High-gain wideband high-power 200-GHz multiple-beam serpentine TWT design

Serpentine traveling-wave tubes (TWTs) are a class of vacuum electronic devices capable of wideband (25%), high power performance. However, their bandwidth depends critically on the path length along a half-period of the serpentine waveguide (from gap to gap). At THz frequencies (200 to 1000 GHz), beam tunnel diameters well below 200 μm are required to achieve bandwidth ~20%. On this scale, fabrication issues, alignment, beam interception, and electron beam thermal effects will dominate and ultimately limit the practical circuit length and, by extension, device gain and output power. Since the gain-bandwidth product is a function of both circuit length and beam current, it is clear that the formation of an intense, well-focused beam is a key factor. Recent advances in high power THz amplifier development have shown that ~100 mA, ~ 20 kV beams with diameters of ~100 ⌈m are feasible. However, even with these stringent beam parameters, we estimate that a single-beam, 220-GHz device with ~40-dB saturated gain and ~20% bandwidth would require a circuit length ~5 cm (assuming half copper surface conductivity). The long circuit length makes this device impractical, as severe beam interception will be unavoidable. We will introduce a novel, multiple-beam approach, which provides a potential means to overcome this fundamental obstacle.