Theoretical Research on Properties of Spatial Harmonics in Corrugated Waveguide

This paper investigates the dependence of the properties of the spatial harmonics on the overmoded ratio of the corrugated waveguide operating in TM01 at 30 GHz with the consideration of the effects of the ripple shape. It is found that as the overmoded ratio grows, the electric fields concentrate towards the ripple surface, which weakens the enhancement of the power handling capacity caused by augmentation of the overmoded ratio. As the overmoded ratio grows, the ratio between the field amplitudes of the -1st spatial harmonic and the fundamental harmonic increases monotonically. Therefore, the total electric fields composed of the spatial harmonics become the spatial beating-wave with the single frequency, whose amplitudes vary along z-axis with the spatial period determined mainly by the -1st spatial harmonic and the fundamental harmonic. Researches on coupling impedances show that with the same overmoded ratio, the corrugated waveguide consisting of the improved rectangular ripples provides better synchronous coupling for the wave-beam interaction than that of the sinusoidal ripples does. Moreover, it is better to choose the fundamental harmonic close to the light line as the synchronous wave for the wave-beam interaction in the low magnetic field. In that case, even if the electron beam keeps far away from the ripple surface, the synchronous wave still provides strong enough coupling for the electron beam.