Plasma may be defined as any statistical system containing mobile charged particles. Work on plasmas was confined to a comparatively few individuals and laboratories until the early 1950s, when intensive work was begun in the United States, the United Kingdom, and the Soviet Union on the realization of controlled release of nuclear fusion energy. The Coulomb force with which the charged particles interact is well known to be a long-range force. As a consequence, the physical properties of plasma exhibit remarkable differences from those of an ordinary neutral gas. The potential field around a charged particle is effectively screened by the cloud of other charged particles; its force range is now confined within a certain characteristic length determined by the density and temperature of the plasma. In the plasma the spatial distribution of charged particles is affected by the presence of such a potential field and deviates from a uniform distribution.