An Electron Beam Modulator And Microwave Source Based On A Split Cavity

An electron beam passsing through a thin pillbox cavity with conducting screen walls is energy neutral with respect to the fundamental resonant electromagnetic mode of the cavity. That is, an oscillation will neither grow nor decay. If, however, the cavity were wide enough that the beam transit time exceeds the period of the mode, an instability results in which the oscillation grows at the expense of the beam's energy. Such a device is called a Transit Time Oscillator (TTO). It may be quite wide. For a 1 GHz pillbox (R=11.5 cm) and a 200 keV beam (v=.7c) the cavity must be almost half a meter wide. We present theoretical and experimental results for a device in which the beam interaction with the fundamental mode of a thin cavity is unstable. It consists of a pillbox with a partial partitioning center screen. The electric field in the new fundamental mode of this geometry reverses sign across the partition. A traversing beam experiences both signs of an electric field spatially, rather than temporally as in the TTO. Because of this, the beam interacts unstably with this mode, which grows at the expense of the beam's energy. Since the beam need not remain in the region for an entire period, the pillbox can be quite narrow. At saturation, when the field is large enough to stop the beam, the exit current is almost fully modulated at the modal frequency. This device, with the electric field of the fundamental mode, is illustrated below. It is called a Split Cavity Oscillator (SCO).