Demonstration of the Improved Rocket Efficiency in Direct-Drive Implosions Using Different Ablator Materials

The success of direct-drive implosions depends critically on the ability to create high ablation pressures ($\sim 100\mathrm{Mbar}$) and accelerating the imploding shell to ignition-relevant velocities ($>3.7\times {10}^7\mathrm{cm}/\mathrm{s}$) using direct laser illumination. This Letter reports on an experimental study of the conversion of absorbed laser energy into kinetic energy of the shell (rocket efficiency) where different ablators were used to vary the ratio of the atomic number to the atomic mass. The implosion velocity of Be shells is increased by 20% compared to C and CH shells in direct-drive implosions when a constant initial target mass is maintained. These measurements are consistent with the predicted increase in the rocket efficiency of 28% for Be and 5% for C compared to a CH ablator.

Figure 1

(a) A gold sphere is illuminated by several laser beams that are delayed with successive 50-ps intervals. To absolutely calibrate the timing of the x-ray framing camera (XRFC) to the laser, the x-ray emission measured by the XRFC, in the time reference (bottom axis) of the XRFC (symbols), is compared to the measured laser pulse (dashed curves), in the time reference (top axis) of the laser. The two images (insets) correspond to the XRFC measurements at $t_{\mathrm{XRFC}}=200\mathrm{ps}$ and $t_{\mathrm{XRFC}}=240\mathrm{ps}$. The timing of the laser ($t=0\mathrm{ps}$) corresponds to the 2% intensity point of the first laser pulse and the timing of the XRFC ($t_{\mathrm{XRFC}}=0\mathrm{ps}$) corresponds to the beginning of the first strip of the XRFC. The dashed white circles correspond to the three beams used in the plot. (b) A single radial lineout (solid curve) of the self-emission image presented in (c) is compared with a lineout obtained from postprocessing the hydrodynamic simulation (dashed curve). The variation of the difference between the half-intensity point and the radius of the best circle is plotted (inset). (c) The self-emission image at $t=2.5\mathrm{ns}$ was obtained from a CH target. The black curve corresponds to the location of the half-intensity point; the dashed white circle corresponds to the best-fit circle.

Figure 2

Comparison of the calculated (curves) and measured velocities of the shells averaged over 200 ps (symbols) in CH (orange solid line and circles), C (blue dashed line and triangles), and Be (green dotted line and squares) ablators. Timing error bars correspond to the absolute timing; the relative timing between points is smaller (5 ps).

Figure 3

(a) and (b) Comparison of the calculated (curves) and measured (symbols) shell trajectories (the different symbols represent different shots). (c) and (d) Comparison of the measured (solid curve) and calculated (dashed curve) unabsorbed scattered light. The results are presented for [(a) and (c)] CH and [(b) and (d)] Be ablators. The laser power is plotted on each figure (dashed black curve) and corresponds to an on-target overlapped intensity of $7\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 4

Comparison of the calculated (a) ablation pressure and (b) percentage of remaining mass at $t=2.3\mathrm{ns}$ for the CH ablator ($⟨A⟩/⟨Z⟩=1.85$), C ablator ($⟨A⟩/⟨Z⟩=2$) and Be ablator $⟨A⟩/⟨Z⟩=2.25$). (c) Comparison of the hydrodynamic efficiency (solid squares) and the rocket efficiency of the shell (open squares) for the three ablators.