CST

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.

References:

[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 31. May 2016 7:50, Article ID 473
URL:

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.

Feedback

4 of 4 people found this article useful

Did you find this article useful?

Other Articles

8th Order Dielectric Resonator Filter with Three Asymmetric Transmission Zeroes

8th Order Dielectric Resonator Filter with Three Asymmetric Transmission Zeroes Document type
The dielectric resonator fi lter (Figure 1) is a high-performance filter design which is well-suited for applications where compactness and power are important. These sorts of filters are widely used in communication systems – for example, in mobile phone base-stations. The number of independent design parameters that need to be optimized makes higher-order dielectric resonator filters challenging to tune. Simulation software can therefore be used to make it easier to design and tune these filters. Read full article..

Simulation of Accelerator Components

Simulation of Accelerator Components
Simulation software can help during the complicated design process in various phases. The webinar will describe different aspects of design in which CST STUDIO SUITE® can be of interest, beginning with simple eigenmode simulations and the choice of the correct solver. Accelerator specific post-processing features are addressed as well as multiphysics approaches. A short look into magnet simulation will be also given. Last but not least, the field of particle dynamics will be shown, including multipaction and impedance analysis. The webinar will include both an introductory overview as well as new features, making it suitable for both new and long-term users. Read full article..

Design of an Ultra-Wideband High-Power-Microwave Traveling-Wave Antenna

Design of an Ultra-Wideband High-Power-Microwave Traveling-Wave Antenna Document type
In this article we discuss the design and implementation of high-power-microwave (HPM) traveling-wave antenna. The antenna is designed to be driven by a high-power, single-shot signal generator with 1 ns pulse-width at the -3 dB power points, and peak voltage of up to 100 kV. Since the signal generator is equipped with an air-filled coaxial-waveguide output, a coaxial-waveguide to parallel-plates transition was also designed and fabricated. Initial theoretical electrical parameters and characteristics along with physical dimensions of the system were solved and derived using MATLAB[1]. Then, the components comprising the antenna were modeled, solved and optimized using CST STUDIO SUITE®[2]. Using the CAD export capabilities in CST STUDIO SUITE, fabrication models and schematics were produced from the simulation model. The antenna was fabricated and measured results agree with simulation results to a great extent. Read full article..

Electroquasistatic Simulation of a High Voltage Insulator

Electroquasistatic Simulation of a High Voltage Insulator
This article demonstrates the application of the CST EM STUDIO™ (CST EMS) electroquasistatic (EQS) solver to the simulation of a high voltage insulator. It also demonstrates the difference between the results obtained from the EQS Solver and Electro-Static Solver. Read full article..

3D EM Simulation of a resistance spot welding gun

3D EM Simulation of a resistance spot welding gun
This article is concerned with the 3D electromagnetic modelling and equivalent circuit parameter extraction of a Resistance Spot Welding (RSW) gun. The parameters for the gun are not as easily established for the gun as for other RSW system components. The current transformer and electronic converter electrical parameters are either known or can be obtained by standard test procedures. This is not the case for the gun where the electrical parameters are unknown and difficult to measure. A 3D EM simulation using CST EM STUDIO® can be used to establish these parameters and complete the system characterisation. Read full article..
Back Back  

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