Ignition on the NIF: Challenges, Achievements & Lessons for Pulsed Power ICF

Abstract

The achievement of Inertial Confinement Fusion (ICF) ignition on the National Ignition Facility (NIF) in December, 2022 was the culmination of decades of scientific research and technology development. In the 30 years between the Key Decision to build NIF and reaching ignition, the national ICF program solved numerous technical challenges, many of which were unanticipated in the 1990’s. Reaching ignition required both multi-year research efforts and serendipitous discoveries, along with an unwavering focus on the fundamentals of ICF. In this talk, we review some of the major technical achievements on the path to ignition. Along the way, we highlight the phenomenal capabilities of the NIF laser, diagnostics, and target fabrication machinery. A major turning point was the switch from plastic capsules to diamond capsules, which resulted in the near elimination of laser backscatter, improved implosion symmetry, and reduced hydrodynamic mix. From 2016-2022, the size of the diamond capsule and the energy coupled to the fuel were steadily increased, using the principals of hydrodynamic scaling. In parallel, the design and target fabrication teams improved the quality of the implosion.

The scaling of diamond ablator designs from marginal alpha-heating at 1 MJ laser energy to ignition at 2 MJ provides a case study for achieving ignition with pulsed power ICF. For many years, the Lawrence Livermore (LLNL) and Sandia National Laboratories (SNL) have collaborated on fusion target design for the SNL Z facility, resulting in numerous innovations; however, Z is not large enough to reach ignition: that requires a larger driver, the Next Generation Pulsed Power (NGPP) facility. Reaching ignition will require increasing the energy delivered to the load by almost 10×—far less than the 50× increase in energy between the NOVA laser and NIF. In addition, we believe that ignition designs for NGPP can be scaled from Z experiments today. Research on Z today should focus on improving the performance of ICF platforms to provide a better basis for driver scaling, incorporating innovations in diagnostic and target technology from the NIF. We have also learned from NIF that many of the hardest challenges are probably not known today and must be tackled on the larger driver. The low cost-per-joule of pulsed power should enable us to design the ignition driver with substantial margin.

Prepared by LLNL under Contract DE-AC52-07NA27344.