Red glow of hydrogen plasma inside EAST tokamak. Hydrogen atoms lose energy by emitting at a wavelength of 656 nm, which corresponds to red light. (Source: Wikipedia.org)

Energy that is emitted from a plasma in the form of electromagnetic radiation. When the plasma is heated, the remaining electrons in the ion shells are excited. When the electrons return to the ground state, they release their energy by emitting a photon. This mechanism makes the hot plasma glow. Ions with a higher atomic number, which are mostly unwanted impurities, have more electrons in the electron envelope and therefore convert more of the supplied energy into electromagnetic radiation. Their excessive concentration can lead to strong radiation losses and plasma cooling.