Design methodology and experimental verification of serpentine/folded-waveguide TWTs

Serpentine or, its cousin, the folded waveguide (FW) traveling-wave tubes (TWTs) are a class of vacuum electronic devices capable of wideband, high-power performance. This was first demonstrated in [1] with several Q-band 2-stage TWTs that produced 135 W of average power and saturated gains of ~30 dB over an impressive 25% instantaneous bandwidth. More recently, a 60 W broadband 220-GHz (G-band) serpentine TWT was designed and demonstrated at NRL [2]. The excellent agreement between prediction and data G-band TWT serves as an experimental validation for our design tools (MAGIC and Neptune particle-in-cell codes, large signal code Tesla, gun/collector code MICHELLE, 3-D electromagnetic code Analyst, etc.) and methodology; a critical step in the design of future amplifiers. This presentation will discuss electromagnetic properties and beam-wave interaction of the serpentine/FW circuit topology, in general, and design methodology to optimize power, gain, and bandwidth for a “zero-drive” stable amplifier, in particular. A novel hybrid-serpentine circuit topology, which combines key advantages of serpentine and FW topologies, will also be presented and discussed.