Usage of double-layer targets consisting of heavy and light material with modulated interface between them provides a way for laser-driven generation of collimated ion beams. With extensive 2D3V PIC simulations we show that this configuration may result in a development of a relativistic instability with Rayleigh-Taylor and Richtmyer-Meshkov like features. Initially small perturbations are amplified during the laser-target interaction leading to the formation of low-density plasma regions and high-density bunches between them, which are accelerated by the laser radiation pressure as whole compact structures. That results in collimated quasi-monoenergetic proton beam with high average energy. The properties of this proton beam such as its low emittance (one order of magnitude lower compared to that of conventional accelerators) and divergence are discussed. Results are compared with similar acceleration schemes such as double-layer target without corrugation and single-layer target.

Control of intense-laser ion acceleration

An ion beam has a unique feature to deposit its main energy inside a human body to kill cancer cells or inside material. In this paper, a future intense laser ion accelerator is discussed to make the laser-based ion accelerator compact and controllable. The issues in the laser ion accelerator include the energy efficiency from the laser to the ions, the ion beam collimation, the ion energy spectrum control, the ion beam bunching, and the ion particle energy control. The energy efficiency from the laser to ions is improved by using a solid target with a fine sub-wavelength structure. The ion beam collimation is performed by larger holes behind the solid target. An ion beam bunching and a post-acceleration are successfully realized by a multi-stage laser-target interaction. The controllable laser ion acceleration would become viable based on the study discussed in this paper. In this paper, we present 3D PIC simulation results for the controllability of laser ion acceleration system.

Characterisation and Modelling of Ultrashort Laser-Driven Electromagnetic Pulses

Recent advances on laser technology have enabled the generation of ultrashort (fs) high power (PW) laser systems. For such large scale laser facilities there is an imperative demand for high repetition rate operation in symbiosis with beamlines or end-stations. In such extreme conditions the generation of electromagnetic pulses (EMP) during high intense laser target interaction experiments can tip the scale for the good outcome of the campaign. The EMP effects are several including interference with diagnostic devices and actuators as well as damage of electrical components. The EMP issue is quite known in the picosecond (ps) pulse laser experiments but no systematic study on EMP issues at multi-Joule fs-class lasers has been conducted thus far. In this paper we report the first experimental campaign for EMP-measurements performed at the 200 TW laser system (VEGA 2) at CLPU laser center. EMP pulse energy has been measured as a function of the laser intensity and energy together with other relevant quantities such as (i) the charge of the laser-driven protons and their maximum energy, as well as (ii) the X-ray Kα emission coming from electron interaction inside the target. Analysis of experimental results demonstrate (and confirm) a direct correlation between the measured EMP pulse energy and the laser parameters such as laser intensity and laser energy in the ultrashort pulse duration regime. Numerical FEM (Finite Element Method) simulations of the EMP generated by the target holder system have been performed and the simulations results are shown to be in good agreement with the experimental ones.

Tumbling object deorbiting using spaceborne laser engagement – A CubeSat case study

This paper demonstrates active space debris removal using spaceborne laser systems. The laser beam and the surface of the target are discretised into multiple rays and finite elements, respectively, for laser-target interaction modelling, in which the laser ablation process is investigated. A high-fidelity attitude/orbit propagator tool is developed to account for both the linear impulse and angular impulse induced by the laser engagement and other perturbations. The laser system is activated only when three switch criteria are satisfied. In numerical simulations, laser pulses from international space station are generated to deorbit a 3U CubeSat with initially tumbling modes. The results validate the effectiveness of deorbiting tumbling CubeSats using spaceborne laser engagement, with the perigee height lowered by approximately 2.4km in around 30min after 2h propagation. It is also found that the laser engagement becomes more effective for an initially faster rotating object.

Species distribution in laser-induced plasma on the surface of binary immiscible alloy

Investigation on species distribution of vapor plasma is of great significance for understanding the laser-target interaction, the plasma dynamics and verifying theoretical models. It is also helpful for optimizing the spectral acquisition to improve the quantitative performance of spatial-resolved laser-induced breakdown spectroscopy (LIBS). In this work, the laser-induced plasma on the surface of binary alloy is analyzed by using spectrally, spatially and temporally resolved dual-wavelength differential imaging to explore the dependence of species distribution in plasma on laser-supported absorption (LSA) wave regime and sample composition. The laser-supported combustion (LSC) and laser-supported detonation (LSD) wave dominated plasmas with different elemental mass ratios are induced from a series of binary immiscible alloys by using low and high irradiance laser pulses, respectively. By comparing the spatial-temporal emissivity images of species, the following conclusions are drawn: 1) laser irradiance can change the species distribution of plasma, while the sample composition cannot; 2) for LSC-wave dominated plasma, the species distribution mainly depends on the melting points of constituted elements in the sample, while for LSD-wave dominated plasma, it mainly depends on the atomic masses of elements. The above behaviours on species distribution in plasma produced from binary immiscible alloys are expected to be suitable for other elements or even multi-element plasmas.

