Design and Numerical Analysis of W-band Oscillators With Hollow Electron Beam

The use of hollow electron beams permits a significant increase in the diameter of the beam tunnel in comparison with the conventional pencil-beam slow-wave electron devices. As a result, the current density and the heating of the microwave structure are decreased, which allows increasing the average radiation power at high frequencies. To demonstrate this capability, two versions of W-band oscillators, namely, an axisymmetric orotron and a backward-wave oscillator (BWO), have been designed. The electron beam with an outer diameter of 1.6 mm and a 0.1-mm thick wall could be produced in a 30 kV/1 A Pierce-like electron gun with the hundredfold magnetic compression. Oscillators use microwave structures with an azimuthally symmetric corrugation, sinusoidal for the orotron and rectangular for the BWO. Simulations based on the averaged equations and 3-D PIC-code predict an output power up to 1 kW for the orotron and 0.7 kW for the BWO, whereas the thermal regime in the BWO is easier for continuous-wave operation. Wideband frequency tuning of the BWO is simulated. To reduce ohmic losses, a smooth downtaper of the corrugation in the orotron and an abrupt cutoff with a specially optimized last tooth in the BWO were designed.