Stefano Atzeni

A 1D simulation design study for fast ignition implosions using the second harmonic of a Nd:glass laser (527 nm) is presented. Designs are obtained by combining particle swarm optimization [Kennedy and Eberhart, Proceedings of International Conference on Neural Networks (ICNN'95), Perth, WA, Australia (IEEE, 1995), Vol. 4, pp. 1942–1948] with radiation hydrodynamic simulations using the code MULTI-IFE [Ramis and Meyer-ter-Vehn, Comput. Phys. Commun. 203, 226 (2016)]. Considerations of limits like the maximum laser intensity, the laser power contrast, and the central gas density are incorporated into the design iterations. A subset of the simulation results is verified with the code DUED [Atzeni et al., Comput. Phys. Commun. 169, 153 (2005)]. The obtained design features a low absorption of the incident laser because outer parts of the beam are being refracted around the imploding shell which lowers the absorption. A single stage of laser beam zooming substantially increases the absorption while decreasing the laser power contrast, the compression energy, and the incident intensity. A comparison to compression designs in literature is drawn, which reveals the efficiency of the described approach.

We present first experimental data obtained at the recently commissioned experimental platform to compare laser plasma instabilities (LPIs) driven by monochromatic and broad spectral bandwidth ( ≈ 0.5 % ) laser pulses with a wavelength of around 527 nm. The evaluation of LPIs at 10 ¹⁵ W cm ⁻² is shown by utilizing a full aperture backscatter diagnostic and side scatter diagnostics. For a broad bandwidth, our study indicates a suppression of stimulated Brillouin back- and side scattering as well as of the two plasmon decay instability, but an increase of stimulated Raman side scattering. An increase in both temperature and numbers of hot electrons is also identified by direct and indirect measurements.

A method to study isochoric compression to mass densities relevant for direct-drive fast ignition schemes is presented. The method is based on the combination of one-dimensional radiation-hydrodynamic simulations using the code MULTI-IFE [Ramis and Meyer-ter Vehn, Comput. Phys. Commun. 203, 226 (2016)] and a particle swarm optimization technique [Kennedy and Eberhart, in Proceedings of ICNN'95 - International Conference on Neural Networks (IEEE, Perth, WA, Australia, 1995), Vol. 4, pp. 1942–1948]. The compression of the fuel is optimized through variations of the incident temporal laser power profiles. Uniform mass density profiles are achieved by using appropriate objective functions that allow comparisons between the fuel assemblies obtained from simulations. Several objective functions were created and evaluated on their merits to yield isochoric compression assembly. Ultimately, such a profile is presented in conjunction with the technique to achieve it. A useful objective function is calculating the deviation of the simulated mass density profile from the ideal uniform mass density profile over a volume of the compressed target up to the radial position of the outgoing shock wave.

Gas-filled plastic spherical shells have been imploded using the 60-beam OMEGA laser. The same laser pulse was used as in a previous experiment, which demonstrated the dynamic formation of a shell by irradiating a homogeneous plastic foam sphere [Igumenshchev et al., Phys. Rev. Lett. 131, 015102 (2023)]. In the present experiment, the shell initially implodes, bounces, and expands; a new shell then forms and undergoes a second implosion. Analysis of x-ray self-emission-streaked images and sequences of 2D x-ray self-emission framing camera images show shell evolution that is in reasonable agreement with 1D simulations. Shell integrity and symmetry are well maintained until the formation of the new shell. The final implosion, instead, is affected by substantial asymmetries.

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.

We report on a study of laser plasma instabilities with 527 nm laser pulses in an intensity range of 0.5×1013−1.1×1015 Wcm−2 and plasma parameters entering a regime that is relevant for direct drive inertial confinement fusion. Using the kilojoule high repetition rate L4n laser at the Extreme Light Infrastructure—Beamlines, more than 1300 shots were collected, and the onset and the growth of stimulated Brioullin scattering (SBS) and stimulated Raman scattering (SRS) were studied with a high confidence level. The measured onset intensities are 0.2×1014 Wcm−2 for SBS and 1.4×1014 Wcm−2 for SRS. At the maximum intensity, the total fraction of backscattered energy reaches 2.5% for SBS and 0.1% for SRS. These results are of high relevance for advanced concepts for inertial fusion energy, which rely on the use of 527 nm laser light to drive the implosion of the fuel target, and in particular, they can be used as a benchmark for advanced simulations.

We report on the commissioning of a full aperture backscatter diagnostics station for the kilojoule, nanosecond high repetition rate L4n laser operating at a wavelength of 527 nm at the Extreme Light Infrastructure (ELI) - Beamlines, Dolni Brezany, Czech Republic. Light scattered back from laser–plasma interaction into the cone of the final focusing lens is captured and split into different channels to measure the signatures of laser plasma instabilities from stimulated Brillouin scattering, stimulated Raman scattering, and two plasmon decay with respect to back scattered energy, its spectrum, and its temporal profile. The performance was confirmed in a commissioning experiment with more than 800 shots at laser intensities ranging from 0.5 × 1013 to 1.1 × 1015 W cm−2. These diagnostics are permanently installed at ELI Beamlines, and can be used to understand the details of laser–plasma interactions in experiments with kJ and 527 nm light. The large number of shots that can be collected in an experimental campaign will allow us to study the details of the laser–plasma interaction with a high level of confidence.

Fuel–shell mix in kinetic plasma conditions is probed using nuclear and x-ray self-emission in shock-driven, D3He-gas-filled inertial confinement fusion implosions. As initial gas fill density decreases, measured nuclear yields and ion temperatures are lower than expected as compared to radiation-hydrodynamic simulations. Spatially and temporally resolved x-ray emissions indicate significant mixing at the fuel–shell interface in implosions with low initial gas fill density. This observed fuel–shell mix introduces a substantial amount of shell ions into the center of the implosion prior to and during shock flash and is the key mechanism needed in the kinetic-ion simulations to match experimental nuclear yields.

We use a subignition scale laser, the 30 kJ Omega, and a novel shallow-cone target to study laser-plasma interactions at the ablation-plasma density scale lengths and laser intensities anticipated for direct drive shock-ignition implosions at National Ignition Facility scale. Our results show that, under these conditions, the dominant instability is convective stimulated Raman scatter with experimental evidence of two plasmon decay (TPD) only when the density scale length is reduced. Particle-in-cell simulations indicate this is due to TPD being shifted to lower densities, removing the experimental back-scatter signature and reducing the hot-electron temperature. The experimental laser energy-coupling to hot electrons was found to be 1%–2.5%, with electron temperatures between 35 and 45 keV. Radiation-hydrodynamics simulations employing these hot-electron characteristics indicate that they should not preheat the fuel in MJ-scale shock ignition experiments.