Abstract
Laser–plasma interaction (LPI) at intensities $10^{15}{-}10^{16}~\text{W}\cdot \text{cm}^{-2}$ is dominated by parametric instabilities which can be responsible for a significant amount of non-collisional absorption and generate large fluxes of high-energy nonthermal electrons. Such a regime is of paramount importance for inertial confinement fusion (ICF) and in particular for the shock ignition scheme. In this paper we report on an experiment carried out at the Prague Asterix Laser System (PALS) facility to investigate the extent and time history of stimulated Raman scattering (SRS) and two-plasmon decay (TPD) instabilities, driven by the interaction of an infrared laser pulse at an intensity ${\sim}1.2\times 10^{16}~\text{W}\cdot \text{cm}^{-2}$ with a ${\sim}100~\unicode[STIX]{x03BC}\text{m}$ scalelength plasma produced from irradiation of a flat plastic target. The laser pulse duration (300 ps) and the high value of plasma temperature ( ${\sim}4~\text{keV}$ ) 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.
Highlights
Laser–plasma interaction (LPI) at intensities ∼1016 W · cm−2 is a regime of interaction dominated by parametric instabilities, where collisional absorption begins to turn off and non-collisional laser-driven instabilities – mainly stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS) and two-plasmon decay (TPD) – begin to dominate the scene
It is well known that such an approach is usually not reliable for estimating HE energy, since the acceleration of electrons at nc/4 usually can occur in different stages[7]; that is, thermal electrons are initially trapped and accelerated at lower densities, and their energy is successively boosted by absolute TPD or hybrid TPD/absolute SRS (aSRS) modes driven near nc/4
LPI of an infrared laser pulse with a multilayer target at shock ignition intensity was characterized in detail
Summary
Laser–plasma interaction (LPI) at intensities ∼1016 W · cm−2 is a regime of interaction dominated by parametric instabilities, where collisional absorption begins to turn off and non-collisional laser-driven instabilities – mainly stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS) and two-plasmon decay (TPD) – begin to dominate the scene. In this paper we investigate LPI and HE generation of the full energy (∼650 J) PALS laser pulse at 1ω0 irradiation (λ0 = 1314 nm), resulting in a maximum intensity of ∼1.2× 1016 W · cm−2, a value seldom reached in experiments In such irradiation conditions the plasma is heated to a temperature in excess of 4 keV, which is crucial for our studies because of the strong temperature dependence of the threshold and the damping of parametric instabilities (and their respective weights), of the density where they are driven, and of the HE energy distribution. It is worth noting that while conventional ICF schemes make use of ultraviolet (3ω) lasers, in the shock ignition scheme 2ω or even 1ω lasers could in principle be considered for the final irradiation spike driving the strong shock[31]
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