The beneficial effects of phase velocity jumping technology in broad band CCTWT

In coupling cavity travelling-wave tube (CCTWT), the gain and efficiency band-widths are not coincident, that is, at the high-frequency there is lower gain and at the low-frequency the efficiency is lower. This restricts the useful range of operating frequencies. Phase velocity jumping technology has been developed as a way to improve the efficiency of CCTWT at first. In this article, we discuss improving bandwidth characteristic of CCTWT by using phase velocity jumping technology, and the conclusion based on Pierce wave theory gives that the gain bandwidth and efficiency bandwidth could be balanced at the high-frequency and low-frequency by choosing a special phase velocity of natural circuit wave for output section, this velocity is lower than the phase velocity of natural circuit wave in input section and mid section. Thus electron beam velocity is synchronized with velocity of natural circuit wave at high-frequency in input section and mid section, but in output section, it is synchronized at low-frequency. In this article, we also discuss the advantages brought to periodic permanent magnet focus system when apply phase velocity jumping. Otherwise, the application of phase velocity jumping can also reduce 2π mode oscillation.