Editing Tokamak (section)

=== Progress in the 1960s ===
In contrast to the other designs, the experimental tokamaks appeared to be progressing well, so well that a minor theoretical problem was now a real concern. In the presence of gravity, there is a small pressure gradient in the plasma, formerly small enough to ignore but now becoming something that had to be addressed. This led to the addition of yet another set of coils in 1962, which produced a vertical magnetic field that offset these effects. These were a success, and by the mid-1960s the machines began to show signs that they were beating the [[Bohm diffusion|Bohm limit]].{{sfn|Bromberg|1982|p=153}}

At the 1965 Second [[International Atomic Energy Agency]] Conference on fusion at the UK's newly opened [[Culham Centre for Fusion Energy]], Artsimovich reported that their systems were surpassing the Bohm limit by 10 times. Spitzer, reviewing the presentations, suggested that the Bohm limit may still apply; the results were within the range of experimental error of results seen on the stellarators, and the temperature measurements, based on the magnetic fields, were simply not trustworthy.{{sfn|Bromberg|1982|p=153}}

The next major international fusion meeting was held in August 1968 in [[Novosibirsk]]. By this time two additional tokamak designs had been completed, TM-2 in 1965, and T-4 in 1968. Results from T-3 had continued to improve, and similar results were coming from early tests of the new reactors. At the meeting, the Soviet delegation announced that T-3 was producing electron temperatures of 1000&nbsp;eV (equivalent to 10 million degrees Celsius) and that confinement time was at least 50 times the Bohm limit.{{sfn|Bromberg|1982|p=151}}

These results were at least 10 times that of any other machine. If correct, they represented an enormous leap for the fusion community. Spitzer remained skeptical, noting that the temperature measurements were still based on the indirect calculations from the magnetic properties of the plasma. Many concluded they were due to an effect known as [[runaway electrons]], and that the Soviets were measuring only those extremely energetic electrons and not the bulk temperature. The Soviets countered with several arguments suggesting the temperature they were measuring was [[Maxwell–Boltzmann distribution|Maxwellian]], and the debate raged.{{sfn|Bromberg|1982|p=166}}

==== Culham Five ====
In the aftermath of ZETA, the UK teams began the development of new plasma diagnostic tools to provide more accurate measurements. Among these was the use of a [[laser]] to directly measure the temperature of the bulk electrons using [[Thomson scattering]]. This technique was well known and respected in the fusion community;{{sfn|Bromberg|1982|p=172}} Artsimovich had publicly called it "brilliant". Artsimovich invited [[Bas Pease]], the head of Culham, to use their devices on the Soviet reactors. At the height of the [[Cold War]], in what is still considered a major political manoeuvre on Artsimovich's part, British physicists were allowed to visit the Kurchatov Institute, the heart of the Soviet nuclear bomb effort.<ref>{{cite web |url=https://www.walesonline.co.uk/news/wales-news/valleys-boy-who-broached-iron-1794244 |title= The Valleys boy who broached the Iron Curtain to convince the USA that Russian Cold War nuclear fusion claims were true |date=3 November 2011 |website=WalesOnline}}</ref>

The British team, nicknamed "The Culham Five",<ref>{{cite magazine |first=Robert |last=Arnoux |url=http://www.iter.org/newsline/102/1401 |title=Off to Russia with a thermometer |magazine=ITER Newsline |issue=102 |date=9 October 2009}}</ref> arrived late in 1968. After a lengthy installation and calibration process, the team measured the temperatures over a period of many experimental runs. Initial results were available by August 1969; the Soviets were correct, their results were accurate. The team phoned the results home to Culham, who then passed them along in a confidential phone call to Washington.{{sfn|Bromberg|1982|p=167}} The final results were published in ''Nature'' in November 1969.<ref name=culham>{{cite journal |first1=N. J. |last1=Peacock |first2=D. C. |last2=Robinson |first3=M. J. |last3=Forrest |first4=P. D. |last4=Wilcock |first5=V. V. |last5=Sannikov |s2cid=4290094 |title=Measurement of the Electron Temperature by Thomson Scattering in Tokamak T3 |journal=[[Nature (journal)|Nature]] |volume=224 |issue=5218 |pages=488–490 |year=1969 |doi=10.1038/224488a0 |bibcode=1969Natur.224..488P }}</ref> The results of this announcement have been described as a "veritable stampede" of tokamak construction around the world.<ref>{{cite magazine |magazine=New Scientist |title=Fusion research - the temperature rises |first=Michael |last=Kenward |url=https://books.google.com/books?id=tbhTdnZsqMUC&pg=PA626 |date=24 May 1979 }}{{Dead link|date=May 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

One serious problem remained. Because the electrical current in the plasma was much lower and produced much less compression than a pinch machine, this meant the temperature of the plasma was limited to the resistive heating rate of the current. First proposed in 1950, [[Spitzer resistivity]] stated that the [[electrical resistance]] of a plasma was reduced as the temperature increased,<ref name="Spitzer 1950">{{cite journal
 |last1=Cohen |first1=Robert S.
 |last2=Spitzer |first2=Lyman Jr.
 |last3=McR. Routly |first3=Paul
 |title=The Electrical Conductivity of an Ionized Gas
 |date=October 1950
 |journal=Physical Review
 |volume=80
 |issue=2
 |pages=230–238
 |url=http://ayuba.fr/pdf/spitzer1950.pdf
 |doi=10.1103/PhysRev.80.230
|bibcode=1950PhRv...80..230C}}</ref> meaning the heating rate of the plasma would slow as the devices improved and temperatures were pressed higher. Calculations demonstrated that the resulting maximum temperatures while staying within ''q'' > 1 would be limited to the low millions of degrees. Artsimovich had been quick to point this out in Novosibirsk, stating that future progress would require new heating methods to be developed.{{sfn|Bromberg|1982|p=161}}

