Energy transfer between lasers in low-gas-fill-density hohlraums

We investigate cross-beam energy transfer (CBET), where power is transferred from one laser beam to another via a shared ion acoustic wave in hohlraums with low-gas-fill density as a tool for late-time symmetry control for long-pulse (greater than $10\mathrm{ns}$) inertial confinement fusion (ICF) and laboratory astrophysics experiments. We show that the radiation drive symmetry can be controlled and accurately predicted during the foot of the pulse (until the rise to peak power), which is important for mitigating areal density variations in the compressed fuel in ICF implosions. We also show that the effective inner-beam drive after CBET is much greater than observed in previous high-gas-filled-hohlraum experiments, which is thought to be a result of less inverse bremsstrahlung absorption of the incident laser light and reduced (by more than 10 times) stimulated Raman scattering (and Langmuir wave heating). With the inferred level of inner-beam drive after transfer, we estimate that more than $1.25$ times larger plastic capsules could be fielded in this platform with sufficient laser-beam propagation to the waist of the hohlraum. We also estimate that a full-scale plastic capsule, $1100\mu \mathrm{m}$ in capsule radius, would require $\sim 1–2\AA$ of $1\omega$ wavelength separation between the outer and inner beams to achieve a symmetric implosion in this platform.

Figure 1

Shown on the left is a schematic of the hohlraum with simulations overlaid showing the trajectory of the inner and outer beams. The bottom right diagram shows the GDP capsule configuration, including the silicon-doped layers. The top right is a graph of the laser powers vs time for the shock timing (N161103, in black) and radiography (N170706, in red) shots. The inset shows the measured cone fractions.

Figure 2

(a) Simulated shock velocities (km/s) vs time in the polar direction (red and green) compared to experimental data (black) for shock timing shot N161103. (b) Simulated (red and green) vs measured (black) shock velocities along the equatorial direction for N161103. The insets are expanded views of the final shock mergers to show model sensitivity. The three experimental curves correspond to the uncertainty range of the measurement.

Figure 3

(a) Simulated radiographs and P2 decompositions (red points) of the dense shell inflight (radius near $200\mu \mathrm{m}$) for shot N170706 vs effective average power increase on the inner beams as a result of CBET (top $x$ axis). The simulations saturate the amplitude of the ion acoustic wave ($\delta n_e^{\mathrm{sat}}/n_e$) to vary the $x$ axis (shown on the bottom $x$ axis). Also shown is the measured radiograph and P2 with a band that denotes the uncertainty. (b) Decompositions of the measured P2 inflight ($P0=200\mu \mathrm{m}$) and near peak compression ($P0\sim 50\mu \mathrm{m}$) for low-gas-fill hohlraum experiments with no intentional CBET (N150826, $\mathrm{\Delta }\mathrm{\Lambda }=0\AA$) and with CBET (N170706, $\mathrm{\Delta }\mathrm{\Lambda }=5.5\AA$).