[Table of Contents]

Plasma and Fusion Research

Volume 4, 007 (2009)

Regular Articles

Theoretical Study of the Electrostatic Lens Aberrations of a Negative Ion Accelerator for a Neutral Beam Injector
Kenji MIYAMOTO and Akiyoshi HATAYAMA1)
Naruto University of Education, 748 Nakashima, Takashima, Naruto-cho, Naruto-shi,Tokushima 772-8502, Japan
1)
Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
(Received 25 July 2008 / Accepted 21 December 2008 / Published 16 March 2009)

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.

Keywords

nuclear fusion device, NBI, accelerator, negative ion beam, aberration, electrostatic lens

DOI: 10.1585/pfr.4.007

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This paper may be cited as follows:

Kenji MIYAMOTO and Akiyoshi HATAYAMA, Plasma Fusion Res. 4, 007 (2009).