Electron Optics and Space Charge in Simple Emission Systems with Circular Symmetry

Properties of a simple type of emission system, such as is used in electron guns of cathode-ray tubes, have been investigated. It was found that the properties depend largely on the spacings between the electrodes of the system. For any grid-anode spacing, there exists a certain grid-cathode spacing referred to as the preference spacing, for which the current density in the focused beam is a maximum. The divergence of the emitted electron beam and the spherical aberration in it both have minimum values at this spacing, near which the crossover of the rays in the system occurs at a maximum distance from the cathode and in general the grid base is least. Some physical explanations of these effects are put forward. The presence of ions produced by traces of residual gas modifies the values of the preference spacings and tends to enhance the effects. From the results given it is seen that a direct application of first-order optical laws is not adequate to describe the crossover in an electron gun.