Studies Of Low Q Slow Wave Structures For Relativistic Backward Wave Devices

The slow wave structure supporting the beam-wave interaction in a relativistic backward wave oscillator (BWO) requires a carefully designed output waveguide in order to couple the electromagnetic wave to a radiating antenna. The coupling between these two sections determines the quality factor (Q) of the slow wave structure and affects the starting current and the saturation processes occurring when the electron beam is injected into the device. For some applications, the end reflection and quality factor are made as low as possible over a wide frequency band. The authors goal has been to study the end reflection and quality factor of a practical slow wave structure both experimentally and numerically. When both ends of the slow wave structure are shorted, a resonant cavity is formed that can have a high quality factor (1,000--3,000) since the only losses are due to ohmic heating of the cavity walls. By introduction of an output section additional diffractive losses are created. The width of the resonances grows larger, i.e. the Q of the cavity has been lowered to 200--300 in some cases. The quality factors for all the axial modes of the TM{sub 01} mode in a closed cavity have been measured as havemore » the quality factors for the same modes when the cavity is coupled to a smooth, linearly tapered output section. From these two sets of data, the magnitude of the reflection coefficient has been calculated. A calculation of the Q-value has also been completed.« less