Centre Lasers Intenses et Applications

Carbon has a central role in biology and organic chemistry, and its solid allotropes provide the basis of much of our modern technology¹. However, the liquid form of carbon remains nearly uncharted², and the structure of liquid carbon and most of its physical properties are essentially unknown³. But liquid carbon is relevant for modelling planetary interiors4,5 and the atmospheres of white dwarfs⁶, as an intermediate state for the synthesis of advanced carbon materials7,8, inertial confinement fusion implosions⁹, hypervelocity impact events on carbon materials¹⁰ and our general understanding of structured fluids at extreme conditions¹¹. Here we present a precise structure measurement of liquid carbon at pressures of around 1 million atmospheres obtained by in situ X-ray diffraction at an X-ray free-electron laser. Our results show a complex fluid with transient bonding and approximately four nearest neighbours on average, in agreement with quantum molecular dynamics simulations. The obtained data substantiate the understanding of the liquid state of one of the most abundant elements in the universe and can test models of the melting line. The demonstrated experimental abilities open the path to performing similar studies of the structure of liquids composed of light elements at extreme conditions.

In direct-drive inertial-confinement fusion, understanding and controlling laser–plasma instabilities (LPIs) is crucial to optimizing energy coupling and achieving high-gain fusion. Herein, we report the experimental investigation of the effects of density scale-length on LPIs. The experiment was performed at the GEKKO-XII Laser facility [C. Yamanaka et al., IEEE J. Quantum Electron. 17, 1639 (1981)], specifically to characterize stimulated Raman scattering (SRS) and two-plasmon decay (TPD), and the effects of density scale-lengths on the relationship between these LPIs and hot-electron generation. The experimental results consistently indicate that the reduction in hot-electron generation with increasing density scale-length is strongly correlated with decreases in both SRS and TPD in high-density regions. Rosenbluth gain analysis implies that pump depletion by stimulated Brillouin scattering is not responsible for the observed reduction in SRS and TPD. Instead, spatial and temporal incoherence of propagating laser beams, driven by filamentation, could have suppressed SRS and TPD, as evaluated by filamentation figure of merit (FFOM). On the other hand, the thermally corrected FFOM suggests that hot spots by random phase plates are the origin of the early growth of SRS in low-density regions.

We present the results of the second commissioning phase of the short-focal-length area of the Apollon laser facility, located in Saclay, France. This phase was conducted using the main laser beam (F1), scaled to a peak power of 2 PW. Under the tested conditions, the F1 beam delivered on-target pulses with a maximum energy of up to 45 J and a duration of 22 fs. Several diagnostics were deployed to assess the facility's performance. Key measurements included the on-target focal spot and its spatial stability, along with characterizations of secondary sources generated by irradiating solid targets. These evaluations aim at assisting users in designing future experiments. The laser-target interactions were thoroughly characterized, with emissions of energetic ions, x rays, and neutrons recorded, demonstrating good laser-to-target coupling efficiency. Additionally, we successfully demonstrated the simultaneous operation of the F1 beam with the auxiliary 0.5 PW F2 beam of Apollon, enabling dual-beam operation. This commissioning phase paves the way for the next stage in 2025, which will involve scaling the F1 beam to 8 PW, progressing toward the ultimate goal of achieving 10 PW power.

This Letter reports the first complete observation of magnetized collisionless shock precursors formed through the compression of Biermann-battery magnetic fields in laser produced plasmas. At OMEGA, lasers produce a supersonic CH plasma flow which is magnetized with Biermann-battery magnetic fields. The plasma flow collides with an unmagnetized hydrogen gas jet plasma to create a magnetized shock precursor. The situation where the flowing plasma carries the magnetic field is similar to the Venusian bow shock. Imaging 2ω Thomson scattering confirms that the interaction is collisionless and shows density and temperature jumps. Proton radiographs have regions of strong deflections and FLASH magnetohydrodynamic (MHD) simulations show the presence of Biermann fields in the Thomson scattering region. Electrons are accelerated to energies of up to 100 keV in a power-law spectrum. OSIRIS particle-in-cell (PIC) simulations, initialized with measured parameters, show the formation of a magnetized shock precursor and corroborate the experimental observables.

Ion acoustic waves in collisionless plasma have a phase speed determined by the adiabatic constants of electrons and protons. Typically, the isothermal equation of state is assumed for electrons, resulting in an adiabatic constant γe=1, while the adiabatic equation of state with one degree of freedom is applied to protons, yielding γp=3. This selection has been experimentally validated in plasmas with hot electrons and cool ions. Here, we investigate whether this remains true in particle-in-cell (PIC) simulations, which generally exhibit noise levels significantly higher than those in real plasma. By comparing the power spectrum of simulation noise to the thermal noise spectrum and the dispersion relation of ion acoustic waves, we confirm that γe=1 and γp=3 are good approximations for the adiabatic constants that determine dispersive properties of ion acoustic waves in unmagnetized PIC simulation plasma with proton temperatures well below the electron temperature.

