Approaching a burning plasma on the NIF*

After the National Ignition Campaign, our program to achieve ignition initiated a more exploratory approach starting from less stressful, lower convergence implosions.   By probing away from a more conservative implosion in-steps towards conditions of higher velocity and compression, Fuel Gain and alpha-heating were obtained. In the process, performance cliffs were identified the most damaging of which were symmetry control of the shell of the implosion and adverse hydro seeded by engineering features that penetrated the shell.  From 2015-2017 we focused on reducing LPI by moving to larger hohlraums with lower gas fill densities to improve symmetry control and on reducing the impact of the fill-tubes and capsule mounting.  A broad parameter space was explored [1] including capsule material and size, laser pulse length, hohlraum size and gas fill and implosion velocity and compression.  The results were much more efficient implosions that obtained the same performance levels, but with less laser energy.  Presently, we have several implosions [2,3] poised to step into a burning plasma state where alpha particle heating is the dominant source of heating.  The data from this exploration supported by focused experiments shed light on key target physics and has provided a data-based road-map to optimize the energy coupled to the capsule and subsequently to the hot spot – our HYBRID (high-yield big radius implosion design) strategy.  Here we describe the key data underpinning our principal conclusions and hypotheses and the foundation of this data-based approach to increasing the energy coupled to the hot spot in an attempt to take the next step to a burning plasma and ultimately ignition.

[1] Callahan, D. et. al., PoP, 25, 056305 (2018)

[2] Casey, D.T, et al., PoP, 25, 056308 (2018)

[3] Le Pape, S., et al., PRL, 120, 045003 (2018)