Modeling parametric scattering instabilities in large-scale expanding plasmas

We present results from two-dimensional simulations of long scale-length laser-plasma interaction experiments performed at LULI. With the goal of predictive modeling of such experiments with our code Harmony2D, we take into account realistic plasma density and velocity profiles, the propagation of the laser light beam and the scattered light, as well as the coupling with the ion acoustic waves in order to describe Stimulated Brillouin Scattering (SBS). Laser pulse shaping is taken into account to follow the evolution of the SBS reflectivity as close as possible to the experiment. The light reflectivity is analyzed by distinguishing the backscattered light confined in the solid angle defined by the aperture of the incident light beam and the scattered light outside this cone. As in the experiment, it is observed that the aperture of the scattered light tends to increase with the mean intensity of the RPP-smoothed laser beam. A further common feature between simulations and experiments is the observed localization of the SBS-driven ion acoustic waves (IAW) in the front part of the target (with respect to the incoming laser beam)

The dependence of spatial autoresonance in SRS on

Spatial autoresonance is investigated as a mechanism for the enhancement of stimulated Raman scattering (SRS) in the kinetic regime (kLλD > 0.29). Autoresonance in 3-wave simulations was demonstrated in a previous study [Chapman et al., Phys. Plasmas 17, 122317 (2010)]. These results are applied to particle-in-cell (PIC) simulations. Good agreement is found between PIC simulations and a 3-wave model using a nonlinear frequency shift beyond the regime usually referred to as “weakly kinetic”. Autoresonance is studied for a range of values of kLλD

The dependence of spatial autoresonance in SRS on kLλD

Spatial autoresonance is investigated as a mechanism for the enhancement of stimulated Raman scattering (SRS) in the kinetic regime (kLλD > 0.29). Autoresonance in 3-wave simulations was demonstrated in a previous study [Chapman et al., Phys. Plasmas 17, 122317 (2010)]. These results are applied to particle-in-cell (PIC) simulations. Good agreement is found between PIC simulations and a 3-wave model using a nonlinear frequency shift beyond the regime usually referred to as “weakly kinetic”. Autoresonance is studied for a range of values of kLλD

Studies on laser beam propagation and stimulated scattering in multiple beam experiments

The propagation and stimulated scattering of intense laser beams interacting with underdense plasmas are two important issues for inertial confinement fusion (ICF). The purpose of this work was to perform experiments under well-controlled interaction conditions and confront them with numerical simulations to test the physics included in the codes. Experimental diagnostics include time and space resolved images of incident and SBS light and of SBS-ion acoustic activity. New numerical diagnostics, including similar constraints as the experimental ones and the treatment of the propagation of the light between the emitting area and the detectors, have been developed. Particular care was put to include realistic plasma density and velocity profiles, as well as laser pulse shape in the simulations. In the experiments presented in this paper, the interaction beam was used with a random phase plate (RPP) to produce a statistical distribution of speckles in the focal volume. Stimulated Brillouin Scattering (SBS) was described using a decomposition of the spatial scales which provides a predictive modeling of SBS in an expanding mm-scale plasma. Spatial and temporal behavior of the SBS-ion acoustic waves was found to be in good agreement with the experimental ones for two laser intensities

Laser-plasma interaction physics for shock ignition

In the shock ignition scheme, the ICF target is first compressed with a long (nanosecond) pulse before creating a convergent shock with a short (∼100 ps) pulse to ignite thermonuclear reactions. This short pulse is typically (∼2.1015–1016 W/cm2) above LPI (Laser Plasma Instabilities) thresholds. The plasma is in a regime where the electron temperature is expected to be very high (2–4 keV) and the laser coupling to the plasma is not well understood. Emulating LPI in the corona requires large and hot plasmas produced by high-energy lasers. We conducted experiments on the LIL (Ligne d'Integration Laser, 10 kJ at 3ω) and the LULI2000 (0.4 kJ at 2ω) facilities, to approach these conditions and study absorption and LPI produced by a high intensity beam in preformed plasmas. After introducing the main risks associated with the short pulse propagation, we present the latest experiment we conducted on LPI in relevant conditions for shock ignition

