Considerations and Requirements for Providing Cryogenic Targets for Direct-Drive Inertial Fusion Implosions at the National Ignition Facility

Modifications to the National Ignition Facility (NIF) Cryogenic Target Positioner (Cryo-TarPos) are needed to provide polar-drive-ignition targets; ideally, these modifications will be completed and tested by 2017, the earliest date anticipated for polar-drive-ignition experiments. The extent of these modifications is defined by the mechanical and thermal requirements needed for the target to conform to the ignition design and the capabilities of the existing equipment. This paper describes the design of the polar-drive target assembly and the surrounding cryogenic environment that meets many of the specifications and requirements for the ignition target. Further work is necessary to optimize the design and provide more-detailed guidance for modifying the NIF Cryo-TarPos; however, there is sufficient information to begin the redesign effort at the conceptual level. A specialized facility has been constructed to test different target assembly and cryogenic hardware designs. The equipment provides the mechanical and cryogenic functionality available at the NIF, making it possible to test different target designs with deuterium in a configuration suitable for integration with the NIF Cryo-TarPos. The polar-drive target assembly has demonstrated a stable ice layer (170 to 350 mu M thick) and the ability to control the thickness to +/- 3 mu m of the desired value. The target is rotatable to fully characterize the D-2 ice surface using shadowgraphy and X-ray phase contrast. Thermal models of the target and its environment indicate that (a) it should be possible to achieve the desired 1-mu m root-mean-square smoothness using D-T, (b) the fill tube has little effect on the ice smoothness, and (c) it is possible to shape the isotherms surrounding the target sufficiently to form an oblate ice layer that may be more desirable for polar-drive implosions.