Tokamak | Research & Encyclopedia Articles

Tokamak

Scientists have known for approximately seven decades that nuclear fusion reactions are a potentially important source of energy. A fusion reaction is one in which two small nuclei, such as protons or deuterons, combine with each other to form one large nucleus. In the fusion process, large amounts of energy are released.

The primary obstacle to achieving fusion is the fact that nuclei are all positively charged and, therefore, repel each other strongly. In order to overcome this force of repulsion, the nuclei must be given very large amounts of kinetic energy. In practical terms, that means heating the nuclei to temperatures in excess of 20,000,000K. Such conditions exist within stars, where fusion is the primary source of energy, but are unknown in any natural condition on the earth.

The first instance in which scientists were able to initiate fusion reactions artificially was the hydrogen (fusion) bomb. The hydrogen bomb is a device that is ignited by the explosion of an atom (fission) bomb. The atom bomb briefly produces temperatures of 20,000,000-40,000,000K or more, sufficient to initiate fusion reactions in hydrogen isotopes that surround the atom bomb.

The development of controlled nuclear fusion has been a far more challenging task. The problem has been to find a way to contain the very hot gasses required for fusion to occur. Obviously, no conventional construction material can survive temperatures of a few million degrees. The most successful method discovered so far is to confine the hot gases within a magnetic field.

A critical problem with this approach is to design a magnetic field with exactly the correct geometric shape. Simply surrounding the reacting gases with a magnetic field is not sufficient since they tend to leak out of the field at its extremities. Designing a controlled fusion reaction becomes a problem in geometry, therefore, as much as it is a problem in chemistry and physics.

The effectiveness of any design is determined by its ability to confine a large number of particles at a high enough temperature for a sufficiently long period of time. The method has to be able to result in the release of more energy than the large quantities used to initiate the reaction.

Perhaps the most promising technique yet developed is called the tokamak, developed largely as the result of research by the Russian physicist, Lev Artsimovich (1909-1973), in the 1950s. The name tokamak is an acronym for "toroid camera with magnetic field." In a tokamak, nuclei are trapped in the middle of a magnetic field that has the shape of a torus, a hollow, doughnut-shaped figure. The torus prevents particles from escaping from the field of reaction, turning them back onto themselves.

At the high temperatures required for fusion reactions, neutral atoms are completely broken apart into positively-charged nuclei and negative charged electrons. The swarm of nuclei and electrons is known as plasma, a state of matter with properties different from those of solids, liquids, and gasses. Some of these special properties of plasma present additional technical problems that must be solved in containing the hot material.

Artsimovich, the tokamak's inventor, was born in Moscow on February 25, 1909. He graduated from the Belorussian State University in Minsk at the age of 19 and took a position at the Leningrad (now St. Petersburg) Physical-Technical Institute. Some of Artsimovich's earliest research dealt with the properties of the recently discovered neutron. He made critical discoveries concerning the ability of nuclei to capture slow-moving neutrons. His later work dealt with methods for accelerating electrons in particle accelerators and for the electromagnetic separation of isotopes.

Artsimovich began teaching at the Leningrad Polytechnical Institute in 1930 and, later, at Leningrad University also. He received a number of honors and awards from the Soviet Union and international scientific organizations. He died in Moscow on March 1, 1973.