First polar direct-drive implosions on the Laser MégaJoule

Abstract We report here the preliminary results of the first polar direct-drive (PDD) implosions carried out on the Laser MégaJoule (LMJ) facility. These implosions used D 2 -filled thin SiO 2 capsules in an exploding-pusher (strong shock) regime, with 10 laser bundles (20 quadruplets, 80 beams) operating at the 100 kJ level total laser energy. In this configuration, a square laser pulse and a thin ablator result in small convergence ratios and large hot spots with high ion and electron temperatures. For optimizing the beam irradiation uniformity of the LMJ facility that is not dedicated to direct-drive implosions, a quad repointing is applied using the PDD technique. Namely, laser quads within the 49° irradiation cone are repointed towards the capsule equatorial plane in order to compensate the energy deficit in this plane compared to the poles. Meanwhile, laser quads within the 33° irradiation cone are repointed towards the capsule poles, and the power balance is set so that the power in the 33° cone is 60% of the power in the 49° cone. Neutron data as well as time-resolved x-ray images are presented and compared to 3D pre-shot hydrodynamics simulations. We find an improvement of implosion symmetry and neutron yield using optimized PDD. We also notify a relative robustness of the neutron yield to the different laser pointing configurations in this implosion regime.