Microfabricated Folded Waveguide for Broadband Traveling Wave Tube Application

As in most other technical areas, microfabrication is becoming more and more popular in the vacuum tube community. The reason is the need for miniaturization when going to higher frequencies. Circuits created by conventional fabrication techniques suffer from fragility. To circumvent this problem the structure suggested and analysed by R. Zheng and X. Chen [1] is dominated by metal and therefore much more robust. 

Structure of the folded waveguide.
Figure 1: Structure of the folded waveguide.

The slow wave structure is realized by a 50 period folded waveguide as shown in figure 1. The structure is fed at the RF input via waveguide ports known from CST MWS. Likewise the obtained output power is recorded at RF output with waveguide ports. The particles are traveling perpendicular to the waveguide as indicated by the arrow in figure 1.

Dispersion diagram of a single period.
Figure 2: Dispersion diagram of a single period.

A cold test simulation of a single period performed with CST MWS Eigenmode solver (see Slow Wave Article) gives the dispersion diagram shown in figure 2 (see also [1]).  The normalized phase velocity in this frequency band is about 0.255. Therefore the particles are emitted from the surface shown in figure 3 with a slightly higher beta of 0.2556 in order to transfer EM power from the electron beam to the RF-structure. The emitted beam current is 50mA.

Particle emission surface.
Figure 3: Particle emission surface.

The input signal is a monofrequent sinus with an input power of 2.5mW and a frequency of 230GHz. The port amplitudes are wave amplitudes and in units sqrt(Power). Therefore the input signal (red) illustrated in figure 4 shows an amplitude of 0.05. The output signal saturates at 480ps with an amplitude of 0.514 which results in a gain of 20.24dB. This agrees quite well to the gain of 20.9dB given by Pierce small signal theory (see page 282 in [2]).

The frequency spectrum of the output signal has a peak as well at 230GHz. The additional ripples are resulting due to a finite simulation time which is in time domain a multiplication with a rectangular pulse. In frequency domain this is equivalent to the convolution with an SI function which is seen in figure 4.

Time signals of RF in and out (left) and frequency spectrum of output signal (right).
Figure 4: Time signals of RF in and out (left) and frequency spectrum of output signal (right).

The particle trajectory is illustrated in figure 5. A zoom into the end section shows very nicely the sections with low and high velocity. This indicates the velocity modulation and the interaction of the beam with the electromagnetic wave which finally amplifies the RF input signal.

Particle trajectory and zoom into end section.
Figure 5: Particle trajectory and zoom into end section.

The small signal analysis has been carried out by R. Zheng and X. Chen [1] for the complete frequency band of interest and compared to Pierce small signal theory. The comparison shows a reasonable agreement with respect to the validity of Pierce theory (see figure 6) which could be violated by space charge effects and electron bunching.

Comparison of CST PS PIC analysis and Pierce small signal theory (courtesy of R. Zheng and X. Chen [1]).
Figure 6: Comparison of CST PS PIC analysis and Pierce small signal theory (courtesy of R. Zheng and X. Chen [1]).

The article shows the cold and hot test simulation of a slow wave structure by means of  CST MWS Eigenmode solver and CST PS PIC solver. The results are in good agreement with theoretical values. Compared to a CST MWS model, which often is already existent after cold test simulations, only slight modifications have to be made to include the particles. The output power is directly provided by waveguide ports known from CST MWS. The signals can conveniently be postprocessed into gain and frequency spectrum inside the CST template based postprocessing.


[1] R. Zheng and X. Chen, "Design and 3-D Simulation of Microfabricated Folded Waveguide for a 220GHz Broadband Travelling-Wave Tube Application", Proceedings of the IVEC 2009, Rome, Italy, April 28-30, pp. 135-136, 2009.

[2] A. S. Gilmour, Jr., "Principles of Travelling Wave Tubes", Artech House, Inc, Norwood, MA, USA, 1994.

CST Article "Microfabricated Folded Waveguide for Broadband Traveling Wave Tube Application"
last modified 26. Jun 2013 4:27
printed 26. Oct 2016 2:19, Article ID 473

All rights reserved.
Without prior written permission of CST, no part of this publication may be reproduced by any method, be stored or transferred into an electronic data processing system, neither mechanical or by any other method.


4 of 4 people found this article useful

Did you find this article useful?

Other Articles

Microwave Plasma Sources

Microwave Plasma Sources
Optimisation of Microwave Plasma Sources with the transient, frequency domain and eigenmode solvers of MAFIA. Read full article..

EM field distribution and SAR in a Human Head with MRI Coil

EM field distribution and SAR in a Human Head with MRI Coil
CST MICROWAVE STUDIO® (CST MWS) was used to aid in the computational investigation of the transverse B1-field homogeneity and SAR values in a 11.7 T / 500 MHz 4-port driven RF head coil loaded with a high-resolution human model (HUGO based on the Visible Human Project®). The simulations show the expected enhancement of the B-field in the centre of the head compared with the unloaded case and no significant changes in the maximum 1g SAR values between 2-port linear and circular polarizations. This work was carried out by CEA Saclay, France and is summarised in this article with the permssion and courtesy of Xavier Hanus and his colleagues. Read full article..

Multiphysics Simulation Medical Applications

Multiphysics Simulation Medical Applications
This webinar will introduce the basics of bio-EM simulations, such as the available body models, the choice of numerical solver and relevant post-processing quantities, as well as advanced workflows for multi-channel systems including EM/circuit co-simulation and some HPC aspects. Finally, the tight coupling of the EM solvers with the advanced bio-heat solvers including human thermo-regulation and spin response solvers for MRI imaging will be covered. All steps will be demonstrated with state-of-the-art examples from applications areas like ultra-high-field MRI, implant safety, microwave imaging, hyperthermia, pacemakers, etc. Read full article..

MIMO Antenna Systems for Advanced Communication

MIMO Antenna Systems for Advanced Communication
Multiple-input, multiple-output (MIMO) systems are a major field of study for researchers interested in achieving high data rate communication in typical urban multi-path environments. Although a fast analysis can be based on S-parameters, this approach has limitations. A more detailed analysis needs to take into account broadband, farfield and antenna properties. These are especially important in presence of the human body. This webinar will show how simulation can be used to calculate the effect of hand and head (e.g. CTIA models) on mobile devices, MIMO for wearable antennas and different power weighting functions for different environments, along with post-processing options for envelope correlation (including spatial power weighting functions), derived quantities diversity gain and multiplexing efficiency. Finally, there will be a demonstration of the link between CST MICROWAVE STUDIO® and Optenni Lab for multiple antenna matching to optimize power transfer to antennas while minimizing cross-coupling. Read full article..

Modeling and Measurement of Shielding Enclosures

Modeling and Measurement of Shielding Enclosures
The shielding effectiveness (SE) of an enclosure has been simulated by using the CST MICROWAVE STUDIO® (CST MWS) and then compared to measurement from a known reference case. Read full article..
Back Back  

Your session has expired. Redirecting you to the login page...