In situ measurement and modeling of hydrogen recycling and transport processes -: the role of molecules

In TEXTOR-94 experiments have been performed which try to elucidate the nature and origin of the hydrogen released from plasma facing graphite surfaces. Density profiles and velocity distributions have been measured by means of passive and active spectroscopy on atomic and molecular line emissions for different surface temperatures and plasma conditions. It was found that until 1100 K hydrogen is predominantly released as PII. Above 1100 K a reduction of the Hz-flux (50% at 1370 K) was determined. No flux density dependence in comparison to ion beam experiments were found. The photon rates for hydrogen atoms are larger than for the two atoms in the molecules. In the presence of a large fraction of molecules this may lead to an underestimation of the total hydrogen influx from Balmer line measurements. Comparisons with EIRENE-code calculations and molecular spectroscopic data show that the molecules are highly vibrationally excited, which leads to low energetic atomic dissociation products. The origin or these kind of molecules is probably not in the plasma but already at the graphite surface. (C) 1999 Elsevier Science B.V. All rights reserved.