Diagnosing ions and neutrals via n = 2 excited hydrogen atoms in plasmas with high electron density and low electron temperature

A.E. Shumack, D.C. Schram, J. Biesheuvel, W.J. Goedheer, G.J. Rooij, van

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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~Te) are obtained from analysis of Ha 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-a 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.
Original languageEnglish
Article number036402
Pages (from-to)036402-12
Number of pages12
JournalPhysical Review E - Statistical, Nonlinear, and Soft Matter Physics
Volume83
Issue number3
DOIs
Publication statusPublished - 2011

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