Electromagnetic properties of periodic cavities coupled to a radiating antenna

The electromagnetic properties of spatially periodic cavities determine both the linear and nonlinear interaction between the waves and the electron beam in high power backward wave oscillators. A corrugated cavity is usually left open at one end for extraction of the useful microwave 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 falling in the lowest pass band of the structure is equal to N+1, an open cavity was found to support only N axial modes. The resonance 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, time-dependent, quasi three-dimensional code. It was also demonstrated that a short interface section between the periodic cavity and the radiating antenna can drastically reduce the quality factors to the diffraction limit over a very wide frequency band. This is expected to substantially increase the Starting current and allow operation at high beam current without degradation of spectral purity.