Improving Energy Coupling Efficiency in National Ignition Facility Cylindrical Hohlraums*

The 1050 Coupling campaign at the National Ignition Facility builds on current MJ-class HYBRID-E designs [1,2], further improving cylindrical hohlraum efficiency by reducing the laser entrance hole (LEH) diameter from 3.1 mm to 2.7 mm and decreasing the hohlraum diameter from 6.4 mm to 6.2 mm. The ~6% extra x-ray flux obtained is used to drive a thicker 1.05 mm inner radius capsule with a 10 micron thinner DT ice layer to similar velocities with the goal of improving performance by increasing the implosion kinetic energy. The smaller case-to-capsule ratio (CCR) and laser pointing restrictions imposed by the smaller 2.7 mm LEH make implosion symmetry control more challenging. With some creative pointing of the outer cone beams to minimize overlapping intensity, we avoid clipping the LEH edge and measure a similar gold bubble velocity as before from polar x-ray images. Leveraging cross-beam energy transfer (CBET) with up to 5Å of inner-outer cone laser wavelength detuning, a prolate implosion shape is achieved in tuning experiments. This work describes the design effort, tuning experiments, and first cryogenic DT high performance experiment in the 1050 Coupling platform, which produced a fusion yield of 500 kJ with some indications of mode 1, equatorial mode 2, and polar mode 4 asymmetry, in addition to signatures of localized mix.

References

[1] A. B. Zylstra et al., Nature 601, 542 (2022)

[2] H. Abu-Shawareb et al. (Indirect Drive ICF Collab.), Phys. Rev. Lett. 129, 075001 (2022)