Exploring the limits of the National Ignition Facility's capsule coupling

The original ignition "point designs" (circa 1992) for the National Ignition Facility (NIF) [J. A. Paisner, E. M. Campbell, and W. J. Hogan, Fusion Technol. 26, 755 (1994)] were made energetically conservative to provide margin for uncertainties in laser absorption, x-ray conversion efficiency and hohlraum-capsule coupling. Since that time, extensive experiments on Nova [J. T. Hunt and D. R. Speck, Opt. Eng. 28, 461 (1989)] and Omega [J. M. Soures et al., Phys. Plasmas 3, 2108 (1996)] and their related analysis indicate that NIF coupling efficiency may be almost "as good as we could hope for." Given close agreement between experiment and theory/modeling, one can credibly explore target enhancements which couple more of NIFs energy to an ignition capsule. These include using optimized mixtures of materials to reduce x-ray wall losses, slightly reduced laser entrance holes, and laser operation strategies which increase the amount of energy one can extract from NIF. It is found that 3-4x increases in absorbed capsule energy appear possible, providing a potentially more robust target and similar to 10x increase in capsule yield. [S1070-664X(00)94805-5].