[en] Molecular hydrogen isotopes in the divertor region of fusion experiments may affect the divertor plasma via dissociation, ionization and recombination processes . These processes depend on the vibrational population of the ground state of the molecules. The vibrational populations of D₂ and H₂ were investigated simultaneously at similar divertor plasma conditions, i.e., degree of detachment. The spectroscopic method used, yields vibrational temperatures of 4000-6000 K in the outer divertor, depending on plasma parameters. The isotopes show tendencies, known from laboratory experiments , especially similar vibrational ground state populations up to v=4. As previously shown for pure hydrogen discharges , the interpretation of experimental data and plasma edge simulations (B2-EIRENE) needs a collisional-radiative (CR) model for H₂. With respect to isotope investigations the measurements and parameter studies show that the transfer of the CR model to D₂ needs an implementation of complete data sets, i.e., rate coefficients, or at least reliable scaling of the input data given

[en] In order to reduce the power load onto the target plates detached divertor conditions are often preferred. These are characterized by volume recombination, i.e. three-body and radiative recombination. Due to low T_e (few eV) hydrogen molecules can penetrate into the plasma and may play a role in divertor dynamics. In particular, it was suggested, that molecules may assist the volume recombination process. The role of molecules in the divertor is examined here by a combination of experimental results with plasma edge simulations (B2-EIRENE) and a collisional-radiative model for hydrogen molecules. Spectroscopic diagnostics of the Fulcher transition carried out at the divertor of ASDEX Upgrade yield estimates of molecular hydrogen fluxes and the vibrational population in the ground state in detached and attached hydrogen plasmas. Good agreement with B2-EIRENE is achieved only if vibrational levels are treated as distinct (metastable) particles in the model and if the collisional-radiative model is applied to the electronically excited levels. On this basis the contribution of molecules to plasma recombination was determined to be in the order of a few 10%. The dominant molecular process is the dissociation process via H₂⁺. As a consequence initially detached divertor plasmas can even re-attach if vibrationally resolved molecules are properly included in plasma edge models. A set of B2-EIRENE calculations carried out for ASDEX Upgrade is discussed. In particular the threshold upstream density for detachment was found to be up to a factor 1.5 higher than that originally expected due to these molecular effects. The transferability of the results to deuterium will be discussed

[en] The determination of molecular hydrogen fluxes in the plasma edge of fusion experiments from molecular radiation requires the precise knowledge of photon efficiencies (S+D)/XB, i.e. the ionization and dissociation processes per emitted photon. Photon efficiencies are calculated by a collisional radiative model for molecular hydrogen which gives dependencies on electron temperature, on electron density and on the vibrational population of the molecule in its ground state. To prove the model, photon efficiencies are measured by gas puffing experiments in the divertor of ASDEX Upgrade at high electron densities (n_e∼10¹⁹ m^-3) and low electron temperatures (T_e∼10 eV). Furthermore, results from laboratory plasmas (n_e∼10¹⁷ m^-3, T_e∼3 eV) are also used for an experimental check of the model. The comparison allows an identification of relevant processes such as dissociative attachment from excited states of hydrogen, thus improving molecular hydrogen diagnostics

[en] Hydrogen and hydrocarbons are some of the most dominant molecular species in edge plasmas of fusion devices. The recombination of hydrogen particles on surfaces leads to the formation of hydrogen molecules, whereas hydrocarbons are released from carbon (graphite) surfaces due to the chemical erosion process. The molecules penetrate into the plasma and can undergo a variety of reactions: vibrational excitation, dissociation, ionization, recombination, charge exchange - to name some of them. An insight in the role of molecules in edge plasma can be obtained by a combination of diagnostics and modelling. The wide parameter range (n_e, T_e) in which edge plasmas can operate requires a description of plasma processes by effective rate coefficients. A summary of the activities at the University of Augsburg is given in the following with emphasis on the progress made since the last meeting in May 2003

[en] A diagnostic method has been evaluated for measuring the relative vibrational ground-state population of molecular hydrogen and deuterium. It is based on the analysis of the diagonal Fulcher bands ·³Π_u→a³Σ_g⁺) and the Franck-Condon principle of excitation. The validity of the underlying assumptions was verified by experiments in microwave discharges and the method is recommended for application in divertor plasmas in controlled fusion experiments. By attributing a vibrational temperature T_vib to the ground-state electronic level (X¹Σ_g⁺) and assuming population via the Franck-Condon principle, the upper Fulcher state vibrational distribution can be derived theoretically with T_vib as parameter. Comparison with experimentally derived upper-state population gives the corresponding T_vib of the ground state. The Franck-Condon factors for the ·³Π<-X¹Σ_g⁺ and ·³Π_u→a³Σ_g⁺ transitions have been calculated for both hydrogen and deuterium from molecular constants using the FCFRKR code. The method has been applied to low pressure H₂/He and D₂/He microwave plasmas, showing good agreement of experimentally and theoretically derived upper Fulcher state vibrational distributions. The vibrational temperatures range from 3200 K to 6800 K for H₂ and 2600 K to 4000 K for D₂· depending on molecular density, pressure and electron temperature, but indicating nearly the same vibrational population for H₂ and D₂ for comparable plasma conditions. (author)

[en] The radiation of hydrogen molecules in the divertor of fusion experiments is proposed to be a sensitive diagnostic method for electron temperature (T_e) in the range between 1 and 7 eV. A collisional-radiative (CR) model for H₂ was used for the prediction of vibrational population in the ground state of the molecule, depending on electron temperature. In addition, the model was modified in order to enable diagnostics for deuterium. Applying the Franck-Condon principle and vibrationally resolved excitation rate coefficients the population was transferred into the upper state of the Fulcher transition (d³Π_u-a³Σ_g⁺). On the other hand, the relative population in the excited state can be derived directly from the emission spectra of the diagonal Fulcher bands. Results of hydrogen discharges in ASDEX Upgrade will be presented, which show that the strong variation in T_e during the discharge correlated with the degree of plasma detachment. The transferability for deuterium will be demonstrated by discussing results of deuterium discharges including hydrogen injections. Due to the spectroscopic technique, the derived T_e represents the volume where the radiation of the molecules originates. (author)