The working gas used to create a magnetically confined plasma for fusion purposes typically consists of light atomic species, such as Hydrogen (H) or Helium (He). Any contaminating, higher atomic number (Z) species entering the plasma are considered 'impurities'.
Impurities tend to have a deleterious effect on heat confinement, as the high-Z atoms have a large number of bound electrons prior to entering the plasma, and upon entering the plasma the ionization process removes energy from the plasma, while the subsequent recombination process radiates away energy in the form of electromagnetic radiation. Thus, the overall effect of impurities is to cool the plasma, reducing the fusion yield. Furthermore, the high-Z species, which do not participate in the fusion reactions, rarify the density of the fusion fuel.
- Divertors are intended, among others, to reduce impurity influx from the wall