How does an atomic bomb ignite?

Answer 1

In a fission bomb:

1. a subcritical mass of fissile material (U-235 and/or Pu-239) is rapidly assembled into a supercritical mass by using conventional explosives
2. the chain reaction is started by firing neutrons from a neutron source through the supercritical mass
3. a dense tamper holds the supercritical mass together for roughly 10 microseconds, then the explosion happens dispersing the supercritical mass and stopping the chain reaction

In a fusion bomb:

1. a fission bomb is fired as the primary stage
2. x-rays from the fission bomb cause a radiation implosion in the secondary stage
3. when compressed enough, neutrons from the primary initiate fission in a plutonium "sparkplug" at the center of the secondary stage
4. neutrons from the "sparkplug" transmute lithium in the lithium-deuteride fusion fuel into tritium, which mixes with the deuterium
5. the deuterium-tritium fuel mix is heated and compressed by the simultaneous radiation implosion and the "sparkplug" explosion until a thermonuclear fusion reaction begins
6. a dense tamper holds the fusion reaction together for a few 10s of microseconds, then the explosion happens dispersing the remaining fusion fuel and stopping fusion when it gets too cold
7. if the dense tamper was made of U-238, then high energy neutrons from the fusion reaction triggers fission in the tamper, releasing as much as 90% of the yield of the bomb as well as a corresponding amount of fallout