Improving target gain in inertial confinement fusion implosions on NIF with increased energy coupling and areal density*

Target gain greater than unity was first attained in an indirect-drive inertial confinement fusion (ICF) implosion at the National Ignition Facility (NIF) on December 5, 2022 [1]. Since then, we have entered an era of routine ignition and target gain on NIF. Increasing the maximum available laser energy on the NIF facility from 1.9 MJ to 2.05 MJ proved critical in producing the first ignition result. This enabled both coupling ~8% more x-ray drive energy to the CVD diamond capsule containing the deuterium-tritium (DT) fuel and fielding a 7% thicker ablator that increased the implosion areal density at stagnation by ~18%. Now, target gains up to 2.4 and yields up to 5.2 MJ have been produced by following the same approach, again enabled by a further increase in laser energy to 2.2 MJ.

This presentation describes the design effort and experimental results from recent high-performing ICF implosions on NIF with enhanced energy coupling. This is accomplished both by leveraging the 2.2 MJ upgraded laser capability and developing more efficient hohlraums using 1.9 MJ laser energy and below. Now that capsules are routinely igniting, increasing the confinement (areal density) is necessary to improve burn efficiency of the DT fuel and further increase fusion performance. One key method for exploiting higher energy coupling to boost areal density is to increase the stagnated mass by starting with a thicker ablator. Doing so requires a longer laser pulse to maintain optimized shock timing and renders implosion symmetry control more challenging. Specifically, symmetry “swings,” or time-varying low-mode asymmetries, impact the uniformity of the compressed DT shell and create thin spots that reduce overall confinement.

Ongoing work to improve equatorial mode 2 and polar mode 4 asymmetries in these higher coupling and higher areal density designs will be discussed, including new mitigations to control symmetry swings by carefully balancing power across the various NIF beams. Detuning the laser wavelengths to drive cross-beam energy transfer remains an important technique for maintaining symmetry control.

[1] H. Abu-Shawareb et al., Phys. Rev. Lett. 132, 065102 (2024)