have on the energy needed from the primary to cause ignition. The simplest change is to replace the LiD fuel with DT gas, removing the need for the spark plug. This allows secondaries of any size – as the secondary shrinks, so does the amount of energy needed for ignition. At the milligram level, the energy levels started to approach those available through several known devices. By the early 1960s, Nuckolls and several other weapons designers had developed ICF's outlines. The DT fuel would be placed in a small capsule, designed to rapidly ablate when heated and thereby maximize compression and shock wave formation. This capsule would be placed within an engineered shell, the hohlraum, which acts like the bomb casing. The hohlraum did not have to be heated by x-rays; any source of energy could be used as long as it delivered enough energy to heat the hohlraum and produce x-rays. Ideally the energy source would be located some distance away, to mechanically isolate both ends of the reaction. A small atomic bomb could be used as the energy source, as in a hydrogen bomb, but ideally smaller energy sources would be used. Using computer simulations, the teams estimated that about 5 MJ of energy would be needed from the primary, generating a 1 MJ beam. To put this in perspective, a small (0.5 kt ) fission primary releases 2 TJ. ICF program, 1970s While Nuckolls and LLNL were working on hohlraum-based concepts, UCSD physicist Keith Brueckner was independently working on direct drive. In the early 1970s, Brueckner formed KMS Fusion to commercialize this concept. This sparked an intense rivalry between KMS and the weapons labs. Formerly ignored, ICF became a hot topic and most of the labs started ICF work. LLNL decided to concentrate on glass lasers, while other facilities studied gas lasers using carbon dioxide (e.g. ANTARES, Los Alamos National Laboratory) or KrF (e.g. Nike laser, Naval Research Laboratory). Throughout these early stages, much of the understanding of the fusion process was the result of computer simulations, primarily LASNEX. LASNEX simplified the reaction to a 2-dimensional approximation, which was all that was possible with the available computing power. LASNEX estimated that laser drivers in the kJ range could reach low gain, which was just within the state of the art. This led to the Shiva laser project which was completed in 1977. Shiva fell far short of