Time evolution of stimulated Raman scattering and two-plasmon decay at laser intensities relevant for shock ignition in a hot plasma

Laser–plasma interaction (LPI) at intensities 1015–1016 W cm2 is dominated by parametric instabilities which can be responsible for a significant amount of non-collisional absorption and generate large fluxes of high-energy nonthermal electrons. Such a regime is of paramount importance for inertial confinement fusion (ICF) and in particular for the shock ignition scheme. In this paper we report on an experiment carried out at the Prague Asterix Laser System (PALS) facility to investigate the extent and time history of stimulated Raman scattering (SRS) and two-plasmon decay (TPD) instabilities, driven by the interaction of an infrared laser pulse at an intensity 1:2 1016 W cm2 with a 100 mm scalelength plasma produced from irradiation of a flat plastic target. The laser pulse duration (300 ps) and the high value of plasma temperature (4 keV) expected from hydrodynamic simulations make these results interesting for a deeper understanding of LPI in shock ignition conditions. Experimental results show that absolute TPD/SRS, driven at a quarter of the critical density, and convective SRS, driven at lower plasma densities, are well separated in time, with absolute instabilities driven at early times of interaction and convective backward SRS emerging at the laser peak and persisting all over the tail of the pulse. Side-scattering SRS, driven at low plasma densities, is also clearly observed. Experimental results are compared to fully kinetic large-scale, two-dimensional simulations. Particle-in-cell results, beyond reproducing the framework delineated by the experimental measurements, reveal the importance of filamentation instability in ruling the onset of SRS and stimulated Brillouin scattering instabilities and confirm the crucial role of collisionless absorption in the LPI energy balance

Identifiable Acetylene Features Predicted for Young Earth-like Exoplanets with Reducing Atmospheres Undergoing Heavy Bombardment

The chemical environments of young planets are assumed to be largely influenced by the impacts of bodies lingering on unstable trajectories after the dissolution of the protoplanetary disk. We explore the chemical consequences of impacts within the context of reducing planetary atmospheres dominated by carbon monoxide, methane, and molecular nitrogen. A terawatt high-power laser was selected in order to simulate the airglow plasma and blast wave surrounding the impactor. The chemical results of these experiments are then applied to a theoretical atmospheric model. The impact simulation results in substantial volume mixing ratios within the reactor of 5% hydrogen cyanide (HCN), 8% acetylene (C2H2), 5% cyanoacetylene (HC3N), and 1% ammonia (NH3). These yields are combined with estimated impact rates for the early Earth to predict surface boundary conditions for an atmospheric model. We show that impacts might have served as sources of energy that would have led to steady-state surface quantities of 0.4% C2H2, 400 ppm HCN, and 40 ppm NH3. We provide simulated transit spectra for an Earth-like exoplanet with this reducing atmosphere during and shortly after eras of intense impacts. We predict that acetylene is as observable as other molecular features on exoplanets with reducing atmospheres that have recently gone through their own "heavy bombardments," with prominent features at 3.05 and 10.5 μm

Investigation on the origin of hot electrons in laser plasma interaction at shock ignition intensities

Shock Ignition is a two-step scheme to reach Inertial Confinement Fusion, where the precompressed fuel capsule is ignited by a strong shock driven by a laser pulse at an intensity in the order of 10 16 W/cm 2 . In this report we describe the results of an experiment carried out at PALS laser facility designed to investigate the origin of hot electrons in laser-plasma interaction at intensities and plasma temperatures expected for Shock Ignition. A detailed time- and spectrally-resolved characterization of Stimulated Raman Scattering and Two Plasmon Decay instabilities, as well as of the generated hot electrons, suggest that Stimulated Raman Scattering is the dominant source of hot electrons via the damping of daughter plasma waves. The temperature dependence of laser plasma instabilities was also investigated, enabled by the use of different ablator materials, suggesting that Two Plasmon Decay is damped at earlier times for higher plasma temperatures, accompanied by an earlier ignition of SRS. The identification of the predominant hot electron source and the effect of plasma temperature on laser plasma interaction, here investigated, are extremely useful for developing the mitigation strategies for reducing the impact of hot electrons on the fuel ignition

Characterization of hot electrons generated by laser-plasma interaction at shock ignition intensities

