Science across the Iron Curtain

Before you read the sources

In 1968 the world was divided by the Iron Curtain, the border between the Soviet Union and its allies in the East and the countries of Western Europe and North America. Travel across it was difficult, and scientists in nuclear research were watched by security services on both sides. In August 1968, Warsaw Pact armies invaded Czechoslovakia to end the reforms of the Prague Spring.

Fusion research had been open since the Geneva conference of 1958, but it was stuck. Most machines lost their plasma much faster than theory predicted, at a rate known as Bohm diffusion. If that rate was a law of nature, a fusion reactor would be impossible. The five sources below follow what happened next. They are short texts written for this course from historical documents; words in quotation marks are the original words.

Source A: Novosibirsk, August 1968

Type: a summary of results presented at a scientific conference. Based on the International Atomic Energy Agency's conference of 1–7 August 1968 and on later accounts.

At the third IAEA conference on plasma physics and controlled fusion research, held in Novosibirsk in Siberia, Lev Artsimovich, the head of the Soviet fusion programme, presented results from the T-3 tokamak at the Kurchatov Institute in Moscow. T-3 had reached an electron temperature of about $1\ \text{keV}$, roughly ten million degrees, and it held its energy for more than $10\ \text{ms}$. This was many times better than the Bohm rate and about ten times better than any other machine in the world. The temperature was not measured directly: it was calculated from the magnetic properties of the plasma.

Source B: Why the West doubted

Type: a summary of the scientific objections, based on histories of the tokamak.

Many Western physicists did not accept the T-3 temperature. Already in 1965, when Artsimovich had shown earlier, smaller improvements, Lyman Spitzer of Princeton had questioned the Soviet measurements. The critics' main argument was physical: the magnetic method measures the total energy of the electrons. A small group of extremely fast runaway electrons, accelerated by the electric field that drives the current, could carry a lot of energy while the bulk of the plasma stayed much colder. Then the true temperature would be far lower than claimed. The Soviet scientists answered that their plasma was in thermal equilibrium, but with their method they could not prove it.

Source C: An invitation across the Iron Curtain

Type: a summary of memories of the people involved, published by the ITER Organization and in a biography of Derek Robinson.

Artsimovich made an unusual offer to Bas Pease, the director of the British fusion laboratory at Culham: a British team could come to Moscow and measure T-3 themselves. Culham had used a new laser method for measuring plasma temperatures for about five years, and the British wanted to test it on the best machine in the world. In December 1968, less than six months after Novosibirsk, the team was chosen: Nicol Peacock, Mike Forrest, Peter Wilcock and Derek Robinson, with a technician, later known together as the Culham Five. British rules said that a person with valuable knowledge could travel to the Soviet Union only if "properly accompanied by a reliable person". In Moscow the team felt watched: once, a broken light bulb that the Robinsons had mentioned at breakfast was replaced before they came back. Forrest later wrote that they felt safe "enjoying the powerful patronage of Lev Artsimovitch" (the original spells his name with -tch; it is the same Artsimovich).

Source D: Five tonnes of equipment and a laser

Type: a summary of the team's own accounts and of their scientific paper (Peacock, Robinson, Forrest, Wilcock and Sannikov, Nature, November 1969).

In the spring of 1969 the team arrived at the Kurchatov Institute with about five tonnes of equipment. They opened the machine to fit windows, built optical benches that did not shake, and placed a powerful ruby laser in the basement under the tokamak. Vibrations from the huge generators that powered T-3 made the floor move "like the skin of a drum". At first the scattered light was lost in the noise, and the team had to make the laser much more powerful. On 22 July 1969 the signal finally appeared, and a short note in Robinson's notebook recorded an electron temperature of about $1\ \text{keV}$. By early August they had measured about 88 plasma shots. Most showed electron temperatures of about ten million degrees, as Artsimovich had said. The results were phoned to Culham. The paper in Nature reported electron temperatures from $100\ \text{eV}$ to $1\ \text{keV}$ and agreed with the other methods where they could be compared. One of its five authors, V. V. Sannikov, was a Soviet scientist.

Source E: The tokamak stampede

Type: a summary of what laboratories did after 1969, based on histories of the tokamak and the stellarator and on the ITER Organization's account.

The British measurement changed fusion research. At Princeton, the stellarator Model C was shut down in December 1969 and rebuilt as a tokamak, the Symmetric Tokamak, which started in May 1970 and within months matched the T-3 results. Laboratories in the United States, France, Japan and Germany also built tokamaks, and the tokamak became the main road to fusion. The Soviet physicist Evgeny Velikhov later said that the British measurement "led to the beginning of active research on tokamaks all over the world". Today's largest fusion experiment, ITER, is a tokamak.

How Thomson scattering works

When laser light passes through a plasma, a tiny fraction of it is scattered sideways by the free electrons. This is Thomson scattering. An electron that moves towards the detector scatters the light with a slightly shorter wavelength, and one that moves away with a slightly longer wavelength, because of the Doppler effect. In a plasma the electrons move in all directions at random, so the scattered light is spread over a range of wavelengths around the laser's own wavelength, $694\ \text{nm}$ for a ruby laser.

Key idea

Scientists judge a result by its evidence: measurements that others can check, made by a method whose limits are understood. The circumstances of a result, such as who made it, in which country and during which political crisis, can explain why people trusted or doubted it, but they are not evidence about nature.

In 1968 the circumstances made Western scientists suspicious: T-3 was a Soviet machine in the middle of the Cold War, and earlier claims such as ZETA's had been wrong. But their doubt also had a scientific reason: the temperature came from an indirect method that runaway electrons could fool. The way to settle the question was independent verification: a different team, with its own equipment and a different method, measuring the same plasma. Thomson scattering measured the bulk electron temperature directly, and it agreed with the Soviet result.

Independent verification matters because every method has hidden weaknesses and every team has hopes. A result that two independent methods agree on is much stronger than a result that one team repeats many times. The T-3 story also shows that cooperation can serve both sides: Artsimovich gained trust for his result, and the British tested their method on the best plasma in the world.

Common misconception

"The British confirmed the result because East–West relations had improved." The political situation was not good in 1968–1969, and the team worked under security rules and surveillance. The result was accepted because the measurement was independent and direct, not because of politics.

A second mistake is to think that repeating a measurement is the same as verifying it. Repeating the same indirect measurement many times does not remove its systematic weakness. A different method does.

Summary

• In August 1968 at Novosibirsk, Artsimovich reported that the T-3 tokamak had reached about $1\ \text{keV}$ (about ten million degrees) and confinement far better than the Bohm rate.
• Western scientists doubted it for political and scientific reasons: the temperature was calculated indirectly, and runaway electrons could have fooled the method.
• Artsimovich invited a British team from Culham. In 1969 they measured T-3 with laser Thomson scattering, and from 22 July their data confirmed temperatures of about $1\ \text{keV}$. The paper appeared in Nature in November 1969 with a Soviet co-author.
• Thomson scattering measures the electron temperature directly from the Doppler width of the scattered laser light: the hotter the electrons, the wider the spectrum.
• After 1969 laboratories around the world built tokamaks; Princeton turned its Model C stellarator into the Symmetric Tokamak in 1970.
• Evidence is a checkable measurement; circumstances explain why people trust or doubt a result. Independent verification with a different method is the strongest test.