Introduction to Fusion

Without energy, our world stops. Each day, millions of people work to harvest energy sources like coal, oil and gas. And thousands of scientists develop new energy sources, that are needed to make the energy we use cleaner and more sustainable.

One of those sources is fusion, the energy source of the sun and the stars. Light atoms, like hydrogen, can fuse together at extremely high temperatures. During this process, a lot of energy is released. Fusion research is aimed at reproducing this process here on earth, and to use fusion as a safe way of producing large-scale energy, without the emission of greenhouse gases.

In principle, fusion can provide mankind with energy for millions of years. But it is not easy: a gas must be kept together sufficiently long at a temperature of 150 million degrees. Since the 50ies, scientists from all over the world have worked on bringing this energy source closer to reality, and with success. In the mean time, the fusion community is ready to construct ITER, a large international experiment that should demonstrate that fusion can be used as an energy source on earth.

Fusing nuclei

In our daily life, we are used to chemical reactions when we deal with energy. The burning of coal, oil and gas is a chemical reaction, where the atoms in the fuel, together with oxygen from the air, form new molecules The atoms form new, more stable combinations, which releases energy. Many fusion reactions are possible: in stars this process starts with the lightest element - hydrogen - and forms ever heavier elements, right up to iron.

Such a reshufling is also possible between the building blocks - the protons and neutrons - in the nucleus of atoms. That can happen in two different ways. In a fission power plant, heavy nuclei like uranium are split in smaller fragments. In the fusion process, light nuclei, such as hydrogen, fuse together to form heavier atoms. Both processes release energy.

Figure 1:Two atoms, here deuteirum
and tritium, fuse together, forming a
helium nucleus, a neutron, and lots
of energy.

Fusion does not just happen. The nuclei of atoms have an electric charge, and equal charges repel. But if two nuclei manage to get close enough together in spite of the repelling force, another force manifests itself: the nuclear force. The nuclear force is extremely powerful, but only acts on very small distances. All of a sudden, the two nuclei are pulled together with a great force, and a new atom is born.

Because the nuclear force is so strong, a single fusion reaction releases an enormous amount of energy, millions of times more than a single chemical reaction. One kilogram of fusion fuel can generate the same amount of energy as 10.000.000 kilograms of coal! An electricity generating power plant working on this principle only needs a very small amount of fuel.

The energy source of the universe

Fusion is the energy source of the universe. In stars like our sun, hydrogen is fused to helium, at a temperature of about 15 million degrees Celcius. Every second, the sun turns 600 million tons of hydrogen into helium, releasing an enormous amount of energy. The energy released in the sun escapes as light, which almost completely disappears in the universe. Only one-billionth part of the light from the sun illuminates the earth, where it provides the energy source for life.

When things get hot: plasma

To make fusion happen, two nuclei must come very close together. That only happens if they collide with a very high speed, which means that the temperature of the gas must be very high. If a gas is heated to a very high temperature, the electrons are separated from the atoms which they belong to, and together they form a gas of charged particles, in which the electrons and nuclei move independently. That state is called a plasma.

Plasma is often called the fourth state of matter, next to solids, fluids and gasses. In the universe, more than 99.9% of all matter exists in the plasma state! The sun, the stars and the nebulae are all examples of plasmas. On earth, a bolt of lightning, a flame and fluorescent lights are examples of plasmas. In industry, plasmas are used in many ways, for example during the production of microchips and for welding purposes. In plasma screens, small plasma discharges produce the colored light that constitute the pixels.

Fusion on earth

The sun keeps together the hot plasma by gravity, which causes a very high pressure in the center of the sun. On earth such conditions cannot be reproduced, so a different technique needs to be used. Moreover, contrary to intuition, the sun "burns" quite slowly. A cubic meter of the center of the sun only produces 30 watt, barely enough to power a lightbulb.

Although many different fusion reactions are possible, only a few of them are interesting for fusion on earth. Those are the reactions that will still occur at a relatively low temperature. The fusion reaction that is easiest to accomplish on earth is the reaction between deuterium and tritium, two isotopes of hydrogen. As shown in the illustration, a deuterium and a tritium nucleus can combine to form a helium nucleus, a neutron, and a lot of energy.

Deuterium is the stable isotope of hydrogen, with one extra neutron in its nucleus, tritium is the unstable istope of hydrogen, and has two extra neutrons. To produce enough fusion reactions, the deuterium-tritium mixture has to be brought to a temperature of 150 million degrees, ten times the temperature of the centre of the sun!

Of course, no single material can withstand such temperatures. Somehow, the plasma must be kept away from the walls of the plasma vessel, because if the plasma would touch the wall, the plasma would cool down, and fusion would stop. The plasma must be contained.

To accomplish this, we can use a property of the plasma: as it consists of charged particles - positive nuclei and negative electrons - a plasma can be influenced with a magnetic field. It is a property of charged particles that they follow magnetic field lines. The magnetic field lines can be organised in such a way that the plasma does not touch the inner wall of a plasma vessel: this technique is called magnetic confinement. In modern fusion experiments, the plasma is confined in a doughnut-shaped vessel with magnetic coils called a tokamak.

Figure 2:The principle of a tokamak. The plasma is contained in a
doughnut-shaped vessel, also called a "torus". Using superconduncting
coils (blue) a magnetic field is generated, which causes the plasma
particles to run around in circles, without touching the vessel wall. In
reality, a number of other coils are present, that produce subtle changes
to the magnetic field.