The National Ignition Facility (NIF) will be configured in its baseline design to achieve ignition and gain using the indirect drive approach. However, the NIF primary criteria and functional requirements require the NIF design “to not preclude” the ability to conduct inertial confinement fusion experiments using the direct drive approach.

The direct drive approach requires symmetrical illumination of an inertial confinement fusion (ICF) capsule where each beam fully subtends the capsule. Therefore, the re-directing of 24 of the 48 NIF beamlines (each consisting of a 2 × 2 beamlet group) from ~30° and ~50° cone angles to ~75° cone angles located near the chamber ‘equator’ is required. This would be accomplished by adjusting intermediate transport mirrors such that the beams would intercept different final mirrors in the Target Bay and be directed into final optics assemblies attached to the chamber at the new port locations. Allowing space to be able to convert from one irradiation scheme to another while fully meeting the mechanical stability requirements for each approach is a significant challenge. Additionally, NIF user needs (features supporting weapons physics, weapons effects, inertial fusion energy, or Basic Energy Sciences) cannot be compromised by direct drive needs.

The target for direct drive, absent a hohlraum, emits much fewer cold x rays than in the indirect drive case. Further, the irradiation scheme, by its nature, may not result in the absorption of all of the 3ω light and therefore could create a unique hazard to the NIF chamber first wall of significant fluences of scattered UV laser light. This paper describes possible design features of the NIF Target Area to allow conversion to a direct drive capability, and discusses some of the differences in post-shot conditions created compared to indirect drive.