Investigation of Laser Plasma Instabilities driven by Coupled High-Power Laser Beams in Magnetized Underdense Plasmas

Abstract

Stimulated Brillouin and Raman scattering (SBS and SRS) are instabilities that affect the propagation of high-power lasers in plasmas. The latter is further affected by Cross-Talk (CT) effects when multiple laser beams are simultaneously propagated in the plasma, as found in the schemes proposed for inertial confinement fusion (ICF). Here we develop a new theoretical model that allows us to evaluate the impact of CT on SBS and SRS in low-density plasmas. As supported by experiments, we demonstrate that CT can lead to a reduction of both SBS and SRS, due to the destabilization of the individually triggered instabilities. We further demonstrate that this destabilization effect is accelerated by applying an externally magnetic field to the plasma, which is also beneficial for the hydrodynamics or fuel heating of ICF. By shedding new light on the promising scheme of magnetized ICF, our findings thus offer beneficial prospects for ICF.

Figures (4)

• Figure 1: (a) Experimental setup, illustrating the arrangement of laser beams inside the magnetic field coil yao2023dynamics, along with the fielded diagnostics (see text). A low-density gas jet is positioned in the center of the coil. The gas flows along the z-axis, and the lasers propagate perpendicularly to the flow. (b) Map of the electron density in the plasma ionized by the lasers. The image is taken 3 ns after the start of the laser propagation in the gas, and is measured by optical probing (see Supplemental Material SupplementalMaterial). The measured plasma density corresponds to the lower end of the plasma densities explored here. The lasers propagates horizontally in the image. (c) Plasma temperature measurement retrieved through Thomson Scattering (TS, see Supplemental Material SupplementalMaterial) off the ion waves. The experimental data is shown as the black solid line, together with a theoretical fit doecode_68245 (red dashed lines) and its associated uncertainty bar (blue dashed lines), which gives $T_e = 60 \pm 10$ eV and $T_i = 0.1 T_e$. what is the density for the TS measurement?
• Figure 2: Experimental measurements of the variation of the reflectivities (R) of (a) backward SRS and (b) backward SBS (both normalized to the input laser energy for each shot), as function of the plasma electron density $n_e$, when driving the plasma with two laser beams, compared to with just one laser beam. The vertical axis is the difference percentage in diode signals between the case of two and one laser beams, with (red squares) or without (blue circles) magnetic field. The error bars, calculated as the standard deviation of two to three shots per data point, are comparable to the size of the data points.
• Figure 3: Experimental measurements of the variation of the reflectivities (R) of (a) backward SRS and (b) backward SBS (both normalized to the input laser energy), as function of the plasma electron density $n_e$, when magnetizing the plasma compared to the unmagnetized case. The vertical axis is the difference percentage in diode signals between the case with and without magnetic field, when using two laser beams (purple triangles) or just one beam (green diamonds). The error bars, calculated as the standard deviation of two to three shots per data point, are comparable to the size of the data points.
• Figure 4: (a) Map of the the square root of the condition for SBS growth, based on our model, in Cross-Talk (CT) conditions, normalized to the Landau damping (as defined in Table S1 of the Supplemental Material SupplementalMaterial). (b) Map of the minimum magnetic field required to achieve a decrease single-beam SRS, and correspondingly a two-beam SRS increase, as a function of the plasma electron density and temperature, when magnetizing the plasma. The red dotted lines correspond to iso-values of the $k\lambda_D$ factor. The colormap is capped at 30 T to enhance readability.