==== US turmoil ====
One of the people attending the Novosibirsk meeting in 1968 was [[Amasa Stone Bishop]], one of the leaders of the US fusion program. One of the few other devices to show clear evidence of beating the Bohm limit at that time was the [[multipole (fusion reactor)|multipole]] concept. Both [[Lawrence Livermore National Laboratory|Lawrence Livermore]] and the [[Princeton Plasma Physics Laboratory]] (PPPL), home of Spitzer's stellarator, were building variations on the multipole design. While moderately successful on their own, T-3 greatly outperformed either machine. Bishop was concerned that the multipoles were redundant and thought the US should consider a tokamak of its own.{{sfn|Bromberg|1982|p=152}}

When he raised the issue at a December 1968 meeting, directors of the labs refused to consider it. [[Melvin B. Gottlieb]] of Princeton was exasperated, asking "Do you think that this committee can out-think the scientists?"{{sfn|Bromberg|1982|p=154}} With the major labs demanding they control their own research, one lab found itself left out. [[Oak Ridge National Laboratory|Oak Ridge]] had originally entered the fusion field with studies for reactor fueling systems, but branched out into a mirror program of their own. By the mid-1960s, their DCX designs were running out of ideas, offering nothing that the similar program at the more prestigious and politically powerful Livermore did not. This made them highly receptive to new concepts.{{sfn|Bromberg|1982|p=158}}

After a considerable internal debate, [[Herman Postma]] formed a small group in early 1969 to consider the tokamak.{{sfn|Bromberg|1982|p=158}} They came up with a new design, later christened [[Ormak (fusion reactor)|Ormak]], that had several novel features. Primary among them was the way the external field was created in a single large copper block, fed power from a large [[transformer]] below the torus. This was as opposed to traditional designs that used electric current windings on the outside. They felt the single block would produce a much more uniform field. It would also have the advantage of allowing the torus to have a smaller major radius, lacking the need to route cables through the donut hole, leading to a lower ''[[aspect ratio]]'', which the Soviets had already suggested would produce better results.{{sfn|Bromberg|1982|p=159}}

==== Tokamak race in the US ====
In early 1969, Artsimovich visited [[Massachusetts Institute of Technology|MIT]], where he was hounded by those interested in fusion. He finally agreed to give several lectures in April{{sfn|Bromberg|1982|p=161}} and then allowed lengthy question-and-answer sessions. As these went on, MIT itself grew interested in the tokamak, having previously stayed out of the fusion field for a variety of reasons. [[Bruno Coppi]] was at MIT at the time, and following the same concepts as Postma's team, came up with his own low-aspect-ratio concept, [[Alcator]]. Instead of Ormak's toroidal transformer, Alcator used traditional ring-shaped magnetic field coils but required them to be much smaller than existing designs. MIT's [[Francis Bitter Magnet Laboratory]] was the world leader in magnet design and they were confident they could build them.{{sfn|Bromberg|1982|p=161}}

During 1969, two additional groups entered the field. At [[General Atomics]], [[Tihiro Ohkawa]] had been developing multipole reactors, and submitted a concept based on these ideas. This was a tokamak that would have a non-circular plasma cross-section; the same math that suggested a lower aspect-ratio would improve performance also suggested that a C or D-shaped plasma would do the same. He called the new design [[Doublet (fusion reactor)|Doublet]].{{sfn|Bromberg|1982|p=164}} Meanwhile, a group at [[University of Texas at Austin]] was proposing a relatively simple tokamak to explore heating the plasma through deliberately induced turbulence, the [[Texas Turbulent Tokamak]].{{sfn|Bromberg|1982|p=165}}

When the members of the Atomic Energy Commissions' Fusion Steering Committee met again in June 1969, they had "tokamak proposals coming out of our ears".{{sfn|Bromberg|1982|p=165}} The only major lab working on a toroidal design that was not proposing a tokamak was Princeton, who refused to consider it in spite of their Model C stellarator being just about perfect for such a conversion. They continued to offer a long list of reasons why the Model C should not be converted. When these were questioned, a furious debate broke out about whether the Soviet results were reliable.{{sfn|Bromberg|1982|p=165}}

Watching the debate take place, Gottlieb had a change of heart. There was no point moving forward with the tokamak if the Soviet electron temperature measurements were not accurate, so he formulated a plan to either prove or disprove their results. While swimming in the pool during the lunch break, he told [[Harold Furth]] his plan, to which Furth replied: "well, maybe you're right."{{sfn|Bromberg|1982|p=167}} After lunch, the various teams presented their designs, at which point Gottlieb presented his idea for a "stellarator-tokamak" based on the Model C.{{sfn|Bromberg|1982|p=167}}

The Standing Committee noted that this system could be complete in six months, while Ormak would take a year.{{sfn|Bromberg|1982|p=167}} It was only a short time later that the confidential results from the Culham Five were released. When they met again in October, the Standing Committee released funding for all of these proposals. The Model C's new configuration, soon named [[Symmetrical Tokamak]], intended to simply verify the Soviet results, while the others would explore ways to go well beyond T-3.{{sfn|Bromberg|1982|p=168}}