Slow wave circuits used in forward wave amplifiers like helix TWT's also support backward waves. A backward wave can interact with the slow and fast space charge waves ofan unmodulated electron beam at frequencies at which the phase velocities of the waves are nearly equal. Under small signal conditions, the interaction of a backward wave with the slow space charge wave produces positive backward wave gain that becomes infinite at the threshold for oscillation, while the interaction of the backward wave with …
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