How and why the Sun shines and stars twinkle are questions that have always intrigued the human mind. For a long time it was believed that the Sun shone by actually burning its own material through combustion - just like a fire in the fireplace. However, by the mid 19th century geologists were suggesting that a few hundred thousand years were needed for the important features on the Earth´s surface to form and it was realised that the Sun would have needed inconceivable amounts of fuel to have kept burning that long.
Lord Kelvin and Helmholtz elaborated a theory that the Sun could generate energy by its own gravitational collapse. But this would only generate energy for 20 million years. In fact, Lord Kelvin argued against Darwin´s theory of evolution because he believed that there was not enough time for it to have taken place. Today the we know that our sun is about five billion years old.
With the arrival of the new understanding of the atomic and nuclear nature of matter at the beginning of the 20th century, the scene was set for a real understanding of fusion as the power source of the stars. The first clues to how the stars function were revealed in Einstein´s deceptively simple equation E = mc2 derived in 1905 as a consequence of his special theory of relativity. This famous equation predicted that a tiny amount of mass could, in principle, be converted into a tremendous amount of energy.
Albert Einstein
What was the connection between Einstein´s equation and the energy source of the Sun? The answer was not obvious. Astronomers did their part by defining the constraints that observations of stars imposed on possible explanations for the generation of stellar energy. In 1919, Henry Norris Russell, a leading theoretical astronomer in the United States, summarised concisely the hints on the nature of the stellar energy source. Russell stressed that the most important clue was the high temperature in the interior of stars.
Francis William Aston discovered the key experimental piece of the puzzle in 1920. He made precise measurements of the masses of many different atoms, among them hydrogen and helium, and found that four hydrogen nuclei were heavier than a helium nucleus.
The importance of Aston´s measurements was recognised immediately by Sir Arthur Eddington, the British astrophysicist. Eddington argued in his 1920 presidential address to the British Association for the Advancement of Science that Aston´s measurement of the mass difference between four atoms of hydrogen and a helium atom meant that the sun could shine by converting hydrogen atoms into helium. This burning of hydrogen into helium would (according to E=mc2) release about 0.7% of the mass equivalent of the energy. In principle, this would allow the sun to shine for about 100 billion years.
Hans Bethe
In 1939, Hans Bethe described a quantitative theory explaining the fusion generation of energy in the stars (including our sun). The results of his calculations presented in a paper entitled "Energy Production in Stars", won him the Nobel prize for Physics in 1968. With the general theory for fusion reactions now understood, experimental efforts to control the release of fusion energy for net energy output could now be progressed and continue today.
The second half of the 20th century exploited this theory in an attempt to reproduce the power of the Sun in a controlled manner on Earth. These experiments, initially in closely guarded, secret national programmes, have evolved into the major international collaborations that we see today.
The first fusion experiments were conducted in the Cavendish laboratory in Cambridge, UK, during the 1930's but results led the eminent scientist Lord Rutherford to pronounce in 1933 that "anyone who looks for a source of power in the transformation of the atom is talking moonshine." However after World War II and the technical success of the Manhattan project that developed the first nuclear weapons, an increased interest in atomic physics and fusion in particular was seen.
There was serious interest in the peaceful use of fusion physics all around the world. In fact, in 1951 scientists in Argentina claimed to have controlled the release of nuclear fusion energy. These claims proved to be false but they acted as a spur to many other research groups.
Figure 1:The Zeta device
at Harwell, UK
© Image: CCFE
In the UK, much of the early work on fusion was undertaken by universities, principally Sir George Thomson´s group at Imperial College and Peter Thonemann´s team at Oxford, before being centred respectively at Harwell and Aldermaston. Sir George Thomson even developed a patent for a fusion reactor. In 1952 Cousins and Ware built a small toroidal pinch device, but the original large-scale experimental fusion device on which most British fusion physicists worked during the 1940s and 50s was housed in a hangar at Harwell and called the Zero Energy Toroidal Assembly (ZETA). ZETA was a stabilised toroidal pinch device and worked from 1954 until 1958 giving results that showed initial promise and gave clues to later larger devices.
In the US, Lyman Spitzer started the Princeton Plasma Physics Laboratory working on a magnetic confinement device called a stellarator. James Tuck, a British physicist, began work at Los Alamos National Laboratory working on magnetic pinch devices and Edward Teller expanded work on the hydrogen bomb at Lawrence Livermore Laboratory to include inertial confinement techniques.
In the Soviet Union, significant fusion research was also being undertaken. At first all these national projects were shrouded in secrecy, but with the temporary thaw in the Cold War created in 1956 by the visit of the Soviet leaders Nikita Khrushchev and Bulganin to the UK, the first attempts at global co-operation were created. The Russians brought their leading fusion expert academician I.V. Kurchatov to give a lecture "The Possibility of Producing Thermonuclear Reactions in a Gas Discharge". This described Soviet work in the field and the UK shared its ZETA experience.
Fusion research also started elsewhere (e.g. France and Germany). International co-operation began under the normal scientific exchange of information as countries declassified fusion research, and an Atoms for Peace conference in Geneva in 1958 sealed the start of the process. In the UK this led directly to the setting up of a custom-built laboratory at Culham that would subsequently become the home of the Joint European Torus (JET).
Almost 10 years later (in 1968) results from the Russians Tamm and Sakharov using a new type of magnetic confinement device called a tokamak caused a major stir. Their experiment ran at temperatures ten times higher (10 million degrees centigrade) than anywhere else in the world with excellent confinement results. The success of the Russians, confirmed by visiting UK scientists in 1969, led to the construction of many tokamak experiments and its position as the dominant technique for fusion research today.