Laser-solid Target Interaction Research Articles

Femtosecond, two-dimensional spatial Doppler mapping of ultraintense laser-solid target interaction

We present measurements of the spatiotemporal evolution of a hot-dense plasma generated by the interaction of an intense 25 fs laser pulse with a solid target, using pump-probe two-dimensional (2D) Doppler spectrometry. Measuring the time-dependent Doppler shifts at different positions across the probe beam, we achieve velocity mapping at hundreds of femtoseconds time resolution simultaneously with a few micrometer spatial resolution across the transverse length of the plasma. Simulations of the interaction using a combination of 2D particle-in-cell and 2D radiation hydrodynamics codes agree well with the experiment.

Direct electron attachment to fast hydrogen in 10−9 contrast 1018 W cm−2 intense laser solid target interaction

The interaction of an ultra-short (<30 fs), high-contrast (<10−9), high-intensity (>1018 W cm−2) laser pulse with a solid target is not generally known to produce and accelerate negative ions. The transient accelerating electrostatic-fields are so strong that they ionize any atom or negative ion at the target surface. In spite of what may appear to be unfavourable conditions, here it is reported that H− ions extending up to 80 keV are measured from such an interaction. The H− ion flux is about 0.1% that of the H+ ions at 20 keV. These measurements employ a recently developed temporally-gated Thomson parabola ion spectrometry diagnostic which significantly improves signal-to-noise ratios. Electrons that co-propagate with the fast protons cause a two-step charge-reduction reaction. The gas phase three-body attachment of electrons to fast neutral hydrogen atoms accounts for the measured H− yield. It is intriguing that such a fundamental gas-phase reaction, involving the attachment of an electron to a hydrogen atom, has not been observed in laboratory experiments previously. Laser-produced plasma offers an alternative environment to the conventional charged particle beam experiments, in which such atomic physics processes can be investigated.

Spatio-temporal analysis of high-power laser micro-structured targets interaction

The present work reports the development of an optimized ultra-intense laser—solid target interaction system which aims to achieve higher intensities for different pulse durations, down to few-cycle regime, considering different micro-structured profiles of the internal cone target walls. A spatio-temporal analysis of the electromagnetic field in the interaction area is elaborated in order to provide a complex description of this approach. The numerical analysis is made by using the finite difference time domain (FDTD) method. The results are explained in correspondence with the extension of the Rayleigh range and it is relevant for a wide range of ultra-short and ultra-intense laser experiments planned at laser facilities in petawatt regimes.

Modeling terahertz emission from the target rear side during intense laser-solid interactions.

Relativistic laser-solid target interaction is a powerful source of terahertz radiation where broadband terahertz radiation is emitted from the front and rear surfaces of the target. Even though several experimental works have reported the generation of subpicosecond duration gigawatt peak power terahertz pulses from the target rear surface, the underlying physical process behind their origin is still an open question. Here we discuss a numerical model that can accurately reproduce several aspects of the experimental results. The model is based on the charged particle dynamics at the target rear surface and the evolution of the charge separation field. We identify the major contributors that are responsible for broadband terahertz emission from the rear surface of the target.

Characterization of fast electron divergence and energy spectrum from modeling of angularly resolved bremsstrahlung measurements

Characteristics of fast electrons generated in an intense laser-solid target interaction are studied by modeling angularly resolved bremsstrahlung measurements with a hybrid Particle-In-Cell code, Large Scale Plasmas. The experiment was performed using the 50 TW Leopard laser at the Nevada Terawatt Facility. A 100 μm thick Cu foil was irradiated by the 15 J, 0.35 ps laser at a peak laser intensity of 2 × 1019 W/cm2. Bremsstrahlung produced by transport and recirculation of the fast electrons in the foil was measured with two differential filter-stack x-ray spectrometers at 22° and 40° from the laser axis. The two-spectrometer signals simultaneously fit by varying single slope temperatures (Thot) and divergence angles (θ) enable for determining Thot and θ to be 1.1 ± 0.3 MeV and 15° ± 8°, respectively. The Thot inferred from the bremsstrahlung signals agrees with that from an escaped electron measurement, suggesting that that the bremsstrahlung is predominantly produced by the transport of the high energy fast electrons in the first pass.

