Study of the relevance of thermal instability caused by impurity radiation to MARFE development in a limiter tokamak

Observations from tokamak TEXTOR indicate that multi-facetted asymmetric radiation from the edge (MARFE) is not preceded by thermal radiative instability due to impurity radiation as it is often assumed. The relevance of this instability under tokamak conditions is re-examined by taking into account the transport of individual charge states of impurity by considering the response of impurity radiation on temperature perturbations. It is demonstrated that the increase in radiation losses induced by a reduction of the temperature is much weaker than in the case when this effect is neglected. An instability triggered by the plasma convection to the inner wall is considered as an alternative mechanism for MARFE formation. (orig.)