First Inertial Confinement Fusion Implosion Experiment in Octahedral Spherical Hohlraum

In inertial confinement approaches to fusion, the asymmetry of target implosion is a major obstacle to achieving high gain in the laboratory. A recently proposed octahedral spherical hohlraum makes it possible to naturally create spherical target irradiation without supplementary symmetry control. Before any decision is made to pursue an ignition-scale laser system based on the octahedral hohlraum, one needs to test the concept with the existing facilities. Here, we report a proof-of-concept experiment for the novel octahedral hohlraum geometry on the cylindrically configured SGIII laser facility without a symmetry control. All polar and equatorial self-emission images of the compressed target show a near round shape of convergence ratio 15 under both square and shaped laser pulses. The observed implosion performances agree well with the ideal spherical implosion simulation. It also shows limitations with using the existing facilities and adds further weight to the need to move to a spherical port geometry for future ignition laser facilities.

Figure 1

(a) Schematic diagrams of the experimental setup with diagnostics and the 32-beam optimum repointing scheme. The gray sphere represents the target chamber with major diagnostic ports; the golden sphere represents the six-hole hohlraum with laser beams entering through two polar and four equatorial ports; the wine sphere at the center represents the capsule. (b) Scenography of the laser beam configuration and the laser spots on the hohlraum wall. (c) Square laser pulses (colored lines) for different shots and $T_{r,\mathrm{cap}}$ (black line) for shot SGIII20181227364, which is the temporal space-averaged x-ray radiation drive on the capsule from postshot simulations with lared-integration. (d) Shaped laser pulses (colored lines) for different shots and $T_{r,\mathrm{cap}}$ (black line) for shot SGIII20190102003. (e) Layered capsule diagram. (f) Scenography of the radiation flux distribution on the capsule ablation surface at 1.5 ns for shot SGIII20190102003.

Figure 2

(a) Target configuration with side-target radiography to measure the implosion trajectory of the capsule via an XSC. (b) Implosion trajectory recorded by an XSC for SGIII20181227364 under the square pulse (upper) and for SGIII20181228366 under the shaped pulse (lower). (c) Comparisons of the observed implosion trajectory $R(t)$ (blue squares) and velocity $v(t)=dR(t)/dt$ (red squares) with the postshot 1D simulations (blue and red dashed lines) from a capsule-only multigroup radiation hydrodynamic code lared-s.

Figure 3

Comparisons of polar (left) and equatorial (right) self-emission images between the experiment and postshot 2D simulation for SGIII20190102003. First row shows time-resolved polar self-emission images from an x-ray framing camera at 4.5, 4.6, and 4.7 ns (left, with system resolution of $\sim 17\mu \mathrm{m}$ and pixel value of $0.9\hspace{0.17em}\hspace{0.17em}\mu \mathrm{m}$ at the object space) and time-integrated equatorial self-emission images from a pinhole camera with different filters for x-ray emissions above 5, 9, and 10 keV (right, with system resolution of $\sim 19\mu \mathrm{m}$ and pixel value of $2.5\hspace{0.17em}\hspace{0.17em}\mu \mathrm{m}$ at the object space). The red spots in the center hottest regions are due to the space resolution. The solid and dotted lines are contours of 20% and 50% peak emissions, respectively, with their contour asymmetries $Y_{20}$ and $Y_{40}$ at the image bottom. Here, $Y_{8m}$ is too small to be analyzed with our present technology. The second row shows polar (left) and equatorial (right) images from postshot 2D simulation with LPI taken into account in the asymmetry calculation. The third row shows superimpositions of the measured contour lines on the simulation images. The fourth row shows polar (left) and equatorial (right) images from postshot 2D simulation without accounting for LPI in the asymmetry calculation.