Y. Maheut

Generation of high pressure shocks relevant to the shock-ignition intensity regime

An experiment was performed using the PALS laser to study laser-target coupling and laser-plasma interaction in an intensity regime ≤1016 W/cm2, relevant for the “shock ignition” approach to Inertial Confinement Fusion. A first beam at low intensity was used to create an extended preformed plasma, and a second one to create a strong shock. Pressures up to 90 Megabars were inferred. Our results show the importance of the details of energy transport in the overdense region.

Recent results from experimental studies on laser?plasma coupling in a shock ignition relevant regime

Shock ignition (SI) is an appealing approach in the inertial confinement scenario for the ignition and burn of a pre-compressed fusion pellet. In this scheme, a strong converging shock is launched by laser irradiation at an intensity Iλ 2 >10 15 Wc m −2 µm 2 at the end of the compression phase. In this intensity regime, laser–plasma interactions are characterized by the onset of a variety of instabilities, including stimulated Raman scattering, Brillouin scattering and the two plasmon decay, accompanied by the generation of a population of fast electrons. The effect of the fast electrons on the efficiency of the shock wave production is investigated in a series of dedicated experiments at the Prague Asterix Laser Facility (PALS). We study the laser–plasma coupling in a SI relevant regime in a planar geometry by creating an extended preformed plasma with a laser beam at ∼7 × 10 13 Wc m −2 (250 ps, 1315 nm). A strong shock is launched by irradiation with a second laser beam at intensities in the range 10 15 –10 16 Wc m −2 (250 ps, 438 nm) at various delays with respect to the first beam. The pre-plasma is characterized using x-ray spectroscopy, ion diagnostics and interferometry. Spectroscopy and calorimetry of the backscattered radiation is performed in the spectral range 250–850 nm, including (3/2)ω, ω and ω/2 emission. The fast electron production is characterized through spectroscopy and imaging of the Kα emission. Information on the shock pressure is obtained using shock breakout chronometry and measurements of the craters produced by the shock in a massive target. Preliminary results show that the backscattered energy is in the range 3–15%, mainly due to backscattered light at the laser wavelength (438 nm), which increases with increasing the delay between the two laser beams. The values of the peak shock pressures inferred from the shock breakout times are lower than expected from 2D numerical simulations. The same simulations reveal that the 2D effects play a major role in these experiments, with the laser spot size comparable with the distance between critical and ablation layers.

Coupled hydrodynamic model for laser-plasma interaction and hot electron generation.

We present a formulation of the model of laser-plasma interaction (LPI) at hydrodynamical scales that couples the plasma dynamics with linear and nonlinear LPI processes, including the creation and propagation of high-energy electrons excited by parametric instabilities and collective effects. This formulation accounts for laser beam refraction and diffraction, energy absorption due to collisional and resonant processes, and hot electron generation due to the stimulated Raman scattering, two-plasmon decay, and resonant absorption processes. Hot electron (HE) transport and absorption are described within the multigroup angular scattering approximation, adapted for transversally Gaussian electron beams. This multiscale inline LPI-HE model is used to interpret several shock ignition experiments, highlighting the importance of target preheating by HEs and the shortcomings of standard geometrical optics when modeling the propagation and absorption of intense laser pulses. It is found that HEs from parametric instabilities significantly increase the shock pressure and velocity in the target, while decreasing its strength and the overall ablation pressure.

Effect of nonthermal electrons on the shock formation in a laser driven plasma

In the laser-driven inertial fusion schemes and specifically in the shock ignition concept, non thermal electrons may be generated. By depositing their energy far from the origin, they can significantly modify the target hydrodynamics. It is shown in this paper that these electrons may affect the laser-driven shock formation and its propagation through the target. These changes are induced by the target heating and depend on the electron energy spectrum. Furthermore, results of some passive diagnostic may be misinterpreted, indicating an apparent different pressure.

Experiment on laser interaction with a planar target for conditions relevant to shock ignition

We report the experiment conducted on the Prague Asterix Laser System (PALS) laser facility dedicated to make a parametric study of the laser–plasma interaction under the physical conditions corresponding to shock ignition thermonuclear fusion reactions. Two laser beams have been used: the auxiliary beam, for preplasma creation on the surface of a plastic foil, and the main beam to launch a strong shock. The ablation pressure is inferred from the volume of the crater in the Cu layer situated behind the plastic foil and by shock breakout chronometry. The population of fast electrons is analyzed by Kα emission spectroscopy and imaging. The preplasma is characterized by three-frame interferometry, x-ray spectroscopy and ion diagnostics. The numerical simulations constrained with the measured data gave a maximum pressure in the plastic layer of about 90 Mbar.

