routinely deliver 1.8 MJ, these designs accommodate variations in laser entrance hole or hohlraum size that might be desirable for easier control of symmetry, or for compensation of the observed variation in stimulated scattering or the uncertainties in x-ray conversion and wall losses. Most plausible variations in the physics database, such as the capsule equation of state or opacity, are performance-neutral. The surface finish on the capsules might also be improved relative to present NOVA capsules if this is required to help suppress hydrodynamic instabilities. In addition, other capsule materials under development, such as Be and B4C, can reduce the growth of hydrodynamic instabilities and are less sensitive to roughness of the cryogenic fuel layer. Operation with somewhat lower-temperature hohlraums, while requiring a laser energy at the upper end of the NIF operating range, would reduce the effects of laser-plasma instabilities.
The major differences between the present NIF design and the concept envisaged in 1990 have resulted from modifications to the target chamber, the target area, and the front end of the NIF to incorporate direct-drive capability. The likelihood of ignition and burn propagation through direct drive has increased significantly since 1990 owing to better understanding of both laser-beam imprinting and the growth of hydrodynamic instabilities. By an optimal choice of port locations for indirect drive, the direct-drive geometric requirements can be met by moving one-half of the NIF beams to 24 new beam ports that have been added to the chamber. The detailed requirements for beam smoothing await the results of experiments over the next several years on OMEGA and NIKE. Space has been provided in the NIF front end to implement one or more of the wide variety of beam-smoothing techniques now being evaluated. A further illustration of the NIF's flexibility is that the direct-drive beam configuration also allows a more nearly spherical (tetrahedral) hohlraum to be used.
The campaign to achieve ignition with the NIF will certainly be the most challenging ever undertaken by the national ICF program. However, given the success in the indirect-drive target physics campaign on NOVA over the past 6 years and the flexibility of the NIF design, the ignition and burn propagation objectives of the NIF will likely be achieved, but cannot be guaranteed.