A Fixed-Focus Electron Gun for Cathode-Ray Tubes

Many cathode-ray tubes in use at the present time employ electrostatic focusing of the electron beam. It is customary to supply these tubes with at least two anode voltages, one of which is varied to bring the beam to a focus. However, an electrostatically focused electron gun can be designed so that only one anode voltage is required, and so that the electron beam remains in focus regardless of that anode voltage. A fixed-foxus electron gun may be made by using a focusing field generated by electrodes at a common anode potential and others at cathode potential. The point at which the beam is brought to a focus is determined by the dimensions of the electrodes which make up the electron lens. Developmental cathode-ray tubes have been built in which the beam was brought to a focus with a single anode voltage, and in which the focus remained substantially constant at any voltage in the test range between 750 and 2500 volts.