Performance and Mix Measurements of Indirect Drive Cu-Doped Be Implosions

The ablator couples energy between the driver and fusion fuel in inertial confinement fusion (ICF). Because of its low opacity, high solid density, and material properties, beryllium has long been considered an ideal ablator for ICF ignition experiments at the National Ignition Facility. We report here the first indirect drive Be implosions driven with shaped laser pulses and diagnosed with fusion yield at the OMEGA laser. The results show good performance with an average DD neutron yield of $\sim 2\times {10}^9$ at a convergence ratio of $R_0/R\sim 10$ and little impact due to the growth of hydrodynamic instabilities and mix. In addition, the effect of adding an inner liner of W between the Be and DD is demonstrated.

Figure 1

Overview of the experimental configuration. (a) Shaped laser pulse shape (black curve) used to produce the hohlraum drive (Omega shape LA232301) and radiation temperature (Tr) obtained by using the DANTE [20, 21] diagnostic (red curve). (b) Schematic of the $D_2$ gas-filled Be capsule. Half of capsules had a $0.2\mu \mathrm{m}$ W inner layer. (c) The hohlraum and capsule are backlit by using Cu $k\text{−}\alpha$ x rays produced by using the OMEGA EP laser with a 0.8 kJ, 10 ps, laser pulse. The backlighter x rays are recorded on an image plate by using a spherical crystal imager, which provides a very narrow bandpass image ($\sim 8\mathrm{eV}$).

Figure 2

(a) A cross-sectional image of the Be capsule before the mandrel is removed by pyrolysis. (b) A cross-sectional image of the Be capsule after the mandrel is removed. (c) Profiles of Cu dopant before and after pyrolysis and of the reminant Ar from the coating process.

Figure 3

(a) View of the capsule with the spherical crystal imager, which is 3° off the hohlraum polar axis. (b) Backlit image of the imploding shell at 2.17 ns using a 10 ps OMEGA EP driven Cu $k\text{−}\alpha$ backlighter. The central feature is the hot-spot self-emission. (c) Backlit image of the imploding shell at 2.43 ns. (d) Time-integrated self-emission image of the capsule. The hot spot is blown up in the lower panel. A 17% intensity contour is illustrated in blue corresponding to a mean hot-spot radius of $38\mu \mathrm{m}$. Note that (b)–(d) are on the same color scale but that (d) shows a target without W, which is not as bright as the targets with W, described in more detail in the text. Note also that the color scale for (b) and (c) are saturated to emphasize the imploding shell (making the apparently hot-spot size appear larger by emphasizing lower contours); however, the image plate data are not saturated.

Figure 4

Performance observations plotted as a function of simulated observable. In all of the panels, blue points represent targets with a $0.2\mu \mathrm{m}$ thick W tracer layer on the gas-shell interface, while red squares are without. On the abscissa, solid symbols show simulations that include turbulent mix, while open symbols show simulations without. (a) Measured DD neutron yield from the OMEGA nTOF diagnostic suite [29] as a function of simulated yield. Note the experimental uncertainties are suppressed for clarity and are of the order of the symbol size. (b) Hot-spot radius measured from the x-ray self-emission radiographs with the SCI. Note the experimental uncertainties are suppressed for clarity and are of the order of the symbol size. (c) DD Neutron bang time measured by using the neutron bang time detector. (d) Burn weighted ion temperature measured by using the Doppler broadened 2.45 MeV DD neutron peak observed with the nTOF diagnostics. (e) $\mathrm{D}{}^3\mathrm{He}$ proton yield, collected with WRFs over three lines of sight, normalized by the DD neutron yield, which is sensitive to the fuel areal density [31]. (f) Secondary $\mathrm{D}{}^3\mathrm{He}$ proton energy measured with three WRFs at different lines of sight, which is sensitive to the total fuel and shell areal densities.

Figure 5

(a) Transmission profiles produced from radiographs of the in-flight shell. (b) Measured (square) radius of minimum transmission as a function of time compared with simulations (open circles) and simulations including mix (closed circles).