"Power holes" and nonlinear forward and backward wave gain competition in helix traveling-wave tubes
The output power of a forward wave linear beam amplifier like a helix traveling-wave tube (TWT) can show unexpected dips as a function of drive signal frequency in certain narrow frequency ranges. These dips, commonly called "power holes," can have serious consequences for the performance of the system of which the amplifier is a part. It is widely believed that these power holes, which only occur under large signal conditions, are due to the amplification of a backward wave space harmonic of a signal at a harmonic of (he drive signal frequency, when the frequency of (hat harmonic happens to fall in the range of frequencies in which backward wave gain occurs. The resulting growth of the backward wave can compete with the forward wave gain, resulting in a reduction of output power at the drive frequency. Power holes can occur under conditions for which the backward wave oscillator (BWO) instability does not occur, i.e., the backward wave gain is finite, not infinite. In this paper, we report on a study of power holes using the large signal helix TWT code, CHRISTINE 3D. In particular we demonstrate the connection between backward wave gain and the depth of the power holes. We show that the frequency range over which backward wave gain occurs can be different than that predicted by small signal BWO theory when a large amplitude forward wave signal is present. This effect is due to the destabilization of the fast beam space charge wave, interacting with the backward wave under large signal conditions. We review commonly used methods to reduce the backward wave gain, and argue that any of these should reduce the depth of the power holes. The choice If one or another gain reduction method will depend on various engineering design constraints, and on cost. We suggest that a code like CHRISTINE 3D can be helpful in making this choice.