Optimizing Hohlraum Efficiency for Ignition and High Gain in Inertial Confinement Fusion

Achieving ignition and gain in inertial confinement fusion hinges on efficient energy conversion, concentration, and containment. Recent experiments at the National Ignition Facility show that enhanced coupling between the laser, hohlraum, and capsule, enabled by advanced cylindrical hohlraum designs, is essential for fusion ignition and target gains above unity. While earlier work focused on symmetry control to produce burning plasmas, those experiments lacked the x-ray drive required for ignition. Newer hohlraum designs reduce energy losses at both the walls and laser entrance holes (LEHs), resulting in significantly higher coupled energy.

We present the development and characterization of these efficient hohlraums, combining simulations and experiments to assess their impact on performance. Initial improvements increased fusion output from approximately 100 kJ to over 1 MJ by reducing LEH losses by ~27%. Continued efforts to further decrease LEH loses, by 24% and wall loses by 3%, improved efficiency but revealed the fundamental trade-off between implosion symmetry and hohlraum energy coupling. Recently, effective control of symmetry was demonstrated through a reduction in hohlraum gas fill, which is typically predicted to decrease symmetry control, raising questions about our understanding of beam propagation and energy deposition within the hohlraum.

These advances leverage new diagnostics, analysis tools, and simulation techniques to probe hohlraum dynamics, gas fill effects, coupling and symmetry, revealing gaps in our understanding. We present models and hypotheses to reconcile discrepancies between simulations and data and show how tuning hohlraum gas fill helps restore symmetry without compromising coupling. These results guide the design of future ignition-scale targets and are directly relevant to next-generation inertial fusion facilities and fusion energy concepts, where higher gains and robustness will be essential.