Implosion Experiments using Glass Ablators for Direct-Drive Inertial Confinement Fusion

Direct-drive implosions with $20\mathrm{\text{−}}\mu \mathrm{m}$-thick glass shells were conducted on the Omega Laser Facility to test the performance of high-$Z$ glass ablators for direct-drive, inertial confinement fusion. The x-ray signal caused by hot electrons generated by two-plasmon-decay instability was reduced by more than $\sim 40\times$ and hot-electron temperature by $\sim 2\times$ in the glass compared to plastic ablators at ignition-relevant drive intensities of $\sim 1\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$, suggesting reduced target preheat. The measured absorption and compression were close to $1D$ predictions. The measured soft x-ray production in the spectral range of $\sim 2$ to 4 keV was $\sim 2\times$ to $3\times$ lower than $1D$ predictions, indicating that the shell preheat caused by soft x-rays is less than predicted. A direct-drive-ignition design based on glass ablators is introduced.

Figure 1

Pulse shapes used in mid- and high-intensity glass implosions.

Figure 2

(a) The measured and predicted spectra of soft x-ray emission at 1.5 ns for one of the high-intensity implosions. Comparison of measured and predicted x-ray signals in two channels with photon energies of (b) 1.6 to 1.8 keV and (c) 3.2 to 4.8 keV.

Figure 3

Measured versus predicted (a) peak-burn areal densities and (b) laser-absorption fractions for midintensity (triangles) and high-intensity (diamonds) implosions.

Figure 4

(a) The schematic of the glass-ablator ignition target with a radius of $1600\mu \mathrm{m}$, a $35\mathrm{\text{−}}\mu \mathrm{m}$-thick outer glass-ablator shell followed by an $80\mathrm{\text{−}}\mu \mathrm{m}$-thick DT-ice shell. The target is driven by a 10-ns pulse with a total laser energy of 1.5 MJ and a peak intensity of $\sim 8.7\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. The predicted gain in this $1D$ ignition design is 27. (b) Comparison of the density profiles of ignition designs with all-DT (dashed curve) and glass-ablator (solid curve) targets taken near the end of their acceleration phases. The profile of the glass-ablator design shows a double ablation front.