What is Fusion?
Fusion is the process at the core of our Sun. What we see as light and feel as warmth is the result of a fusion reaction: hydrogen nuclei collide, fuse into heavier helium atoms and release tremendous amounts of energy in the process.
Fusion is the energy source of the Universe, occuring in the core of the Sun and stars.
In the stars of our universe, gravitational forces have created the necessary conditions for fusion. Over billions of years, gravity gathered the hydrogen clouds of the early Universe into massive stellar bodies. In the extreme density and temperature of their cores, fusion occurs.
How does fusion produce energy?
Atoms never rest: the hotter they are, the faster they move. In the core of our Sun, temperatures reach 15,000,000° Celsius. Hydrogen atoms are in a constant state of agitation, colliding at very great speeds. The natural electrostatic repulsion that exists between the positive charges of their nuclei is overcome, and the atoms fuse. The fusion of two light hydrogen atoms (H-H) produces a heavier element, helium.
The mass of the resulting helium atom is not the exact sum of the two initial atoms, however—some mass has been lost and great amounts of energy have been gained. This is what Einstein's formula E=mc² describes: the tiny bit of lost mass (m), multiplied by the square of the speed of light (c²), results in a very large figure (E), which is the amount of energy created by a fusion reaction.
Every second, our Sun turns 600 million tons of hydrogen into helium, releasing an enormous amount of energy. But without the benefit of gravitational forces at work in our Universe, achieving fusion on Earth has required a different approach.
Fusion on Earth
Twentieth-century fusion science has identified the most efficient fusion reaction to reproduce in the laboratory setting: the reaction between two hydrogen (H) isotopes deuterium (D) and tritium (T). The D-T fusion reaction produces the highest energy gain at the 'lowest' temperatures. It requires nonetheless temperatures of 150,000,000° Celsius to take place—ten times higher than the H-H reaction occurring at the Sun's core.
Three, two, one ... We have plasma! Inside the European JET Tokamak, both before and during operation. Photo: EFDA, JET.
At extreme temperatures, electrons are separated from nuclei and a gas becomes a plasma—a hot, electrically charged gas. In a star as in a fusion device, plasmas provide the environment in which light elements can fuse and yield energy.
In ITER, the fusion reaction will be achieved in a tokamak device that uses magnetic fields to contain and control the hot plasma. The fusion between deuterium and tritium (D-T) will produce one helium nuclei, one neutron, and energy.
The helium nucleus carries an electric charge which will respond to the magnetic fields of the tokamak and remain confined within the plasma. However, some 80 percent of the energy produced is carried away from the plasma by the neutron which has no electrical charge and is therefore unaffected by magnetic fields. The neutrons will be absorbed by the surrounding walls of the tokamak, transferring their energy to the walls as heat.
In ITER, this heat will be dispersed through cooling towers. In the subsequent fusion plant prototype DEMO and in future industrial fusion installations, the heat will be used to produce steam and—by way of turbines and alternators—electricity.