James A. Dayton

Accurate cold-test model of helical TWT slow-wave circuits

Recently, a method has been established to accurately calculate cold-test data for helical slow-wave structures using the three-dimensional (3-D) electromagnetic computer code, MAFIA. Cold-test parameters have been calculated for several helical traveling-wave tube (TWT) slow-wave circuits possessing various support rod configurations, and results are presented here showing excellent agreement with experiment. The helical models include tape thickness, dielectric support shapes and material properties consistent with the actual circuits. The cold-test data from this helical model can be used as input into large-signal helical TWT interaction codes making it possible, for the first time, to design a complete TWT via computer simulation.

Computational investigation of experimental interaction impedance obtained by perturbation for helical traveling-wave tube structures

Conventional methods used to measure the cold-test interaction impedance of helical slow-wave structures involve perturbing a helical circuit with a cylindrical dielectric rod placed on the central axis of the circuit. It has been shown that the difference in resonant frequency or axial phase shift between the perturbed and unperturbed circuits can be related to the interaction impedance. However, because of the complex configuration of the helical circuit, deriving this relationship involves several approximations. With the advent of accurate three-dimensional (3-D) helical circuit models, these standard approximations can be fully investigated. This paper addresses the most prominent approximations made in the analysis for measured interaction impedance by Lagerstrom (1957) and investigates their accuracy using the 3-D simulation code MAFIA. It is shown that a more accurate value of interaction impedance can be obtained by using 3-D computational methods rather than performing costly and time consuming experimental cold-test measurements.

Effect of helical slow-wave circuit variations on TWT cold-test characteristics

Recent advances in the state of the art of computer modeling offer the possibility for the first time to evaluate the effect that slow-wave structure parameter variations, such as manufacturing tolerances, have on the cold-test characteristics of helical traveling-wave tubes (TWTs). This will enable manufacturers to determine the cost effectiveness of controlling the dimensions of the component parts of the TWT, which is almost impossible to do experimentally without building a large number of tubes and controlling several parameters simultaneously. The computer code MAFIA is used in this analysis to determine the effect on dispersion and on-axis interaction impedance of several helical slow-wave circuit parameter variations, including thickness and relative dielectric constant of the support rods, tape width, and height of the metallized films deposited on the dielectric rods. Previous computer analyzes required so many approximations that accurate determinations of the effect of many relevant dimensions on tube performance were practically impossible.

Applying microfabrication to helical vacuum electron devices for THz applications

A new class of helical THz vacuum electron devices is under development using unconventional applications of microfabrication technology, modern computer modeling, and novel materials. The resulting slow wave circuits consist of a coil of gold wire, smaller in outside diameter than a human hair, supported by a thin diamond sheet and suspended within a diamond box. This configuration will extend the operating range of the helical slow wave circuit into the THz frequency band. Previously, the advantages of the wide bandwidth and high efficiency of the helical slow wave circuit have been available only for operation at frequencies below 50 or 60 GHz because of the difficulty of winding small coils of wire and because it is impossible to transmit a significant beam current through the small aperture offered by the center of the helix. These obstacles are overcome by fabricating the helices lithographically and by passing the electron beam around the outside of the helix. The design and fabrication of a 650 GHz backward wave oscillator (BWO) will be described as well as proposed applications of this technology to traveling wave tubes (TWTs) operating at frequencies as high as 1.0 THz. A THz amplifier, possibly with multioctave bandwidth, would have a wide range of important applications.

95 GHz helical TWT design

The helical slow-wave circuit is an attractive choice for traveling wave tube amplifiers (TWTAs) because of its inherently large bandwidth and relatively high RF efficiency. Unfortunately, as the operational frequency increases beyond Q-or V-band, its use has been limited by conventional fabrication techniques, and by the difficulty of passing enough current through the center of such a small structure. This paper describes the design and fabrication status of a 95 GHz TWT using microfabrication technology to create and assemble the helix. The electron beam propagates as two kidney shaped beamlets between the helix outer diameter and barrel.

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.

Microfabrication of diamond-based slow-wave circuits for mm-wave and THz vacuum electronic sources

Planar and helical slow-wave circuits for THz radiation sources have been made using novel microfabrication and assembly methods. A biplanar slow-wave circuit for a 650 GHz backward wave oscillator (BWO) was fabricated through the growth of diamond into high aspect ratio silicon molds and the selective metallization of the tops and sidewalls of 90 μm tall diamond features using lithographically created shadow masks. Helical slow-wave circuits for a 650 GHz BWO and a 95 GHz traveling wave tube were created through the patterning of trenches in thin film diamond, electroplating of gold half-helices, and high accuracy bonding of helix halves. The development of new techniques for the microfabrication of vacuum electronic components will help to facilitate compact and high-power sources for terahertz range radiation.

Fabrication and testing of the 0.650 THz helical BWO

Teraphysics Corporation has assembled and tested a 0.65 THz backward wave oscillator (BWO) with a helical slow wave circuit. The helix is a coil of gold wire, which is smaller in outside diameter than a human hair. It is supported by a thin diamond sheet and mounted within a micromilled copper block.

A 650 GHz helical BWO

The operational frequency of helical vacuum electron devices has always been limited by the ability of conventional technology to fabricate very small helices, and by the difficulty of passing a meaningful current through the center of such a small structure. Microfabrication technology now offers the possibility of creating helices smaller in outside diameter than a human hair. These helices are not fabricated by wrapping wire or tape around a mandrel, they are grown. We will use such small helices to create a new class of vacuum electron devices that will operate by passing an electron beam around the outside of the helix, rather than through the center. This technology not only offers the opportunity to make meaningful incursions into a previously unused portion of the electromagnetic spectrum, it offers the possibility to do so inexpensively and in large quantities. This paper will discuss the design rationale and fabrication plan to be employed for the first device of this new class, a 650 GHz helical backward wave oscillator (BWO). A companion paper will describe the computational simulation of the 650 GHz BWO.

Interaction simulations of two 650 GHz BWOs using MAFIA

Teraphysics Corp. is currently developing two novel, 650 GHz backward wave oscillators (BWOs). The computations of BWO output power have depended on the classic literature for a round electron beam in a helical structure [1, 2]. This may not be directly applicable to these novel devices. Therefore, we have computed the performance of biplanar interdigital, and helical BWOs using the 3D particle-in-cell (PIC) code, MAFIA. The results indicate MAFIA simulations are in fairly good agreement with the classic approximations.