The interpretation of plasma edge conditions in tokamaks

The modelling used in the boundary layer depends on a number of factors including whether energy loss is dominated by convection through the sheath, by conduction, or by radiation from impurities. A number of criteria are derived for assessing these factors and are then compared with the experimental data from a range of tokamaks. The comparison is limited to tokamaks operating with limiters, although the criteria also apply to tokamaks with divertors. In most tokamaks, under normal operating conditions, the dominant impurity process is sputtering. Impurities from the wall and limiter enter as neutral atoms and undergo ionization at radial positions which depend on the boundary plasma density and temperature profiles. When impurity densities are sufficiently high they can reduce the edge temperature by increasing the radiation level resulting in a feedback effect on impurity production. The impurity source profiles and impurity transport in the boundary are discussed.