Directional enhancement of selected high-order-harmonics from intense laser irradiated blazed grating targets.

Relativistically intense laser solid target interaction has been proved to be a promising way to generate high-order harmonics, which can be used to diagnose ultrafast phenomena. However, their emission direction and spectra still lack tunability. Based upon two-dimensional particle-in-cell simulations, we show that directional enhancement of selected high-order-harmonics can be realized using blazed grating targets. Such targets can select harmonics with frequencies being integer times of the grating frequency. Meanwhile, the radiation intensity and emission area of the harmonics are increased. The emission direction is controlled by tailoring the local blazed structure. Theoretical and electron dynamics analysis for harmonics generation, selection and directional enhancement from the interaction between multi-cycle laser and grating target are carried out. These studies will benefit the generation and application of laser plasma-based high order harmonics.

Modelling K shell spectra from short pulse heated buried microdot targets

K shell X-ray emission measurements have been used to diagnose plasma conditions in short-pulse heated buried microdot targets on the Orion high power laser. These experiments have been used to validate simulations of short pulse laser-solid interaction that combine hybrid PIC modelling of the laser absorption with radiation-hydrodynamics simulations including an electron transport model. Comparison of these simulations with streaked K shell spectroscopy show the importance of including radial gradients in fitting the spectra. An example is presented of the emission of sulphur from a 50µm diameter microdot sample buried in a plastic foil. Previously agreement between simulation and experiment was obtained only by treating the absorbed energy, electron temperature and beam divergence as fitting parameters. The good agreement obtained in this work used the measured laser energy and laser pulse length and calculated the laser-solid target interaction from first principles.

Dynamics of bulk electron heating and ionization in solid density plasmas driven by ultra-short relativistic laser pulses

The dynamics of bulk heating and ionization is investigated both in simulations and theory, which determines the crucial plasma parameters such as plasma temperature and density in ultra-short relativistic laser-solid target interactions. During laser-plasma interactions, the solid density plasma absorbs a fraction of laser energy and converts it into kinetic energy of electrons. A portion of the electrons with relativistic kinetic energy goes through the solid density plasma and transfers energy into the bulk electrons, which results in bulk electron heating. The bulk electron heating is finally translated into the processes of bulk collisional ionization inside the solid target. A simple model based on the Ohmic heating mechanism indicates that the local and temporal profile of bulk return current is essential to determine the temporal evolution of bulk electron temperature. A series of particle-in-cell simulations showing the local heating model is robust in the cases of target with a preplasma and without a preplasma. Predicting the bulk electron heating is then benefit for understanding the collisional ionization dynamics inside the solid targets. The connection of the heating and ionization inside the solid target is further studied using Thomas-Fermi model.

Picosecond metrology of laser-driven proton bursts.

Tracking primary radiation-induced processes in matter requires ultrafast sources and high precision timing. While compact laser-driven ion accelerators are seeding the development of novel high instantaneous flux applications, combining the ultrashort ion and laser pulse durations with their inherent synchronicity to trace the real-time evolution of initial damage events has yet to be realized. Here we report on the absolute measurement of proton bursts as short as 3.5±0.7 ps from laser solid target interactions for this purpose. Our results verify that laser-driven ion acceleration can deliver interaction times over a factor of hundred shorter than those of state-of-the-art accelerators optimized for high instantaneous flux. Furthermore, these observations draw ion interaction physics into the field of ultrafast science, opening the opportunity for quantitative comparison with both numerical modelling and the adjacent fields of ultrafast electron and photon interactions in matter.

