The transition of a gas into the plasma state involves various processes of interparticle interaction. These processes involve collisions of particles between themselves or interactions of particles with radiation. First we should mention the process called ionization, i.e., the stripping of electrons from atoms or molecules. Without ionization it would be impossible to obtain a plasma. Ionization occurs in two ways: in a dense plasma, by electron collisions, and in a highly rarefied plasma, by the action of radiation (light, ultraviolet, or x-rays). In principle, ionization can also be accomplished by atom— atom or atom— ion collisions, but considerably higher energies are necessary.