Experimental demonstration of low laser-plasma instabilities in gas-filled spherical hohlraums at laser injection angle designed for ignition target

Octahedral spherical hohlraums with a single laser ring at an injection angle of ${55}^{\circ }$ are attractive concepts for laser indirect drive due to the potential for achieving the x-ray drive symmetry required for high convergence implosions. Laser-plasma instabilities, however, are a concern given the long laser propagation path in such hohlraums. Significant stimulated Raman scattering has been observed in cylindrical hohlraums with similar laser propagation paths during the ignition campaign on the National Ignition Facility (NIF). In this Rapid Communication, experiments demonstrating low levels of laser-driven plasma instability (LPI) in spherical hohlraums with a laser injection angle of ${55}^{\circ }$ are reported and compared to that observed with cylindrical hohlraums with injection angles of $28.5^{\circ }$ and ${55}^{\circ }$, similar to that of the NIF. Significant LPI is observed with the laser injection of $28.5^{\circ }$ in the cylindrical hohlraum where the propagation path is similar to the ${55}^{\circ }$ injection angle for the spherical hohlraum. The experiments are performed on the SGIII laser facility with a total $0.35\text{−}\mu \mathrm{m}$ incident energy of 93 kJ in a 3 nsec pulse. These experiments demonstrate the role of hohlraum geometry in LPI and demonstrate the need for systematic experiments for choosing the optimal configuration for ignition studies with indirect drive inertial confinement fusion.

Figure 1

The illustrations of the spherical hohlraum (a) and the cylindrical hohlraum (b). The red, green, cyan, and purple poles denote laser beams injecting at ${55}^{\circ }$, $49.5^{\circ }$, ${35}^{\circ }$, and $28.5^{\circ }$, respectively. As shown in the figure, the backscattered light is measured from the lower portion of the hohlraums at ${55}^{\circ }$ for the spherical hohlraum and $28.5^{\circ }$ and ${55}^{\circ }$ for the cylindrical hohlraum.

Figure 2

Schematic diagrams of the experimental setup. Inset is contour lines of $n_e$ in the plane of sphere radius and laser energy, which gives the initial design of the sphere radius by using the extended plasma-filling model. The yellow star is the design point.

Figure 3

The laser backscatter fractions (including both SRS and SBS) measured by using FABS and NBS diagnostic on one $28.5^{\circ }$ innermost beam (upper) and on one ${55}^{\circ }$ outermost beam (lower) for all shots. The circles are for the spheres, and the squares are for the cylinders. In our experiment, the error bar is about $30\%$ for SBS and SRS, respectively, and a little higher for SRS than for SBS.

Figure 4

Observed evolutions of SRS spectrum for both configurations filled with gas: without capsule (upper) and with capsule (lower). From left to right: at ${55}^{\circ }$ for sphere, and at ${55}^{\circ }$ and $28.5^{\circ }$ for cylinder.

Figure 5

Spatial distributions of $N_e$ and $T_e$ at 2.3 ns from lared for the gas-filled sphere (left) and cylinder (right) without (upper) or with (lower) a capsule inside. The black lines are boundaries of the laser beam at $28.5^{\circ }$, and the red are that at ${55}^{\circ }$. The regions with letters of SRS denote the places with high gain of SRS.