Local effects of gas fuelling and their impact on transport processes in the plasma edge of the tokamak TEXTOR

Deuterium fuelling through a carbon test limiter has been applied to maximize the plasma density in the Radiative Improved Mode. The impact of the fuelling on the local plasma edge properties has been investigated, by analyzing the spectral emission of both deuterium atoms and molecules, which indicates the creation of a cold and dense plasma cloud with a local electron density at the last closed flux surface up to 8 × 10¹⁹ m^-3, about 4 times higher than the electron density far away from the puffing location at the same plasma radius. The local electron temperature decreases to less than 10 eV. The experimental data can be reproduced by a model for the development of the cold plasma cloud and the critical fuelling rate to initiate the process based on the heat balance in the cloud. The correlation of the resulting local perturbation with the global confinement properties is discussed.