Secret beginnings: fusion research to the 1950s

From the stars to the laboratory (1920–1939)

The story of fusion starts with a question about the stars. In August 1920 the British astronomer Arthur Eddington suggested in a lecture in Cardiff that the Sun shines by building helium out of hydrogen. Four hydrogen atoms have a little more mass than one helium atom, and the missing mass becomes energy, $E = mc^2$. Eddington even hoped that people might one day control this energy.

Physicists then had to explain how nuclei can join when their positive charges repel each other. In 1928 George Gamow showed with quantum mechanics that a particle can tunnel through such a barrier. In 1929 Robert Atkinson and Fritz Houtermans used this idea to calculate fusion rates inside stars. In 1938–1939 Hans Bethe worked out the reaction chains that power the Sun and heavier stars.

At the same time, particle accelerators brought nuclear reactions into the laboratory. In 1932 John Cockcroft and Ernest Walton at Cambridge fired fast protons at lithium. In 1934 Mark Oliphant, Paul Harteck and Ernest Rutherford fired deuterium nuclei at deuterium and saw the first fusion reactions made by people, producing tritium and helium-3. But a beam of fast particles mostly loses its energy in collisions, and only a tiny fraction of the nuclei fuse. In 1933 Rutherford called the idea of getting useful power from atoms "moonshine".

War, bombs and secrecy (1938–1952)

In December 1938 Otto Hahn and Fritz Strassmann in Berlin found that uranium nuclei can split. Lise Meitner and Otto Frisch explained this fission, and physicists soon saw that a chain reaction could make a bomb. During the Second World War the American Manhattan Project built the atomic bomb, which destroyed Hiroshima and Nagasaki in August 1945. From then on, nuclear physics was a matter of national security.

Fusion was part of this. A fission bomb can heat hydrogen isotopes so much that they fuse: this is the hydrogen bomb. The United States tested its first one, Ivy Mike, on 1 November 1952. Controlled fusion was a different aim, a slow and steady reaction for electricity, but it used the same nuclear physics. A fusion device also makes neutrons, and neutrons can turn uranium into plutonium, the fuel of bombs.

In Britain, George Paget Thomson and Moses Blackman applied in 1946 for a patent on a ring-shaped fusion reactor. Soon after 1950, when the physicist Klaus Fuchs admitted that he had passed atomic secrets to the Soviet Union, British fusion work became classified: secret, and open only to people with security clearance. In the Soviet Union, Igor Tamm and Andrei Sakharov, who worked on the Soviet hydrogen bomb, wrote a proposal in 1950 to hold a hot plasma in a ring of magnetic field. On 5 May 1951 Stalin signed the decision to start a fusion laboratory under Lev Artsimovich. In the United States, fusion research became a secret programme of the Atomic Energy Commission, later called Project Sherwood.

An island in Patagonia (1948–1952)

On 24 March 1951 the President of Argentina, Juan Perón, told the world press that his country had produced controlled fusion. The scientist behind the claim was Ronald Richter, an Austrian who had come to Argentina in 1948. Perón had given him a laboratory on Huemul Island, in a lake near San Carlos de Bariloche, and a very large budget. Perón even said that energy might one day be sold in bottles like milk.

The news reached newspapers everywhere. Most physicists were sceptical, because Richter gave almost no details and no measurements that others could check. In September 1952 a team of Argentine physicists led by José Antonio Balseiro inspected the laboratory. Their reports found no sign of a nuclear reaction, and in November 1952 the project was closed. Richter had probably seen hot gas in an electric arc, far too cold for fusion.

The claim was false, but it had real effects. It worried governments that another country might be ahead, and it helped to start fusion programmes in other countries. In the United States, it started the work of an astronomer at Princeton.

Pinches, stellarators and a Soviet ring

A fusion fuel must be heated to many millions of degrees, so it becomes a plasma of ions and electrons. No wall can touch it, but a magnetic field can push on moving charges. In the early 1950s three main ideas competed.

