Laser-Beam Zooming to Mitigate Crossed-Beam Energy Losses in Direct-Drive Implosions

Spherically symmetric direct-drive-ignition designs driven by laser beams with a focal-spot size nearly equal to the target diameter suffer from energy losses due to crossed-beam energy transfer (CBET). Significant reduction of CBET and improvements in implosion hydrodynamic efficiency can be achieved by reducing the beam diameter. Narrow beams increase low-mode perturbations of the targets because of decreased illumination uniformity that degrades implosion performance. Initiating an implosion with nominal beams (equal in size to the target diameter) and reducing the beam diameter by $\sim 30\%–40\%$ after developing a sufficiently thick target corona, which smooths the perturbations, mitigate CBET while maintaining low-mode target uniformity in ignition designs with a fusion gain $\gg 1$.

Figure 1

Simulated (a) fusion gain $G$, (b) implosion velocity $V_{\mathrm{imp}}$, (c) laser-absorption fraction $f_{\mathrm{abs}}$, and (d) implosion hydrodynamic efficiency $\eta$ for the 1.5-MJ spherically symmetric direct-drive implosion design [7] as functions of $R_b/R_t$. The black circles and solid lines correspond to simulations including CBET. The open diamonds and dashed lines show the performance in the nominal configuration (assuming $R_b/R_t=1$) without CBET. The open circles represent the improvements in the nominal configuration with CBET, when CBET is mitigated employing two-color laser light [8].

Figure 2

Low-adiabat ($\alpha =2$), 24-kJ triple-picket cryogenic OMEGA implosion design (shot 66613) used in 2D draco simulations. (a) Target geometry and (b) laser power history. A spherically symmetric simulation of this design without CBET predicts a yield of $3.4\times {10}^{14}$ neutrons.

Figure 3

Simulated distributions of the density at the peak neutron production for the implosion design shown in Fig. 2. (a) Nominal-size beams, $R_b/R_t=1$; (b) reduced-size beams, $R_b/R_t=0.6$; (c) zooming beams from $R_b/R_t=1$ to 0.6 at $t=0.4$ ns [after the first picket; see Fig. 2b]; (d) same zooming beams as in (c), but at $t=0.9\mathrm{ns}$ (after the second picket).