Boosted fission weapon
A boosted fission weapon is a type of nuclear weapon that makes use of a small amount of fusion fuel to increase the rate, and thus yield, of a nuclear fission reaction. The fast neutrons released by the fusion reactions add to the fast neutrons released by the fission reactions, allowing for more neutron-induced fission reactions to take place. The rate of fission is thereby greatly increased such that much more of the fissile material undergoes fission before the core explosively disassembles. The fuel is commonly a 50-50 deuterium-tritium gas mixture. The fusion process itself adds only a small amount of energy to the process, perhaps 1%.
The primary benefit of boosting is further miniaturization of nuclear weapons as it reduces the minimum inertial confinement time required for a supercritical nuclear explosion by providing a sudden influx of fast neutrons before the critical mass would blow itself apart. While the bulky Fat Man had a diameter of 5 feet (1.5 m) and required 3 tons of high explosives for implosion, a boosted fission primary can be fitted on a small nuclear warhead (such as the W88) to ignite the thermonuclear secondary.
The idea of boosting was originally developed between late 1947 and late 1949 at Los Alamos National Laboratory. It was boosting, mainly, that was responsible for the 100-fold increase in the efficiency of fission weapons between 1947 and 1971.