Terahertz backward wave oscillator circuits

by Oviedo Vela, Guillermo Antonio, Ph.D., THE UNIVERSITY OF UTAH, 2010, 102 pages; 3428280


This work focuses on increasing the electric field mode-electron beam interaction in terahertz backward-wave oscillators through increasing the interaction impedance of the slow wave circuit. In a backward wave oscillator (BWO) or a traveling wave tube (TWT), the electric field traveling in the waveguide interacts with an electron beam grazing or piercing the electric field of a slow wave circuit and transfers energy from the beam to the circuit mode. The mechanism of this interaction is analyzed and the traditional model is adapted to provide one that is scalable to terahertz frequencies. The efficiency of the BWO can be enhanced by improving the interaction between the beam and the circuit modes, utilizing beam sources with greater current densities and using larger magnetic fields. This work presents the results on the modeling, fabrication and performance of the mode-piercing and mode-grazing slow wave circuits studied.

Meandering folded waveguides and interdigital lines were modeled to evaluate their interaction impedance and electron beam requirements at terahertz frequencies. The models were verified against published results. Because of their low interaction impedance, terahertz meandering waveguides would need to be excited with large current density electron beams which in turn would require large magnets for their control. Interdigital lines were found to be the slow-wave circuit of choice in this work because their high interaction impedance and their ability to operate with low voltage, and low current density electron beams.

Finite element numerical calculations were used to design, optimize and scale a terahertz free interdigital line (FIDL) circuit. The designed free interdigital line was fabricated using microfabrication technologies. Characterization of a free interdigital line (FIDL) circuit was performed in a tube body with a dispenser cathode and a three-anode Pierce gun. The FIDL measured output was compared with the calculated result and its performance met the simulated behavior. The resulting vacuum tube had an output of ≈ 1 μW a total efficiency of ≈ 0:05% at a center frequency of 1.25 THz.

Because of its high interaction impedance, the use of confined electron beams in this FIDL BWO was not necessary. The limit of using magnet-free electron beams in traveling-wave tubes (TWTs) at terahertz frequencies (500-3000 GHz) was explored. The study showed that there is a minimum interaction impedance limit where this approach will not work, due to the diverse losses in the system.

This work concludes that the high interaction impedances from beam-grazing circuits should enable a new class of terahertz-frequency tubes for signal sources and amplification. The short high impedance interdigital line demonstrated here enables magnet-free tubes at higher frequencies. Devices realized in this newly-identified design space should empower the advancement of a broad spectrum of fields, from chemical detection, to imaging, to communication, among others.

AdviserMark S. Miller
Source TypeDissertation
SubjectsElectrical engineering; Electromagnetics
Publication Number3428280

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