Particle Interchange Reactions Involving Plasma Impurity Ions and H2, D2, and HD

Abstract

The edge plasma in a tokamak nuclear fusion reactor is characterized by low plasma temperatures and high plasma densities. In the divertor region, the plasma temperature may be as low as a few electron volts, and plasma densities may be as high as 1017 cm-3.1 An important consequence of the low edge plasma temperature is that molecular species are present in this region, resulting either from plasma-wall interactions (e.g., hydrocarbons) or from recycling and plasma fueling (molecular hydrogen and its isotopes). In addition to the primary constituents of the plasma, for example D, T, and He (in D-T plasmas) and H, D, and He in the present tokamaks, a relatively large variety of atomic impurities (at concentrations between 0.1 and 10%) are present. The most common impurity-generating processes are particle-surface interaction processes, mainly desorption, physical sputtering, and evaporation. The main impurities in the plasma edge of most of the present-generation tokamaks and fusion reactor designs are carbon (≤10%), oxygen (≤5%), various metallic (and related) impurities originating from structural materials (Ti, V, Cr, Fe, Ni, Cu, Al, Mo, Mn, Mg, B, Si, Ge, Nb, Ag; with a concentration of ≤2%), and various diagnostic species (Li, Ne, Ar). Under these plasma conditions, a wide range of atomic and molecular processes that are not important in the core plasma become relevant.