Designing Gradient-Density Pusher-Shell Targets for Laser-Direct-Drive Fusion*

We have performed laser-direct-drive (LDD) fusion target designs with a high-Z gradient-density pusher shell (GDPS) through 1-D and 2-D radiation-hydrodynamic simulations. These studies show that ignition with moderate gain is feasible with these GDPS targets, even when the detrimental cross-beam energy transfer (CBET) effect is still present. Compared with the conventional DT-push-on-DT targets, the robustness of such GDPS implosions can be attributed to the following facts: (1) the high-Z pusher shell can be placed on a very high adiabat (α = 6 to 10), while the DT fuel may still be in a low-entropy state; (2) the GDPS target ignition only needs a relatively lower implosion velocity of vimp = 250 to 300 km/s, which can be ~30% lower than the minimum implosion velocity (vimp = ~370 km/s) required for conventional DT-push-on-DT targets; (3) these GDPS implosions only require a relatively smaller convergence (CRhs = ~22 and CRPS = ~17) to ignite; and (4) the high-Z layer serves as a heat insulator to reduce thermal conduction loss, plus the possible radiation “recycling/trapping” by the high-Z pusher. Two-dimensional DRACO simulations show that ignition with producing neutrons of 4 to ~10 MJ energy could be possible with a driven laser of 1.9 to ~2.5 MJ in a symmetric LDD configuration, even when CBET is still present. In the future, once CBET is mitigated in future facilities, these GDPS targets will have an even larger margin that could potentially result in 10- to ~20-MJ neutron yields for the same laser energy.