ELECTROMAGNETIC PROPERTIES OF PERIODIC CAVITIES COUPLED TO A RADIATING ANTENNA

The electromagnetic properties of spatially periodic cavities determin es both the linear and nonlinear interaction between the waves and the electron beam in high-power backward-wave oscillators. A corrugated c avity is usually left open at one end for extraction of the useful mic rowave energy; however, reflections at the open end are large so that a cavity is still formed. In contrast to a previously studied N period closed cavity where the number of axial modes with frequencies fallin g in the lowest pass band of the structure is equal to N + 1, an open cavity was found to support only N axial modes. In this paper the reso nance frequencies, quality factors and the field patterns of the axial modes in an open cavity were all investigated experimentally and the results are in very good agreement with those obtained using a new, ti me-dependent, quasi-three-dimensional code. It was also demonstrated t hat a short interface section between the periodic cavity and the radi ating antenna can drastically reduce the quality factors to the diffra ction limit over a very wide frequency band. This is expected to subst antially increase the starting current and allow operation at high-bea m current without degradation of spectral purity.