Alarm clock (nuclear weapon)
In the development of thermonuclear weapons, the first technique considered was surrounding a fission device at the center with a supply of light isotopes for fusion. It was believed that the expanding shock wave would heat and compress the fusion fuel, but the only plausible nuclear reaction that would undergo fusion under these conditions was deuterium-tritium. Tritium, however, is prohibitively expensive to be used for fuel, although it has an important role in other designs. This turns out to have significant limitations, but has been used at least experimentally by several countries, on the way to the Teller-Ulam design. 
This design was reinvented independently several times:
- In the United States by Edward Teller, called “Alarm Clock” and "Method A". A 1948 refinement was called the "Booster".
- In the Soviet Union by Andrei Sakharov and Vitalii Ginzberg, termed "Layer Cake”
- In Great Britain by Keith Roberts, called “tamper boosting”
Teller called the design "Alarm Clock" because it would "wake up the world." It was an improvement, however, only over the original and more infeasible "tube" concept of a nuclear bomb setting off a cylinder of deuterium.
A single concentric layer, as originally conceived by Teller and Sakharov, would have compressed by a factor of 7 to 16, being effective only with D-T.
Ginzberg leading to the "layer cake" referred to the assumption that there might be several concentric shells of fusion and fission material. "Fast neutrons generated in thermonuclear reactions were expected to initiate fissions in neighbouring layers of fissionable materials, which would result in a considerably higher energy yield. "