Three dimensional effects on proton acceleration by intense laser solid target interaction

Multi-dimensional effects on ion acceleration by a normally incident linearly polarized intense laser pulse interacting with a thin solid target have been investigated numerically, where the laser has the peak intensity of 1.37×1020 W/cm2, focused spot size of 6 μm, pulse duration of 33 fs, and total pulse energy about 3 J, which are commercially available now. We have checked the effects of simulation geometries by running one, two, and three dimensional (1D, 2D, 3D) particle-in-cell simulations. 3D simulation results show that, in the case of using a relatively thick target (in the opaque regime, i.e., 2 μm) with the so-called target normal sheath field acceleration mechanism, electrons spread almost uniformly along two transverse directions. While in the case of using an ultra-thin target (in the relativistic-induced transparent regime, i.e., 100 nm) with the so-called break-out afterburner mechanism, electrons spread more quickly along the direction orthogonal to the laser polarization direction especially at the early stage. The transverse spreading of electrons strongly decreases the electron density at the rear side of the target. Such an effect causes different estimation of electron temperatures in different simulation geometries. Usually, 1D and 2D simulations overestimate the temperature; and as a result, the maximum proton energy observed in 1D and 2D simulations is, respectively, about 3 and 2 times of that observed in 3D simulation.

X-ray source produced by laser solid target interaction at kHz repetition rate

利用kHz激光与固体靶相互作用产生了平均流强为1.3&#215;107 photons&#183;sr-1&#183;s-1的X射线源, 研究了激光的对比度和能量对激光与固体靶相互作用产生的X射线能谱及Kα 产额的影响, 使用刀边成像技术测量了X射线源的源尺寸, 并进行了初步的成像实验. 实验中观察到对于非相对论级别的激光脉冲, 降低激光的对比度有利于提高Kα 产额, 而使用高对比度高强度激光, 更有利于获得高通量高信噪比X射线源.

Equation of state studies of warm dense matter samples heated by laser produced proton beams

Heating of matter by proton beams produced by short pulse, laser-solid target interaction has been demonstrated over the last ten years by a number of workers. In the work described in this paper heating by a pulse of laser produced protons has been combined with high-resolution soft x-ray radiography to record the expansion of thin wire targets. Analysis of the radiographs yields material properties in the warm dense matter regime. These measurements imply initial temperatures in the experimental samples over a range from 14 eV up to 40 eV; the sample densities varied from solid to a tenth solid density. Assuming an adiabatic expansion after the initial proton heating phase isentropes of the aluminium sample material were inferred and compared to tabulated data from the SESAME equation of state library. The proton spectrum was also measured using calibrated magnetic spectrometers and radiochromic film. The accuracy of the technique used to infer material data is discussed along with possible future development.

Precision X-ray spectroscopy of intense laser-plasma interactions

Polarisation sensitive emission spectroscopy measurements are reported for a petawatt laser-solid target interaction at intensities up to 5 × 10 20 W cm −2. These measurements were single-shot and used pairs of highly-orientated graphite spectrometers to resolve the sulphur Ly-α doublet. The sulphur Ly-α 1 component shows a large positive polarisation indicative of a low energy electron beam in the plasma, the Ly-α 2 component acts as a cross-spectrometer calibration. The measurements show a significant anisotropic or beam-like component to a cold return current.

Extreme ultraviolet diagnostics of preformed plasma in laser-driven proton acceleration experiments

Proton acceleration experiments involving a 5μm thick Ti foil target irradiation are carried out with the femtosecond Ti:sapphire laser JLITE-X. The plasma emission at 13.5nm is recorded employing concave multilayer mirrors, which image the front- and rear-side plasmas onto the sensitive surfaces of a fast x-ray photodiode and a backside-illuminated charge coupled device. Online time-of-flight fast-particle measurements are performed simultaneously with the extreme ultraviolet (XUV) measurements. A strong correlation is observed between the energetic proton signal and the spatiotemporal behavior of the XUV plasma emission. In particular, the longer duration of the prepulse-produced XUV plasma emission indicates a lowering of the maximum proton energy. This allows using the XUV emission for the diagnostics of the high-intensity laser–solid-target interaction.

