Hairpin-filament electron guns for low-energy use

A long series of simulations of hairpin-filament guns as used in electron s pectrometers was performed for filament-anode distances from 2 to 10 mm and acceleration voltages from 10 to 100 V using Child's law approximation, sp ace-charge effects and rotational symmetry. The role of the Wehnelt electro de is clarified. Two distinct operation modes, the laminar (with positive W ehnelt voltages) and scrambled (with negative Wehnelt voltages) modes, are described and, for each, typical equations for the dependence of the curren t on the filament-anode distance and anode voltages are given.