Backward wave gain due to nonlinear interaction with the fast space charge wave [forward wave amplifiers]
Slow wave circuits used in forward wave amplifiers like helix TWTs also support backward waves. A backward wave can interact with the slow and fast space charge waves of an unmodulated electron beam at frequencies at which the phase velocities of the waves are nearly equal. We have identified the positive backward wave gain due to the interaction of the fast wave and the first backward wave space harmonic in a helix TWT as being the primary physical mechanism responsible for so-called 'power holes' - dips in the output power vs. frequency curves in TWTs observed under large signal conditions. We speculate that this effect may be used as a diagnostic, in a way analogous to the use of the Kompfner dip.