Backward Wave Oscillator Experiment Research Articles

Analysis of multimode radiation from 0.4 THz backward wave oscillator

Abstract A terahertz backward wave oscillator (BWO) is a high-power terahertz wave source based on the Cherenkov interaction between an electron beam and a slow backward wave. Excitation of slow wave modes with different mode numbers degrades the performance of BWO due to mode competition. Analyzing the mode spectrum is a difficult challenge to confirm single mode operation. In this study, the excitation of the slow waves in a 0.4 THz cylindrical corrugated waveguide is examined. In the corrugated waveguide, azimuthal modes become slow backward waves with evanescent fields. We found the superposition of the modes with different azimuthal indices has an asymmetric field intensity and the excitation of the superposed modes produces the terahertz radiation with the asymmetric pattern. We conduct a BWO experiment using a 0.4 THz corrugated waveguide. The experimental radiation pattern is obtained and corresponds well to the calculated asymmetric pattern. The symmetry of the radiation can be regarded as an indicator of the mode control.

Rapid startup in relativistic backward wave oscillator by injecting external backward signal

Investigation on accelerating the building up of oscillation and achieving a rapid startup in powerful relativistic backward wave oscillator by injecting a weak external backward signal is carried out in this paper. Synchronizing the signal with the backward wave excited by intense electron beam extracting with slow wave structure, the initial noise is greatly reduced and mode competition is restrained. The analysis is demonstrated by high power X-band backward wave oscillator experiment, in which a plasma switch is designed to realize the backward signal injection. The results show that the significant reduction of microwave output delay is attained and the start time of oscillation is ahead of 10 ns with the energy conversion efficiency increases about 62%.

Slow Cyclotron Instability in a High-Power Backward-Wave Oscillator

The linear dispersion relation of a backward-wave oscillator (BWO), derived earlier by the authors, is modified to include effects of RF surface current at the beam-vacuum interface. This modified dispersion relation results in an unstable interaction between the slow cyclotron mode (SCM) and the structure mode in addition to the conventional Cherenkov instability caused by the slow space charge mode. Numerical analysis is then carried out using parameters of a BWO experiment at University of Maryland. Fine structure of the SCM instability is elucidated. The analysis indicates that BWO radiation would not be suppressed near cyclotron absorption in an infinitely long system.

Equilibrium and confinement of bunched annular beams

The azimuthally invariant cold-fluid equilibrium is obtained for a periodic, strongly bunched charged annular beam with an arbitrary radial density profile inside of a perfectly conducting cylinder and an externally applied uniform magnetic field. The self-electric and self-magnetic fields, which are utilized in the equilibrium solution, are computed self-consistently using an electrostatic Green’s function technique and a Lorentz transformation to the longitudinal rest frame of the beam. An upper bound on the maximum value of an effective self-field parameter for the existence of a bunched annular beam equilibrium is obtained. As an application of the bunched annular beam equilibrium theory, it is shown that the Los Alamos National Laboratory relativistic klystron amplifier experiment is operating slightly above the effective self-field parameter limit, and a discussion of why this may be the cause for their observed beam loss and microwave pulse shortening is presented. The existence of bunched annular beam equilibria is also demonstrated for two other high-power microwave (HPM) experiments, the relativistic klystron oscillator experiment at Air Force Research Laboratory and the backward wave oscillator experiment at the University of New Mexico. In general, the results of the equilibrium analysis will be useful in the determination of the stability properties of strongly bunched annular beams in HPM devices.