Design of a Hollow-Beam Electron Optics System With Control Focus Electrodes

The design of a hollow electron beam (HEB) electron optics system for a 10-kW Ka-band extended interaction klystron is presented. To lower the maximum cathode current density and realize the fast switch of the current emission, an HEB with two focus electrodes (FEs) is chosen, which attains to a high area compression ratio of over 100. A 25-kV, 2.5-A beam is produced by a curved spherical segment cathode with the area of 0.321 cm
²
and an average beam loading of 8 A/cm
²
, namely, the high area compression ratio is necessary to reduce the cathode emission current density. The HEB with an area compression ratio of 105 can propagate 40 mm long with good laminarity confined by a 0.7-T uniform magnetic field. The beam current can be quickly shut off when the voltages of the FEs are biased to -4100 V. The results were simulated with two simulation codes: EGUN and OPERA 3-D. A sensitivity analysis with respect to critical parameters such as the displacement of FE1 and FE2 in the radial direction is provided. The simulation results of OPERA 3-D/SCALA and EGUN show an excellent agreement.