In an experiment carried out at the Prague Asterix Laser System at laser intensities relevant to shock ignition conditions (I > 1016 W/cm2), the heating and transport of hot electrons were studied by using several complementary diagnostics, i.e., Kα time-resolved imaging, hard x-ray filtering (a bremsstrahlung cannon), and electron spectroscopy. Ablators with differing composition from low Z (parylene N) to high Z (nickel) were used in multilayer planar targets to produce plasmas with different coronal temperature and collisionality and modify the conditions of hot-electron generation. The variety of available diagnostics allowed full characterization of the population of hot electrons, retrieving their conversion efficiency, time generation and duration, temperature, and angular divergence. The obtained results are shown to be consistent with those from detailed simulations and similar inertial confinement fusion experiments. Based on the measured data, the advantages, reliability, and complementarity of the experimental diagnostics are discussed

Laser-driven quasi-static B-fields for magnetized high-energy-density experiments

We present measurements of magnetic fields generated in laser-driven coil targets irradiated by laser pulses of nanosecond duration, 1.053 μm wavelength, 500 J energy, and ∼ 10 15 W / cm 2 intensity, at the LULI2000 facility. Using two perpendicular probing axes, proton deflectometry is used to characterize the coil current and static charge at different times. Results reveal various deflection features that can be unambiguously linked to a looping quasi-steady current of well-understood polarity or to a static charging of the coil surface. Measured currents are broadly consistent with predictions from a laser-driven diode-current source and lumped circuit model, supporting the quasi-steady assessment of the discharges. Peak magnetic fields of ∼ 50 T at the center of 500-μm-diameter coils, obtained at the moderate laser intensity, open up the use of such laser-driven coil targets at facilities worldwide to study numerous phenomena in magnetized high-energy-density plasmas, and its potential applications

Multi-energy ion implantation from high-intensity laser

The laser-matter interaction using nominal laser intensity above 1015 W/cm2 generates in vacuum non- -equilibrium plasmas accelerating ions at energies from tens keV up to hundreds MeV. From thin targets, using the TNSA regime, plasma is generated in the forward direction accelerating ions above 1 MeV per charge state and inducing high-ionization states. Generally, the ion energies follow a Boltzmann-like distribution characterized by a cutoff at high energy and by a Coulomb-shift towards high energy increasing the ion charge state. The accelerated ions are emitted with the high directivity, depending on the ion charge state and ion mass, along the normal to the target surface. The ion fluencies depend on the ablated mass by laser, indeed it is low for thin targets. Ions accelerated from plasma can be implanted on different substrates such as Si crystals, glassy-carbon and polymers at different fluences. The ion dose increment of implanted substrates is obtainable with repetitive laser shots and with repetitive plasma emissions. Ion beam analytical methods (IBA), such as Rutherford backscattering spectroscopy (RBS), elastic recoil detection analysis (ERDA) and proton-induced X-ray emission (PIXE) can be employed to analyse the implanted species in the substrates. Such analyses represent ‘off-line’ methods to extrapolate and to character the plasma ion stream emission as well as to investigate the chemical and physical modifications of the implanted surface. The multi-energy and species ion implantation from plasma, at high fluency, changes the physical and chemical properties of the implanted substrates, in fact, many parameters, such as morphology, hardness, optical and mechanical properties, wetting ability and nanostructure generation may be modified through the thermal-assisted implantation by multi-energy ions from laser-generated plasma

Shock dynamics induced by double-spot laser irradiation of layered targets

We studied the interaction of a double-spot laser beam with targets using the Prague Asterix Laser System (PALS) iodine laser working at 0.44 μm wavelength and intensity of about 1015 W/cm2. Shock breakout signals were recorder using time-resolved self-emission from target rear side of irradiated targets. We compared the behavior of pure Al targets and of targets with a foam layer on the laser side. Results have been simulated using hydrodynamic numerical codes

Shock dynamics and shock collision in foam layered targets

Abstract We present an experimental study of the dynamics of shocks generated by the interaction of a double-spot laser in different kinds of targets: simple aluminum foils and foam–aluminum layered targets. The experiment was performed using the Prague PALS iodine laser working at 0.44 μm wavelength and irradiance of a few 1015 W/cm2. Shock breakouts for pure Al and for foam-Al targets have been recorded using time-resolved self-emission diagnostics. Experimental results have been compared with numerical simulations. The shocks originating from two spots move forward and expand radially in the targets, finally colliding in the intermediate region and producing a very strong increase in pressure. This is particularly clear for the case of foam layered targets, where we also observed a delay of shock breakout and a spatial redistribution of the pressure. The influence of the foam layer doped with high-Z (Au) nanoparticles on the shock dynamics was also studied