Observation and modelling of hydrogen atomic and molecular ions on divertor simulator

In a divertor plasma of fusion reactors, vibrationally excited hydrogen molecules H2(v) persist in dissociation and ionization processes of the plasma volume. However, atomic and molecular processes with H2(v) molecules have not been reported clearly for high-density plasma. We have carried out the experimental observation and modelling of atomic and molecular ions in hydrogen high-density plasma on a divertor simulator,TPD-SheetIV. The hydrogen plasma was generated at a discharge current of 30-100 A. Electron density and electron temperature were measured using a planar Langmuir probe, which were located 3 cm in front of the endplate. An omegatron mass analyzer, situated behind a small hole in the endplate with a differential pumping system, is used for analyzing ion species. The relative densities of the molecular and atomic ions were determined from the collector current of the mass analyzer. To model the ion density in this experiment, a simple zero dimensional model is developed for solving the system of rate balance equations for ion and gas species. In the reactions involving H2, H2+, a ground-state vibrational temperature of hydrogen molecule is Tvib=3000 K in the model. The ion density ratio of H+ is larger than that of H2+ or H3+ in the low gas pressure (<6 mtorr) at the discharge current Id of 50 A in the hydrogen plasma. On the other hand, the ion density ratio of H3+ rapidly increases from 0.1 to 0.5 with increasing gas pressure P and saturated up to P=10 mtorr. From a zerodimensional model using the relevant rate balance equations, the calculated molecular ion densities of H2+, and H3+ was found to predict the observed dominant ion density ratio, demonstrating the importance of vibrational temperature of hydrogen molecule. (orig.)