Photon efficiency (S+D)/XB of hydrogen molecules at low electron temperatures

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 (ne≈1019 m−3) and low electron temperatures (Te≈10 eV). Furthermore, results from laboratory plasmas (ne≈1017 m−3, Te≈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.