AVS 53rd International Symposium
    Plasma Science and Technology Friday Sessions
       Session PS2-FrM

Paper PS2-FrM11
Energy Distribution and Flux of Fast Neutrals and Residual Ions Extracted out of High Aspect-Ratio Holes in a Neutral Beam Source

Friday, November 17, 2006, 11:20 am, Room 2011

Session: Diagnostics
Presenter: A. Ranjan, University of Houston
Authors: A. Ranjan, University of Houston
V.M. Donnelly, University of Houston
D.J. Economou, University of Houston
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The energy distribution and flux of fast neutrals and residual ions extracted from a neutral beam source were measured. Positive ions generated in an inductively coupled argon plasma were extracted through a metal grid with high aspect ratio holes. Ions suffering grazing angle collisions with the inside surface of the grid holes turned into fast neutrals. The neutral energy distribution shifted to lower energies compared to the corresponding residual ion energy distribution. The neutralization efficiency increased with power, decreased with the imposed plasma potential (controlled with a boundary voltage) and, for thin neutralization grids, was almost independent of plasma gas pressure. The residual ion flux decreased with increasing hole diameter and hole aspect ratio. The fast neutral flux first increased and then dropped as the hole diameter was increased. These results were explained based on plasma molding inside the grid holes. The effect of surface roughness of the grid walls on the energy distribution and flux of fast neutrals and residual ions was also studied. A nearly atomically smooth grid was fabricated from a closely-spaced stack of polished Si (100) wafer strips. With this grid, a small fraction of fast neutrals was observed at energies nearly equal to the maximum ion energy. For the metal grids, with rougher surfaces, the highest energy neutrals were well below the maximum ion energy. These observations will be discussed in terms of the type of scattering (specular vs. non-specular) that occurs when ions are converted into fast neutrals.