Fusion Plasma Physics Essentials

What is fusion?

In technical terms fusion here means the combination of light element nuclei in order to produce exothermic nuclear reactions. In practical terms fusion reactions provide the life-sustaining energy of the sun, and is the most fundamental energy source of the universe.

The goal of fusion research is therefore, symbolically, to design a man-made sun. Nuclear fission and fossile fuel power should be substituted with an environmentally superior form of energy that would be available for millions of years.

The technological development of fusion is, however, a task of extreme complexity and difficulty. The sun holds together its 10 million K hot gas, or plasma, by gravitational forces. This force would be far too weak for terrestrial purposes. Instead fusion research has developed mainly along two different confinement principles, those of magnetic and inertial confinement.

Laboratory experiments started already in the 1940's but fusion reactors are presently not expected to be commercially available until the 2040's. Apart from physical and technological difficulties, the development is hampered by a relatively modest worldwide effort. The present global investments into fusion amounts to 1-2 billion dollars annually, and the Swedish contribution is about 7 million dollars. In Europe there are about 2000 active researchers.

Why then are the requirements for fusion so hard to satisfy?

A fusion reactor plasma must fulfill the following basic two conditions:

1.  The product of the plasma density and the confinement time of the plasma thermal energy must be at least of the order 10²⁰ m^-3s.
2. The plasma temperature must be at least 100 million K.

The latter condition is clearly understood: high temperature means high average ion velocity. The fastest ions in the distribution function can penetrate the Coulomb barrier in head-on collisions to produce the fused nuclei, which quickly disintegrate into new particles. The cross-section for these fusion reactions increases strongly with temperature, and a minimum value must be exceeded for a given plasma. The first condition expresses the requirement for a sufficient number of particles to react and compensate for the external energy used for heating and maintaining the plasma.

How can electricity be obtained from fusion reactions?

Different light elements can be used for fusion. The reaction which requires the lowest temperature and confinement product is that between deuterium and tritium. These fusion reactions generate helium ions and neutrons of high energy. By surrounding the plasma with a "blanket", the neutron kinetic energy is converted to heat during slowing down. The heat is taken away from the blanket by a coolant, which is used to run an electric generator.

How can the plasma be heated to such large temperatures?

The most obvious way to attain fusion would be the Bennett pinch: run a large electrical current between two electrodes in a suitable gas. The plasma is heated by ohmical dissipation (like a light bulb) and the closed magnetic field lines encircling the discharge provides the required confinement. Of course, nature would not allow this, however. Just like a house of cards the configuration easily becomes unstable; the plasma deforms and the discharge is confined only for a small fraction of the required time.

The ohmical heating mechanism is instead used on more stable configurations. Unfortunately, the mechanism becomes inefficient at high temperatures and additional heating methods are often used. The most successful are neutral beam heating (neutral particles with high energies are fired into the plasma to become ionized near the core region) and radio frequency heating (similar to heating in house-hold microwave ovens).

How is the plasma confined in the reactor?

In magnetic confinement fusion the tendency of charged particles (a plasma consists of fully detached ions and electrons) to follow magnetic field lines is used to contain the plasma in a "magnetic vessel". A fusion plasma would be rapidly cooled if it came into contact with material walls.

In inertial confinement fusion the underlying strategy is to decrease the required time for fusion to the time the plasma is kept together by inertial forces alone. This requires enormous pulses of energy from a number of focused lasers or particle beams; the order of magnitude is 10 MJ during a ns. The target, a spherical shell of frozen deuterium-tritium mixture, should be compressed to increase its density by a factor 1000.

What type of reactors will finally be used?

During the years a number of magnetic confinement schemes have been developed. The today most promising in terms of stability and confinement are the so-called Tokamak, the Stellarator and the Reversed Field Pinch.

Both the Tokamak and laser-driven inertial fusion have made impressive progress during the last two decades. These schemes have generated significant amounts of fusion reactions at reactor-relevant conditions. The technological problems remaining to be solved (large superconducting coils, high neutron wall loads etc.) are, however, severe.

If the technology problems of these major lines would be solved today, certain difficulties would remain. By necessity, standard Tokamak and laser fusion reactors would be so complex and so large that the capital investment would be similar to or larger than that of todays fission power plants. The desire to create alternative, more compact and less expensive lines of approach, comes naturally. At the Alfvén laboratory in Stockholm we pursue experimental and theoretical research on one of these alternatives, the Reversed Field Pinch.

Why is fusion power attractive from the environmental viewpoint?

Clearly, the first fusion power reactors will not be free from radiation. The strong neutron flux in the reactor core will contaminate the reactor walls and blanket, and long-time storage will be required for this material. By using advanced structural materials (like Ti, Al or SiC) these problems will be restricted in the future. Also one of the plasma components, tritium, is a radioactive gas needing special precautions.

The total amount of radioactive fuel in a fusion reactor is, however, very small in comparison with a fission reactor. It cannot lead to uncontrolled modes of reaction. Uranium is not required. Finally there is hope for utilising more advanced fuels, which would yield fewer, or even no neutrons as reaction products.