In 1951 Lyman Spitzer, a Princeton astronomer, read about Richter's claim and doubted it. He then designed a different magnetic bottle, the stellarator: a tube bent into a figure-eight, so that the slow sideways drift of the particles in one half of the loop cancels the drift in the other half. The Atomic Energy Commission gave him 50,000 dollars, and his secret Project Matterhorn later became the Princeton Plasma Physics Laboratory. Its first small machine, Model A, ran in 1953. In Moscow, Tamm and Sakharov's ring became the tokamak, a ring with both a strong magnetic field from outside coils and a current in the plasma. The first true tokamak, T-1, started at the end of 1958.

ZETA: a headline and a correction

The largest fusion machine of the 1950s was ZETA, a ring-shaped pinch at Harwell in England. It started in August 1957, and within weeks it produced about a million neutrons in each pulse of deuterium plasma. Neutrons are made when two deuterium nuclei fuse, so many people hoped that ZETA had reached fusion. At a press conference on 24 January 1958, John Cockcroft, the director of Harwell, said he was "90 percent certain" that the neutrons came from fusion. Newspapers called it a breakthrough.

Geneva 1958: the doors open

By 1958 all the secret programmes had met the same problem: their plasmas were unstable and lost heat much faster than expected. No country had a reactor to hide. From 1 to 13 September 1958 the second United Nations conference on the peaceful uses of atomic energy met in Geneva. The United States, the Soviet Union and Britain declassified their fusion research and presented it openly. Scientists from the rival countries found that they had tried many of the same ideas and met the same problems. From then on, controlled fusion became one of the most open fields of physics, with conferences and journals that everyone could read.

Key idea

After 1945, governments treated controlled fusion as secret for several reasons at once:

• Weapons physics. The physics of hot fusion fuel is close to the physics of the hydrogen bomb.
• Neutrons. A fusion device makes neutrons, and neutrons can be used to make plutonium for bombs.
• Advantage and prestige. A country with fusion power would have cheap energy and be admired. False claims such as Huemul made governments fear that they were behind.
• Spies. After Klaus Fuchs admitted to spying, Britain and the United States trusted openness even less.

Secrecy had a cost. Teams in different countries repeated the same experiments, and few outside scientists could check a result. When results were finally shown, as with ZETA in January 1958, the pressure to announce success was strong, and a claim could reach the newspapers before it had been tested. In science, a result counts only when the evidence can be checked: Balseiro's inspection ended Huemul, and Rose's direction measurement ended the ZETA claim. In September 1958 the three nuclear powers declassified their work, because the plasma problems had turned out to be much harder than anyone had hoped, and openness now helped everyone more than secrecy.

Common misconception

"The research was secret because a fusion reactor could explode like a hydrogen bomb." This is not why. A controlled fusion device holds only a tiny amount of fuel, and when anything goes wrong the plasma cools and the reactions stop. The secrecy was about weapons knowledge, neutrons for bomb fuel, and national advantage.

A second mistake is to think that neutrons prove fusion in a hot plasma. A few fast ions can also make neutrons. The proof needs a test that tells the two causes apart, such as the directions of the neutrons or a direct measurement of the plasma temperature.

Summary

• From 1920 to 1939 physicists explained the energy of the stars (Eddington, Gamow, Bethe) and made the first fusion reactions with accelerators (Oliphant, 1934), but in accelerator beams only a tiny fraction of the nuclei fuse, so they use far more energy than they give.
• After the atomic bomb, nuclear physics became secret. Around 1950–1951 Britain, the Soviet Union and the United States all started secret, government-funded programmes on controlled fusion.
• The reasons were the link with the hydrogen bomb, neutrons that can make bomb fuel, and the hope of an advantage; Richter's false Huemul claim of 1951 added pressure.
• Three designs competed: pinches (Z-pinch, theta pinch; kink and sausage instabilities), Spitzer's stellarator (1951) and the Soviet tokamak (Tamm and Sakharov, 1950; T-1 in 1958).
• ZETA's neutrons of 1957–1958 did not come from a hot plasma: they flew mostly in one direction, and the claim was corrected on 16 May 1958.
• In September 1958 at Geneva, the three countries declassified fusion research. Everyone faced the same unstable plasmas, and fusion became an open, international science.