|Fig. 1: Teller-Ulam Explosion Sequence. (Source: Wikimedia Commons)|
A thermonuclear weapon or more commonly known as a hydrogen bomb/H-bomb is a two-stage nuclear weapon that uses the energy from a principal nuclear fission reaction to ignite a secondary reaction. It is referred to as a hydrogen bomb because it primarily employs the hydrogen fusion process. This results in an increased explosion power when compared to other single stage devices. Such weapons are currently considered as those that have the largest megaton yield.
The first thermonuclear weapon was developed and used in 1952 and has since been employed by most of the world's nuclear weapons.  The contemporary design and configuration of United States thermonuclear weapons is known as the Teller-Ulam configuration. Aptly named for its two founders, Edward Teller and Stanislaw Ulam, who developed it in 1951, for the United States. 
The characteristics of a Teller-Ulam thermonuclear weapon typically include; A two-stage system including a triggering (primary) explosive and a powerful (secondary) explosive.  The first stage is commonly known as the compression of the secondary stage through X-rays coming from the nuclear fission reaction of the first stage (radiation implosion).  And finally heating of the second stage, after a cold compression, caused by a second fission explosion inside the second stage. 
The energy released in the primary stage is transferred to the secondary stage; the exact mechanism in which this happens is still considered a secret.  This energy causes the compression of the fusion fuel in the second stage. This compressed becomes critical and undergoes a fission reaction, in which further heating of the compressed fuel induces fusion, ultimately supplying sufficient neutrons to react in the second stage to create a fuel for fusion.  The compressed fuel of the second stage is sometimes shrouded by a depleted uranium that cannot sustain chain reactions. It is however, fissionable when showered by the high-energy neutrons released by fusion in the secondary stage. This provides a considerable amount of energy yield, about half of the total yield of large devices.  (See Fig. 1)
|Fig. 2: Castle Bravo Blast. (Source: Wikimedia Commons)|
The concept of a thermonuclear fusion bomb ignited by a smaller fission bomb was initially suggested by Enrico Fermi to Edward Teller in 1941during the inception of the Manhattan Project.  This idea caused Teller to spend most of his time during the Manhattan Project attempting to figure out how to make this concept work; neglecting his assigned work on the Manhattan Project. Teller, was able to convince his colleague Stanislaw Ulam, to join him on this idea provided by Fermi. Ulam, made the first key steps towards a workable fusion design, rendering the idea of a fusion bomb practical.  He noted that compression of the thermonuclear fuel before extreme heating was a practical path towards the conditions needed for fusion.  This spawned the idea of staging and placing a separate thermonuclear component outside the primary fission component.  From this leap Teller then proved that the gamma radiation produced in the primary stage could be transferred and used in the second stage to create a successful implosion.  This work lead to various projects ("operations") that ended in many different thermonuclear weapons testing.
The first of these operations was "Operation Greenhouse." On May 9, 1951 the basic concept of Teller and Ulams idea was tested in a very small scale.  It was a successful, uncontrolled release of nuclear fusion energy. That produced a small fraction of the 225kt yield.  This ultimately raised prospects that the concept would work. Then on November 1, 1952, the first Teller-Ulam configuration was tested at full scale under Ivy Mike.  Ivy Mike was deployed upon an island in the Enewetak Atoll, and yielded approximately 10.4 megatons.  Ivy Mike used an extra-large fission bomb as a "trigger" and liquid deuterium, as its fusion fuel. 
Ivy Mike, however was impractical for as a deployable weapon, because of the liquid deuterium fuel. This spurred development for a next generation weapon, "Castle Bravo" that used solid lithium deuteride as a fusion fuel.  In 1954, "Castle Bravo" was deployed in the Bikini Atoll, and yielded approximately 15 megatons and is considered the largest U.S. bomb ever tested.  (See Fig. 2)
With successful testing the United States sought to shift its sight towards developing miniaturized version of these goliath weapons. This development spawned an outfit that easily integrated onto intercontinental ballistic missiles and submarine-launched ballistic missiles.  By 1960, the W47 warhead was deployed on Polaris ballistic submarines.  The W47 warhead is 18 inches in diameter and weighs 720 pounds.  Unfortunately, testing found that the Polaris warhead did not work reliably and had to be redesigned. Further development in miniaturizing the warheads lead to the successful adaptation of the W88 warhead to submarine launched ICBMs.  The Mark 45 anti-submarine torpedo, or MK 45 ASTOR, was another derivative of Teller-Ulam's configuration for miniaturization. The ASTOR is a submarine-launched nuclear torpedo designed by the United States Navy for high- speed, deep-diving, enemy submarines.  The ASTOR is a 19-inch diameter torpedo fitted with a W34 nuclear warhead.  The torpedo controlled via wire connection until detonation. The design of this torpedo was completed in 1960, which lead to the manufacturing of 600 torpedoes, and the replacement of the Mark 48 torpedo. The W34 nuclear warhead used on ASTOR has an explosive yield of 11 kilotons and range of 5 to 8 miles (8.0 to 12.9 km). 
Further innovation in miniaturizing warheads continued through the mid-1970s, however, much of the Teller-Ulam design for these weapons and older are considered a top nuclear secret. And is not discussed in much detail by any official publications. Most information in the public domain about these designs is relegated to a few terse statements by the Department of Energy and the work of a few individual investigators.
© Daniel Hernandez. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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