Precision Shock Tuning on the National Ignition Facility

Ignition implosions on the National Ignition Facility [J. D. Lindl et al., Phys. Plasmas 11, 339 (2004)] are underway with the goal of compressing deuterium-tritium fuel to a sufficiently high areal density ($\rho R$) to sustain a self-propagating burn wave required for fusion power gain greater than unity. These implosions are driven with a very carefully tailored sequence of four shock waves that must be timed to very high precision to keep the fuel entropy and adiabat low and $\rho R$ high. The first series of precision tuning experiments on the National Ignition Facility, which use optical diagnostics to directly measure the strength and timing of all four shocks inside a hohlraum-driven, cryogenic liquid-deuterium-filled capsule interior have now been performed. The results of these experiments are presented demonstrating a significant decrease in adiabat over previously untuned implosions. The impact of the improved shock timing is confirmed in related deuterium-tritium layered capsule implosions, which show the highest fuel compression ($\rho R\sim 1.0\mathrm{g}/{\mathrm{cm}}^2$) measured to date, exceeding the previous record [V. Goncharov et al., Phys. Rev. Lett. 104, 165001 (2010)] by more than a factor of 3. The experiments also clearly reveal an issue with the 4th shock velocity, which is observed to be 20% slower than predictions from numerical simulation.

Figure 1

NIF laser power history for the tuning series. Shot N110513 (red), N110515 (green), N110517 (blue), N110521 (magenta), and N110526 (cyan). Inset shows the hohlraum geometry for these experiments.

Figure 2

VISAR streak images of three sequential tuning shots. Arrival time of each of the four shocks in liquid ${\mathrm{D}}_2$ is indicated.

Figure 3

Comparison of measured vs predicted tuning sensitivities. (a) time of 1st shock breakout into liquid ${\mathrm{D}}_2$, (b) 2nd shock velocity, (c) depth of shock 3–4 merger, (d) 4th shock velocity.

Figure 4

(a) Shock velocity history for four shots in the tuning series. Black curve is VISAR data. Red curve is simulated VISAR from 2D integrated-hohlraum simulations. The individual shocks are labeled in (b).

Figure 5

Shock trajectories ($r$, $t$) in liquid ${\mathrm{D}}_2$ for the 5 shot tuning series.