Assessment of the Feasibility of Shock Ignition in Indirect Drive

We report on research with modern, low-gas-fill hohlraums with higher energy coupling efficiency that might permit variants of shock ignition to be obtained on the National Ignition Facility in indirect drive. The idea is to assemble a thick, low velocity shell of fuel that is then separately ignited by a late-time laser pulse. By partial decoupling of the compression from the ignition, more fuel mass could be assembled and ignited relative to conventional (fast-compression) ignition, potentially resulting in higher fusion yields and gains. Moreover, these thicker, lower-aspect-ratio shells may be less affected by symmetry and stability perturbations. Analogous research using the “Big Dipper” laser pulse shape to precondition the ablation plasma for a subsequent drive shock has identified this intermediate drive regime as “shock-augmented ignition”. Indirect drive shock ignition and the Big Dipper are members of the same continuum of pulse shapes and appear to offer similar shell dynamics and performance. Current simulation efforts are focused on whether the potential stability of these thicker, lower IFAR, low-convergence-ratio capsules with lower late-time coast times, can offset their lower ignition margins resulting from inherently lower velocities and their longer pulse lengths (that results in more hohlraum plasma filling). NIF experiments are planned to assess whether predicted hohlraum radiation temperature ramp rates, and resulting capsule dynamics can be realized. These approaches could be viewed as just indirect drive ignition using rather different laser pulse shapes, hence their testability in the near term.