Impurity-induced desorption mechanism for releasing light impurities in Tokapole II

The release mechanism for light impurities in Tokapole II, a tokamak with a four-node poloidal divertor, has been characterized through impurity gas puffing experiments. The desorption process responsible for the light impurity concentrations during the current-rise phase of the discharge depends on the mass of the particles in the discharge. During switch-over from H2 to D2 filling gas the oxygen concentration increases as the deuterium-to-hydrogen ratio increases. Puffing with small but increasing amounts of any gas studied (H2, D2, He, N2, O2, Ne, Ar, Kr, Xe) usually produces linear increases in the early-time radiation of O, C, and N. These impurity increases are proportional to the mass of the injected impurity as might be expected for a momentum transfer process. O and N signals increase faster than linearly with concentration of O2 and N2, respectively, since both dopant-induced desorption and direct population of the plasma and wall occur. A model for early-time desorption is discussed. (orig.)