Numerical modelling of MARFE phenomena near the bumper limiter of TEXTOR-94

It has been observed on TEXTOR-94 that the spontaneous development of MARFEs, poloidally localized intensively radiating regions of cold dense plasma, is caused by plasma cooling due to impurity radiation and due to the hydrogen recycling at the inner bumper limiter of TEXTOR tokamak. In this paper we present results of numerical modelling performed in the specific geometry of TEXTOR-94 in order to study the relative importance of different physical mechanisms in the MARFE development. Calculations were done by using several 2D transport codes (B2-EIRENE and TECXY) to judge the significance of particular processes. The present extensive calculations confirm the idea that a MARFE development can be triggered solely by localized recycling, when the last closed magnetic flux surface is close enough to the bumper limiter and certain critical conditions are fulfilled. (orig.)