Generation of intense quasi-electrostatic fields due to deposition of particles accelerated by petawatt-range laser-matter interactions

We demonstrate here for the first time that charge emitted by laser-target interactions at petawatt peak-powers can be efficiently deposited on a capacitor-collector structure far away from the target and lead to the rapid (tens of nanoseconds) generation of large quasi-static electric fields over wide (tens-of-centimeters scale-length) regions, with intensities much higher than common ElectroMagnetic Pulses (EMPs) generated by the same experiment in the same position. A good agreement was obtained between measurements from a classical field-probe and calculations based on particle-flux measurements from a Thomson spectrometer. Proof-of-principle particle-in-cell simulations reproduced the measurements of field evolution in time, giving a useful insight into the charging process, generation and distribution of fields. The understanding of this charging phenomenon and of the related intense fields, which can reach the MV/m order and in specific configurations might also exceed it, is very important for present and future facilities studying laser-plasma-acceleration and inertial-confinement-fusion, but also for application to the conditioning of accelerated charged-particles, the generation of intense electric and magnetic fields and many other multidisciplinary high-power laser-driven processes.

Shock Waves in Laser-Induced Plasmas

The production of a plasma by a pulsed laser beam in solids, liquids or gas is often associated with the generation of a strong shock wave, which can be studied and interpreted in the framework of the theory of strong explosion. In this review, we will briefly present a theoretical interpretation of the physical mechanisms of laser-generated shock waves. After that, we will discuss how the study of the dynamics of the laser-induced shock wave can be used for obtaining useful information about the laser–target interaction (for example, the energy delivered by the laser on the target material) or on the physical properties of the target itself (hardness). Finally, we will focus the discussion on how the laser-induced shock wave can be exploited in analytical applications of Laser-Induced Plasmas as, for example, in Double-Pulse Laser-Induced Breakdown Spectroscopy experiments.

A platform for nuclear physics experiments with laser-accelerated light ions

A novel platform for nuclear physics experiments has been developed on the high-energy, short-pulse OMEGA EP Laser System. Planar foil targets are irradiated with a 10-ps, 1-kJ infrared beam focused to an intensity of the order of 1018 W/cm2. Relativistic electrons generated in the laser–target interaction escape the target, generating a very large electrostatic field, which extracts ions from the target’s back surface and accelerates them to MeV energies. The energetic ion flow from the back side of this primary target creates neutrons and charged particles through nuclear reactions in a secondary target placed in the ion flow. Charged-particle detectors were used to measure the energy spectra of the ions. The energy spectrum of the neutrons generated in the secondary target was measured using scintillator-based neutron time-of-flight detectors. First experiments to validate the performance of this setup studied d(d,n)3He and 9Be(d,n)10B reactions. This experimental platform is suitable especially for survey-type studies of nuclear reactions and for reactions that involve rare or radioactive ions like tritium.

Highly-collimated, high-charge and broadband MeV electron beams produced by magnetizing solids irradiated by high-intensity lasers

Laser irradiation of solid targets can drive short and high-charge relativistic electron bunches over micron-scale acceleration gradients. However, for a long time, this technique was not considered a viable means of electron acceleration due to the large intrinsic divergence (∼50° half-angle) of the electrons. Recently, a reduction in this divergence to 10°–20° half-angle has been obtained, using plasma-based magnetic fields or very high contrast laser pulses to extract the electrons into the vacuum. Here we show that we can further improve the electron beam collimation, down to ∼1.5° half-angle, of a high-charge (6 nC) beam, and in a highly reproducible manner, while using standard stand-alone 100 TW-class laser pulses. This is obtained by embedding the laser-target interaction in an external, large-scale (cm), homogeneous, extremely stable, and high-strength (20 T) magnetic field that is independent of the laser. With upcoming multi-PW, high repetition-rate lasers, this technique opens the door to achieving even higher charges (>100 nC).

Features of the generation of fast particles from microstructured targets irradiated by high intensity, picosecond laser pulses

AbstractThe use of targets with surface structures for laser-driven particle acceleration has potential to significantly boost the particle and radiation energies because of enhanced laser absorption. We investigate, via experiment and particle-in-cell simulations, the impact of micron-scale surface-structured targets on the spectrum of electrons and protons accelerated by a picosecond laser pulse at relativistic intensity. Our results show that, compared with flat-surfaced targets, structures on this scale give rise to a significant enhancement in particle and radiation emission over a wide range of laser–target interaction parameters. This is due to the longer plasma scale length when using micro-structures on the target front surface. We do not observe an increase in the proton cutoff energy with our microstructured targets, and this is due to the large volume of the relief.

