skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Surface-plasma generation of negative ions in gas discharges without cesium

Abstract

Surface-plasma generation (SPG) of negative ions in gas discharges is extremely sensitive to small admixture of uncontrolled impurities. An efficiency of SPG was greatly improved by injection into discharge a small amount of cesium or other substances with low ionization potential. For explanation of anomalistic H{sup -} generation in discharges without the addition of cesium it is important to identify the sources of important impurities. Generation of H{sup -} ion in different ion sources is analyzed. Sources of impurities catalyzing SPG are proposed. A higher probability of cold H{sup -} extraction is outlined and explained. A proposal for enhanced H{sup -} generation in discharges without cesium is presented. Further experiments for identification of the dominant mechanism of H{sup -} generation are discussed.

Authors:
;  [1]
  1. Brookhaven Technology Group, Inc., Setauket, New York 11733 (United States)
Publication Date:
OSTI Identifier:
20779019
Resource Type:
Journal Article
Resource Relation:
Journal Name: Review of Scientific Instruments; Journal Volume: 77; Journal Issue: 3; Conference: 11. international conference on ion sources, Caen (France), 12-16 Sep 2005; Other Information: DOI: 10.1063/1.2176689; (c) 2006 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS; BEAM EXTRACTION; CESIUM; ELECTRIC DISCHARGES; HYDROGEN; HYDROGEN IONS 1 MINUS; ION SOURCES; IONIZATION POTENTIAL; PLASMA; PLASMA IMPURITIES; PROBABILITY; SURFACES

Citation Formats

Dudnikov, Vadim, and Farrell, J. Paul. Surface-plasma generation of negative ions in gas discharges without cesium. United States: N. p., 2006. Web. doi:10.1063/1.2176689.
Dudnikov, Vadim, & Farrell, J. Paul. Surface-plasma generation of negative ions in gas discharges without cesium. United States. doi:10.1063/1.2176689.
Dudnikov, Vadim, and Farrell, J. Paul. Wed . "Surface-plasma generation of negative ions in gas discharges without cesium". United States. doi:10.1063/1.2176689.
@article{osti_20779019,
title = {Surface-plasma generation of negative ions in gas discharges without cesium},
author = {Dudnikov, Vadim and Farrell, J. Paul},
abstractNote = {Surface-plasma generation (SPG) of negative ions in gas discharges is extremely sensitive to small admixture of uncontrolled impurities. An efficiency of SPG was greatly improved by injection into discharge a small amount of cesium or other substances with low ionization potential. For explanation of anomalistic H{sup -} generation in discharges without the addition of cesium it is important to identify the sources of important impurities. Generation of H{sup -} ion in different ion sources is analyzed. Sources of impurities catalyzing SPG are proposed. A higher probability of cold H{sup -} extraction is outlined and explained. A proposal for enhanced H{sup -} generation in discharges without cesium is presented. Further experiments for identification of the dominant mechanism of H{sup -} generation are discussed.},
doi = {10.1063/1.2176689},
journal = {Review of Scientific Instruments},
number = 3,
volume = 77,
place = {United States},
year = {Wed Mar 15 00:00:00 EST 2006},
month = {Wed Mar 15 00:00:00 EST 2006}
}
  • The relative contribution of volume and surface-plasma generation in emission of H- ions in gas discharge sources is analyzed. At the present time, it is generally accepted that surface-plasma generation of extracted H- ions dominates over volume processes in discharges with admixture of cesium or other catalysts with low ionization potential. We will attract attention to the evidence, that surface-plasma generation can be significantly enhanced in high density discharges without cesium after electrode activation by high temperature conditioning in discharge. With this optimization of conditions for surface-plasma generation of emitted H-, an emission current density was increased up to {approx}1more » A/m2 in discharges without cesium. Diffusion of impurities with low ionization potential can be the reason for the observed enhancement of H- emission. Such optimization allows considerable improvement of H-/D- source characteristics. Volume generation of extracted H- in high density discharges is suppressed by the high level of gas dissociation.« less
  • The results of theoretical and experimental studies of physical processes in the source of hydrogen negative ions are presented. The source is based on reflective discharge with incandescent cathode and H{sup -} ions extraction across magnetic field. Calculations of gas discharge plasma parameters for given current and energy of electrons emitted from the cathode are performed in theoretical part of the proceeding. Plasma parameters, including H{sup -} ions concentration, are determined on a basis of Boltzman equation solution taking into consideration principal collision reactions in a volume and at a surface of the discharge chamber.
  • The presence of high Rydberg states in a negative-ion discharge will increase the concentration of negative ions on the order of order of one per cent compared to those generated via dissociative attachment to ground-state vibrationally-excited molecules. {copyright} {ital 1996 American Institute of Physics.}
  • Detailed measurements of gas fluxes in the pressure of plasma discharge, to determine the gas efficiency of surface-plasma negative hydrogen ion sources, have been carried out with a differential ionization gauge having an improved signal-noise ratio.
  • An optimized tandem two-chamber negative-ion source system is discussed. In the first chamber high energy (E>20 eV) electron collisions provided for H/sub 2/ vibrational excitation, while in the second chamber negative ions are formed by dissociative attachment. The gas density, electron density, and system scale length are varied as independent parameters. The extracted negative ion current density passes through a maximum as electron and gas densities are varied. This maximum scales inversely with system scale length, R. The optimum extracted current densities occur for electron densities near nR = 10/sup 13/ electrons cm/sup -2/ and for gas densities, N/sub 2/R,more » in the range 10/sup 14/ to 10/sup 15/ molecules cm/sup -2/. The extracted current densities are sensitive to the atomic concentration in the discharge. The atomic concentration is parametrized by the wall recombination coefficient, ..gamma.., and scale length, R. As ..gamma.. ranges from 0.1 to 1.0 and for system scale length of one centimeter, extracted current densities range from 8.0 to 80. mA cm/sup -2/.« less