Plasma
The term plasma refers to a condition of matter sufficiently different from solids, liquids, and gases to have earned the description, "the fourth state of matter. " The state develops when a gas is heated to such a high temperature that all atoms in the gas are ionized. In this state, matter consists of positively charged ions and electrons in apparently random motion. The name plasma was given to this state by the American chemist Irving Langmuir in 1920.
The study of plasma-like materials actually goes as far back as the 1830s when the English physicist, Michael Faraday, passed electrical discharges through gases at low pressures. Faraday's research was extended and expanded by William Crookes in the 1870s. Crookes was apparently the first scientist to suggest that the ionized gas within his glass tubes might be a fourth state of matter.
Plasma research during the 1920s and 1930s was largely a matter of interest to astronomers. It was apparent that, at temperatures present in stars, matter almost certainly exists as plasma. Understanding the composition and properties of stars, therefore, required some understanding of the nature of plasma.
Some of the earliest breakthroughs in plasma research were accomplished by Hannes Olof Göst Alfvén, a Swedish astrophysicist. Alfvén developed a theory to explain the behavior of plasma in the presence of magnetic fields. His work forms the basis of the modern science known as magnetohydrodynamics (MHD). For this research, Alfvén was awarded a share of the 1970 Nobel Prize for physics.
Research on nuclear fusion in the 1940s shifted the focus of plasma research from the stars to laboratories on Earth. Nuclear fusion reactions--the combination of two small nuclei to produce one larger nucleus--occur only at very high temperatures, greater than 10 million°C. At such temperatures, similar to those present in the core of a star, matter exists as a plasma.
Scientists attempting to find ways to use fusion reactions as a practical source of energy discovered that trapping plasma inside magnetic fields was the best way to control such reactions. Finding the most practical method of achieving such controlled reactions, however, is an enormously difficult task, one that has still not been perfected after a half century of research.
One of the earliest suggestions for solving the problem of plasma confinement was offered by the Russian physicist Igor Evgenevich Tamm in 1950. Tamm outlined a model by which the magnetic field surrounds the plasma and "pinches" it together. Another approach was proposed by the American astrophysicist, Lyman Spitzer, Jr. Spitzer's interest in controlled fusion grew out of his earlier research on fusion reactions in the stars. His model calls for a twisting magnetic field to be wrapped around the hot plasma in an arrangement that came to be known as a stellerator.
The most common mechanism for controlling plasma reactions today is called a tokamak, originally designed by the Russian physicist Lee Artsimovich in the late 1950s. A tokamak consists of a toroidal (hollow, doughnut-shaped) tube in which the hot plasma is contained by a strong magnetic field.
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