Key Point: A crazy weapon for a crazy time.
Design work for a minimum-size atomic warhead called the XW-51 began at the University of California Radiation Laboratory in the mid 1950s. It later shifted to the Los Alamos Scientific Laboratory, which was also working on compact warheads, and re-designated the W-54 in January 1959. Initially intended to be used for lightweight thermonuclear weapons, the prototype subsequently was adapted for a variety of tactical uses by the armies in the field.
XW-51 precursors, as well as compact LASL designs, were first evaluated as part of Operation Plumbbob at the Nevada Test Site in 1957. The most powerful of these Plumbbob prototypes was a boosted, plutonium-cored implosion device that produced a 9.7-kiloton yield. This device incorporated the latest technological advances, prominent among them the powerful, plastic-bonded explosives (PBX) that could be machined into lenses that shaped a perfectly symmetrical shock wave to compress the fissile core to the density needed to sustain an atomic chain reaction. Deuterium-Tritium gas-boosting was also used, a technology first evaluated with a sealed beryllium core during Operation Teapot in 1955. Known as fusion boosting, this process used neutrons from the fusion of a D-T gas mixture blown into a hollow core just before detonation to greatly accelerate the chain reaction.
Because it was fusion-boosted, the core was beryllium-encased. Beryllium reflects stray neutrons back into the core to provide additional acceleration of the chain reaction and thus eliminates heavy tampers and the bulk of levitation. A thickness of an inch or two can reduce the radius of a core by about 50%. Because beryllium is only one-tenth as dense as plutonium, this results in additional mass savings. A final important advance was an electronic, external neutron initiator that replaced the older, internal mechanical initiator. Colloquially called “zippers,” external initiators were first used during Operation Teapot in 1955. These devices relied on a miniature linear particle accelerator called a “pulse neutron tube” that collided deuterium and tritium nuclei together to generate high energy neutrons through a fusion reaction and could be placed anywhere within the warhead.