Periodic Eigenmode Simulation of a Travelling Wave Tube
This example demonstrates an eigenmode calculation using periodic boundaries in z-direction. The phase shift of the periodic boundary is defined as a parameter which is swept from 5 degrees to 175 degrees with a step size of ten degrees. CST MWS's Eigenmode solver is ideal for this task.
Figure 1: Geometry reduced to a single helix turn
The frequency range is defined up to 10 GHz. The boundary conditions are set to "electric" except for the two boundary conditions in the z direction, which are defined as "periodic" in order to model the periodicity of the helix. A parameter "phase" is assigned to the periodic boundary, so that the phase shift can be used in a parameter sweep.
Figure 2: E, H fields and surface currents obtained from the periodic eigenmode solution
All Fields for periodic phase shift may be plotted as in figure 2 where the E and H Fields are shown.
Figure 3: Dispersion characteristics obtained from parameter sweep of the phase
Figure 3 shows the dispersion curves obtained from the parameter sweep via post-processing templates. The phase velocity is shown here as a function of frequency. The eigenmode solver delivers any aribitrary number of desired modes, 3 of which are shown in the plot.
Figure 4: Pierce Impedance extracted via template-based post-processing of the parameter sweep results
Figure 4 shows the Pierce Impedance obtained as a post-processing step. The powerflow in the tube is also an additional post-processing quantity that may be calculated.
CST Article "Periodic Eigenmode Simulation of a Travelling Wave Tube"
last modified 30. Apr 2013 9:56
printed 1. Sep 2015 2:05, Article ID 123
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.