title: Diagnosing ions and neutrals via n=2 excited hydrogen atoms in plasmas with high electron density and low electron temperature.
abstract: Ion and neutral parameters are determined in the high electron density, magnetized, hydrogen plasma beam of an ITER divertor relevant plasma via measurements of the n=2 excited neutrals. Ion rotation velocity (up to 7 km/s) and temperature (2-3 eV~T_{e}) are obtained from analysis of Hα spectra measured close to the plasma source. The methodology for neutral density determination is explained whereby measurements in the linear plasma beam of Pilot-PSI are compared to modeling. Ground-state atomic densities are obtained via the production rate of n=2 and the optical thickness of the Lyman-α transition (escape factor ~0.6) and yield an ionization degree >85% and dissociation degree in the residual gas of ~4%. A 30% proportion of molecules with a rovibrational excitation of more than 2 eV is deduced from the production rate of n=2 atoms. This proportion increases by more than a factor of 4 for a doubling of the electron density in the transition to ITER divertor relevant electron densities, probably because of a large increase in the production and confinement of ground-state neutrals. Measurements are made using laser-induced fluorescence (LIF) and absorption, the suitability of which are evaluated as diagnostics for this plasma regime. Absorption is found to have a much better sensitivity than LIF, mainly owing to competition with background emission.
authors.affiliation: FOM-Institute for Plasma Physics Rijnhuizen, Association EURATOM-FOM, Trilateral Euregio Cluster, Nieuwegein, The Netherlands.
authors.forename: A E
authors.initials: AE
authors.lastname: Shumack
authors.forename: D C
authors.initials: DC
authors.lastname: Schram
authors.forename: J
authors.initials: J
authors.lastname: Biesheuvel
authors.forename: W J
authors.initials: WJ
authors.lastname: Goedheer
authors.forename: G J
authors.initials: GJ
authors.lastname: van Rooij