Thermonuclear fusion of light nuclei is the process by which the stars produce energy. In the sun a complex series of fusion reactions results in the fusion of four hydrogen nuclei to helium. Thereby, the mass difference is converted into energy. The necessary reaction conditions are maintained by the balance between the gravitational pressure and the inner gas or plasma pressure. At the end of the star development, when the fuel is exhausted, depending on the mass of the star stands a white dwarf, a neutron star or a black hole. On earth there are basically two possibilities to generate thermonuclear fusion with a positive energy balance. Inertial fusion is based on the ignition of a small fuel pellet. For a short moment the plasma pressure is balanced by the inertia of the fuel mass itself. To avoid hydrodynamic instabilities the energy has to be supplied very symmetrically. Besides, the pellet has to have a minimum size to fulfill the reaction requirements. But it must not become too large because the produced energy has to remain controllable. Magnetic fusion is based on the stationary confinement of a fusion plasma by strong magnetic fields. In contrast to inertial fusion which produces conditions similar to those in the core of the sun, magnetic fusion requires significantly higher temperatures of about 100 Mio Kelvin. However, the plasma pressure lies only in the range of a few atmospheres. Leading experiments, aiming at producing burning fusion plasmas for the first time, are the National Ignition Facility (NIF) in the US and the ITER experiment in the south of France.
The lecture introduces the basics of thermonuclear fusion. Starting from the description of the fusion processes in the stars and their life cycle to the point of supernovae, which are made responsible for the synthetisation of heavy elements, the possibilities to employ fusion as an energy source are discussed. Many physical processes, as they can be observed in these laboratory plasmas, are also important for understanding of the phenomena in the stars.