The  largest nuclear bombs, and the backbone of the nuclear arsenal of the  United States and Russia, are hydrogen bombs.  These bombs work using a  two-stage design whereby a fission bomb "primary" is detonated using the  conventional implosion method, which then compresses a fusion fuel  "secondary" and ignites a uranium "spark plug" which fissions and  subjects the fusion fuel to the heat necessary to begin a chain reaction  -- about 20 million degrees F.  The light nuclei fuse together into  heavier elements, releasing tremendous energy from the strong nuclear  force that binds their constituent particles together.
As a result of using fusion rather than fission, the bomb  yield is much higher.  In fission bombs, approximately 0.1% of the mass  of the fuel is converted directly into energy, while in fusion bombs,  this ratio is slightly improved, on the order of 0.3%.  The first time  the principle of a hydrogen bomb was tested was on 9 May 1951 by the  United States military, during the George shot of Operation  Greenhouse at the Pacific Proving Grounds.  Most of the yield of this  test came from fission fuel, but it tested the idea that a fission bomb  could be used as a stepping stone to something even more destructive.  A  similar test, Item, took place on 25 May 1951.
The first true hydrogen bomb test, Ivy Mike, was on 1 November  1952, detonated at Enewetak Atoll in the Pacific, as part of Operation  Ivy.  The bomb exploded with a force equivalent to 10.4 megatons of TNT  (over 450 times more powerful than the atomic bomb dropped on Nagasaki  during WWII).  Using liquid deuterium as fuel, this hydrogen bomb  required 18 tonnes of refrigeration equipment to keep the deuterium in  liquid form.  A later test, Castle Bravo, used solid lithium deuteride instead, decreasing the weight of the device and making it a practical weapon.  The Castle Bravo shot is the most powerful nuclear weapon ever tested by the United States, with a yield of 15 megatons.
Today, all of humanity is still put at risk by a nuclear winter initiated by hydrogen bombs. 
 



