Modulated cusp electron gun for millimeter-wave gyro-amplifiers

The design and optimization of a gridded cusp electron gun for millimeter wave gyro-amplifiers is reported. Development of gyro-amplifiers operating in the millimetre wave band in recent years has shown great progress from designs using periodically loaded waveguides driven by MIG type electron beams using a modulating anode. Gyro-travelling wave amplifiers using helically corrugated interaction regions, however, favour the use of a large-orbit, axis-encircling electron beam. These devices have shown less progress, at least partially because the cusp electron gun required to produce an axis-encircling electron beam is less well understood and only cathode-modulated operation has been demonstrated. In this paper, we show that optimization of the cathode, anode, and control electrode geometries as-well-as the profile of the magnetic field can result in a high-quality electron beam. The optimized electron gun operated at 60 kV with a current of 1.8 A and <10% alpha spread.