A brief history
How and why the Sun shines and the distant stars twinkle have been questions that have intrigued the human mind since the dawn of history. But only in the last 100 years have fusion researchers all over the world truly started to understand the nature of the Sun's energy, and only during the past 50 years have made the first steps to reproduce the power of the Sun here on Earth.
Einstein provided the first clues on how the Sun works in 1905 with his famous E=mc² equation derived from his special theory of relativity. This simple equation predicted that the conversion of a small amount of mass (m) could yield a very large amount of energy (E) with the conversion factor being the square of the speed of light (c = 3 x 108 metres per second).
But how did that relate to processes in the Sun? The key experimental observation was made in 1920 by British chemist Francis William Aston who took precise measurements of the masses of atoms. This work was seized upon by Sir Arthur Eddington, a British astrophysicist, who realised that by burning hydrogen into helium, the Sun would release around 0.7 % of the mass into energy. In 1939, German physicist Hans Bethe completed the picture with a quantitative theory explaining the generation of fusion energy in stars.
First fusion experiments
Some early and unsuccessful experiments were conducted in the Cavendish laboratory in Cambridge, UK, during the 1930s, but it was only after World War Two and the development of nuclear fission weapons that interest in fusion, and nuclear technologies in general, increased.
The original large-scale experimental fusion device was built in the late 1940s and early 1950s at Harwell in the UK. The Zero Energy Toroidal Assembly (ZETA) worked from 1954 to 1958 showing initial promise and producing useful results for later devices.
Research on fusion quickly became an international area of science with experimental devices developed in France, Germany, the Soviet Union and the US. Even during the depth of the Cold War, scientific exchange on fusion was encouraged. In 1958 an Atoms for Peace conference in Geneva formally sealed the start of truly international collaboration that would in time lead to today's ITER experiment in southern France.
A major step forward occurred in 1968 when the Russian scientists Tamm and Sakharov announced results from a new type of magnetic confinement device called a tokamak. It was able to run at temperatures ten times higher than other current experiments. Today, the tokamak is the dominant experimental technique for studying fusion.
In 1978 the Joint European Torus (JET) project was launched in Europe and came into operation in 1983, about the same time as the Tokamak Fusion Test Reactor (TFTR) in the USA. The Japanese tokamak JT-60 came online in 1985. In 1991, JET produced for the first time in the world a significant amount of power (1.7 megawatts-MW) from controlled nuclear fusion. Subsequently, in 1993 TFTR produced 10 MW of power. The current world record for fusion power was regained by JET in 1997 when it hit 16 MW.
The international project ITER will be the largest tokamak ever constructed and it will build on the experience and knowledge gained by its predecessors as it takes the next big steps on the road to fusion as a worldwide energy source.