Achieving highest fusion yields in direct-drive ICF through improved energy coupling

The fusion yield of ICF implosions is strongly dependent on the shell implosion velocity. A careful balance between velocity and hydrodynamic stability must be maintained to achieve the highest yields. In direct drive, the implosion velocity can be increased by augmenting the laser-to-target energy coupling. This is accomplished by reducing the effects of Cross-Beam Energy Transfer (CBET) and by increasing the energy absorbed by the target. CBET is mitigated by decreasing the laser beam radius (R_b) relative to the target radius (R_t), thereby reducing the crossing of edge rays from one beam with the central rays of another. Laser absorption can be improved by adding mid-Z dopants to the ablator to enhance inverse bremsstrahlung. With the OMEGA beam geometry, reducing the R_b/R_t ratio leads to larger seeds of mid-mode (l>10) nonuniformities and the shell in-flight adiabat must be increased to maintain stability at higher velocities. Therefore, the design of such implosions must be carefully optimized to account for these different effects [Williams, C. A., et al. Physics of Plasmas 28.12 (2021): 122708.]. Using new statistical prediction capabilities [Gopalaswamy et al, Nature 565 (2019) 581–586; A. Lees et al, Phys. Rev. Lett. 127, 105001 (2021)], targets were designed with multilayer CH ablators with outer Si-doped overcoats. To further enhance energy coupling, the OMEGA multipulse-driver configuration (MPD) was used where SSD smoothing is ON during the initial picket pulse and OFF during the main drive. With SSD-OFF, the beam radius shrinks, and the laser coupling is improved. These new target designs were fielded on OMEGA leading to record neutron yields above 3x10¹⁴, or about 1kJ of fusion energy, and a hydroscaled, normalized Lawson parameter about 80% of the value required for ignition at 2MJ of symmetric laser drive.