Interplay of light and heavy impurities in a fusion device

Breathing' activity observed in the Large Helical Device stellarator is characterized by macro-scale oscillations of diverse plasma parameters such as the radiation losses from heavy (iron) and light (carbon and oxygen) impurities, electron density, temperature and the power absorbed in the plasma from neutral beam. They provide an example of a complex behavior in fusion plasmas triggered by the synergy effects from impurities of different species. A one-dimensional non-stationary model, describing the transport across flux surfaces in the plasma of the main and impurity neutral and charged particles, as well as of the thermal energy with the heat absorption from the neutral beam, radiation of high-Z ions from the plasma core and of low-Z impurities from the edge, is elaborated. The model is numerically realized, by applying the finite volume and 'progonga' methods to integrate the system of non-linearly coupled transport equations. The results of simulations presented reproduce qualitatively and essentially quantitatively the observations. The model allows prediction of the plasma and impurity environment conditions under which one has to expect 'breathing' oscillations