Semiconductor crystals driven by strong mid-infrared pulses offer advantages for studying many-body physics and ultrafast optoelectronics via high-harmonic generation. While the process has been used to study solids in the presence strong mid-infrared fields, its potential as an attosecond light source is largely underexplored. We demonstrate that high-harmonics emitted from zinc-oxide crystals produce attosecond pulses, measured through spectroscopy of alkali metals. Using a cross-correlation approach, we photoionize Cesium atoms with vacuum-ultraviolet high-harmonics in the presence of a mid-infrared laser field. We observe oscillations in the photoelectron yield, originating from the instantaneous polarization of atoms by the laser field. The phase of these oscillations encodes the attosecond synchronization of the high-harmonics and is used for attosecond pulse metrology. This source opens new spectral windows for attosecond spectroscopy, enabling studies of bound-state dynamics in natural systems with low ionization energies, while facilitating the generation of non-classical entangled light states in the visible-VUV.

We present a novel scheme for rapid quantitative analysis of debris generated during experiments with solid targets following relativistic laser–plasma interaction at high-power laser facilities. Results are supported by standard analysis techniques. Experimental data indicate that predictions by available modelling for non-mass-limited targets are reasonable, with debris of the order of hundreds of μg per shot. We detect for the first time two clearly distinct types of debris emitted from the same interaction. A fraction of the debris is ejected directionally, following the target normal (rear and interaction side). The directional debris ejection towards the interaction side is larger than on the side of the target rear. The second type of debris is characterized by a more spherically uniform ejection, albeit with a small asymmetry that favours ejection towards the target rear side.

We demonstrate that pulsed THz radiation produced in air by a focused ultrashort laser pulse can be steered to large angles or even in the backward direction with respect to the laser propagation axis. The emission angle is adjusted by the flying focus technique, which determines the speed and direction of the ionization front created by the single-color laser pulse. This easily adjustable THz source, being well separated from the intense laser, opens exciting applications for remote THz spectroscopy.

We used the PW high-repetition laser facility VEGA-3 at Centro de Láseres Pulsados in Salamanca, with the goal of studying the generation of radioisotopes using laser-driven proton beams. Various types of targets have been irradiated, including in particular several targets containing boron to generate α-particles through the hydrogen–boron fusion reaction. We have successfully identified γ-ray lines from several radioisotopes created by irradiation using laser-generated α-particles or protons including ⁴³ Sc, ⁴⁴ Sc, ⁴⁸ Sc, ⁷ Be, ¹¹ C and ¹⁸ F. We show that radioisotope generation can be used as a diagnostic tool to evaluate α-particle generation in laser-driven proton–boron fusion experiments. We also show the production of ¹¹ C radioisotopes, $\approx 6 \times 10^{6}$ , and of ⁴⁴ Sc radioisotopes, $\approx 5 \times 10^{4}$ per laser shot. This result can open the way to develop laser-driven radiation sources of radioisotopes for medical applications.

Understanding the physics of electromagnetic pulse (EMP) emission and nozzle damage is critical for the long-term operation of laser experiments with gas targets, particularly at facilities looking to produce stable sources of radiation at high repetition rates. We present a theoretical model of plasma formation and electrostatic charging when high-power lasers are focused inside gases. The model can be used to estimate the amplitude of gigahertz EMPs produced by the laser and the extent of damage to the gas jet nozzle. Looking at a range of laser and target properties relevant to existing high-power laser systems, we find that EMP fields of tens to hundreds of kV/m can be generated several metres from the gas jet. Model predictions are compared with measurements of EMPs, plasma formation and nozzle damage from two experiments on the VEGA-3 laser and one experiment on the Vulcan Petawatt laser.

The majority of studies on laser-driven proton–boron nuclear reaction is based on the measurement of α-particles with solid-state nuclear tracks detector (Cr39). However, Cr39's interpretation is difficult due to the presence of several other accelerated particles which can bias the analysis. Furthermore, in some laser irradiation geometries, cross-checking measurements are almost impossible. In this case, numerical simulations can play a very important role in supporting the experimental analysis. In our work, we exploited different laser irradiation schemes (pitcher–catcher and direct irradiation) during the same experimental campaign, and we performed numerical analysis, allowing to obtain conclusive results on laser-driven proton–boron reactions. A direct comparison of the two laser irradiation schemes, using the same laser parameters is presented.

Coulomb explosion is an established momentum imaging technique, where the molecules are ionized multiple times on a femtosecond time scale before breaking up into ionized fragments. By measuring the momentum of all the ions, information about the initial molecular structure is theoretically available. However, significant geometric changes due to multiple ionizations occur before the explosion, posing a challenge in retrieving the ground-state structure of molecules from the measured momentum values of the fragments. In this work, we investigate theoretically and experimentally such a connection between the ground-state geometry of a polyatomic molecule (OCS) and the detected momenta of ionic fragments from the Coulomb explosion. By relying on time-dependent density functional theory (TDDFT), we can rigorously model the ionization dynamics of the molecule in the tunneling regime. We reproduce the energy release and the Newton plot momentum patterns of an experiment in which OCS is ionized to the 6+ charge state. Our results provide insight into the behavior of molecules during strong field multiple ionization, opening a way toward precision imaging of real-space molecular geometries using tabletop lasers.

