Thermonuclear fusion is the process that occurs when two atoms combine to make a larger atom, creating a whole lot of energy. Fusion already happens naturally in stars — including the sun — when intense pressure and heat fuse hydrogen atoms together, generating helium and energy.

Thermionic emission: thermally induced flow of charge
Nuclear fission: nuclear reaction or a radioactive decay process
Photoelectric emission: emission of electrons or other free carriers when light shines on a material