Wall atoms ejected from the wall of the plasma vessel by plasma particles can enter the plasma and contaminate it.
Unlike light hydrogen atoms, however, the heavy atoms of the elements iron, nickel, chromium, oxygen, and the like, are not completely ionised even at the high fusion temperatures. The higher the atomic number of these impurities, the more electrons are still bound to the atom rumps; the more strongly they remove energy from the plasma and re-emit it as ultraviolet radiation or X-radiation. In this way they cool the plasma, rarefy it, and thus reduce the fusion yield.
In order to protect the vessel from particles out of the plasma and, conversely, the plasma from impurities out of the wall, a special magnetic field directs the plasma boundary layer to specially equipped areas of the vessel wall, viz. the divertor plates. It is thus possible to remove the disturbing impurities from the plasma. At the same time, the vessel wall is spared and good thermal insulation of the plasma achieved.