Designs for the National Ignition Facility Using Current and Optimized Phase Plates*

The three-shell Revolver target[1] proposed for direct-drive ignition at the National Ignition Facility has a diameter (~6 mm) much larger than that of conventional designs. Simulations carried out using the 2-D hydrodynamics code SAGE[2] illustrate the advantages and challenges of the design. The absorption is almost 100%, the absorption occurs at a significant distance from the ablation surface, and the average intensity reaching the quarter-critical surface is low (just over 1 x 1014 W/cm2). However, uniformity presents a challenge, especially if the current phase plates are used. An optimized design, taking into account the full 3-D energy-deposition pattern, results in an implosion velocity nonuniformity of ~2.5% rms with the nonuniformity modes dominated by the small beam sizes. This can be improved to ~1.2% using custom phase plates with larger beam spots. Further improvement should be realized through thermal smoothing of azimuthal variations, not accounted for in the simulations.

[1] K. Molvig et al., Phys. Rev. Lett. 116, 255003 (2016).

[2] R. S. Craxton and R. L. McCrory, J. Appl. Phys. 56, 108 (1984).