Jeffrey D. Wilson

Design of high-efficiency wide-bandwidth coupled-cavity traveling-wave tube phase velocity tapers with simulated annealing algorithms

The output circuit section of a traveling-wave tube (TWT) routinely contains an RF phase velocity taper for the purpose of increasing RF output power and efficiency. By slowing the RF phase velocity in approximate synchronism with the decelerating electron beam bunches, the taper increases power transfer from the beam to the RF wave. Recently, the computational optimization technique of simulated annealing was shown to be very effective in the design of an RF phase velocity taper that significantly increased computed RF power and efficiency of a coupled-cavity TWT. In this paper, two new broadband simulated annealing algorithms are presented that optimize (1) minimum saturated efficiency over a frequency bandwidth and (2) simultaneous bandwidth and minimum efficiency over the frequency band with constant input power. The algorithms were incorporated into the NASA 2.5-dimensional (2.5-D) coupled-cavity TWT computer model and used to design optimal phase velocity tapers using a 59-64 GHz coupled-cavity TWT as a baseline model. Compared to the baseline taper design, the computational results of the first broadband algorithm showed an improvement of 73.9% in minimum saturated efficiency. The second broadband algorithm indicates an improvement of 272.7% in minimum RF efficiency with constant input power drive and an increase in simultaneous bandwidth of 0.5 GHz over that calculated for the baseline TWT.

Simulation of cold-test parameters and RF output power for a coupled-cavity traveling-wave tube

Procedures have been developed which enable the accurate computation of the cold-test (absence of an electron beam) parameters and RF output power for the slow-wave circuits of coupled-cavity traveling-wave tubes (TWTs). The cold-test parameters, which consist of RF phase shift per cavity, impedance, and attenuation, are computed with the three-dimensional electromagnetic simulation code MAFIA and compared to experimental data for an existing V-band (59-64 GHz) coupled-cavity TWT. When simulated in cylindrical coordinates, the absolute average differences from experiment are only 0.3% for phase shift and 2.4% for impedance. Using the cold-test parameters calculated with MAFIA as input, the NASA Coupled-Cavity TWT Code is used to simulate the saturated RF output power of the TWT across the V-band frequency range. Taking into account the output window and coupler loss, the agreement with experiment is very good from 60-64 GHz, with the average absolute percentage difference between simulated and measured power only 3.8%. This demonstrates that the saturated RF output power of a coupled cavity TWT can be accurately simulated using cold-test parameters determined with a three dimensional electromagnetic simulation code. >

A simulated annealing algorithm for optimizing RF power efficiency in coupled-cavity traveling-wave tubes

Decreasing the radio frequency (RF) phase velocity in the output section of a traveling-wave tube (TWT) is a technique commonly used to increase RF power efficiency. In order to optimize the profile of the phase velocity, a simulated annealing algorithm has been developed and implemented into the NASA multidimensional large-signal coupled-cavity TWT computer model. This algorithm allows the determination of the lengths of the individual cavities at the end of the output section necessary to provide the optimized phase velocity profile. The resulting nonlinear computer-generated phase velocity profile provides a design with optimized RF efficiency. In this paper, the optimization algorithm is described and computational results are shown. These results indicate an increase in center-frequency RF efficiency from 7.1 to 13.5% for a V-band coupled-cavity TWT.

Advances in Space Traveling-Wave Tubes for NASA Missions

Significant advances in the performance and reliability of traveling-wave tubes (TWTs) utilized in amplifying space communication signals for NASA missions have been achieved over the last three decades through collaborative efforts between NASA and primarily L-3 Communications Electron Technologies, Inc. (L-3 ETI). This paper summarizes some of the key milestones during this period and includes development of TWTs for the Communications Technology Satellite, Cassini, and Lunar Reconnaissance Orbiter missions. Technical advances in computer modeling, design techniques, materials, and fabrication have enabled power efficiency to increase by almost 40% and the output power/mass figure-of-merit to increase by an order of magnitude during this period.

