Impact of asymmetries on fuel performance in inertial confinement fusion

Low-mode asymmetries prevent effective compression, confinement, and heating of the fuel in inertial confinement fusion (ICF) implosions, and their control is essential to achieving ignition. Ion temperatures ($T_{\mathrm{ion}}$) in ICF experiments are inferred from the broadening of primary neutron spectra. Directional motion (flow) of the fuel at burn also impacts broadening and will lead to artificially inflated “$T_{\mathrm{ion}}$” values. Flow due to low-mode asymmetries is expected to give rise to line-of-sight variations in measured $T_{\mathrm{ion}}$. We report on intentionally asymmetrically driven experiments at the OMEGA laser facility designed to test the ability to accurately predict and measure line-of-sight differences in apparent $T_{\mathrm{ion}}$ due to low-mode asymmetry-seeded flows. Contrasted to chimera and xrage simulations, the measurements demonstrate how all asymmetry seeds have to be considered to fully capture the flow field in an implosion. In particular, flow induced by the stalk that holds the target is found to interfere with the seeded asymmetry. A substantial stalk-seeded asymmetry in the areal density of the implosion is also observed.

Figure 1

(a) Picture of CH-shell capsule used in the OMEGA experiment mounted to the stalk with a glue spot with characterized length and diameter. (b) Asymmetry A and (c) Asymmetry B contour maps of laser intensity as a function of polar and azimuthal angle relative to the average laser intensity, illustrating the imposed mode 2. The plus signs represent the locations of the primary nTOF detectors; from left to right, 15.8 mntof, 12 mntof, and 5.0 mcvd. The purple star represents the location of the stalk.

Figure 2

Average $T_{\mathrm{ion}}$ for shots driven with (a) Asym. A and (c) Asym. B, minus $T_{\mathrm{ion}}$ for the symmetric reference shot. Points with error bars represent measured values in the three lines of sight using the 12 mntof, 15.8 mntof, and 5.0 mcvd detectors; black crosses 3D chimera-simulated values for the same lines of sight. The gray arrows below the plots indicate whether a detector is located parallel or perpendicular to the axis of the imposed mode 2 asymmetry. Also shown are cartoons illustrating the angle between the imposed mode 2 and the stalk for the two drive configurations, Asym. A in (b) and Asym. B in (d); the thin black line represents the asymmetry axis.

Figure 3

(a) Measured shell trajectories as inferred from self-emission x-ray images for Asym. A (red triangles) and Asym. B (blue squares), contrasted to shell trajectories as inferred from chimera-simulated x-ray images (thick red dashed and solid blue lines for the two cases). Also shown is the laser pulse shape (dotted black) and the burn history for the two implosions (red dashed line for Asym. A, broken blue line for Asym. B). (b) Mode 2 amplitude inferred from measured and simulated x-ray images, with the same color coding as in (a).

Figure 4

Measured (points with error bars) and 2D-xrage-simulated (solid green line) ρR for the symmetric reference shot as a function of angle to the stalk. While the xrage simulation underestimates the overall ρR magnitude, it captures the shape of the asymmetry remarkably well: The dashed green line representing the xrage simulation renormalized to match the amplitude of the data matches the measurement with ${\chi }_{\mathrm{red}}^2=0.5$. In contrast, using a flat ρR (broken black curve) to describe the data gives ${\chi }_{\mathrm{red}}^2=1.6$.