Delayed-drive laser pulse shapes on the National Ignition Facility: The potential to trade some ignition margin for improved stability and lower adiabats

The National Ignition Facility (NIF) is investigating strategies for enhanced fusion yields over 10 MJ through: (a) understanding and improving the stability of present targets; (b) alternative target platforms via, e.g, alternate laser pulse shapes, different hohlraum geometries for enhanced coupling, different ablator materials, etc; and (c) augmented laser energy. This work targets the prospects for higher fusion yields through alternate laser pulse shapes and potentially enhanced stability. Shock-ignition-like and big-dipper-like (R.Scott et al., Phys Rev Lett, 129, 195001 (2022)) drives are members of this class of so-called “delayed-drive” pulse shapes. A major feature is the potential amelioration of the N+1 (and N+n’s) shocks and a thicker inflight shell. Relative to regular indirect drive with a conventional fast rise to a constant peak power, delayed-drive introduces a temporary reduction in laser power after attaining the initial peak and potentially offers: (1) enhanced stability: IFAR, convergence ratio, ablator mass remaining, coast time and Atwood number, (2) better potential for symmetry control (prolate without tune), but (3) typically lower 1D ignition margins (which may be compensated by better stability). Such modified pulse shapes may allow us to operate at a lower adiabat relative to regular indirect drive (and thus realize higher yields) or, at the same adiabat, realize a higher fraction of clean-1D yield, providing, of course, the capsule has sufficient margin to ignite