Laser-direct-drive fusion target design with a high-Z gradient-density pusher shell

Laser-direct-drive fusion target designs with solid deuterium-tritium (DT) fuel, a high -Z gradient-density pusher shell (GDPS), and a Au-coated foam layer have been investigated through both 1D and 2D radiation -hydrodynamic simulations. Compared with conventional low -Z ablators and DT-push-on-DT targets, these GDPS targets possess certain advantages of being instability-resistant implosions that can be high adiabat (alpha >= 8) and low hot-spot and pusher-shell convergence (CRhs approximate to 22 and CRPS approximate to 17), and have a low implosion velocity (v(imp) < 3 x 10(7) cm/s). Using symmetric drive with laser energies of 1.9 to 2.5 MJ, 1D LILAC simulations of these GDPS implosions can result in neutron yields corresponding to greater than or similar to 50-MJ energy, even with reduced laser absorption due to the cross-beam energy transfer (CBET) effect. Two-dimensional DRACO simulations show that these GDPS targets can still ignite and deliver neutron yields from 4 to similar to 10 MJ even if CBET is present, while traditional DT-push-on-DT targets normally fail due to the CBET-induced reduction of ablation pressure. If CBET is mitigated, these GDPS targets are expected to produce neutron yields of >20 MJ at a driven laser energy of similar to 2 MJ. The key factors behind the robust ignition and moderate energy gain of such GDPS implosions are as follows: (1) The high initial density of the high -Z pusher shell can be placed at a very high adiabat while the DT fuel is maintained at a relatively low-entropy state; therefore, such implosions can still provide enough compression rho R >1 g/cm(2 )for sufficient confinement; (2) the high -Z layer significantly reduces heat-conduction loss from the hot spot since thermal conductivity scales as similar to 1/Z; and (3) possible radiation trapping may offer an additional advantage for reducing energy loss from such high -Z targets.