Igor A. Chernyavskiy

Large-Signal Multifrequency Simulation of Coupled-Cavity TWTs

We describe a steady-state large-signal model of coupled-cavity traveling-wave tubes in which the input and output signals are periodic functions of time that may be represented by Fourier series of finite length. The model includes both linear and nonlinear effects including circuit dispersion, reflections, intermodulation, and harmonic generation. The model uses a lumped element representation of the circuit and a 1-D disk model of the beam. Several favorable comparisons of model predictions with experimental measurements, including gain versus frequency and power transfer characteristics, are illustrated. The inclusion of nonlinear effects in this multifrequency model enables predictions of intermodulation products, as functions of the input power. An example of the computation of C3IM is illustrated.

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

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 Maxwells 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.

Parallel Simulation of Independent Beam-Tunnels in Multiple-Beam Klystrons Using TESLA

We present an extension of the klystron simulation code TESLA to model multiple-beam klystrons (MBKs) in which interaction parameters may vary significantly from beam-tunnel to beam-tunnel. In earlier work, the single-beam code was applied to model the MBK by assuming that all electron beams and beam-tunnels were identical and all electron beams interacted identically with the fields of the resonant cavities, using averaged values of R/Q to represent interaction with each resonant cavity. To overcome the limitations of this approach and to take into account the effects from nonidentical beams and/or beam-tunnels, we have modified the code to use a parallel algorithm for multiple beams. The implementation of the parallel version of TESLA is based on the latest Fortran-95 version of the serial code and uses the message-passing interface library for communication. For testing and verification purposes, the new version of the code is applied to simulate an experimental four-cavity, eight-beam klystron amplifier, which was designed and successfully tested last year at the Naval Research Laboratory. The results of modeling using the new parallel TESLA and their comparison with experimental data are discussed in detail.

Demonstration of a High Power, Wideband 220-GHz Traveling Wave Amplifier Fabricated by UV-LIGA

We present the first vacuum electronic traveling wave amplifier to incorporate an interaction circuit fabricated by ultraviolet (UV) photolithography and electroforming, demonstrating over 60 W of output power at 214.5 GHz from a 12.1 kV, 118 mA electron beam. The tube also achieved an instantaneous bandwidth of ~15 GHz in G-band in the small signal regime. The all-copper circuit was fabricated in two layers using a UV-transparent polymer monofilament embedded in the photoresist to form the beam tunnel prior to electroforming. Effects arising from fabrication errors and target tolerances are discussed. This microfabrication technique and demonstration paves the way for a new era of vacuum electron devices that could extend into the 1-2 THz range with advances in high-current-density electron guns.

Simulation of Klystrons With Slow and Reflected Electrons Using Large-Signal Code TESLA

The problem of modeling slow and reflected electrons that appear during operation of high-power klystrons is discussed. These electrons are not only problematic for the operation of real klystrons but also present a numerical challenge for simulation codes that solve the equations of motion with axial distance rather than time as the independent parameter. To meet this challenge, particles with axial velocity below some threshold are treated using an alternative algorithm. This approach has been implemented in the simulation code TESLA. The method is verified by comparison with PIC simulation. The effect of these particles on the efficiency of a hypothetical device is also presented.

Demonstration of a high power, wideband 220 GHz serpentine waveguide amplifier fabricated by UV-LIGA

We present the hot test results of a 220 GHz, serpentine waveguide vacuum electron amplifier showcasing a novel embedded monofilament microfabrication technique based on UV-LIGA. The instantaneous operating bandwidth exceeds 15 GHz and the small signal gain of the circuit is over 14 dB. By varying the voltage slightly, an operating bandwidth of almost 40 GHz is realizable with a minimum circuit gain of 7 dB across the band. A maximum power of just over 60 W was obtained at the output flange of the device, corresponding to a power of almost 80 W generated in the circuit.

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

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 CCTWTs, including the VTA-6430N1 prototype which achieved over 700-W (879-W maximum power) over a 5-GHz range in Ka-band.

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

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.

Transmission Line Model for Folded Waveguide Circuits

We describe an equivalent circuit model for traveling wave structures based on folded or serpentine waveguides (F/SWs), including a beam tunnel. The model consists of a combination of transmission line segments and lumped electrical elements. Values of the five parameters that characterize the model are found by minimizing the differences between the dispersion and coupling impedance as calculated by the model and those obtained from a 3-D finite element code. We find that the model reproduces the dispersion to better than 0.1% and the coupling impedance to better than 1% over a broad frequency range in different examples in which the dependence of the impedance on frequency tends either to zero or to infinity near the band edge at 2π. The model is also able to capture the effects of asymmetric fields and misalignment of the beam tunnel. We conclude that the model can be used with confidence in codes that simulate the beam-wave interaction in FW and SW traveling wave tubes.

1-D Large Signal Model of Folded-Waveguide Traveling Wave Tubes

A recently published hybrid circuit model for folded-waveguide slow wave structures has been implemented in the 1-D large signal code CHRISTINE. The resulting code is applied to a design for a G-band (220 GHz) folded-waveguide traveling wave tube. Results of small and large signal gain are compared with those from TESLA, a 2-D code using the same circuit model. Conditions for accuracy of the 1-D model are illustrated and explained. The effects of an offset beam tunnel on circuit dispersion and amplifier stability are illustrated using the CHRISTINE code.