The need for long-term energy sources, in particular for our highly technological society, has become increasingly apparent during the last decade. One of these sources, of tremendous poten­ tial importance, is controlled thermonuclear fusion. The goal of controlled thermonuclear fusion research is to produce a high-temperature, completely ionized plasma in which the nuclei of two hydrogen isotopes, deuterium and tritium, undergo enough fusion reactions so that the nuclear energy released by these fusion reactions can be transformed into heat and electricity with an overall gain in energy. This requires average kinetic energies for the nuclei of the order of 10 keV, corresponding to temperatures of about 100 million degrees. Moreover, the plasma must remain confined for a certain time interval, during which sufficient energy must be produced to heat the plasma, overcome the energy losses and supply heat to the power station. At present, two main approaches are being investigated to achieve these objectives: magnetic confinement and inertial con­ finement. In magnetic confinement research, a low-density plasma is heated by electric currents, assisted by additional heating methods such as radio-frequency heating or neutral beam injection, and the confinement is achieved by using various magnetic field configurations. Examples of these are the plasmas produced in stellarator and tokamak devices.