DIMOHA is a many-particle time-domain model offering a good balance between exhaustive physics and time efficiency. We apply this model to study the effects of defects and reflections in the slow wave structure (SWS) of a traveling wave tube (TWT). To assess DIMOHA’s validity, we set up a TWT with varying reflection coefficients at the tube’s end. A ripple effect in saturated output power with respect to frequency is measured and simulated self-consistently with a rough agreement.

This work aims to present cross sections, for electron removal processes in neutral-atom collisions with water molecules, computed within the CDW-EIS (Continuum Distorted Wave-Eikonal Initial State) approximation. For describing these reactions, two corrections are taken into account in the present model: the mean projectile charge as a function of the impact energy and the dynamic projectile charge depending on the momentum transfer during the interaction. The theoretical data, in terms of the double differential and the total cross sections, for target ionization and projectile electron loss processes is compared with existing data.

Very high-average optical enhancement cavities (OECs) are being used both in fundamental and applied research. The most demanding applications require stable megawatt level average power of infrared picosecond pulses with repetition rates of several tens of MHz. Toward reaching this goal, we report on the achievement of 710 kW of stable average power in a two-mirror hemispherical optical enhancement cavity. This result further improves the state of the art. So far, in compact high-power systems, cavity geometry optimization has been driven by the need to limit the deformation of radii of curvatures due to thermal effects. Here we explicitly demonstrate that thermal lensing must be accounted for, too, and that it can be used to assess the absorption of coatings. Experimental observations are matched with a simple model of thermal effects in the mirror’s coatings. These results set a further stage for designing an optimized optical system for several applications where very high-average power enhancement cavities are expected to be operated.

Interactions between magnetic fields advected by matter play a fundamental role in the Universe at a diverse range of scales. A crucial role these interactions play is in making turbulent fields highly anisotropic, leading to observed ordered fields. These in turn, are important evolutionary factors for all the systems within and around. Despite scant evidence, due to the difficulty in measuring even near-Earth events, the magnetic field compression factor in these interactions, measured at very varied scales, is limited to a few. However, compressing matter in which a magnetic field is embedded, results in compression up to several thousands. Here we show, using laboratory experiments and matching three-dimensional hybrid simulations, that there is indeed a very effective saturation of the compression when two independent parallel-oriented magnetic fields regions encounter one another due to plasma advection. We found that the observed saturation is linked to a build-up of the magnetic pressure, which decelerates and redirects the inflows at their encounter point, thereby stopping further compression. Moreover, the growth of an electric field, induced by the incoming flows and the magnetic field, acts in redirecting the inflows transversely, further hampering field compression.

Blast waves have been produced in solid target by irradiation with short-pulse high-intensity lasers. The mechanism of production relies on energy deposition from the hot electrons produced by laser–matter interaction, producing a steep temperature gradient inside the target. Hot electrons also produce preheating of the material ahead of the blast wave and expansion of the target rear side, which results in a complex blast wave propagation dynamic. Several diagnostics have been used to characterize the hot electron source, the induced preheating and the velocity of the blast wave. Results are compared to numerical simulations. These show how blast wave pressure is initially very large (more than 100 Mbar), but it decreases very rapidly during propagation.

The ongoing improvement in laser technology and target fabrication is opening new possibilities for diagnostic development. An example is x-ray phase-contrast imaging (XPCI), which serves as an advanced x-ray imaging diagnostic in laser-driven experiments. In this work, we present the results of the XPCI platform that was developed at the OMEGA EP Laser-Facility to study multi-Mbar single and double shocks produced using a kilojoule laser driver. Two-dimensional radiation-hydrodynamic simulations agree well with the shock progression and the spherical curvature of the shock fronts. It is demonstrated that XPCI is an excellent method to determine with high accuracy the front position of a trailing shock wave propagating through an expanding CH plasma that was heated by a precursor Mbar shock wave. The interaction between the rarefaction wave and the shock wave results in a clear signature in the radiograph that is well reproduced by radiation-hydrodynamic simulations.

This study focuses on the optimization of beam chamber geometry designs for future direct-drive laser facilities. It provides a review of leading target chamber geometries, with a particular emphasis on random errors. Through comprehensive solid-sphere illuminations and analysis, we identify an optimized beam geometry design, highlighting its robustness and performance under realistic experimental conditions. Three major sources of random errors are evaluated, closely linked to experimental evaluations at OMEGA. The findings underscore the importance of optimizing the irradiation system alongside beam pattern considerations to enhance the efficiency and reliability of inertial confinement fusion experiments. We conclude that for a desired illumination uniformity of 1% in the presence of system errors, the split icosahedron design is the most robust. However, for a 0.3% uniformity goal, the charged-particle, icosahedron, and t-sphere methods exhibit similar performance.