Suppression of Laser Nonuniformity Imprinting Using a Thin High-$Z$ Coating

Imprinting of laser nonuniformity is a limiting factor in direct-drive inertial confinement fusion experiments, particularly when available laser smoothing is limited. A thin ($\sim 400\AA$) high-$Z$ metal coating is found to substantially suppress laser imprint for planar targets driven by pulse shapes and intensities relevant to implosions on the National Ignition Facility while retaining low adiabat target acceleration. A hybrid of indirect and direct drive, this configuration results in initial ablation by x rays from the heated high-$Z$ layer, creating a large standoff for perturbation smoothing.

Figure 1

Laser intensity on target (inset) and the experimental setup showing face-on radiography using monochromatic x-ray imaging.

Figure 2

Raw streaked face-on radiographs with 1.86 keV x rays (top row) and the resulting areal mass fluctuations in the range of $10–100\mu \mathrm{m}$ (bottom row) for uncoated (left) and 400 Å Au coated (right) targets. The coating suppresses the imprint to below the measurement noise level.

Figure 3

Areal mass fluctuations for uncoated, 400 Å Pd, and 600 Å Pd coated targets. For the 600 Å coating, the imprint is reduced to below measurement noise.

Figure 4

rms of areal mass perturbations vs time for uncoated, 200 Å Au, 400 Å Pd, 400 Å Au, and 600 Å Pd overcoat targets. Two uncoated shots are shown with larger and smaller rms values in the nonlinear stage of growth, representative of extremes of shot-to-shot variation at that stage. Baseline noise level for the measurement is generally $0.2\mathrm{mg}/{\mathrm{cm}}^2$, except for the 200 Å Au case due to weaker backlighting on that shot.

Figure 5

Evolution of areal mass perturbations started by a front surface ripple ($0.5\mu \mathrm{m}$ amplitude, $30\mu \mathrm{m}$ wavelength) for uncoated and 400 Å Au overcoat targets. The ripple development is very similar in both cases.

Figure 6

rms of areal mass perturbations for the shots shown in Fig. 5. The RT growth rate is unchanged by the coating, except for an approximately 0.3 ns time delay (for comparison, the dashed line shows the coated case shifted 0.3 ns earlier).

Figure 7

Streaked side-on backlighting at 1.86 keV of a 600 Å Pd coated target. The laser is coming from the left. Though the target is $50\mu \mathrm{m}$ thick, the shadow prior to shock breakout is $60\mu \mathrm{m}$ wide due to a slight rotation with respect to the diagnostic plane and the $\approx 2\mathrm{mm}$ width of the target. No motion of the rear of the target is evident prior to the shock breakout.

Figure 8

Absolutely calibrated x-ray spectra show a marked increase in soft x-ray emission for Au and Pd coated targets as compared to the uncoated case. The emission peaks soon after the start of the main pulse (main pulse half rise is at $t=0.7\mathrm{ns}$) for the coated targets, decaying to the level of the uncoated ones during the pulse. The spectra are measured by the individual photodiode channels of a transmission grating spectrometer [27], with an approximately 0.4 ns time resolution.

Figure 9

Large separation between the high-$Z$ layer and the underlying CH as evidenced from side-on radiography at 1.86 keV. Shown is the line-integrated density of Au (left panel) and Pd (right panel) prior to and in the early stages of the laser pulse. The line density is obtained from absorption of the 1.86 keV x rays in the high-$Z$ layer (the ablated CH opacity is too low to be seen; Au and Pd lose opacity once the main drive comes on). These shots had pulse shape with a $4\mathrm{TW}/{\mathrm{cm}}^2$ foot starting at $t=-2\mathrm{ns}$ and a $1\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ main drive at $t=0$. During the pulse, the ablation of the underlying CH moves the high-$Z$ layer toward the laser (coming in from the right). Both Au and Pd are preexpanded by the low-level prepulse prior to the start of the laser pulse; while Pd hugs the original target surface (the shadow of which is shown as the gray vertical strip) prior to the laser pulse, Au is pushed off by the underlying CH vaporization. Imprint suppression is effective in both cases, however.