Filter Helices in Low-Frequency Traveling-Wave Tubes

This work deals with the improvement of tube performance by using a novel method of second-harmonic suppression. The common techniques of harmonic suppression inside the tube are introduced and discussed with respect to their suitability for modern space applications. A filter helix delay line is presented as a useful approach for harmonic suppression in these applications and its design parameters are introduced. Its effect in traveling-wave tube operation is simulated with the code ¿MVTRAD-Reflections¿ proprietary to Thales Electronic Systems. Three different filter helix designs are developed and tested, one in an L-band tube and two in S-band tubes. Improvement of the output power, i.e. increase of the first and suppression of the second harmonic as well as a higher energy of the electron beam at the output are the main design goals. The influence of the filter helix design parameters on the tube output is investigated. They are optimized to achieve the designated design goals. In simulation and measurement, the performance of the tubes is analyzed and compared to that of the tubes without a filter helix. In addition, the influence of the reflection at the output coupler on filter helix performance is investigated.