Chapter 11 - Microwave Tubes

Abstract

In an M-type magnetron, the electron beam travels in a circular path, shock exciting a series of resonant cavities. The RF wave set up across the entrances to the cavities interacts with the electron beam to force the electron cloud into a characteristic rotating spoked-wheel shape. A coupling loop or waveguide slot is used to extract RF power from the magnetron. The crossed-field amplifier (CFA) is a magnetron like device that uses crossed electric and magnetic fields, and a negatively charged sole plate electrode to force the electron beam into a curved path that takes it past a slow wave or delay line structure. The RF wave extracts energy from the electron beam, causing it to be amplified. O-type microwave devices differ from M-type in that the magnetic field is parallel to the electric field, rather than at right angles to it. The interaction causes velocity modulation of the beam, and the velocity modulation is converted into density modulation in the drift space between input and output cavities. RF power is extracted from the output cavity. The traveling-wave tube (TWT) is an O-type microwave amplifier that uses the parallel magnetic field to focus the electron beam into a thin stream.