Polar direct drive on the National Ignition Facility

Abstract

Three recent developments in direct-drive target design have enhanced the possibility of achieving high target gain on the National Ignition Facility (NIF): (1) Laser absorption was increased by almost 50% using wetted-foam targets. (2) Adiabat shaping significantly increased the hydrodynamic stability of the target during the acceleration phase of the implosion without sacrificing target gain. (3) Techniques to reduce laser imprint using pulse shaping and radiation preheat were developed. These design features can be employed for direct-drive-ignition experiments while the NIF is in the x-ray-drive configuration. This involves repointing some of the beams toward the equator of the target to improve uniformity of target drive. This approach, known as polar direct drive (PDD), will enhance the capability of the NIF to explore ignition conditions. PDD will couple more energy to the fuel than x-ray drive. The compressed fuel core can be more easily accessed for high-ρR diagnostic development and for fast-ignitor studies. Polar direct drive is examined in this manuscript using two-dimensional hydrodynamic simulations to determine the level of target performance that can be achieved.

Investigation of ion kinetic effects in direct-drive exploding-pusher implosions at the NIF

“…Exploding-pusher implosions at the National Ignition Facility (NIF) 23 were conducted with ∼192 laser beams pointed in the polar direct drive configuration, 16 delivering 40 to 130 kJ onto a capsule in a 1.4-or 2.0-ns ramp pulse. The experiments used 2.2 g/cm 3 SiO 2 shells with a 1530 to 1680-µm diameter and a thickness of 4.1 to 4.6 µm, filled with 10 to 12 atm.…”

“…To further investigate whether the trends observed in these prior experiments apply generally, under quite different experimental conditions (larger capsules, asymmetric illumination, oblate implosions), data were obtained on polar-direct-drive (PDD) 16 exploding pusher shots at the NIF that were conducted for diagnostic development and calibration. [17][18][19] In addition, given the scarcity of NIF shots for non-programmatic purposes, it is important to cull as much information and physics insight possible from these diagnostic development shots.…”

Symmetry issues in Directly Irradiated Targets

“…It is known that displacing the beam axis a distance from target centre, the so called Polar Direct Drive (PDD) [11], the illumination symmetry can be improved. By varying the value of , an optimum irradiation is obtained.…”

Optimal laser intensity profiles for a uniform target illumination in direct-drive inertial confinement fusion

“…In this case the higher intensity is situated between It is worth noticing that these optimum intensities are located off-centre. This could be regarded as the application of the polar direct-drive [15] (PDD) technique where a centred laser intensity profile moves towards the equator to compensate the over-irradiation of the polar zones [16] . In addition, it has been shown [17,18] that for the configurations C and D elliptical intensity profiles provide a more uniform irradiation than circular ones.…”