First Observation of Cross-Beam Energy Transfer Mitigation for Direct-Drive Inertial Confinement Fusion Implosions Using Wavelength Detuning at the National Ignition Facility

Cross-beam energy transfer (CBET) results from two-beam energy exchange via seeded stimulated Brillouin scattering, which detrimentally reduces ablation pressure and implosion velocity in direct-drive inertial confinement fusion. Mitigating CBET is demonstrated for the first time in inertial-confinement implosions at the National Ignition Facility by detuning the laser-source wavelengths ($\pm 2.3\AA$ UV) of the interacting beams. We show that, in polar direct-drive, wavelength detuning increases the equatorial region velocity experimentally by 16% and alters the in-flight shell morphology. These experimental observations are consistent with design predictions of radiation-hydrodynamic simulations that indicate a 10% increase in the average ablation pressure.

Figure 1

(a) The effect of CBET in PDD predominantly affects the equatorial region; (b) successful CBET mitigation benefits the same region.

Figure 2

The NIF PDD target design for wavelength detuning with cone swapping to induce a wavelength difference across the equator. (Inset) The warm plastic (CH), $1160\text{−}\mu \mathrm{m}$-radius, $100\text{−}\mu \mathrm{m}$-thick shell with a 20-atm ${\mathrm{D}}_2$ gas fill; (red) the total 590-kJ design pulse; (blue) the 45-kJ backlighter pulse.

Figure 3

NIF quad-port hammer projections for the wavelength-detuning CBET mitigation scheme. (a) Indirect-drive mapping where the colored symbols indicate relative wavelength; (b) PDD repoint mapping that achieves hemispheric detuning, typical northern hemisphere repointing, and southern hemisphere cone swapping.

Figure 4

Comparison of backlit radiographs from postshot draco simulations and NIF experimental results near the end of the laser pulse at $t=8.5\mathrm{ns}$. The dashed lines indicate the outer shell surface extracted from each image defined by the steepest gradient in the inward radial direction.

Figure 5

Equatorial shell trajectories from postprocessed simulated (solid lines) and experimental (symbols) backlit radiographs. The red lines and symbols represent the baseline zero-detuning experiment (N160405). The blue lines and symbols represent the average of the two detuning experiments (N160821 and N170102). The inset shows superimposed extracted surfaces from the experimental radiographs of Fig. 4, exemplifying the equatorial mitigation.