Simulated impact of increased hohlraum radius on symmetry control in igniting and burning fusion platforms at the NIF

The Hybrid E platform at the National Ignition Facility has demonstrated target gain above unity^[1], but experimental data and simulations predict low mode asymmetries are still significantly impacting performance, motivating further optimization. In this work, we investigate the impact of increasing the hohlraum diameter from 6.40 to 6.72 mm as a strategy to mitigate residual kinetic energy (RKE, or kinetic energy that remains unconverted to hot spot compression and heating) at stagnation due to these low mode asymmetries. The larger hohlraum radius provides several avenues for improved symmetry control: it increases the propagation time of the inner beams before being intercepted by gold plasma blowoff from the hohlraum wall, enhances smoothing of the radiation drive, and enables operation at a lower Δλ for cross-beam energy transfer (CBET), which has been correlated with reduced RKE and backscattered laser energy. While this approach allows for greater symmetry control, it introduces a tradeoff in the form of reduced implosion velocity. We present integrated simulation results, initially tuned to reproduce the N230729 igniting capsule design at 2.05 MJ of input laser energy, and highlight the expected effects on P2 and P4 symmetry modes in both the x-ray drive and the capsule. We discuss design choices for reduced time-dependent P2 asymmetry “swing” and strategies to mitigate the impact of reduced implosion velocity. Results show a reduced P2 asymmetry swing and corresponding decrease in RKE, with implications for improved capsule performance and design flexibility on the Hybrid-E platform.

[1] A. L. Kritcher et al., Phys. Rev. E 109, 025204 (2024)