archers found that confining fusion plasma in a "magnetic trap" was far more challenging than they had anticipated. Plasma must be heated to tens of millions of degrees Kelvin or higher to induce and sustain the thermonuclear reaction required to produce usable amounts of energy. At temperatures this high, the nuclei in the plasma move rapidly enough to overcome their mutual repulsion and fuse. It is exceedingly difficult to contain plasmas at such a temperature level because the hot gases tend to expand and escape from the enclosing structure. The work of the major American, British, and Soviet fusion programs was strictly classified until 1958. That year, research objectives were made public, and many of the topics being studied were found to be similar, as were the problems encountered. Since that time, investigators have continued to study and measure fusion reactions between the lighter elements and have arrived at more accurate determinations of reaction rates. Also, the formulas developed by nuclear physicists for predicting the rate of fusion-energy generation have been adopted by astrophysicists to derive new information about the structure of the stellar interior and about the evolution of stars. The late 1960s witnessed a major advance in efforts to harness fusion reactions for practical energy production: the Soviets announced the achievement of high plasma temperature (about 3,000,000 K), along with other physical parameters, in a tokamak, a toroidal magnetic confinement system in which the plasma is kept generally stable both by an externally generated, doughnut-shaped magnetic field and by electric currents flowing within the plasma itself. (The basic concept of the tokamak had been first proposed by Andrey D. Sakharov and Igor Y. Tamm around 1950.) Since its development, the tokamak has been the focus of most research, though other approaches have been pursued as well. Employing the tokamak concept, physicists have attai...
