High-Adiabat High-Foot Inertial Confinement Fusion Implosion Experiments on the National Ignition Facility

This Letter reports on a series of high-adiabat implosions of cryogenic layered deuterium-tritium (DT) capsules indirectly driven by a “high-foot” laser drive pulse at the National Ignition Facility. High-foot implosions have high ablation velocities and large density gradient scale lengths and are more resistant to ablation-front Rayleigh-Taylor instability induced mixing of ablator material into the DT hot spot. Indeed, the observed hot spot mix in these implosions was low and the measured neutron yields were typically 50% (or higher) of the yields predicted by simulation. On one high performing shot (N130812), 1.7 MJ of laser energy at a peak power of 350 TW was used to obtain a peak hohlraum radiation temperature of $\sim 300\mathrm{eV}$. The resulting experimental neutron yield was $(2.4\pm 0.05)\times {10}^{15}$ DT, the fuel $\rho R$ was $(0.86\pm 0.063)\mathrm{g}/{\mathrm{cm}}^2$, and the measured $T_{\text{ion}}$ was $(4.2\pm 0.16)\mathrm{keV}$, corresponding to 8 kJ of fusion yield, with $\sim 1/3$ of the yield caused by self-heating of the fuel by $\alpha$ particles emitted in the initial reactions. The generalized Lawson criteria, an ignition metric, was 0.43 and the neutron yield was $\sim 70\%$ of the value predicted by simulations that include α-particle self-heating.

Figure 1

Laser power vs time is shown for a representative low-foot pulse shape (black) and high-foot pulse shapes (N130710 in blue, N130501 in green, and N130812 in red). The salient features of the high-foot pulse shape are higher picket and trough powers and a shorter pulse duration, as well as the removal of the second laser impulse (seen in the low-foot case near 13 ns).

Figure 2

Hot spot and down-scattered images from cryogenic layered DT implosion shot N130812. From left-to-right, the figure shows x-ray images of emission intensity in the equatorial and polar view, and neutron images in the equatorial view for direct (13–15 MeV) neutrons, and down-scattered (6–12 MeV) neutrons. The color scale reflects emission intensity [red (dark gray) is high intensity, blue (light gray) is low]. The upper row shows the images from the experiment, and the lower row shows results from post-shot 2D hydra simulations. Notably, both experiment and simulation showed oblate x-ray hot spot images viewed from the equator and a toroidal shaped hot spot when viewed from the pole.

Figure 3

A scatter plot of total DT neutron yield versus fuel areal density $\rho R(g/{\mathrm{cm}}^2)$ for cryogenic layered DT implosions on NIF for the low-foot series (gray circles) and high-foot series (green squares). Contours of α-heating multiplication are shown as the blue curves. The shots that are the discussed in this Letter are labeled by shot number: N130501, N130710, and N130812. A key point to note is that shot N130812 achieved a 50% boost in neutron yield due to $\alpha$-particle heating of the hot spot, as denoted by $Y_{\alpha }/Y_{no\text{−}\alpha }\sim 1.5$.

Figure 4

Plots of observed neutron yield versus simulation and GLC. (a) The total experimentally determined yield (vertical axis) versus the yield predicted from 1D simulations (horizontal axis). The diagonal blue lines show contours of experimental yield over clean (YOC) 1D simulation which include α heating. (b) The experimentally observed DT neutron yields vs the generalized Lawson criterion. Notably, shot N130812 reached GLC $\sim 0.43$. The vertical dashed line at $\text{GLC}=0.54$ corresponds to the threshold of $Y_{\alpha }/Y_{n0-\alpha }=2$, corresponding to a yield doubling due to α heat deposition in the hot spot.