Editing Stanisław Ulam (section)

===Teller–Ulam design===
On 29 August 1949, the [[Soviet Union]] tested its first fission bomb, the [[RDS-1]]. Created under the supervision of [[Lavrentiy Beria]], who sought to duplicate the US effort, this weapon was nearly identical to [[Fat Man]], for its design was based on information provided by spies [[Klaus Fuchs]], [[Theodore Hall]], and [[David Greenglass]]. In response, on 31 January 1950, [[President of the United States|President]] [[Harry S. Truman]] announced a crash program to develop a fusion bomb.<ref name='DUNCAN'/>

To advocate an aggressive development program, [[Ernest Lawrence]] and [[Luis Walter Alvarez|Luis Alvarez]] came to Los Alamos, where they conferred with [[Norris Bradbury]], the laboratory director, and with [[George Gamow]], [[Edward Teller]], and Ulam. Soon, these three became members of a short-lived committee appointed by Bradbury to study the problem, with Teller as chairman.<ref name='VITA'/> At this time, research on the use of a fission weapon to create a [[Nuclear fusion|fusion reaction]] had been ongoing since 1942, but the design was still essentially the one originally proposed by Teller. His concept was to put [[tritium]] and/or [[deuterium]] in close proximity to a fission bomb, with the hope that the heat and intense flux of neutrons released when the bomb exploded, would ignite a self-sustaining [[Nuclear fusion|fusion reaction]]. Reactions of these [[isotopes of hydrogen]] are of interest because the energy per unit mass of fuel released by their fusion is much larger than that from fission of heavy nuclei.<ref name="RHODES"/>

[[File:IvyMike2.jpg|thumb|300px|left|[[Ivy Mike]], the first full test of the Teller–Ulam design (a [[Nuclear weapon design#Two-stage thermonuclear weapons|staged]] fusion bomb), with a [[TNT equivalent|yield]] of 10.4 megatons on 1 November 1952|alt=A mushroom cloud lights up the dawn sky]]
Because the results of calculations based on Teller's concept were discouraging, many scientists believed it [[History of the Teller–Ulam design#Teller's "Super"|could not lead to a successful weapon]], while others had moral and economic grounds for not proceeding. Consequently, several senior people of the Manhattan Project opposed development, including Bethe and Oppenheimer.<ref name= "HEWLETT AND DUNCAN"/> To clarify the situation, Ulam and von Neumann resolved to do new calculations to determine whether Teller's approach was feasible. To carry out these studies, von Neumann decided to use electronic computers: ENIAC at Aberdeen, a new computer, [[MANIAC I|MANIAC]], at Princeton, and its twin, which was under construction at Los Alamos. Ulam enlisted Everett to follow a completely different approach, one guided by physical intuition. [[Françoise Aron Ulam|Françoise Ulam]] was one of<ref name= "ULAM 215"/> a cadre of women "[[Human computers#Wartime computing and the invention of electronic computing|computers]]" who carried out laborious and extensive computations of thermonuclear scenarios on [[mechanical calculator]]s, supplemented and confirmed by Everett's [[slide rule]]. Ulam and Fermi collaborated on further analysis of these scenarios. The results showed that, in workable configurations, a thermonuclear reaction would not ignite, and if ignited, it would not be self-sustaining. Ulam had used his expertise in [[combinatorics]] to analyze the chain reaction in deuterium, which was much more complicated than the ones in uranium and plutonium, and he concluded that no self-sustaining chain reaction would take place at the (low) densities that Teller was considering.<ref name='GALISON'/> In late 1950, these conclusions were confirmed by von Neumann's results.<ref name='FULAM'/><ref name="RHODES 422-424"/>

In January 1951, Ulam had another idea: to channel the mechanical shock of a nuclear explosion so as to compress the fusion fuel. On the recommendation of his wife,<ref name='FULAM'/> Ulam discussed this idea with Bradbury and Mark before he told Teller about it.<ref name='MARKBIO'/> Almost immediately, Teller saw its merit, but noted that soft [[X-rays]] from the fission bomb would compress the thermonuclear fuel more strongly than mechanical shock and suggested ways to enhance this effect. On 9 March 1951, Teller and Ulam submitted a joint report describing these innovations.<ref name='LAMS1225'/> A few weeks later, Teller suggested placing a [[fissile]] rod or cylinder at the center of the fusion fuel. The detonation of this "spark plug"<ref name='LAMS1230'/> would help to initiate and enhance the fusion reaction. The design based on these ideas, called staged radiation implosion, has become the standard way to build thermonuclear weapons. It is often described as the "[[Teller–Ulam design]]".<ref name="RHODES 455-464"/>

[[File:Ivy Mike Sausage device.jpg|thumb|The ''Sausage'' device of [[Ivy Mike|Mike]] nuclear test (yield 10.4 Mt) on [[Enewetak Atoll]]. The test was part of the [[Operation Ivy]]. The Sausage was the first true H-Bomb ever tested, meaning the first [[Thermonuclear weapon|thermonuclear]] device built upon the [[Teller–Ulam design|Teller-Ulam]] principles of staged radiation implosion.|alt=Tiny men and a large silver cylindrical object connected to a lot of scaffolding and tubes]]
In September 1951, after a series of differences with Bradbury and other scientists, Teller resigned from Los Alamos, and returned to the University of Chicago.<ref name="HEWLETT AND DUNCAN 554-556"/> At about the same time, Ulam went on leave as a visiting professor at Harvard for a semester.<ref name="ULAM 220-224"/> Although Teller and Ulam submitted a joint report on their design<ref name='LAMS1225'/> and jointly applied for a patent on it,<ref name='LANLBIO'/> they soon became involved in a dispute over who deserved credit.<ref name='MARKBIO'/> After the war, Bethe returned to [[Cornell University]], but he was deeply involved in the development of thermonuclear weapons as a consultant. In 1954, he wrote an article on the history of the H-bomb,<ref name='BETHE'/> which presents his opinion that both men contributed very significantly to the breakthrough. This balanced view is shared by others who were involved, including Mark and Fermi, but Teller persistently attempted to downplay Ulam's role.<ref name = 'UCHII'/> "After the H-bomb was made," Bethe recalled, "reporters started to call Teller the father of the H-bomb. For the sake of history, I think it is more precise to say that Ulam is the father, because he provided the seed, and Teller is the mother, because he remained with the child. As for me, I guess I am the midwife."<ref name=SCHWEBER/>

With the basic fusion reactions confirmed, and with a feasible design in hand, there was nothing to prevent Los Alamos from testing a thermonuclear device. On 1 November 1952, the first thermonuclear explosion occurred when [[Ivy Mike]] was detonated on [[Enewetak Atoll]], within the US [[Pacific Proving Grounds]]. This device, which used liquid deuterium as its fusion fuel, was immense and utterly unusable as a weapon. Nevertheless, its success validated the Teller–Ulam design, and stimulated intensive development of practical weapons.<ref name="ULAM 220-224"/>