Hydrogen bombs, or H-bombs, are the most destructive weapons ever built. The first generation of atomic bombs, or A-bombs, used nuclear fission to produce explosive energies measured in kilotons, equivalent to thousands of tons of conventional explosive. In contrast, H-bombs are measured in megatons, or millions of tons. The fission effect is amplified by means of another type of nuclear reaction, fusion, using hydrogen isotopes as fuel. America won the race to detonate the first fusion weapon, but the Soviets got the H-bomb less than a year later.
The Start of the Arms Race
The first Soviet A-bomb was detonated on August 29, 1949 -- a little over four years after the first American test on July 16, 1945. This was just the start of the nuclear arms race, though. The principle of nuclear fusion was known to both sides, but numerous practical difficulties had to be overcome before a true H-bomb could be developed. An intermediate step was the boosted fission device -- essentially an A-bomb in which a small amount of additional energy was provided by fusion. The United States tested boosted fission weapons on May 1951. Russia tested the fission weapons on September 24, 1951.
The First H-Bomb Test
The output of boosted fission weapons was still only measured in kilotons, not megatons. In America, the first megaton-class H-bomb design was produced by Edward Teller and Stanislaw Ulam. Their design was tested on November 1, 1952, resulting in an explosive yield in excess of 10 megatons. Although the test was successful, the design still had serious limitations as a practical weapon. It used cryogenically cooled liquid hydrogen as a fuel and weighed 80 tons, which meant it was too large and cumbersome to be deployed from an aircraft.
The Soviet Layer-Cake Bomb
Soviet physicist Andrei Sakharov approached the problem in a different way from Teller and Ulam, and came up with an H-bomb design he called the "layer cake." It was tested on August 12, 1953, resulting in a yield of 400 kilotons. This was smaller than the U.S. design, but significantly higher than a boosted fission weapon. The layer cake can be considered the first Soviet H-bomb. It was also a more practical design than the one the Americans tested, being small enough that it could be delivered by an aircraft.
A more compact version of the Teller-Ulam design, using solid rather than liquid fuel, was tested by the U.S. on March 1, 1954. The following year, on November 22, 1955, The Soviets tested a similar device, referring to it as "Sakharov's Third Idea." By this point, the Soviets had effectively caught up with the Americans in the arms race. In fact, the largest nuclear weapon ever tested was a Russian one. On October 30, 1961, the Tsar Bomb, dropped over a test area by a Soviet aircraft, produced an explosion measuring over 50 megatons.