Second harmonic suppression in helix traveling-wave tubes
A filter structure to suppress the second harmonic component in the RF signal of a traveling-wave tube (TWT) was developed and integrated into a helical delay line. Its effect is shown in simulation and measurement.
Filter Helix for Harmonic Suppression in Traveling Wave Tubes
Increasing the efficiency of traveling wave tubes (TWTs) is a basic development strategy. In this paper, second harmonic suppression is introduced as a possible mean. For this purpose, a filter consisting of periodic pitch discontinuities is implemented in the helix transmission line (TL) of a tube. A method is developed to obtain the reflection coefficients of adjustable pitch discontinuities. A filter design for a low frequency TWT is tested in a helical TL without electron beam and implemented in the tube. Both simulation and measurement confirm a stopband at the second harmonic and thus a significant reduction of this harmonic at the output. An additional filter is designed and tested at S-band. It is shown to improve both the output power and the linearity of the tube.
Stability of traveling-wave amplifiers with reflections
The maximum achievable gain in individual sections of helix-type traveling-wave amplifiers is limited by the requirement that the device be stable with respect to the excitation of spurious modes. The excited modes may be of two types: backward waves, which are absolutely unstable, or forward waves, which are unstable in the presence of reflections and regenerative amplification. Whether a specific device is unstable depends on a number of details that must be computed numerically. We will present a model, used in the traveling-wave tube simulation code CHRISTINE, in which stability is determined including a number of important effects. These are: the placement of severs, their reflection and transmission coefficients, the profile of attenuation along the interaction length, the presence of a driven signal, and the coupling of forward and backward waves due to asymmetries in the helix support structure. Asymmetries result in a stopband near the "/spl pi/" point formed by the coupling of the forward and backward waves. For this case, a model is developed to evaluate the maximum stable length.