# Multipacting Resonances in Waveguide (multipactingResonancesT.pre)

Keywords:

multipacting , multipactingResonancesT

## Problem description

Multipacting, which is the resonant build up of secondary electrons, is often a concern in microwave devices. Anytime there is an oscillating electromagnetic field across a gap between two surfaces there exists the possibility that for the right voltage across the gap a resonance condition will exist allowing the exponential build up of secondary electrons. A coaxial waveguide is such a structure where these conditions can exist.

This simulation can be performed with the VSimMD license.

## Opening the Simulation

The Multipacting Resonances example is accessed from within VSimComposer by the following actions:

• In the resulting Examples window expand the VSim for Microwave Devices option.
• Expand the Multipacting (text-based setup) option.
• Select “Multipacting Resonances in Waveguide (text-based setup)” and press the Choose button.
• In the resulting dialog, create a New Folder if desired, and press the Save button to create a copy of this example.

The basic variables of this problem should now be alterable via the text boxes in the left pane of the Setup Window, as shown in Fig. 420.

Fig. 420 Setup Window for the Multipacting Resonances example.

## Input File Features

The input file sets the number of cells along the propagation (x) direction to resolve the wavelength. The electrons are seeded in the middle of the waveguide once the wave has passed. A special electron species is used that allows scans over power to be done in a single simulation. The time step is chosen to be at 90% of the CFL (stability) limit.

## Running the simulation

After performing the above actions, continue as follows:

• Proceed to the Run Window by pressing the Run button in the left column of buttons.
• To run the file, click on the Run button in the upper left corner. of the window. You will see the output of the run in the right pane. The run has completed when you see the output, “Engine completed successfully.” This is shown in Fig. 421.

Fig. 421 The Run window at the end of execution.

## Visualizing the results

After performing the above actions, continue as follows:

• Proceed to the Visualize Window by pressing the Visualize button in the left column of buttons.

To view growth in the number of electrons:

• Select Phase Space from the Data View drop down
• Select scaleFactor for the X-axis
• Select weight forthe Y-axis
• Move the Dump slider to the far right then click on Draw.

Fig. 422 Visualization of the resonance bands.

## Further Experiments

Try seeing how changing the gap voltage or the frequency of the wave changes the multipacting resonances.