Plasma and Fusion Research
Online ISSN : 1880-6821
ISSN-L : 1880-6821
Regular Articles
Theoretical Study of the Electrostatic Lens Aberrations of a Negative Ion Accelerator for a Neutral Beam Injector
Kenji MIYAMOTOAkiyoshi HATAYAMA
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2009 Volume 4 Pages 007

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Abstract

Aberrations due to the electrostatic lenses of a negative ion accelerator for a neutral beam injector and the space charge effect are theoretically investigated. A multi-stage extractor/accelerator is modeled and the aberration coefficients are numerically calculated using the eikonal method, which is conventionally used in electron optics. The aberrations are compared with the radii of a beam core with good beam divergence and a beam halo with poor beam divergence. H- beamlet profile measurements give the 1/e radii of the beam core and beam halo of 5.8 mm (beam divergence angle: 6 mrad) and 11.5 mm (beam divergence angle: 12 mrad), respectively. When the beam divergence angle of the beam core is 5 mrad and the beam energy is 406 keV, the aberrations due to the electrostatic lenses are less than a few millimeters, thus are less than the radii of the beam core and beam halo. The geometrical aberrations due to the space charge effect (negative ion current density: 10 mA/cm2 ), however, are estimated to be much larger than the radius of the beam halo. Although the aperture radii of the grids are not taken into account in this estimation, the results indicate that the space charge effect is an important factor in the aberration or beam halo in a negative ion accelerator.

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© 2009 by The Japan Society of Plasma Science and Nuclear Fusion Research
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