Design Optimization of a Microfabricated Ladder TWT

Problem Description

The common helix shape traveling-wave tube (TWT) presents problems as frequency increases towards the TeraHertz (THz) range (necessary to avoid radiation damage) because the device becomes smaller making it difficult to manufacture, difficult to dissipate heat and fragile. The microfabricated ladder type TWT presents an ideal solution due to the fact that it is a symmetric device requiring fewer pieces and simpler assembly so it can easily be fabricated at low cost. However, important features of a THz device such as this, like nonlinear gain and bandwidth, are not well understood.

vsimmd ladder structure
Figure 1: VSim geometry showing the 3D model of the ladder structure in red and the dielectric in blue (left), and the longitudinal phase space plot of the electrons at (right). VSim enabled researchers to determine nonlinear power saturation (seen by the curling of the electrons on the far right). Click on figure to view full size image.


VSim was used by researchers at the University of Colorado at Colorado Springs (UCCS) to reliably predict the beam optics behavior of a microfabricated ladder type TWT for the first time. They extended the frequency range up to near 50 GHz where the conventional type TWT can no longer be manufactured. Using VSim, they found the optimal length of the ladder as well as the optimal power. The 3D VSim results will be used to design the new ladder type TWT at UCCS.


Why VSim?

The microfabricated ladder TWT is in intricate device requiring many different physics models including particles which most standard electromagnetic models don't include. VSim also has a Dey-Mittra cut-cell algorithm in order to model complex geometries in a non stair-stepped manner leading to smooth surfaces over which to solve Maxwell's equations. The complexity in the physics modeled and the geometry of the design required 3D simulations utilizing the distributed parallel computing capabilities of VSim to cut down simulation time to 4 days on 32 cores.


VSim has been invaluable to us in designing our ladder type TWT. No other code available to us could give the nonlinear beam dynamics of this kind of device. Without the distributed computing capabilities of VSim, these runs would have taken months, making them effectively impossible. Also, the Tech-X support staff was a great help with setting up our problem and getting us to a solution.
– Professor Heather Song, University of Colorado at Colorado Springs
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