The National Ignition Facility is scheduled to field indirect drive cryogenic targets in 2004. The basic target is a film supported 2 mm diameter spherical capsule centered in a cylindrical gold hohlraum. The hohlraum has nominal length of 10 mm, diameter of 6 mm, and wall thickness of 30 µm. The DT fuel must be layered very uniformly and smoothly inside the capsule. Inside the hohlraum, the layering requirement requires very precise control of hohlraum wall temperature gradients. Alternatively, the DT layer is formed and characterized with the capsule supported in a spherical layering shroud, and subsequently assemble and shoot the target in a time short compared with the time required to distort the layer after the hohlraum is assembled. Removing the hohlraum, during the initial capsule processing, offers several important advantages. During the DT fill of the capsule, interactions between hohlraum materials and the DT gas are eliminated and the volume of the DT fill vessel is minimized. During layering process, this approach maximizes the space available around the capsule to include apparatus for enhanced smoothing of the fuel layer and it provides an isothermal environment for uniform layer production. In addition, the solid angle around the capsule is almost un-occluded, providing for better viewing for characterizing the fuel layer. Automatic cryogenic hohlraum assembly has significant challenges. Since the fuel layer quickly conforms to the new isotherms provided by the hohlraum, the time allowable from assembly start until the target is shot, is approximately 10 s. The hohlraum must be assembled with excellent alignment accuracy. Since thermal perturbations can damage the fuel layer and the fuel will migrate under the influence of temperature gradients, the heat generated by the assembly process must be low. The assembled hohlraum must seal in about 60 KPa of gas. This can put significant stress into the fastening materials. The 19 K operating temperature further complicates the design of the equipment.

A number of concepts suitable for automatic joining of the hohlraum halves are examined. These include electrostatic, mechanical fastening, and laser welding. Analysis shows that electrostatic method can provide the force required to hold the hohlraum together against the internal gas pressure. It generates very little heat and very little force must be applied. The mechanical fastening method requires application of force but generates little heat. Analysis of the laser welding method indicates that it heats the target to unacceptably high temperatures.