Multifocal flat lens with left-handed metamaterial

We show experimental results demonstrating multiple focal lengths at microwave frequencies in a flat lens constructed of left-handed metamaterial (LHM). In contrast to conventional lenses, which are constructed of materials with positive index and require a curved surface or inhomogeneous structures to focus light or other electromagnetic radiation, no curvature is needed with a LHM because of a negative effective index of refraction. Such a flat lens has the advantage of being capable of changing the focal length by simply changing the distance between the electromagnetic source and the lens.

Computationally generated velocity taper for efficiency enhancement in a coupled-cavity traveling-wave tube

A computational routine has been created to generate velocity tapers for efficiency enhancement in coupled-cavity traveling-wave tubes (TWTs). Programmed into the NASA multidimensional large-signal coupled-cavity TWT computer code, the routine generates the gradually decreasing cavity periods required to maintain a prescribed relationship between the circuit phase velocity and the electron-bunch velocity. Computational results for several computer-generated tapers are compared to those for an existing coupled-cavity TWT with a three-step taper. Guidelines are developed for prescribing the bunch-phase profile to produce a taper for high efficiency. The resulting taper provides a calculated RF efficiency 45% higher than the step taper at center frequency and at least 37% higher over the bandwidth. >

Wearable Wireless Telemetry System for Implantable Bio-MEMS Sensors

In this paper, a telemetry and contact-less powering system consisting of an implantable bio-MEMS sensor with a miniature printed square spiral chip antenna and an external wearable garment with printed loop antenna is investigated. The implantable chip antenna and the wearable garment pick-up antenna are in close proximity to each other and hence couple inductively through their near-fields and behave as the primary and the secondary circuits of a transformer, respectively. The numerical and experimental results are graphically presented, and include the design parameter values as a function of the geometry and the relative magnetic near-field intensity as a function of the angle, for the implantable chip antenna

Ultra-High Power and Efficiency Space Traveling-Wave Tube Amplifier Power Combiner With Reduced Size and Mass for NASA Missions

In the 2008 IEEE Microwave Theory and Techniques Society International Microwave Symposium Digest version of our paper, recent advances in high power and efficiency space traveling-wave tube amplifiers for NASAs space-to-Earth communications are presented. The RF power and efficiency of a new K-band amplifier are 40 W and 50% and that of a new K-band amplifier are 200 W and 60%. An important figure-of-merit, which is defined as the ratio of the RF power output to the mass (W/kg) of a traveling-wave tube (TWT), has improved by a factor of 10 over the previous generation Ka-band devices. In this paper, a high power high efficiency Ka -band combiner for multiple TWTs, based on a novel hybrid magic-T waveguide circuit design, is presented. The measured combiner efficiency is as high as 90%. In addition, at the design frequency of 32.05 GHz, error-free uncoded binary phase-shift keying/quadrature phase-shift keying (QPSK) data transmission at 8 Mb/s, which is typical for deep-space communications, is demonstrated. Furthermore, QPSK data transmission at 622 Mb/s is demonstrated with a low bit error rate of 2.4 times10-8, which exceeds the deep-space state-of-the-art data rate transmission capability by more than two orders of magnitude. A potential application of the TWT combiner is in deep-space communication systems for planetary exploration requiring transmitter power on the order of a kilowatt or higher.

High power and efficiency space traveling-wave tube amplifiers with reduced size and mass for NASA missions

Recent advances in high power and efficiency space traveling-wave tube amplifiers (TWTAs) for NASA’s space-to-Earth communications are presented in this paper. The RF power and efficiency of a new K-Band amplifier is 40 Watts and 50% and that of a new Ka-Band amplifier is 200 Watts and 60%. An important figure-of-merit, which is defined as the ratio of the RF power output to the mass (W/kg) of a TWT has improved by a factor of ten over the previous generation Ka-Band devices.

Traveling-wave tube cold-test circuit optimization using CST MICROWAVE STUDIO

The internal optimizer of CST MICROWAVE STUDIO (MWS) was used along with an application-specific Visual Basic for Applications (VBA) script to develop a method to optimize traveling-wave tube (TWT) cold-test circuit performance. The optimization procedure allows simultaneous optimization of circuit specifications including on-axis interaction impedance, bandwidth or geometric limitations. The application of MWS to TWT cold-test circuit optimization is described below.