The thermonuclear bomb design used in the vast majority of current nuclear weapons. The thermonuclear bomb in the Teller-Ulam design consists of two parts, primary and secondary. The primary part contains a fission bomb. The secondary stage is filled with tamper, an envelope made from uranium that contains fusion fuel and a plutonium spark plug in the middle.
The bomb’s detonation begins with a fission explosion that starts to intensely emit thermal X-rays. They fill the cavity between the primary and secondary and start to compress the whole secondary stage. In its middle, the density of the plutonium sparkplug rises so high that it undergoes fission. Energy from the fission product heats the already highly compressed fusion fuel to fusion temperatures, and the fusion reaction starts. Neutrons produced by fusion help split fission fuel. The yield of the bomb is thus dramatically enhanced as most of the fusion and fission fuel is consumed during the blast.