Abstract
Engineering of the eigenmode dispersion of slow-wave structures (SWSs) to achieve desired modal characteristics, is an effective approach to enhance the performance of high power traveling wave tube (TWT) amplifiers or oscillators. We investigate here for the first time a new synchronization regime in TWTs based on SWSs operating near a third order degeneracy condition in their dispersion. This special three-eigenmode synchronization is associated with a stationary inflection point (SIP) that is manifested by the coalescence of three Floquet-Bloch eigenmodes in the SWS. We demonstrate the special features of "cold" (without electron beam) periodic SWSs with SIP modeled as coupled transmission lines (CTLs) and investigate resonances of SWSs of finite length. We also show that by tuning parameters of a periodic SWS one can achieve an SIP with nearly ideal flat dispersion relationship with zero group velocity or a slightly slanted one with a very small (positive or negative) group velocity leading to different operating schemes. When the SIP structure is synchronized with the electron beam potential benefits for amplification include (i) gain enhancement, (ii) gain-bandwidth product improvement, and (iii) higher power efficiency, when compared to conventional Pierce-like TWTs. The proposed theory paves the way for a new approach for potential improvements in gain, power efficiency and gain-bandwidth product in high power microwave amplifiers.
Highlights
The classical approach for designing high power microwave amplifiers provides for an efficient energy transfer from high energy electron beams to electromagnetic fields at radio and microwave frequencies [1], [2]
As shown in the dispersion diagram of the fundamental Brillouin zone in Fig. 1(c), two different stationary inflection point (SIP) are possible at ωSIP : (i) an SIP in the region 0 < kSIPd < π that exhibits positive group velocity for frequencies higher and lower than the SIP frequency and (ii) another SIP in the region π < −kSIPd + 2π < 2π that is associated to negative group velocity for frequencies higher or lower than the SIP frequency
We have reported a new regime of operation for slow-wave structure (SWS) with a third order degeneracy interacting with an electron beam with possible application in traveling wave tube (TWT) amplifiers
Summary
The classical approach for designing high power microwave amplifiers provides for an efficient energy transfer from high energy electron beams to electromagnetic fields at radio and microwave frequencies [1], [2]. A traveling wave tube (TWT) amplifier is a conventional high power device comprising of a slow-wave structure (SWS) whose interacting mode has a synchronous phase velocity to the average electron’s velocity of the electron beam [1]–[4] Pierce and his contemporaries [3], [5]–[8] developed a ubiquitous framework and design procedure for such devices through circuit theory. The theory in [31]–[33], [39] describes four Floquet-Bloch eigenmodes synchronous scheme of oscillators operating with a fourth order degeneracy at the degenerate band edge (DBE) In this premise, instead, we investigate amplification in TWTs operating near the SIP in their dispersion relation. Throughout this paper, we assume linear operation of the TWT and assume that the time-dependence is implicit in the form of e jωt and suppressed
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