Showing papers by "Stefano Atzeni published in 2019"

Quantitative phase contrast imaging of a shock-wave with a laser-plasma based X-ray source

Abstract: X-ray phase contrast imaging (XPCI) is more sensitive to density variations than X-ray absorption radiography, which is a crucial advantage when imaging weakly-absorbing, low-Z materials, or steep density gradients in matter under extreme conditions. Here, we describe the application of a polychromatic X-ray laser-plasma source (duration ~0.5 ps, photon energy >1 keV) to the study of a laser-driven shock travelling in plastic material. The XPCI technique allows for a clear identification of the shock front as well as of small-scale features present during the interaction. Quantitative analysis of the compressed object is achieved using a density map reconstructed from the experimental data.

Collisionless Shocks Driven by Supersonic Plasma Flows with Self-Generated Magnetic Fields

Abstract: Collisionless shocks are ubiquitous in the Universe as a consequence of supersonic plasma flows sweeping through interstellar and intergalactic media. These shocks are the cause of many observed astrophysical phenomena, but details of shock structure and behavior remain controversial because of the lack of ways to study them experimentally. Laboratory experiments reported here, with astrophysically relevant plasma parameters, demonstrate for the first time the formation of a quasiperpendicular magnetized collisionless shock. In the upstream it is fringed by a filamented turbulent region, a rudiment for a secondary Weibel-driven shock. This turbulent structure is found responsible for electron acceleration to energies exceeding the average energy by two orders of magnitude.

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

Abstract: Laser–plasma interaction (LPI) at intensities ) 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.

X-ray phase-contrast imaging for laser-induced shock waves

Abstract: X-ray phase-contrast imaging (XPCI) is a versatile technique with applications in many fields, including fundamental physics, biology and medicine. Where X-ray absorption radiography requires high density ratios for effective imaging, the image contrast for XPCI is a function of the density gradient. In this letter, we apply XPCI to the study of laser-driven shock waves. Our experiment was conducted at the Petawatt High-Energy Laser for Heavy Ion EXperiments (PHELIX) at GSI. Two laser beams were used: one to launch a shock wave and the other to generate an X-ray source for phase-contrast imaging. Our results suggest that this technique is suitable for the study of warm dense matter (WDM), inertial confinement fusion (ICF) and laboratory astrophysics.

Observations of Multiple Nuclear Reaction Histories and Fuel-Ion Species Dynamics in Shock-Driven Inertial Confinement Fusion Implosions.

Abstract: Fuel-ion species dynamics in hydrodynamiclike shock-driven DT^{3}He-filled inertial confinement fusion implosion is quantitatively assessed for the first time using simultaneously measured D^{3}He and DT reaction histories. These reaction histories are measured with the particle x-ray temporal diagnostic, which captures the relative timing between different nuclear burns with unprecedented precision (∼10 ps). The observed 50±10 ps earlier D^{3}He reaction history timing (relative to DT) cannot be explained by average-ion hydrodynamic simulations and is attributed to fuel-ion species separation between the D, T, and ^{3}He ions during shock convergence and rebound. At the onset of the shock burn, inferred ^{3}He/T fuel ratio in the burn region using the measured reaction histories is much higher as compared to the initial gas-filled ratio. As T and ^{3}He have the same mass but different charge, these results indicate that the charge-to-mass ratio plays an important role in driving fuel-ion species separation during strong shock propagation even for these hydrodynamiclike plasmas.

Laser-driven strong shocks with infrared lasers at intensity of 1016 W/cm2

Abstract: We present the results of an experiment on laser-driven shock waves performed at the Prague Asterix Laser system (PALS), where the fundamental frequency of the laser (1315 nm) is used to launch a strong shock in planar geometry. The experiment aims to characterize both shock waves and hot electrons generated at intensities of ≃ 10 16 W / cm 2. It is shown that, in these interaction conditions, hydrodynamics is strongly impacted by noncollisional mechanisms, and the role of the hot electrons, generated by parametric instabilities, is essential in determining shock dynamics.

Hydrodynamic studies of high gain shock ignition targets: effect of low- to intermediate-mode asymmetries

Abstract: Shock-ignition (SI) is a direct-drive laser fusion scheme, in which the fuel is imploded at velocity somewhat smaller than in conventional schemes. The hot spot required for ignition is attained thanks to a converging shock-wave generated by an intense final spike of the laser pulse. Earlier studies show potentials of SI for high gain at driver energy of the order of 1 MJ, provided that laser-plasma instabilities do not degrade laser absorption and do not preheat the fuel. However, also hydrodynamic aspects need investigation. A crucial issue is the design of target-pulse concepts with sufficiently large 1D safety margin (ITF, ignition threshold factor), capable of tolerating unavoidable departures from design specifications. In the present paper we use 2D simulations to investigate the effect of low- to intermediate-mode implosion asymmetries (modes l = 1 − 8) on the performance of targets with different ITF’s. As expected, tolerance to perturbations increases with ITF. For given perturbation amplitude at beginning of deceleration, modes l = 1 − 4 are substantially more damaging than mode l = 8. By comparing simulations with and without alpha-particle heating we find a clear relation between tolerable perturbation amplitudes in simulations with alpha-particle heating and yield degradation in simulations without alpha heating.

Probing ion species separation and ion thermal decoupling in shock-driven implosions using multiple nuclear reaction histories

Abstract: Simultaneously measured DD, DT, and D3He reaction histories are used to probe the impacts of multi-ion physics during the shock phase of inertial confinement fusion implosions. In these relatively hydrodynamiclike (burn-averaged Knudsen number ⟨NK⟩ ∼0.3) shock-driven implosions, average-ion hydrodynamic DUED simulations are able to reasonably match burnwidths, nuclear yields, and ion temperatures. However, kinetic-ion FPION simulations are able to better simulate the timing differences and time-resolved reaction rate ratios between DD, DT, and D3He reactions. FPION simulations suggest that the D3He/DT reaction rate ratio is most directly impacted by ion species separation between the 3He and T ions, whereas the D3He/DD reaction rate ratio is affected by both ion species separation and ion temperature decoupling effects.

Propagation-based imaging phase-contrast enhanced imaging setup for single shot acquisition using laser-generated X-ray sources

Abstract: The development of new diagnostics is important to improve the interpretation of experiments. Often well-known physical processes and techniques originally developed in unrelated fields of science can be applied to a different area with a significant impact on the quality of the produced data. X-ray phase-contrast imaging (XPCI) is one techniques which has found many applications in biology and medicine. This is due to its capability to emphasise the presence of strong density variations normally oriented with respect to the X-ray propagation direction. With the availability of short energetic X-ray pulses XPCI extends to time-resolved pump-probe measurements of laser-matter interaction where strong density gradient are also present. In this work we present the setup for XPCI tested at the laser PHELiX at GSI in Germany.