High harmonic generation in the relativistic limit

The generation of extremely bright coherent X-ray pulses in the femtosecond and attosecond regime is currently one of the most exciting frontiers of physics–allowing, for the first time, measurements with unprecedented temporal resolution1,2,3,4,5,6. Harmonics from laser–solid target interactions have been identified as a means of achieving fields as high as the Schwinger limit2,7 (E=1.3×1016 V m−1) and as a highly promising route to high-efficiency attosecond (10−18 s) pulses8 owing to their intrinsically phase-locked nature. The key steps to attain these goals are achieving high conversion efficiencies and a slow decay of harmonic efficiency to high orders by driving harmonic production to the relativistic limit1. Here we present the first experimental demonstration of high harmonic generation in the relativistic limit, obtained on the Vulcan Petawatt laser9. High conversion efficiencies (η>10−6 per harmonic) and bright emission (>1022 photons s−1 mm−2 mrad−2 (0.1% bandwidth)) are observed at wavelengths <4 nm (the `water-window' region of particular interest for bio-microscopy).

Experimental study on energy distribution of the hot electrons generated by femtosecond laser interacting with solid targets

This paper reports the results of the experiment of hot electron energy distribution during the femtosecond laser-solid target interaction. The hot electrons formed an anisotropic energy distribution. In the direction of the target normal, the energy spectrum of the hot electron was a Maxwellian-like distribution with an effective temperature of 206keV, which was due to the resonance absorption. In the direction of the specular reflection of laser, there appeared a local plateau of hot electron energy spectrum at the beginning and then it was decreased gradually, which maybe produced by several acceleration mechanisms. The effective temperature and the yield of hot electrons in the direction of the target normal is larger than those in the direction of the specular reflection of laser, which proves that the resonance absorption mechanism is more effective than others.

Measurements of ultrastrong magnetic fields during relativistic laser–plasma interactions

Measurements of magnetic fields generated during ultrahigh intensity (&amp;gt;1019 W cm−2), short pulse (0.7–1 ps) laser–solid target interaction experiments are reported. An innovative method is used and the results are compared with particle-in-cell simulations. It is shown that polarization measurements of the self-generated harmonics of the laser can provide a convenient method for diagnosing the magnetic field—and that the experimental measurements indicate the existence of peak fields greater than 340 MG and below 460 MG at such high intensities. In particular, the observation of the X-wave cutoffs and the observed induced ellipticity of the harmonics can provide a reliable method for measuring these fields. These observations are important for evaluating the use of intense lasers in various potential applications and perhaps for understanding the complex physics of exotic astrophysical objects such as neutron stars.

Modeling stimulated Raman scattering for smoothed laser–solid target interaction at 0.53 μm

The behavior of stimulated Raman backscattering in an inhomogeneous plasma at subcritical density is investigated in the context of inertial confinement fusion. A refined independent hot spots model has been introduced in a laser interaction postprocessor to hydrodynamical code simulations. The model is shown to reproduce most experimentally observed trends, provided that the Langmuir waves damping is reduced on axis, due to the presence of trapping, inverse bremsstrahlung heating, or perpendicular self-generated magnetic fields. The experiments were carried out at 0.53 μm at the Phébus laser facility, and Raman scattering was measured as a function of laser intensity and relative bandwidth, as well as solid target composition.

Picosecond optical probing of ultrafast energy transport in short pulse laser solid target interaction experiments

This paper reports observations of rapid energy transport effects resulting from high intensity laser heating of fused silica targets. Picosecond optical probing of these interactions provides information on the kinematics of supersonic ionization fronts driven into the targets. Studies have been conducted as a function of laser intensity, wavelength, and target angle. Additionally, targets with metallic surface layers have been investigated. Characterization of the laser absorption has enabled plasma and radiation hydrodynamics energy transport simulations to be implemented. Although consideration has been given to several energy transport mechanisms, including thermal and suprathermal electron transport, the kinematics are best explained with a radiation transport model. This is confirmed by angled and high and medium Z coated target experiments.

Modeling of dynamical processes in laser ablation

Various physics and computational approaches have been developed to globally characterize phenomena important for film growth by pulsed-laser deposition of materials. These include thermal models of laser-solid target interactions that initiate the vapor plume, plume ionization and heating through laser absorption beyond local thermodynamic equilibrium mechanisms, hydrodynamic and collisional descriptions of plume transport, and molecular dynamics models of the interaction of plume particles with the deposition substrate.