Investigation of the hydrogen fluxes in the plasma edge of W7-AS during H-mode discharges

In the stellarator W7-AS the H-mode is characterized by an edge transport b arrier which is localized within a few centimeters inside the separatrix. T he corresponding L-H transition shows well-known features such as the steep ening of the temperature and density profiles in the region of the separatr ix, With a so-called sniffer probe the temporal development of the hydrogen and deuterium fluxes has been studied in the plasma edge during different I-I-mode discharges with deuterium gas puffing. Prior to the transition a s ignificant reduction of the deuterium and also the hydrogen fluxes can be o bserved. This fact confirms the assumption that the steepening of the densi ty profiles starts at the outermost edge of the plasma. Moreover, sniffer p robe measurements in the plasma edge could therefore identify a precursor f or the L-H transition. The analysis of the hydrogen neutral gases shows a d istinct change of the hydrogen isotope ratio during the transition. This ob servation is in agreement with the change in the particle fluxes onto the t argets and can also be seen in the reduced H-alpha signals from the limiter s. It is further demonstrated that significant improvement in the time reso lution of the measured data can be obtained by deconvolution of the data wi th the apparatus function using Bayesian probability theory and the Maximum Entropy method with adaptive kernels. (C) 2001 Elsevier Science B.V. All r ights reserved.