Abstract：In directly heated electron guns, the cathode current generates a magnetic field that will change trajectories of electrons. The deflection of the electron beam caused by a cathode magnetic field can be estimated with an equivalent plane diode analytical model. It is shown that such a magnetic field really can affect the practical operating characteristics of an electron gun. The equations suggested for engineering calculation agree well with results of computer simulation for the real construction of a three-electrode gun. It is shown that a magnetic field which changes with time leads to a dispersive electron beam and causes a substantial decrease in the accuracy of positioning of the beam on the object, irremovable by the static alignment of the device.
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