Mechanism for negative-ion production in the surface-plasma negative-hydrogen-ion source

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  • J. R. Hiskes
    Lawrence Livermore Laboratory, University of California, Livermore, California 94550
  • A. Karo
    Lawrence Livermore Laboratory, University of California, Livermore, California 94550
  • M. Gardner
    Lawrence Livermore Laboratory, University of California, Livermore, California 94550

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<jats:p>The system parameters and surface adsorption conditions of the surface-plasma negative-hydrogen-ion source are reviewed. A mechanism is developed for the production of negative ions by incident energetic hydrogen atoms backscattering from a cesiated tungsten surface. The active electronic level during the course of the collision is approximated as the sum of the negative electron affinity of the cesium hydride negative molecular ion and the image potential. The model predicts negative-ion formation for atomic collisions with all alkali-coated surfaces for alkalis from sodium through cesium. In the case of lithium, the model does not apply due to the breakdown of the image potential approximation close to the surface. Negative-ion formation is also expected to occur for incident positive ions which backscatter from the substrate tungsten as neutrals. The energy thresholds for incident atoms are established.</jats:p>

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