Tuning the Implosion Symmetry of ICF Targets via Controlled Crossed-Beam Energy Transfer

Radiative hydrodynamics simulations of ignition experiments show that energy transfer between crossing laser beams allows tuning of the implosion symmetry. A new full-scale, three-dimensional quantitative model has been developed for crossed-beam energy transfer, allowing calculations of the propagation and coupling of multiple laser beams and their associated plasma waves in ignition hohlraums. This model has been implemented in a radiative-hydrodynamics code, demonstrating control of the implosion symmetry by a wavelength separation between cones of laser beams.

Figure 1

(a) Contour plot of a half NIF hohlraum’s electron density (gray scale), and of the intensity of one pair of laser beams at 30° and 50° from the hohlraum axis; the green arrows represent the plasma flow, and the dashed blue lines the limits of the simulation box. (b) Normalized coupling coefficient $\mathrm{Im}[\gamma ]/k_0$ [cf. Eq. (3)] along the central bisector line as a function of $z$ (distance along that line) and $\delta \lambda$ (wavelength shift between the two laser beams, in Å). A $\delta \lambda =1.3\AA$ shift (dashed black line) cancels the net transfer for that pair of beams; the red dotted lines represent the bandwidth induced by 2.2 Å of SSD on each beam.

Figure 2

Relative energy gain of the inner and outer cones as a function of the wavelength shift between the cones of beams, $\Delta \lambda ={\lambda }_{\mathrm{inner}}-{\lambda }_{\mathrm{outer}}$, with all smoothing techniques (solid lines), CPP with PS and CPP only (dashed curves, shown for the inner cone only). Also plotted is $P2/P0$ from Lasnex simulations including crossed-beam energy transfer for $\Delta \lambda =0$, 0.6 and 1.2 Å.

Figure 3

(a) Hohlraum electron density from Lasnex at peak laser power, with schematics of the inner and outer laser cones (in units of $I_{15}=I/{10}^{15}\mathrm{W}/{\mathrm{cm}}^2$) and their respective wavelengths. (b) Capsule density and fusion yield at ignition for $\Delta \lambda =0$, 0.6 and 1.2 Å from Lasnex calculation including crossed-beam transfer (same spatial scale on each image).