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Event: SPIE Optics + Optoelectronics, 2023, Prague, Czech Republic; High-harmonic generation (HHG) in gaseous media is a workhorse tool in attosecond science. Its description origins from the microscopic scale of a single atom or molecule interacting with a driving IR-laser pulse. However, its practical realization usually employs a macroscopic volume of the gas. The macroscopic aspect aggregates all the microscopic emitters and brings novel physical mechanisms that drive the generation. One of the key mechanisms within the macroscopic scale is the shaping of the driving pulse due to the non-linear response of the medium. We present a comprehensive numerical model describing and coupling the physics on both scales. The model consists of different modules that provide different levels of approximation to choose an optimal trade-off between accuracy and computational cost. We then use it to address two generation schemes, where we provide a detailed picture together with experimental realizations. The first scheme uses a long medium homogeneously pre-ionized by an electrical discharge to optimize the phase-matching of the harmonic signal. This scheme allows, in particular, for optimizing HHG in long media where the control is difficult because the driving pulse undergoes strong reshaping and defocusing due to the non-linear response after the entrance to the medium. The second scheme introduces a mechanism to control the divergence of the harmonic beam in thin targets. The divergence is driven by shaping the wavefront of the driving pulse. This allows for spectrally selective focusing of the harmonic beams without the use of optics, which leads to inevitable losses in the XUV region.

International audience

International audience; S 04 Atelier général PNPS Radiative shocks are present at different stages of stellar evolution, for instance in the stellar infancy when matter is accreted from the stellar disk to the photosphere of the young star and is also ejected in the form of stellar jets, up to the final stages as supernovae and the interaction of their remnants with the interstellar medium. Such shocks are strongly influenced by the radiation through its coupling with hydrodynamics. Thus their topology and dynamics are quite complex. Generating such hypersonic shocks in the laboratory, with controlled conditions, is thus an adequate tool to study the influence of radiation and to compare them with numerical simulations. So far, such laboratory astrophysics studies are performed on large-scale laser facilities, addressing hydrodynamic radiative shocks, in Xenon with very high velocity (50 - 150 km/s) and moderate pressure (0.1 - 1 bar). These experiments advance the understanding of the effect of radiation on the different shock components (radiative precursors, shock collapse, walls heating etc.). In continuation of these laboratory experiments, we recently performed experiments at PALS laser facility to study a fairly new topic focusing on the interaction of two counter propagating shocks. The objective is to understand the influence of one radiative precursor onto another, to achieve higher temperature in the compressed medium. In the astrophysical context, aforementioned studies are relevant for instance, collision of supernovae remnants and more indirectly the interaction of supernovae remnants with dense molecular clouds and the impact of accretion flows on a stellar atmosphere. The experiments have been able to launch shocks with different shock speeds (~30-55 km/s and 10-25 km/s), in varying gases (Ar, Xe) and pressures (~0.1-0.3 bar). Optical interferometry allowed us to estimate several physical parameters such as shock speed and electron density in the precursor, XUV spectroscopy allowed to give information about the shock temperature. We will present some preliminary results together with numerical simulations.

International audience; Radiative shocks are present in various astrophysical contexts and can be proxy of fundamental accretion processes, for instance X-ray signatures from accretion shocks can be related to the rate of mass accretion onto forming stars. Thus the study of hypersonic shocks (Mach number >> 1) in the laboratory Under controlled conditions is of primary interest in order to study the influence of radiation, and to compare with numerical simulations. In the past decade, several experiments on radiative shocks have been performed on various large-scale laser facilities, demonstrating the formation of shocks in Xenon with velocities ~50 - 150 km/s in background gas pressures ~0.1 - 1 bar (Bouquet et al. 2004, Gonzalez et. al. 2006, Reighard et. al. 2007, Stehlé et al. 2010, Doss et al. 2011, Drake et al. 2011, Dizière et al. 2012, Stehlé et al. 2012, Chaulagain et al. 2015). Many advances have been achieved in understanding the effect of radiation on the different shock components (radiative precursor, shock collapse, wall heating etc.), however, these studies were focused solely on the case of a single radiative shock. We have recently conducted experiments at the PALS laser facility, looking at the collision between two counter streaming radiative shocks. Besides providing a new experimental platform, these experiments aimed at studying how one radiative precursor is influenced by the presence of another. The experiments launched shocks with different shock speeds (~30-55 km/s and 10-25 km/s), at a range of different pressures (~0.1-0.3 bar), and with different gases (Ar, Xe). Optical interferometry allowed us to estimate several physical parameters such as shock speed and electron density in the precursor, whereas the use of time and spatially resolved optical spectroscopy led to a number of spectral signatures. We will present preliminary results together with numerical simulations.

International audience; affiche P53 Being able to produce and diagnose radiative shocks under controlled laboratory conditions is a key aspect to understand their role in astrophysical processes such as accretion in stellar formation. The ongoing study of radiative shocks using high-power lasers has significantly advanced our understanding on the physics of these complex flows. In particular, experiments at the Prague Asterix Laser System (PALS) have demonstrated the formation of radiative shocks in Xenon with a characteristic velocity of 50-60 km/s. Latest results on the generation on radiative shocks at PALS demonstrate the feasibility of fielding novel probing techniques such as time-resolved imaging using an X-ray laser at 21.2 nm, allowing to probe simultaneously the high-density shock-front together with the lower-density radiative precursor in front of the shock. These results will be compared with 2-D radiative-hydrodynamic simulations using the code ARWEN. Future experimental campaigns will focus on spectroscopic measurements of the different shock regions together with new experimental configurations such as the interaction of counter-streaming radiative shocks. These prospects will be presented and discussed from both experimental and simulation point of view. This work is supported by french ANR STARSHOCK (grant 08-BLAN-0263-07), Programme National de Physique Stellaire" (PNPS) of CNRS/INSU, France, Observatoire de Paris and labex Plas@Par ( ANR-11-IDEX-0004-02 )

International audience; Radiative shocks are ubiquitous in stellar environments and are characterized by high temperature plasma emitting a considerable fraction of their energy as radiation. The physical structure of these shocks is complex and experimental benchmarks are needed to provide a deeper understanding of the physics at play. In addition, experiments provide unique data for testing radiation hydrodynamics codes which, in turn, are used to model astrophysical phenomena. Radiative shocks have been studied on various high-energy laser facilities for more than a decade, highlighting the importance of radiation on the plasma dynamics. Particularly the PALS facility has focused in producing radiative shocks with typical velocities of ∼50–60 km s−1 in xenon at a fraction of a bar. In addition PALS has the unique capability of producing the most powerful XUV laser available today (21.2 nm (58.4 eV), 0.15 ns), opening the door to new diagnostics of dense plasmas. Here we present results of XUV imaging of the precursor and post-shock structure of radiative shocks generated in xenon in this facility, together with time-and-space resolved measurements of the XUV self-emission using fast diode. The experimental results are interpreted with the help of 2D ARWEN radiative hydrodynamics simulations and state-of-the art monochromatic opacities.