The general electromagnetic properties and design methodology for serpentine/folded-waveguide (FW) amplifiers are presented. In addition, hybrid-waveguide circuit topologies, which permit greater design flexibility than the basic serpentine/FW topologies, are also introduced, and their dispersion characteristics are discussed. Experimental validation of design methodology and tools is provided via test results of the recently demonstrated wideband 220-GHz serpentine amplifier, which embodies the design methodology described herein. Particular attention will be paid to the comparison between code prediction and experimental data, which are in excellent agreement.

Design Methodology and Experimental Verification of Serpentine/Folded-Waveguide TWTs

A new computationally efficient 2-D model of the beam-wave interaction in coupled-cavity traveling-wave tubes (CC-TWTs) has been developed. The model provides self-consistent time-dependent solutions of Maxwell's equations together with a fully relativistic solution of the electron equations of motion. The model is based on different treatments of the RF fields in the region occupied by an electron beam and in the region of the coupled-cavity structure. The RF fields inside the beam tunnel are represented as a sum of eigenmodes of the local cross section of the beam tunnel. The fields outside the beam tunnel are represented as a superposition of modes of an equivalent circuit with lumped capacitors, inductors, and resistors. The model has been implemented in the TESLA-CC code. The results of the code predictions agree well with measured data for a wideband CC-TWT operating in the Ka-band. The code also shows good agreement with predictions of the 1-D code CHRISTINE-CC in regimes in which a 1-D approximation is applicable. A numerical study of CC-TWT operation shows that, in the small-signal regime, the code is able to predict a gain enhancement due to transverse motion at focusing magnetic fields comparable with Brillouin equilibrium values, which is the major 2-D effect. In the large-signal regime, the code is also capable of treating cases in which the transverse displacement of electrons becomes large and of determining the dependence of the spent beam energy distribution on radial position.

A Computationally Efficient Two-Dimensional Model of the Beam–Wave Interaction in a Coupled-Cavity TWT

We describe our progress on the development of a Ka-band TWT driver-booster combination to produce >1kW over a 5 GHz band centered at 35 GHz. The driver is an existing > 500 Watt broadband coupled-cavity TWT
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, employing a 2 stage depressed collector. The power booster is a newly designed sever-less folded waveguide TWT designed to provide 3-4 dB of additional gain. The booster also uses a 2 stage depressed collector and a slightly modified version of the same electron gun used in the driver. A photo of the driver and booster under test is shown in Fig. 1. The principal challenges that had to be met in a booster design were: (1) achieving the required output power and bandwidth, (2) ensuring stable booster operation under both small and large signal conditions and (3) presenting a good output match to the driver so that the driver remains stable.

Multi-kW 35 GHz TWT power booster development

To meet the need to transmit increasingly massive volumes of data, both the defense and commercial sectors are turning to higher operational frequencies to take advantage of larger signal bandwidths while concurrently requiring increased amplifier power to achieve the necessary signal-to-noise ratios over large transmission distances. In response to these needs, the last decade has seen a leap in performance of a variety of millimeter-wave devices. The Naval Research Laboratory (NRL) is the principal U.S. Department of Defense R&D center focused on the development of the science and technology behind new millimeter-wave high power solid-state and vacuum electronic devices. Selected examples of NRLs research projects are described with an emphasis on high power millimeter-wave vacuum electronic devices.

Sheet electron beam millimeter-wave amplifiers at the Naval Research Laboratory

Drive-induced oscillation (DIO) is an instability that limits the achievable output power in wideband coupled-cavity traveling wave tubes (CC-TWTs). It occurs under large-signal conditions, when the electron beam is slowed as a result of its interaction with the circuit at the drive frequency. DIO occurs at a frequency very near the $2\pi $ point (band edge) of the lower branch of the cold circuit dispersion diagram. It is commonly accompanied by a sudden increase in the body current and a flattening of the power transfer curve at the drive frequency. The accurate prediction of the conditions for onset of DIO would be useful to the designers of CC-TWTs. In this paper, we present an approach to the prediction of DIO thresholds using the multifrequency large-signal simulation code TESLA-CC.

Simulation of Drive-Induced Oscillation in Coupled-Cavity TWTs

A series of Ka-band Coupled-Cavity Traveling-Wave Tubes (CCTWTs) has been successfully developed, built, and tested at Communications and Power Industries (CPI) in collaboration with the Naval Research Laboratory (NRL) and SAIC. These devices represent a significant advance in the state-of-the-art of millimeter-wave CCTWTs, exploring the limits of power, bandwidth, and stability. We discuss the design and successful demonstration of the series of CCTWT's, including the VTA-6430N1 prototype which achieved over 700-W (879-W maximum power) over a 5-GHz range in Ka-band.

Alexander T. Burke

Papers

Wide band Ka-band Coupled-Cavity Traveling Wave Tube (CCTWT) development