National direct-drive program on OMEGA and the National Ignition Facility

Abstract

A major advantage of the laser direct-drive (DD) approach to ignition is the increased fraction of laser drive energy coupled to the hot spot and relaxed hot-spot requirements for the peak pressure and convergence ratios relative to the indirect-drive approach at equivalent laser energy. With the goal of a successful ignition demonstration using DD, the recently established national strategy has several elements and involves multiple national and international institutions. These elements include the experimental demonstration on OMEGA cryogenic implosions of hot-spot conditions relevant for ignition at MJ-scale energies available at the National Ignition Facility (NIF) and developing an understanding of laser-plasma interactions and laser coupling using DD experiments on the NIF. DD designs require reaching central stagnation pressures in excess of 100 Gbar. The current experiments on OMEGA have achieved inferred peak pressures of 56 Gbar (Regan et al 2016 Phys. Rev. Lett. 117 025001). Extensive analysis of the cryogenic target experiments and two-and three-dimensional simulations suggest that power balance, target offset, and target quality are the main limiting factors in target performance. In addition, cross-beam energy transfer (CBET) has been identified as the main mechanism reducing laser coupling. Reaching the goal of demonstrating hydrodynamic equivalence on OMEGA includes improving laser power balance, target position, and target quality at shot time. CBET must also be significantly reduced and several strategies have been identified to address this issue.

Impact of imposed mode 2 laser drive asymmetry on inertial confinement fusion implosions

“…The 3D code ASTER, which has been previously used to model the impact of asymmetries on direct-drive OMEGA implosions, 4,[9][10][11]57 has been used to model one of our implosions, 79363, with Asym. A imposed.…”

“…17 On OMEGA, on the other hand, large LOS variations in T ion have been routinely observed. 4,57 On the NIF, implosions are expected to be impacted by a mode 2 in the laser, 62,63 a mode 4 from the capsule support mount ("tent"), [64][65][66] and a mode 1 from the fill tube. 62 With the interplay hypothesis in mind, it is easy to imagine how interference of flows from these various sources could give rise to large flows but small observable T ion asymmetries.…”

Measurement of apparent ion temperature using the magnetic recoil spectrometer at the OMEGA laser facility

“…[16][17][18] These low-mode asymmetries induce flows in the burning fuel and can manifest as T ion asymmetries. nTOF measurements of the primary DT neutron spectrum at OMEGA show evidence of such asymmetries, 19 which are important to control and understand.…”

Simulation and analysis of time-gated monochromatic radiographs of cryogenic implosions on OMEGA

“…Layered cryogenic DT targets are the baseline approach to achieving ignition in directdrive [1] inertial confinement fusion (ICF) [2] experiments [3]. Steady progress has been made in experiments with hydrodynamically equivalent [4], energy-scaled implosions [5][6][7][8][9][10] on the OMEGA Laser System [11].…”