2.g. ionization, recombination and radiation barriers

Because of the intense line radiation from low ionization stages of a given element, it is important that ionization is rapid and effective even at relatively low electron densities, i.e., occurs relatively early in the discharge and reasonably close to the boundary. These requirements lead for given electron impact ionization rates to upper bounds on impurity fluxes from the wall. Assuming these bounds will not be exceeded in future devices, it is then possible to estimate permissible impurity concentrations from stationary solutions of the ionization-recombination balance and impurity transport equations. In the interior of the plasma, the concentration should be small enough that the radiative power density remains at a fraction of the deposited α-particle power. Enhanced recombination, e.g., by charge transfer with injected neutrals, may lead to more stringent conditions for plasma purity