A quasi-static model for hot-electron interaction with self-generated magnetic fields

The time evolution of the target temperature and ionization degree during the laser-target interaction is of primary importance to understanding the transition between solid and plasma. When the interaction lasts a few tens of femtoseconds, the target resistivity is not well known as in the Spitzer regime, and therefore approximated information must be used from experiments and/or from models. The calculation of the target temperature and the magnetic fields produced inside the target after the propagation of a fast electron current is performed in this paper accounting for the pulse temporal envelope and making use of a complete resistivity model. Analytic calculations of temporal and spatial varying magnetic fields are also presented. Finally, a novel interpretation of the beam hollowing phenomenon is given based on the outcomes of the model developed.

Wavelength-dependent variations of the electron characteristics in laser-induced plasmas: A combined hydrodynamic and adiabatic expansion modelling and time-gated, optical emission imaging study

The spatial and temporal evolution of the absolute electron densities and temperatures in plasmas formed by nanosecond pulsed laser ablation of silicon in vacuum at two wavelengths (1064 and 532 nm), at similar irradiances, have been explored by complementary simulation (using combined hydrodynamic and adiabatic models) and experiment. Modelling the laser-target and laser-plume interactions with the POLLUX code reveals the evolving composition and dynamics of the laser induced plasma (LIP) during the incident laser pulse: 532 nm irradiation causes more ablation, but the LIP formed by 1064 nm excitation has a higher average charge state and expands faster. The experimental data, from the analysis of Stark broadened line shapes of SiIII and SiIV cations in time-gated, position- and wavelength-resolved images of the plume emission, allow characterisation of the plume dynamics at later times. These dynamics are compared with predictions from two forms of adiabatic expansion model. Both take as input parameters the plume properties returned by the POLLUX simulations for the end of the laser pulse, but differ according to whether the initial plasma is assumed isothermal or isentropic. The study illustrates the important λ-dependences of the target absorption coefficient (in establishing the ablated material density) and of electron–ion inverse bremsstrahlung absorption (in coupling laser radiation into the emergent plasma); the extents to which these interactions, the relative ablation yields, and the plume expansion dynamics depend on λ; and the importance of identifying appropriate initial conditions for adiabatic expansion modelling of LIP in vacuum.

Ionization dynamics of dense matter generated by intense ultrashort X‐ray pulses

We investigate a pump‐probe X‐ray Thomson scattering (XRTS) experiment that might be carried out at a free electron laser facility to study warm‐to‐hot states of dense matter. Ultrashort and intense X‐ray pulses with different energies (1,560–1,830 eV) heat a 1 µm thick Al target isochorically and create homogeneous and uncompressed warm‐to‐hot states of dense matter. A second pulse with variable delay probes this heated state via XRTS. The X‐ray laser–target interaction is modelled within radiation‐hydrodynamic simulations applying the HELIOS‐CR code. The HELIOS‐CR results qualitatively agree with Monte‐Carlo simulations, where the laser pulse absorption is simulated based on a uniform random sequence of events. The electron feature in the simultaneously observed X‐ray scattering spectrum is a function of the degree of ionization and the target temperature. Therefore, the temporal evolution of the plasmon peak measures the ionization dynamics on ultra‐short time scales. The XRTS spectrum is calculated based on the Chihara formula utilizing the Born‐Mermin approximation for the free electron dynamic structure factor. The proposed experiment will reveal important details of the ionization dynamics on ultra‐short time scales as well as of the relaxation on ps time scales.

Spectral characterization by CVD diamond detectors of energetic protons from high-repetition rate laser for aneutronic nuclear fusion experiments

In this work, we describe the results of experiments performed at the femtosecond ECLIPSE laser facility at CELIA in Bordeaux. We irradiated targets constituted by aluminum foils of various thicknesses and characterized the accelerated protons with energies in the range of interest for p+11B nuclear fusion. Time-of-flight measurements were performed by using Chemical Vapor Deposition (CVD) monocrystalline diamond detectors, which are specifically designed to operate in harsh environments where large ElectroMagnetic Pulses (EMPs) are generated during laser-target interaction. The 1 Hz high repetition rate used for the ECLIPSE laser allowed the collection of a large number of similar shots, giving therefore a large statistics to accurately characterize the energy spectrum of the laser-plasma accelerated protons. The laser repetitivity at a relatively small intensity permits to improve the signal-to-noise ratio of the detection of the products of low cross-section reactions, such as the p+11B one, by their collection over a large number of similar shots.