BEAM FOCUSING BY APERTURE DISPLACEMENT IN 2-STAGE ACCELERATION SYSTEM

The effects of beam focusing and the influence of total beam deflection on injection efficiency are estimated numerically for the neutral beam injectors of large tokamaks such as the JT-60. It is shown that beam focusing is important in improving injection efficiency in such a large device. As one method of focusing, beam focusing by aperture displacement was investigated in the two-stage acceleration system on the basis of a thin lens approximation. The simultaneous displacement of apertures in the plasma and gradient grids (or suppressor and exit grids) was found to be adequate for beam focusing, because the focal point hardly depends on the field intensity ratio which determines the two-stage ion beam optics. © 1979 The Japan Society of Applied Physics.