Focusing Procedures for Electrostatic Accelerators

Abstract

Ion optics for Van de Graaff accelerators are reviewed in elementary terms of geometric optics with particular reference to the Oak Ridge 3-Mv accelerator. This machine produces well-focused beams of over 100 μa and pulsed peak currents of several hundred μa. The beam diameter measured at a distance of 2.7 tube lengths from the accelerator tube exit decreases from 0.26 in. at 0.38 Mv to 0.10 in. at 2.47 Mv. The beam divergence at 1.8 tube lengths from the exit is nearly constant, ≃2.8×10−3 rad. The system is analyzed in terms of five separate lenses: (1) einzel lens, (2) gap lens, (3) accelerator tube, (4) 90-degree analyzing magnet, and (5) strong-focusing lens. It is shown that the crossovers formed in the gap lens and near the exit of the analyzing magnet are of fundamental importance. Brief reference is made to the small energy spread, ≃0.025%, for the beams obtained from this accelerator in spite of the large control slit spacing demanded by a large crossover.