Control of low-mode drive asymmetry in an efficient long-pulse low gas-fill density Hohlraum

Laser-driven Hohlraums filled with gas at lower densities (<0.6mg/cc) have higher efficiency compared to original >= 0.96mg/cc fill because of reduced backscatter losses [Hall et al., Phys. Plasmas 24, 052706 (2017)]. However, using low-density filled Hohlraums with longer drive required for lower adiabat implosions, and hence potentially higher inertial confinement fusion gain designs, has been challenging since the Hohlraum wall blow-off is less tamped, thus altering the laser beam absorption regions and drive symmetry. A series of NIF experiments using optimized pulse shaping, beam pointing, and temporal phasing have demonstrated, through imaging of the Hohlraum and capsule dynamics, that a symmetric implosion using a 14-ns low-adiabat drive pulse {2x longer than high-density-carbon ablator designs using low gas-fill density Hohlraums [Divol et al., Phys. Plasmas 24, 056309 (2017)]} is possible in a low backscatter loss 0.45mg/cc He-filled Hohlraum. The ingress of the Hohlraum walls was mitigated by revisiting the adiabat-shaped design [Clark et al., Phys. Plasmas 21, 112705 (2014)] that uses a low-power (1 TW) trough that delays the wall expansion. Low-mode P-2 and P-4 drive asymmetry swings caused by the drift of the laser spots were essentially zeroed out by employing temporal beam phasing between cones of beams [Turner et al., Phys. Plasmas 7, 333 (2000)]. The results also indicate an improved coupling efficiency of similar to 30% compared to an earlier design using higher density filled Hohlraums and pave the way for revisiting low-adiabat, high convergence drives using CH ablators.