During the time that has passed since the first test in Alamogordo, thousands of explosions of fission charges have thundered, in each of which precious knowledge about the peculiarities of their functioning has been obtained. This knowledge is similar to elements of a mosaic canvas, and it turned out that the “canvas” is limited by the laws of physics: the kinetics of slowing down of neutrons in the assembly puts a limit to the reduction of the size of the ammunition and its power, and the achievement of an energy release significantly exceeding a hundred kilotons is impossible due to nuclear physics and hydrodynamic limitations of the permissible dimensions of the subcritical sphere. But it is still possible to make ammunition more powerful if, together with fission, nuclear fusion is made to work.
The largest hydrogen (thermonuclear) bomb is the Soviet 50-megaton "Tsar Bomb", detonated on October 30, 1961 at a test site on Novaya Zemlya Island. Nikita Khrushchev joked that it was originally supposed to detonate a 100-megaton bomb, but the charge was reduced so as not to break all the glass in Moscow. There is some truth in every joke: structurally, the bomb was really designed for 100 megatons and this power could be achieved by simply increasing the working fluid. They decided to reduce the energy release for safety reasons - otherwise the landfill would be too damaged. The product turned out to be so large that it did not fit into the bomb bay of the Tu-95 carrier aircraft and partially protruded from it. Despite the successful test, the bomb did not enter service; nevertheless, the creation and testing of the superbomb was of great political importance, demonstrating that the USSR had solved the problem of achieving almost any level of megatonnage of the nuclear arsenal.
Fission plus fusion
Heavy isotopes of hydrogen serve as fuel for the synthesis. When deuterium and tritium nuclei merge, helium-4 and a neutron are formed, the energy yield is 17.6 MeV, which is several times higher than in the fission reaction (per unit mass of the reactants). In such a fuel, under normal conditions, a chain reaction cannot occur, so that its amount is not limited, which means that the energy release of a thermonuclear charge has no upper limit.
However, in order for the fusion reaction to begin, it is necessary to bring the nuclei of deuterium and tritium closer together, and this is hindered by the forces of Coulomb repulsion. To overcome them, you need to accelerate the nuclei towards each other and push them. In a neutron tube, during the stripping reaction, a large amount of energy is spent on the acceleration of ions by a high voltage. But if you heat the fuel to very high temperatures of millions of degrees and maintain its density for the time necessary for the reaction, it will release energy much more than that spent on heating. It is thanks to this method of reaction that weapons began to be called thermonuclear (according to the composition of the fuel, such bombs are also called hydrogen bombs).