H01J25/00—Transit-time tubes, e.g. klystrons, travelling-wave tubes, magnetrons
H01J25/02—Tubes with electron stream modulated in velocity or density in a modulator zone and thereafter giving up energy in an inducing zone, the zones being associated with one or more resonators
H01J25/22—Reflex klystrons, i.e. tubes having one or more resonators, with a single reflection of the electron stream, and in which the stream is modulated mainly by velocity in the modulator zone
H01J25/24—Reflex klystrons, i.e. tubes having one or more resonators, with a single reflection of the electron stream, and in which the stream is modulated mainly by velocity in the modulator zone in which the electron stream is in the axis of the resonator or resonators and is pencil-like before reflection