Toward a realistic and tractable model for negative ion extraction from volume sources

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A new negative ion source extraction model has been formulated and implemented that explicitly considers the motion of positive ions and the volume generation of negative ions. It is found that (1) for high-beam currents, the beam current is limited by a transverse space charge limit, meaning that an increase in negative ion density at the extraction sheath will result in a lower beam current; (2) there is a saddle point with a potential barrier preventing most volume produced negative ions from being extracted; (3) introduction of cesium may cause an increase in the transverse space charge limit; (4) cesium ... continued below

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21 p.

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Whealton, J.H.; Olsen, D.K. & Raridon, R.J. December 1, 1997.

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A new negative ion source extraction model has been formulated and implemented that explicitly considers the motion of positive ions and the volume generation of negative ions. It is found that (1) for high-beam currents, the beam current is limited by a transverse space charge limit, meaning that an increase in negative ion density at the extraction sheath will result in a lower beam current; (2) there is a saddle point with a potential barrier preventing most volume produced negative ions from being extracted; (3) introduction of cesium may cause an increase in the transverse space charge limit; (4) cesium also results in an increase in the fraction of volume produced negative ions which are extracted; (5) cesium may also result in reduction of extracted electrons by producing a less negative bias on the plasma electrode with respect to the plasma, thus allowing the transverse space charge limit budget to be taken up virtually totally by the ions; (6) a strong ion time scale sheath instability due to violation of the Bohm criteria produces an anomalous ion temperature which increases with beam current as routinely seen in measurements; and (7) introduction of cesium may result in a reduction in this instability. These insights may lead to improvements in volume negative ion sources.

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21 p.

Notes

INIS; OSTI as DE98000797

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  • 7. international conference on ion sources, Taormina (Italy), 7-13 Sep 1997

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  • Other: DE98000797
  • Report No.: ORNL/CP--95178
  • Report No.: CONF-970958--
  • Grant Number: AC05-96OR22464
  • Office of Scientific & Technical Information Report Number: 554841
  • Archival Resource Key: ark:/67531/metadc689749

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  • December 1, 1997

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  • Aug. 14, 2015, 8:43 a.m.

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  • Jan. 22, 2016, 6:55 p.m.

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Whealton, J.H.; Olsen, D.K. & Raridon, R.J. Toward a realistic and tractable model for negative ion extraction from volume sources, article, December 1, 1997; Tennessee. (digital.library.unt.edu/ark:/67531/metadc689749/: accessed August 20, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.