# 3D Stripline Multipacting (striplineMultipactingT.pre)

Keywords:

striplineMultipactingT

## Problem description

This VSimMD example launches a wave into a stripline geometry and demonstrates multipacting effects. The use of “field scaled” particles allows the scanning of multiple power levels in one run. Each particle has a scaling parameter that multiplies the electromagnetic field, allowing multiple power or voltage levels to exist simultaneously. This feature allows for quick finding of the cut-off resonance in the stripline.

This simulation can be performed with a VSimMD license.

## Opening the Simulation

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

• Select the New From Example… menu item in the File menu.
• In the resulting Examples window expand the VSim for Microwave Devices option.
• Expand the Multipacting (text-based setup) option.
• Select “3D Stripline Multipacting (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. 423.

Fig. 423 Setup Window for the Stripline Multipacting example.

## Input File Features

The Stripline Multipacting example has the following input parameters:

• TIMESTEP_FACTOR multiplies the numerical stability limit on the time step to compute the actual time step. TIMESTEP_FACTOR should be a real number greater than 0 and less than 1.
• START_VOLTAGE specifies the starting voltage for the field scale particles in Volts across the plates in the stripline. START_VOLTAGE should be a positive real number.
• END_VOLTAGE specifies the ending voltage for the field scale particles in Volts across the plates in the stripline. END_VOLTAGE should be a positive real number.
• NUMBER_OF_INC specifies the number of increments to use for the voltage between START_VOLTAGE and END_VOLTAGE.
• FREQ specifies the frequency in Hertz of the signal in the stripline. FREQ should be a positive real number.
• INCLUDE_PARTICLES = 1 loads electrons into the simulation and INCLUDE_PARTICLES = 0 does not load electrons into the simulation.
• LOADTIME specifies the time in seconds at which electrons are loaded into the simulation. LOADTIME should be a non-negative real number.

There are also several input parameters to specify the secondary electron yield based on the equation:

$$SEY_0*(1 - exp(-A*(x/U_0)^B))/C/((x/U_0)^D)$$

## 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. 424 below.

Fig. 424 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 the exponential increase in electrons as in Fig. 425:

• Select History under the Data View drop down
• Set Graph 2 to numRealElectrons
• Set Graphs 3 & 4 to None

Fig. 425 The numRealElectrons history shows exponential growth in the number of electrons.

To view the voltage at which multipacting occurs in the stripline, as seen in Fig. 426:

• Select Phase Space under the Data View drop down
• Set the X-axis to electrons_fieldScaleParameter
• Set the Y-axis to electrons_weight
• Set the Color to electrons_weight
• Press Draw.
• Move the slide to the right to see how the multipacting growth is occurring.

Fig. 426 The multipacting occurs at a scaling value of around 2.5. You can use this to multiply the value of START_VOLTAGE to find out the voltage at which multipacting occurs. In this case that is around 60.0 Volts

## Further Experiments

Try varying the starting and ending voltages as well as the number of increments to see if multipacting occurs in different ranges.