Observed dynamics of inertial confinement fusion implosions approaching unity gain at the National Ignition Facility

In indirect drive inertial confinement fusion experiments conducted with deuterium and tritium fuel, to achieve strong self-heating and energy gain (E_fusion ≥ E_laser_driver), the rate of heating from fusion product alpha particles must outpace all losses. The difference between heating and power losses and the duration over which this condition can be met, is important for setting the achievable energy production from such systems. Previous experiments have achieved a net power balance >0 near the minimum radius achieved by the implosion. Here, new experimental results will be presented where increased levels of alpha heating lead to increases the hot spot temperature, pressure and yield while the hot spot is expanding. This dynamic is a key signature associated with self-heating in the ignition regime and shown to be strongly correlated to the variability in fusion energy production, change in apparent hot spot size and areal density observed over a set of repeat implosions. Using the observed expansion dynamics together with analytic models and simulations the relative balance between heating and cooling will be examined.