Multipacting Growth in Spherical PEC Cavity Using Prescribed Fields (multipactingGrowthPrescribedFields.sdf)



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. This example simulates multipacting growth in a spherical PEC cavity. The fundemental mode profile for the spherical PEC cavity is imported onto the VSim grid, then advanced in time by a single frequency time signal.

This simulation can be performed with the VSimMD or VSimPD license.

Opening the Simulation

The Multipacting Growth Prescribed Fields example is accessed from within VSimComposer by the following actions:

  • Select the NewFrom Example… menu item in the File menu.
  • In the resulting Examples window expand the VSim for Microwave Devices option.
  • Expand the Multipacting option.
  • Select “Multipacting Growth Using Prescribed Fields” 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.

All of the properties and values that create the simulation are now available in the Setup Window as shown in Fig. 417. You can expand the tree elements and navigate through the various properties, making any changes you desire. The right pane shows a 3D view of the geometry, if any, as well as the grid, if actively shown. To show or hide the grid, expand the Grid element and select or deselect the box next to Grid.

Setup Window

Fig. 417 Setup Window for the Multipacting Growth example.

Simulation Properties

This example contains a number of Constants to allow for easy manipulation of the device. Those include:

  • SPHERE_RADIUS: radius of spherical cavity
  • LENGTH_METAL: Length of metal box in each dimension (must be larger than 2*SPHERE_RADIUS)
  • RESOLUTION: The number of cells per wavelength in each dimension

SpaceTimeFunctions are used to create expressions defining the drive frequency and amplitude of the applied field.

The amplitude and frequency of this driving function is defined in Parameters:

  • MODE_FREQ: frequency at which the mode profile oscillates.
  • MODE_AMP: amplitude that is applied to the mode profile each time step.

CSG is used to create the spherical PEC cavity by subtracting sphere from a cube.

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. 418
Run Window

Fig. 418 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, as shown in Fig. 419, do the following:

  • Select History from the Data View pull down menu
  • Set Graphs 2-4 to “None”
  • Set Graph 1 to numElectrons

The overall trend in the number of electrons is an exponential growth with an ocsillatory signal that corresponds to the frequency of the time signal.

Visualize Window

Fig. 419 Visualization of the exponential growth of the electrons due to multipacting.

Further Experiments

Try changing the parameters MODE_AMP and MODE_FREQ to see if one can take the simulation in and out of resonance.