1952

Ivy Mike Hydrogen Bomb

Richard Garwin (b. 1928)

A nuclear bomb uses nuclear fission of a material like U-235 or plutonium to create a massive amount of explosive force. What if engineers want to create an even bigger bomb? In that case they would use hydrogen fusion rather than fission. But getting hydrogen nuclei to fuse is not an easy task. It takes gigantic heat and pressure, mimicking conditions inside the sun. One way to create these conditions: a conventional nuclear bomb.

So the Ivy Mike hydrogen bomb was engineered in the following way: a conventional explosion brings a critical mass of plutonium together to create a nuclear explosion, and that nuclear explosion drives enough hydrogen atoms together to create a hydrogen fusion explosion. For this to work, engineers have to overcome two big problems. First, they need a collection of the right kind of hydrogen atoms in the right place at the right time. Second, they need to hold the whole thing together long enough to initiate the fusion process, while the nuclear explosion is trying to rip the bomb apart.

The first hydrogen bomb—the Ivy Mike device, designed by American physicist Richard Garwin in 1952—used liquid deuterium in a vacuum flask. While this worked, it was not practical for a reliable bomb. The breakthrough was to use lithium deuteride—a solid that decomposes to tritium when bombarded by neutrons.

There is no real magic in the casing—it is just extremely strong steel a foot (30 cm) thick. A key insight is to understand that radiation and neutrons from the fission explosion outrace the blast wave. Directing them to the secondary fusion bomb allows it to detonate prior to the bomb’s destruction. Not everything is understood because of military secrecy, but we do know that engineers figured it out based on the success of test bombs.

It is possible to chain another stage into the process in the time available, so the fusion bomb explodes and ignites another fusion bomb. This was the mechanism used in the Soviet Union’s Tsar bomb—the largest explosion ever created by humans at approximately 50 megatons of TNT.

SEE ALSO Trinity Nuclear Bomb (1945), Uranium Enrichment (1945), International Thermonuclear Experimental Reactor (ITER) (1985).

Aerial view of the cloud produced during the detonation of the Ivy Mike hydrogen bomb.