Study of shock waves generation, hot electron production and role of parametric instabilities in an intensity regime relevant for the shock ignition

We present experimental results at intensities relevant to Shock Ignition obtained at the sub-ns Prague Asterix Laser System in 2012. We studied shock waves produced by laser-matter interaction in presence of a pre-plasma. We used a first beam at 1ω (1315 nm) at 7 x 1013 W/cm2 to create a pre-plasma on the front side of the target and a second at 3ω (438 nm) at ~ 1016 W/cm2 to create the shock wave. Multilayer targets composed of 25 (or 40 µm) of plastic (doped with Cl), 5 µm of Cu (for Kα diagnostics) and 20 µm of Al for shock measurement were used. We used X-ray spectroscopy of Cl to evaluate the plasma temperature, Kα imaging and spectroscopy to evaluate spatial and spectral properties of the fast electrons and a streak camera for shock breakout measurements. Parametric instabilities (Stimulated Raman Scattering, Stimulated Brillouin Scattering and Two Plasmon Decay) were studied by collecting the back scattered light and analysing its spectrum. Back scattered energy was measured with calorimeters. To evaluate the maximum pressure reached in our experiment we performed hydro simulations with CHIC and DUED codes. The maximum shock pressure generated in our experiment at the front side of the target during laser-interaction is 90 Mbar. The conversion efficiency into hot electrons was estimated to be of the order of ~ 0.1% and their mean energy in the order ~50 keV.

Studies on laser–plasma interaction physics for shock ignition

We realized a series of experiments to study the physics of laser–plasma interaction in an intensity regime of interest for the novel “Shock Ignition” approach to Inertial Fusion. Experiments were performed at the Prague Asterix Laser System laser in Prague using two laser beams: an “auxiliary” beam, for pre-plasma creation, with intensity around 7 × 1013 W/cm2 (250 ps, 1ω, λ = 1315 nm) and the “main” beam, up to 1016 W/cm (250 ps, 3ω, λ = 438 nm), to launch a shock. The main goal of these experiments is to study the process of the formation of a very strong shock and the influence of hot electrons in the generation of very high pressures. The shock produced by the ablation of the plastic layer is studied by shock breakout chronometry. The generation of hot electrons is analyzed by imaging Kα emission.

X-ray absorption radiography for high pressure shock wave studies

The study of laser compressed matter, both warm dense matter (WDM) and hot dense matter (HDM), is relevant to several research areas, including materials science, astrophysics, inertial confinement fusion. X-ray absorption radiography is a unique tool to diagnose compressed WDM and HDM. The application of radiography to shock-wave studies is presented and discussed. In addition to the standard Abel inversion to recover a density map from a transmission map, a procedure has been developed to generate synthetic radiographs using density maps produced by the hydrodynamics code DUED. This procedure takes into account both source-target geometry and source size (which plays a non negligible role in the interpretation of the data), and allows to reproduce transmission data with a good degree of accuracy.

Short-wavelength experiments on laser pulse interaction with extended pre-plasma at the PALS-installation

The paper is a continuation of research carried out at Prague Asterix Laser System (PALS) related to the shock ignition (SI) approach in inertial fusion, which was carried out with use of 1ω main laser beam as the main beam generating a shock wave. Two-layer targets were used, consisting of Cu massive planar target coated with a thin polyethylene layer, which, in the case of two-beam irradiation geometry, simulate conditions related to the SI scenario. The investigations presented in this paper are relat e d to the use of 3ω to create ablation pressure for high-power shock wave generation. The interferometric studies of the ablative plasma expansion, complemented by measurements of crater volumes and K α emission, clearly demonstrate the effect of changing the incident laser intensity due to changing the focal radius on efficiency of laser energy transfer to a shock wave and fast electron emission. The efficiency of the energy transfer increases with the radius of the focused laser beam. The pre-plasma does not significantly change the character of this effect. However, it unambiguously results in the increasing temperature of fast electrons, the total energy of which remains very small (