Laser-plasma interaction physics for shock ignition

In the shock ignition scheme, the ICF target is first compressed with a long (nanosecond) pulse before creating a convergent shock with a short (∼100 ps) pulse to ignite thermonuclear reactions. This short pulse is typically (∼2.1015–1016 W/cm2) above LPI (Laser Plasma Instabilities) thresholds. The plasma is in a regime where the electron temperature is expected to be very high (2–4 keV) and the laser coupling to the plasma is not well understood. Emulating LPI in the corona requires large and hot plasmas produced by high-energy lasers. We conducted experiments on the LIL (Ligne d'Integration Laser, 10 kJ at 3ω) and the LULI2000 (0.4 kJ at 2ω) facilities, to approach these conditions and study absorption and LPI produced by a high intensity beam in preformed plasmas. After introducing the main risks associated with the short pulse propagation, we present the latest experiment we conducted on LPI in relevant conditions for shock ignition

Effects of hydrodynamics on Stimulated Brillouin Scattering in multiple plasma interaction

In this paper, an experiment carried out on LULI2000 facility is presented. It was designed to investigate how two successive plasmas interact through hydrodynamic coupling and electromagnetic radiations. Contributions of both effects have been successfully identified and the effects of hydrodynamic coupling on Stimulated Brillouin Scattering has been observed

Numerical investigation of spallation neutrons generated from petawatt-scale laser-driven proton beams

Due to their high cost of acquisition and operation, there are still a limited number of high-yield, high-flux neutron source facilities worldwide. In this context, laser-driven neutron sources offer a promising, cheaper alternative to those based on large-scale accelerators, with, in addition, the potential of generating compact neutron beams of high brightness and ultra-short duration. In particular, the predicted capability of next-generation petawatt (PW)-class lasers to accelerate protons beyond the 100 MeV range should unlock efficient neutron generation through spallation reactions. In this paper, this scenario is investigated numerically through particle-in-cell and Monte Carlo simulations, modeling, respectively, the laser acceleration of protons from thin-foil targets and their subsequent conversion into neutrons in secondary heavy-ion targets. Laser parameters relevant to the 1 PW LMJ-PETAL and 1-10 PW Apollon systems are considered. Under such conditions, neutron fluxes exceeding $10^{23}\,\rm n\,cm^{-2}\,s^{-1}$ are predicted, opening up attractive fundamental and applicative prospects

Enhanced ion acceleration using the high-energy petawatt PETAL laser

The high-energy petawatt PETAL laser system was commissioned at CEA’s Laser Mégajoule facility during the 2017–2018 period. This paper reports in detail on the first experimental results obtained at PETAL on energetic particle and photon generation from solid foil targets, with special emphasis on proton acceleration. Despite a moderately relativistic (<1019 W/cm2) laser intensity, proton energies as high as 51 MeV have been measured significantly above those expected from preliminary numerical simulations using idealized interaction conditions. Multidimensional hydrodynamic and kinetic simulations, taking into account the actual laser parameters, show the importance of the energetic electron production in the extended low-density preplasma created by the laser pedestal. This hot-electron generation occurs through two main pathways: (i) stimulated backscattering of the incoming laser light, triggering stochastic electron heating in the resulting counterpropagating laser beams; (ii) laser filamentation, leading to local intensifications of the laser field and plasma channeling, both of which tend to boost the electron acceleration. Moreover, owing to the large (∼100 μm) waist and picosecond duration of the PETAL beam, the hot electrons can sustain a high electrostatic field at the target rear side for an extended period, thus enabling efficient target normal sheath acceleration of the rear-side protons. The particle distributions predicted by our numerical simulations are consistent with the measurements