When the nuclear bomb was being developed, no detonator precise or reliable enough to set it off existed. So, researchers at Los Alamos National Labs built one that was and dubbed it the Exploding-Bridgewire Detonator.
An exploding-bridgewire detonator (EBW) is a precision-timed detonator used to initiate the explosion using an electric current, similar to a blasting cap. It was invented by Luis Alvarez and Lawrence Johnston and incorporated into the design of the Manhattan Project's Fat Man bombs. Specifically, they were developed to precisely compress the bomb's plutonium pit and initiate the reaction. (Its counterpart, Little Boy, used a much simpler mechanism.)
Traditional detonators allow for variations in the detonation timing ranging in the hundreds of milliseconds, which is more than enough time for the plutonium pit to be pushed out of the reaction chamber by the blast waves of the earlier charges. An EBW detonator, on the other hand, varies in the ballpark of 0.1 microsecond. That allows enough time for the detonation to move at most 1 millimeter with as little as a 0.2 mm variation in the detonation wave.
EBW detonators are typically constructed of either gold, platinum, or an alloy of the two, and are activated with the application of a strong electrical current—about 1000 amperes per microsecond—typically from a Marx generator. This current heats the metal so quickly and in such a small area that the liquid cannot flow off but instead vaporizes. A few nanoseconds after vaporization, the wire explodes, creating a shock wave and releasing the contained thermal energy, igniting the rest of the reaction. In A-bombs, this set up is used to produce the 12.5J to power the neutron trigger or, in the case of the Fat Man, compress the fissionable material to its critical mass.
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