Role of impurities in current tokamak experiments

The authors discuss the possible consequences of the presence of multiply charged impurities in a tokamak plasma. The first such consequence is cooling of the plasma column periphery due to line radiation, which may lead to a sharpening of the electrical conductivity profile σ(r) and hence of the current density profile j(r), as is observed at high values of ne. Concentration of highly ionized impurities towards the plasma column axis is a possible competing process, leading to flattening of σ(r) and j(r) or even to a skin effect. From the results of soft (3–10 keV) X-radiation measurements the authors derive conclusions about the behaviour of impurities at the centre of the plasma. They find that the concentration of impurities can be interrupted by the development of plasma instabilities – especially kink and "disruptive" instabilities. With increasing ne, Te and discharge duration, the role of impurities in the shaping of j(r) necessarily increases. At some level, this may cause an additional deterioration in the thermal insulation and stability of the plasma column.