NIF Invited: Experiments and observations that enabled target gain >1 from inertial confinement fusion implosions at the National Ignition Facility*

Creating a controlled fusion reaction that produces more energy than supplied to initiate it (i.e. target gain >1) is a grand scientific challenge with broad societal implications [1,2]. A challenge for every approach pursuing this goal is to create plasma conditions which exceed Lawson's criteria[3], where the power of fusion self-heating exceeds all the power losses of the system, to the point where target gain >1 is achieved. After decades of research and technological advances the first laser indirect-drive inertial confinement fusion[4,5] experiment to meet the Lawson Criteria was performed, achieving a target gain of 0.72[6-8]. Since then, observations from nuclear and x-ray diagnostics have been used to identify the principal sources of fusion yield variability that can arise from variations in target quality and laser power balance. Experimental observations were evaluated within a semi-analytic framework to quantify the impact of degradations that limit the fusion energy production. The two main sources of degradations were found to be low mode implosion asymmetries and enhanced radiative loss from higher atomic number contaminants that mix into the reacting deuterium tritium plasma. The observations were also used to develop an analytical framework for the burn dynamics and power balance of the reacting plasma. The experimental observations, consistent with the analytic framework, indicate that for the N221204 design, a 7% increase in laser energy to drive a 7% thicker capsule led to increased peak radiation drive, target areal density and energy coupled to the reacting plasma, enabling a higher fusion yield and target gain of ~1.5, marking the first time a target gain >1 was achieved in the laboratory.