Construction and Calibration of a Cylindrical Electrostatic Analyzer

Abstract

A cylindrical electrostatic deflector has been constructed with average radius 15 cm, spacing 0.5 cm and deflection 90°, in which the deflecting potential can be in the neighborhood of 50 kv. Thus protons up to 0.75 Mev can be focused. Movable slits have been incorporated in the design, and the geometry of the entrance and exit apertures of the deflecting channel is such that the stray field may be calculated analytically. This has been done for the case in which the deflecting plates are symmetrically charged above and below zero potential. The predictions have been compared with experiments on the focusing of electron beams by the instrument under controlled conditions. Agreement with the calculations is demonstrated within the accuracy of the experiments, which is about 0.3 percent. Experiments with asymmetric charging of the electrodes are reported and shown to agree with a plausible extension of the theory. Further experiments investigated the effect of small displacements of the slits from the computed focal positions. A set of factors, or calibration constants, has been measured so that, given the coordinates of the centers of the entrance and exit slits, and the potentials applied to the deflecting plates, the kinetic energy of the focused ion beam, as it left its source at zero potential, can be calculated to within 0.3 percent.